MEMO - Prism Marketing Group

SIKA CORPORATION • ROOFING 100 Dan Road ∙ Canton, MA 02021 ∙ USA Phone: 781-828-5400 ∙ Fax: 781-828-5365 ∙ usa.sarnafil.sika.com

MEMO TO Sika Sarnafil Outside and Inside Sales Staff; Manufacture Reps CC Sika Sarnafil Technical Staff FROM Roofing Technical Department PAGES 4 DATE January 28, 2014 Subject: Risks of Concrete Decks

Over the years, the construction industry has been aware of moisture issues due to freshly poured concrete as well as the ability of concrete to hold and absorb great amounts of water. Over time this water may migrate into the roof system, saturating the insulation and cover boards or causing adhered systems to become dis-bonded and potentially cause corrosion to metal components. Many papers and articles have been written discussing the issues of moisture and concrete. These papers identify some of the reasons and issues related to the moisture in concrete, and why they appear to be more prevalent than in the past, such as eliminating vapor retarders, especially ones that are adhered to the concrete deck and the practice of keeping the concrete forms in place, which are typically metal pans.

The most common ways excess water in concrete is generated includes;

• Mixing and pouring new concrete decks/slabs

• Interior finish work, including new concrete pours, water based construction materials including paint, plaster, and drywall application among others and heating the interior with propane or oil burners

• Concrete decks that are exposed to standing water which may come from various sources including exposure to long term leakage into existing roofs, rain or snow and other sources

CONCRETE AND WATER

Concrete is a combination of cement, aggregate (fine and coarse) and water, which typically has about 10–15% cement, 60–75 % aggregate and 15–20 % water. Studies have shown that there may be between 0.9 to 2.6 quarts (0.85 to 2.5 l)of excess water per square foot of concrete surface present in a one month old, 6 inch thick concrete roof deck. This does not include possible water from rain or snow or water from the curing process. This excess water may migrate into a roof system after the concrete has reached sufficient strength or cure which typically is 28 days. With this large amount

Sika Corporation • Roofing 2/4

of free water available it must be noted that cure time (28 days) does not mean the concrete is dry enough to cover.

In addition to normal weight structural concrete (NWSC), there is more lightweight structural concrete (LWSC) being used. The differences between the two structural concretes are the “in place density” and the type of aggregate used. LWSC has a density between 90 and 115 lb/ft3 (1440 to 1840 Kg/m3) and NWSC has a density range of 140 to150 lb/ft3 (2240 to 2400 Kg/m3). The LWSC can achieve the low “in place” densities by using a lightweight porous aggregate filled with air voids. This aggregate will absorb water so it must be saturated before mixing so as to not affect the cement to water ratio causing issues with the final concrete product. The LWSC aggregate can absorb 5 to 25% of its mass with water. To put this in perspective, the Portland Cement Association Engineering Bulletin 119 states the dry down time for LWSC is many months more than NWSC. To achieve a 75% relative humidity for NWSC it will take approximately three month. To achieve the same 75% relative humidity for LWSC it will take twice as long, almost six months according to testing noted in the PCA Engineering bulletin 119. The test was conducted with an 8 inch (20 cm) slab that had both the top and bottom sides exposed to air to dry. Consider, if a roof membrane is installed over the top surface and the bottom surface is a steel form deck (as is very common), the ability of the concrete to dry will be severely affected. The laboratory ideal conditions for the LWSC drying at six months will be much greater under field conditions.

CONSTRUCTIN GENERATED MOISTURE

Various construction activities such as newly poured concrete, water based construction materials including paint, plaster, and drywall application among others will generate and contribute to the accumulation of moisture within an enclosed building space. Additional moisture will be generated when propane or oil burning heaters are used to condition the interior of the building. This heating of the interior may be to help dry the new construction materials or make the interior space more comfortable. To put this moisture accumulation into perspective a 4 inch (10 cm) thick concrete floor slab will generate approximately 1 ton of water for every 1000 square feet of concrete. For every gallon of oil burned there will be 1 gallon of water produced and a 200 pound tank of propane will produce 30 gallons of water. All of this moisture produced and trapped in an enclosed space will affect the roofing system. Should these conditions exist, the project designer and/or the construction manager/general contractor must take steps to properly vent the moisture out of the enclosed space, or provide for a vapor retarder.

WATER ABSORBED INTO CONCRETE

Water, sitting on the concrete deck, as precipitation on new decks, or through long term leakage into existing systems being re-roofed, will typically be absorbed into the concrete deck. The top surface may appear dry, giving a false sense that a roof system can be installed. After the installation of the roof membrane, which will act as a vapor


retarder, the moisture within the concrete will migrate into the roof system. The rate of the water migration will depend on the local climate. Often the migration of the water out of the concrete will be greater than the moisture vapor passing through the roof membrane. The accumulation of water may affect moisture sensitive products such as adhesives, paper faced insulation boards and gypsum boards.

DETERMINING MOISTURE CONTENT

The main issue our industry has regarding water and moisture in concrete is there is not a good, practical, consistent and viable test to determine the moisture content or relative humidity of a concrete roof deck. The plastic film test (ASTM D 4263, Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method) is no longer considered a good valid test, especially with LWSC. This is also true for the calcium Chloride test (ASTM F 1869). Independent testing has shown these test methods often give misleading results.

The flooring industry, which has concerns with moisture in concrete, uses a moisture probe test, ASTM F 2170 Standard Test Method for Determining Humidity in Concrete Floor Slab using In-Situ Probes to determine if the moisture in the concrete slab has reached a level where the flooring material can be adhered. This test uses probes that are set into cores of the concrete slab and sealed for 72 hours. This test works relatively well for flooring due to the more consistent indoor temperatures and humidity. For concrete slabs that are exposed to the weather, such as roof decks, the temperature and humidity will vary, which will affect the readings from the probes. The conditioning section for ASTM 2170 states;

“9.1 Concrete floor slabs shall be at service temperature and the occupied air space above the floor slab shall be at service temperature and service relative humidity for at least 48 h before making relative humidity measurements in the concrete slab.”

Based on the conditioning statement, this test is not viable for concrete slabs exposed to the weather.

Furthermore, even if the amount of moisture could be measured easily and accurately in-situ, the industry has not determined or defined what acceptable moisture content in concrete decks is for the installation of a roofing system.

CONCLUSION

Moisture and concrete decks will continue to be an issue for the roofing industry, based on current practices of not including vapor barriers and leaving the metal pan/forms in place. In some sense we may see more issues as there are energy savings realized when the LWSC is used (reduced transportation costs, handling and weight) which may be used to accumulate some LEED points. As noted above, there is currently


no acceptable test method to determine the moisture content or relative humidity of a concrete deck that is exposed to the weather.

The 28 day “cure” time commonly referenced with structural concrete is for testing the design compressive strength of the concrete and has no correlation with the moisture content or drying time.

The Portland Cement Association has testing that shows it takes up to 3 months to reach a 75% relative humidity level with NWSC and twice as long with LWSC. Their test was done in a laboratory setting, constant temperature and humidity levels and all sides of the concrete exposed, and without any additional moisture, which often occurs in the field due to precipitation.

Although surface dryness can generally easily be determined the remaining free moisture that is within the concrete slab cannot readily be assessed. The decision of when a concrete deck may be roofed should include the project designer, general contractor, the concrete contractor and suppliers as they will have more knowledge of the concrete formulation, and moisture release rates. This design and management group should communicate with the roofing contractor when they can proceed. The designer of projects that include concrete decks, should strongly consider including in the roofing specification an adequate vapor retarder on the top side of the deck to prevent any water that may be retained in the concrete from migrating into the roofing system over time.

ATTACHMENTS

• SPRI Industry Info. Bulletin No. 2-13 • Moisture in concrete roof decks, M. Graham NRCA • What You Can’t See Can Hurt You, S. Condren SGH • Reducing The Risk Of Moisture Problems From Concrete Roof Decks, G. Doelp SGH

NO:

Industry Alert SPRI, RCI, and PIMA would like you to be aware that:

The roofing industry is increasingly experiencing roof system performance issues

when roof systems are installed over lightweight structural concrete roof decks.

The potential for high moisture content in this type of deck, coupled with the need for

extended drying times, can pose significant risk to long‐term performance and

possible premature roof failure.

This risk can be significantly increased by the standard practice of installing these

decks over non‐removable, form deck or other non‐permeable substrates.

These moisture issues are not unique to the roofing industry. The flooring industry

has experienced parallel moisture issues with lightweight structural concrete, and

those slabs are not subject to periodic rewetting from being exposed to weather, as

roof decks are.

Roofing stakeholders, including designers, property owners, roofing contractors, and

roofing manufacturers can be at significant risk when installing roofing systems over

lightweight structural concrete roof decks with elevated moisture levels.

Determining when a deck is ready for roofing

Test methods include (but are not limited to):

The spot application of hot bitumen;

Electrical impedance;

ASTM D4263 (Plastic Sheet);

ASTM F1869 (calcium chloride); and

ASTM F2170 (relative humidity probes).

Latent moisture However, latent moisture in the deck material may still be present:

Latent moisture may not be measured by the tests noted above and can affect the

long term performance of roofing systems placed over lightweight structural concrete

decks.

There is no industry agreement concerning methods to detect this latent moisture or

level of moisture that may be tolerable.

Loss of adhesion Experience has shown that high moisture content can lead to compromised adhesion:

Adhesive applied or self‐adhering products may show acceptable adhesion, but can be

comprimised due to high/elevated moisture content and upward vapor drive.

Exposed to high/elevated levels of moisture, insulation facers can deliminate from the

substrate or the insulation core and membranes that appear to be initially adhered

can lose adhesion due to moisture migration.

Date: 07/31/2013

No: 2-13

INDUSTRY INFORMATION BULLETIN

To: Roofing stakeholders, including designers, property owners,

roofing contractors, and roofing manufacturers

Topic: Moisture Concerns in Roofing Systems Applied Over Lightweight

Structural Concrete Roof Decks

Industry Info. Bulletin No. 2-13 Date: 7/31/13

Loss of R Value Upward vapor drive that results in entrapped moisture in insulation can:

Result in significant loss of insulation value; and Possibly increase a buildings energy use.

Mold growth potential

Mold growth can occur:

High/elevated moisture levels can create conditions consistent with mold growth

within the roof system.

Water-based adhesive curing issues

Elevated moisture in these roof decks:

Could compromise the cure time of adhesive; or

Cause rewetting of water–based (low‐VOC) adhesives.

Corrosion of roof fasteners and other ferrous-containing roof components

Mechanical fasteners used to attach roof insulation and membranes to lightweight structural concrete roof decks.

There is the potential for the occurrence of fastener and steel plate corrosion due to the presence of elevated moisture levels.

FM Global FM Global has not specifically addressed the moisture in lightweight structural concrete issues:

It is important to note the lightweight structural concrete does not meet FM’s definition of “structural concrete”.

In the June 2012 version of the FM 4470 standard, FM’s defines structural concrete as having a “density of approximately 150 lbs/ft3”.

Lightweight structural concrete has a density of 90 – 120 lbs/ft3.

Conclusion Because of these performance issues and the potential risk for roof system failure, SPRI, RCI, and PIMA urge building designers to select roofing components and system with great care. Our organizations are continuing to study possible roofing solutions which mitigate the risks associated with the use of lightweight structural concrete. We hope to provide further guidance for proper roof design in the future.

\' ~ r :-·

Moisture in concrete roof decks

Concrete's curing and drying rates can affect roof systems

by Mark S. Graham

lATELY, NRCA has experienced an increase

in reports of moisture-related problems

with low-slope membrane roof systems

applied to newly poured, normal-weight

or lightweight structural concrete roof

decks.

In the reported instances, significant

amounts of water have been found within

roof systems within several months to up

to three years after construction. In most

of the situations reported, it was deter

mined the roof membrane was watertight

and not the source of moisture infiltration.

Nevertheless, NRCA has some recommen

dations for avoiding such problems.

Concrete decks

When mixed, poured and formed, normal

weight and lightweight structural concrete

contain significant amounts of water. As

concrete cures and hardens, it consumes

large amounts of this water through hy

dration and evaporation. For example,

a 4-inch-thick concrete slab will release

about 1 quart of water for each square

foot of surface area.

Historically, the roofing industry has

used a minimum 28-day period as a guide

line for applying roofing materials over

newly poured concrete roof decks. The

28-day period coincides with the curing

time of concrete before it is tested for de

sign compression strength. There is little

correlation between this 28-day period

and concrete's true "dryness."

Professional Roofing February 20 l 0

In some instances, a plastic sheet test has

been used to determine concrete's dryness.

With this test, a plastic sheet (4-mil-thick

polyethylene) is taped to the concrete surface

and the plastic sheet's underside is moni

tored for the presence of condensation.

Up to the publication of The NRCA

Roofing and Waterproofing Manual, Fourth

Edition in 1996, NRCA recommended

the plastic sheet test as a method for deter

mining a concrete surface's dryness.

However, with the publication of The

NRCA Roofing and Waterproofing Manual,

Fifth Edition in 2001 and continuing with

the publication ofThe NRCA Roofing

Manual this year, NRCA no longer con

siders the plastic sheet test a viable assess

ment of concrete's dryness.

Similar to the roofing industry, the con

crete industry has seen significant advances

in technology regarding concrete mix de

sign, placement and curing.

For example, the use of concrete additives

in concrete mix designs and curing com

pounds during concrete placement greatly

can accelerate or retard concrete's curing and

release of free moisture. Similarly, weather

conditions, covering newly poured concrete,

timing of concrete form removal, and tem

porary heating or ventilating of a build

ing's interior after concrete placement can

affect the rate of concrete's upward or

downward release of free moisture.

For these reasons, NRCA no longer sup

ports the 28-day drying period or plastic

sheet test.

Tech Today

NRCA's recommendations

NRCA considers the decision of when it

is appropriate to cover a newly poured

concrete substrate to be beyond roofing

contractors' control. Because of the nu

merous variables associated with concrete

mix design, placement, curing and drying, roofing contractors are not privy

to and may not be knowledgeable of

the information necessary to make

such a decision.

Also, though a roofing contractor can

assess the dryness of concrete's topmost

surface, he or she cannot readily assess

any remaining free moisture within

the concrete and its likely direction

of release.

NRCA recommends the decision of

when a newly poured concrete substrate is

ready to be covered with a new roof sys

tem be made with the project or roof sys

tem designer, roof system manufacturer

and roofing contractor. It also would be

useful for designers to consult structural

engineers, general contractors, concrete

suppliers and concrete placement contrac

tors who likely have more knowledge of

concrete's curing and moisture-release

rates.

Additional information regarding con

crete roof decks is contained in The NRCA

Roofing Manual: Membrane Roof Systems-

2007. ~·1ft.

MarkS. Graham is NRCA's associate

executive director of technical services.

23

, •. ;' :~··.:cpy~,§lfl!f-' 'lf' .• . JJIII::r - .

1~!- 1:'!".~l':'l"~ l,!;~·WJ L!r·~.f-'WJ ~ .. . . ~·-~~ ll!L·:::::i: ·lll.. \'(1 •. ..

. ,,~ · :...· .. ,~·· ··:~··

~:w ·~,·~ r:: ~ Total cementitious material

650 710 710 779 pounds per cubic yard (lb/ cy)

Woter~cxementitious material ratio 0.39 0.43 0.43 0.50

Added botch water (lb/cyl 242 308 308 384

1 percent fine aggregate 11 13 13 n/o

absorption (lb woter/cy)

Coarse aggregate 10 73 145 n/o absorption (lb woter/cy)

Total water in botch (lb/cyl 263 394 466 384

Water consumed in 163 178 178 195

hydration reactions (lb/cy)

Water remaining after hydration 100 216 288 189

(lb/cy)

Water remaining in 6-inch con-0.9 1.9 2.6 1.7

crete deck (quart per square foot)

Figure 1 : Water added to and remaining in example concrete mixes

Ll' . -W ~: ~ ~·~· ·-:.!'•· !I!' ·;.II 1~·- . =····· . ::..U.· ~· ~r:Ll.llD II"'. .. [· .:Ir· ::.il I :: llLil) \1' ·:~

.. m. . . . I u;; " 11'· ... ·::JI ~;.;.·:~;:'lr" ·...:...· ~ ~~·

... ,.. Cold-moist climate 0.7 2.2 0.5 International Falls, Minn. (80 percent) (87 percent) (3 3 percent)

Worm-humid climate 0.7 2.2 0 .6

Miami (80 percent) ( 1 87 percent) (35 percent)

Worm-dry climate 0.6 2.1 0.3

Phoenix (70 percent) (83 percent) ( 15 percent)

Figure 2 : Water remaining in concrete roof decks aker one month of exposure

Times have cha nged

Although many modern roof membranes and insulations

are moisrure-resisranr, modern insulation board ofren is

faced with moisture-sensitive paper facers and is frequenrly

adhered ro substrates wirh moisture-sensitive adhesives.

The change ro maisrure-sensirive adhesives is a result of

recent vola rile organic compound (VOC) regulations

rhar caused manufacturers ro convert ro either I 00 per

cent solids, low-VOC or warer-based formulations.

Moisture migrating into roof sysrems from concrete

decks also carries alkaline salrs and high pH from rhe

concrete. When the moisture, salrs and high pH react

with moisture-sensitive materials, rhe paper facers may

grow mold, decay and lose cohesive strength; the water

based adhesives may revert to a liquid; and rhe curing of

foam-based adhesives can be altered or delayed. Gypsum

30 www.professionolroofing.net AUGUST 2012

and wood-fiber based cover boards may lose cohesive

strength. Mold rends to develop when rhe relative humid

ity is above 80 percenr and temperatures are above 41 F. Inrernal condensation can occur daily or seasonally as

roofing materials are exposed ro varying temperatures.

Condensed water can degrade adhesives and facers on

insulation boards, as wel l as gypsum-based and other

water-soluble materials. High moisture levels in cold

regions can lead to freeze-thaw damage of some roofing

materials. Concrete mixes also have changed. In general, concrete

mixes made wirh Ay ash or slag are more durable and may

contain admixtures (chemicals added to the concrete to

cause a specific performance change) rhat reduce the water

content. These improvements in concrete technology result in concrete mixes being less permeable, which slows

rhe drying of moisture from rhe concrete. Increased use

of lightweight structural concrete, with irs pre-wetted

aggregates, contributes additional moisture ro concrete that further extends a concrete deck's drying period.

p

In addition, roof deck design has changed. Cast-inlace concrete roof deck design may now include com

osil:e.CQ.O.St.tl.iCUOILwiclw:hc:.s.teeLfoans..(mer.aldeckUefr.. n place. The meral deck prevenrs the drying of moisture

rom rhe deck's underside. Although some metal decks

contain perforations along the ribs (Aures) on rhe corru

n

i

fJ

g

c a

ated forms, rhe perforation area is only 0.25 (o 1.5 per

ent of rhe meral deck surface, allowing only a minimal

mounr of moisture evaporation through the forms.

And because currenr construction schedules usually

require a roof sysrem be installed within three ro four

weeks of placing rhe concrete roof deck, once a modern

roof system is installed, a majority of rhe concrete mois

ture will remain entrapped within the completed roof

system. A metal deck form inhibits moisture loss from rhe

concrete deck's underside, and the roof membrane inhib

its moisture from leaving through the top of rhe roof

system. Modern roof systems are susceptible to internal

condensation, and rhe wer materials will be ar risk of

moisture degradation, mold and decay.

Concrete moisture content

Concrete is a mixture of portland cemenr, aggregates, air

voids, warer and other additives. A portion of the mix warer reacts wirh rhe cement and any poz.zolanic additives

present to form rhe hydra red binder of rhe hardened

ce

y

s

concrete. Additional water provides the fluidity for place

ment and finishing of freshly mixed concrete. Although

chemical admixtures may reduce the amount of addi

tional water required, all freshly placed concrete will

contain surplus water that is not consumed in hydration

reactions.

The mix water added to the concrete mixture is ex

pressed as the water-to-cementitious material ratio (w/cm).

Most concrete mixes are hatched at 0.40 to 0.55 w/cm.

The cement hydration process, most of which occurs

with in the fi rst month, consumes about 0.25 w/cm.

T his leaves an additional 0 . I 5 to 0.30 w/cm, which will

remain in the concrete as liquid water.

In addition, the aggregates absorb water- 0 .1 to 2

percent for normal weight aggregates and between 10

and 30 percent for lightweight aggregates. T his water

also will be contained within the concrete after it has

been placed .

To demonstrate how much water remains, we calcu

lated the tree water in concrete after hydration for typical

roof deck concretes (see Figure 1) .

Our calculations show 0.9 to 2.6 quarts of excess water

per sq uare foot of concrete surface will be present in a

typical one-month-old 6-inch concrete roof deck with

out accounting for any additional water from the curing

process or rain. This is the estimated water that will be

available to migrate into a roof system installed after the

concrete has developed design strength.

Concrete drying

The freshly placed concrete's interior voids (pores, capil

laries and aggregate porosity) initially are saturated with

water. When d rying begins, concrete loses water by evap

oration from the deck's surface. The concrete surface may

appear dry, but it may be an illusion. As the surface loses

water, d iffusion begins, drawing water and alkaline salts

from the wet interior to the surface. W hen roofing mate

rials are installed, the water evaporating from the con

crete's top surface becomes sealed within the roof system.

Our company used WUFI, a one-dimensional, validat

ed heat and moisture transport software application, to

estimate the drying of concrete decks cast over vented

metal decks during the one month before roof system

installation and during the two-week exposure as required

by the cellular concrete manufacturers. We selected three

climate zones across the U.S. as benchmarks for our cal

culations and assumed there was no added water from a

curing process or rain during the initial drying period. To

estimate the best conditions fo r drying, we based the d ry

ing simulatio n on local climate data beginning July I .

We found the amount of water evaporating from

exposed concrete roof decks during the month before

installing a roof system, with the exception of cellular

concrete, is only a small portion of the available excess

water remaining in the concrete roof decks. The water

that remains in the deck after this initial period of evapo

ration is shown in Figure 2.

Our results show there is no location in the' U.S. chat

will dry all the excess water contained in che concrete

within 30 days after placement.

100 r 95 90 85

] 80 75 t i 70

r ~ 65 ... 60 J:! 55 ~ 1 1

50 45

I 0 40 1l 35 ~

30 £ 25 20 15 10

C.a...lcucOI'IC.r.,. lntefno~ fols, M.nn. ,

I ! Cc-!tvklr con<re.,., Miomi

Notmol weight Con<tdt Mioft'li

tigf'ltwefght (Oft(t~hr MOotroi

I~<J .. rwhQhl co•~• ~ !rkofMhonnl ~,., .. ~ M· 11

I --------------------~0-3~0 60 90 120 150 180 210 2...,.40~2~7~0-3~0~0-330 360 Number of days

Note: Cellular concrete must be roofed over between three and 14 days (value in figure reported at 14 days)

Figure 3 : Water loss by evaporation from vented concrete decks exposed for a year (assuming a July 1

summer costing dote and no rain during the entire year)

Figure 3 shows how much d rying can occur if the

evaporation time is extended to one year. The graphs are

based on the assumption that a deck can be left exposed

for one year without precipitation, an unlikely condition

without providing temporary protection. Normal weight

and lightweight concrete decks will contain water if al

lowed to dry for one year in all regions.

In the real world, water from curing and precipitation

must be taken into account, which means the free water

content within the concrete can be expected ro remain

significantly higher for prolonged time periods.

To evaluate the effect free water in a concrete deck

has on roofing materials, we need to consider the vapor

AUGUST 20 1 2 PROFESSIONAL ROOFING 31

ON tin· WEB

To read more about

moisture in concrete roof

decks, log on to www

. professianalroofing. net.

Phoenix (hot-dry)

International Foils, Minn. (cold-moist)

Miami (hot-humid)

drive of rhe moisture into rhe roofing materials from the

concrete. We use relative humidity to evaluate the vapor

drive. When concrete begins drying, the relative humid

ity within the concrete is 100 percent. As drying contin

ues, the relative humidity decreases.

After roofing materials are installed, the moisture in

the concrete raises the relative humidity of the air trap

ped within the roof system above the concrete deck and

begins to cause problems. Typical organic roofing com

ponents are susceptible to mold and ocher decay organ

isms when the relative humidity is greater than 80 per

cent for prolonged rime periods. Relative humidity also

is important for determining when condensation may

occur within a roof system and when condensed water

may begin deteriorating moisture-sensitive roofing

components.

To demonstrate the moisture drive from concrete to

roofing materials, we calculated the relative humidity

withi n one-month-old concrete at the nme the roofing

materials are installed. We found the relative humidity

within concrete was above 95 percent in all locations (see

Figure 4), indicating the roofing materials will be exposed

to deleterious amounts of m-oisture.

94 98 98

97 99 99

97 99 99

Figure 4: Relative humidity of one-month-old concrete roof deck

Because most roof decks receive a roof system within

one month of placement (when concrete is still wet), an

alternative approach is needed to address the entrapped water.

Testing for moisture

Roofing contractors have limited ability to evaluate rhe

amount of water present in roof deck concrete and relate

that moisture ro potential effects on the proposed roof

system's durability.

There are several hand-held surface meters that purport

to measure concrete's moisture content. These meters

supply numeric values, and some indicate the values

ro be percentages of moisture. This data is best used as

arbitrary numbers to compare readings with a concrete

32 www.professionolroofing.net AUGUST 2012

surface known robe dry. The problem is char though the

meters may d isplay an acceptable value (suggesting a con

crete surface might be dry), the concrete's interior is likely

to be wet, and this moisture will migrate to the surface

after rhe deck is covered with roofing materials. Moisture

meter readings at the surface can provide a false impres

sion concrete is dry.

More generally, there are no adequate rests that can

determine acceptable moisture content of roof deck con

cretes for installing specific roof systems. ASTM Inter

national provides standard test methods for determining

moisture-related properties of concrete under controlled

interior conditions, such as interior concrete floors. These

methods include ASTM Standard F 1869, "Standard Test

Method for Measuring Moisture Vapor Emission Rare of

Concrete Subfloor Using Anhydrous Calcium Chloride,"

and ASTM Standard F2170, "Standard Test Method for

Determining Relative Humidity in Concrete Floor Slabs

Using in situ Probes." T hese methods are difficult to use

on roof decks because of solar radiation, temperature

fluctuations and precipitation.

Another standard, ASTM 04263, "Standard Test

NfeiliOc!TorTI1dicaring Moisture in Concrete by Plastic

Sheer Method ," determines the presence of capillary

moisture buc will not provide moisture levels. The results

of rhe ASTM International rests are extremely .difficult

to interpret for evaluating the amount of moisture in

roof deck concrete rhar will migrate into a roof system

as the roof system undergoes substantial temperature

fluctuations.

Currently, NRCA and rhe Midwest Roofing Contrac

tors Association (MRCA) state a concrete deck is likely to

contain levels o f water that could be detrimental to a roof

system. Methods for resting moisture are discussed in

their publications; however, they acknowledge the roof

ing industry has no acceptable moisture values for the

safe application of roofing. MRCA recommends a roof

system should include a complete vapor retarder seal.

NRCA cautions the decision of when it is appropriate to

cover newly placed concrete is beyond roofing contrac

tors' control.

We believe the only reliable method to predict the

potential for excessive moisture within a roof system is

ro perform hygrorhermal modeling simulations for the

specific environment and roof system being evaluated.

Several computer modeling progranlS are available for

conducting these simulations.

e

1 ·

ly

se

St

,f

:s

Hygrothermal rnodeli11g We conducted hygrothermal modeling of modern roof

systems in three climate zones using WUFI. The pro

gram model~ building envelope behavior on an hourly

basis (transient) according to interior and exterior cli

mate conditions. We modeled our roof systems for two

to five years. To develop an effective model, we consid

ered the local climate because changes in temperature

play a major role in moisture migrating out of the con

crete and into roofing materials.

Our studies show concrete deck moisture diffuses into

roofing materials under all condirions of exterior temp

erature and humidity in all the geographical locations

studied. In all cases, moisture diffused out of the concrete

faster than it could diffuse out of the roof system when a

vapor retarder is not included. This trend causes moisture

to accumulate within the roof system and could result in

the deterioration of moisture-sensitive materials in the

roof system.

The moisture diffusion in the modeled systems per

formed differently because the rate at which moisture dif

fuses out of concrete is based on the temperature profile

across the deck system, and the rate at which moisture

diffuses from the roof system is based on roof membrane

permeance. Other factors include membrane color (which

affects solar loads and radiant cooling), type, thickness and

the length of the condensation cycles a roof system experi

ences. Regardless, all the roof systems modeled became wet.

The next step of our hygrothermal modeling included

studying the same roof systems but including a vapor

------ retar er:'.()ur goal was tO mm1m1ze the potennaJ of any

components within the roof system from reaching pro

longed relative humidity levels greater than 80 percent.

Keeping the relative humidity levels below 80 percent

would minimize the potential for mold and condensation

within the roof system. Our modeling considered the

concrete deck's initial moisture content, the roof mem

brane's moisture permeance, and the temperature profile

between exterior and interior surfaces the system will

experience during its service life.

We varied the materials and components in the roof

system until we found systems that will perform in the

specific climates. We reduced the moisture emission out

of concrete by adjusting the vapor retarder's permeance

of the vapor retarder at the concrete surface. We found

a vapor retarder reduces rhe moisture collection within

the roof system to acceptable levels but only if the vapor

retarder's permeance is less than the roof membrane's

Water condensing on the underside of the membrane dripped bock onto the concrete deck.

This roof did not leak.

Roofing material was applied to a concrete deck without a vapor retarder in the mid·

Atlantic region. The membrane lost bond at the wet insulation facer. This roof did notleok.

permeance. Vapor retarder selection must be coordinated

with the type of roof membrane, and the system must be

evaluated for the influence of the local climate.

We did not find systems that will perform equally in

all climates. In other words, a system that maintains low

moisture within the roof system in a hot-humid climate

may not work in a hot-dry climate or cold-moist climate.

AUGUST 2012 PROFESSIONAL ROOFING 33

We found each geographical location required some fine

tuning of.rhe roof system ro make it work.

StL iy conclusions

Typical concrete roof decks installed on steel forms will

conrain I ro 2\12 quarts of free water per square foot of

deck area after rhe decks have cured . Our moisture analy

sis demonstrates much of rhis warer will remain in the

concrete deck following roof system installation.

Because of construction schedules, roof systems are

being installed onro wet concrete even though rhe con

crete's surface may appear dry. Over time, the internal

warer in the concrete will migrate to the roof system .

Without a vapor retarder, a roof membrane will perform

as a vapor retarder and trap moisture within the roof

system. The enrrapped moisture will degrade moisture

sensitive materials. T he rate and amounr of water migrat

ing inro the roof system will depend on the local climate.

Effective roof systems incorporating a vapor retarder can

be designed for specific installations and climatic condi

tions to minimize the derrimenral influence of concrete

------moisture-on roofingmaterial . Dcvcloping"fhese d-esigns

Apparent mold growth on insulation facers in a roof located in the Northeast. The roofing material was applied to a concrete deck without a vapor retarder and became loose. This roof did not leak.

' ' ,,

....

. ...... ' ' ..

"" '- .... ' V•

' .....

. -- { <

' .

Deterioration in a gypsum·based cover boord in a roof system in the M idwest. This roof did not leak.

34 www.prafessianalraofing.net AUGUST 2012

requires systematic evaluation of all componenrs in the

roof and involves hygrorhermal modeling calculations.

T he numbers presenred are based on the id~al condi

tions of placing, finishing and curing the concrete without

adding water during construction or rewerring by rain .

In reality, cement will nor be fully hydrated and a deck

likely will be wet-cured or subjecred to some precipita

tion before application of roofing materials. These condi

tions will increase the amount of water entrapped in the

deck by the rime roofing materials are applied .

F ecomrnendol ions

More research is needed ro determine the maximum

amounr of moisture roofing materials can safely toler

are. In the meantime, roof systems should be designed

and installed with vapor retarders to mitigate moisture

entrapped in concrete roof decks. "•"~~-

STEPHEN J. CONDREN, P.E., is a senior project manager

at national engineering fi rm Simpson Gumpertz & Heger Inc.,

Boston; JOSEPH P. PINON, P.E., is a senior project man·

ager at Simpson Gumpertz & Heger, San Francisco; PAUL C. SCHEINER, Ph.D., is staff consultant at Simpson Gumpertz & Heger, Boston.

REDUCING THE RISK OF MOISTURE PROBLEMS FROM CONCRETE

ROOF DECKS

by

Gregory R. Doelp, P.E. and Philip S. Moser, P.E.

ABSTRACT

In recent years, the roofing industry has become increasingly aware of the problems caused by

moisture in concrete roof decks that migrates into the roofing system. Installing a vapor retarder

over the concrete deck is the primary method of addressing this problem. This paper summarizes

some of the challenges associated with incorporating a vapor retarder into the roofing system.

For example, selecting a vapor retarder of the appropriate vapor resistance is challenging due to

the shortage of published data on the acceptable moisture limits of roofing materials. We explore

the question of acceptable moisture limits through an extensive review of published literature,

product-specific recommendations from manufacturers, and some preliminary laboratory testing

of some common roof cover boards. This paper is based on the authors’ experience as designers

and investigators of roofing systems, literature review, and laboratory testing.

1. BACKGROUND

1.1 Consequences of moisture

While the primary function of a roofing system is to prevent water from passing through it into

the building or structure below, water or moisture vapor that collects within the roofing system

can also be detrimental, both to the roofing system’s immediate performance and its long-term

durability. Besides leakage to the interior, moisture in roofing systems can have numerous

negative consequences including the following:

Reduced thermal resistance of insulation.

Loss of strength of the insulation, cover board (Photos 1 and 2), adhesive, or fasteners

(Photo 3); leaving the roofing system vulnerable to uplift damage from wind or

crushing from foot traffic or hail.

Photo 1- moisture damaged gypsum cover board.

Photo 2 – moisture damaged fiberboard cover board.

Photo 3 – corrosion of roofing fastener

Deterioration of the structural deck.

Dimensional changes in the substrate, which can in turn damage the roof membrane.

Blistering or weakening of the roof membrane itself, especially BUR.

Mold growth.

1.2 Sources of Moisture

Moisture in the roofing system can come from a variety of sources, such as:

Installation of wet materials (i.e., insulation that was not properly protected from

weather while stored on site).

Water leakage through the roof membrane, flashing, or adjacent construction.

Moisture vapor from interior humidity –

Moisture vapor from interior humidity may migrate into the roofing system by

diffusion if no vapor retarder is installed. The need for a vapor retarder to limit

the migration of interior moisture into the roofing system is generally

acknowledged to depend on the local climate and the interior conditions of the

building. Later sections of this paper include additional information about

determining when a vapor retarder is needed.

Moisture vapor from interior humidity may be carried into the roofing system

by air leakage if there is no air barrier in the roofing system, particularly if the

building is positively-pressurized due to stack effect or the operation of the

HVAC system. This is generally addressed by including an air barrier in the

roofing system and connecting it to the air barrier in the wall system to provide

a continuous barrier.

Concrete deck – When a new concrete deck is poured, some of the mix water is used up

in chemical reactions as the concrete cures and some evaporates, but the rate of

evaporation is slow, so large quantities of water remain stored within the pore structure

of the concrete for extended periods of time. While the concrete itself is generally not

damaged by this moisture, the moisture may migrate into the roofing system where it is

absorbed by materials that are more sensitive to moisture. Historically, roofing systems

were adhered to concrete decks in hot asphalt; the hot asphalt (often with reinforcing

felts) provided the additional benefit of limiting the rate of moisture migration from the

concrete into the roofing system. However, modern single-ply roofing systems are

often installed today without any vapor retarder on the concrete deck.

Comparison can be made to the flooring industry, which has also suffered detrimental

effects from moisture diffusing out of concrete, and as a result has developed consensus

test methods for measuring the internal relative humidity (RH) of concrete or moisture

evaporating out of concrete. Flooring manufacturers typically specify acceptable RH or

moisture vapor emission limits for the concrete as a condition of their warranty. By

contrast, the roofing industry, while it has begun to focus more attention on this issue1,

had not (as of 2011) “established any benchmarks or acceptance levels” for moisture in

concrete2.

A 2012 research update3 proposes that “Until we have more data, 75% relative humidity

appears appropriate for normal weight concrete”, and recommends monitoring the RH

of concrete according to ASTM F21704 to determine when it is “dry” enough to roof

over. However, one limitation of F2170 testing is that the standard requires

conditioning both the concrete slab and the air above it to a constant “service

temperature” and relative humidity for at least 48 hrs before making measurements, but

constant temperature and RH do not exist for an in-service roof – the conditions vary

constantly with the weather. Furthermore, the effect that concrete moisture has on the

roofing system will depend on the specifics of the roofing system and the local climate,

so it may be difficult to establish an industry-wide “acceptable” level of moisture for all

concrete roof decks.

Another recent study5 found that concrete retains significant amounts of water after

months of drying, and therefore, high moisture levels are likely to still be present when

the roofing system is installed. In most roofing installations, it is impractical to wait for

the concrete deck to “dry” fully; it is often faster and more reliable to install a vapor

retarder over the concrete deck to inhibit the migration of moisture from the concrete

into the roofing system. Specifying the vapor retarder presents several challenges, as

discussed in the next section.

2. VAPOR RETARDER CHALLENGES

2.1 Wind Uplift Rating

In many roofing systems installed over concrete decks, the roof insulation is adhered to the

concrete (often with ribbons of low-rise foam adhesive). Incorporating a vapor retarder into the

roofing system means adding another layer that needs to be adhered to the concrete, and to which

the insulation needs to be adhered. In this situation, the vapor retarder can affect the wind uplift

resistance, so it is crucial that the vapor retarder be part of the tested assembly. Most roofing

system manufacturers now offer tested assemblies that include adhered vapor retarders, but the

relative number of options for this system is more limited than those without a vapor retarder.

1 Graham, Mark S. Moisture in Concrete Roof Decks, Professional Roofing, February 2010

2 Structural Lightweight Concrete Roof Decks, MRCA T&R Advisory Bulletin 1/2011. September 2011.

3 Dupuis, Rene. Research Continues on Moisture in SLC Roof Decks, Midwest Roofer, June 2012

4 ASTM F2170-11, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs

Using in situ Probes. ASTM International, 2011. 5 Condren, S; Piñon, J; and Scheiner, P. What you Can’t See Can Hurt You – Moisture In Concrete Roof

Decks Can Result in Premature Roof System Failure. Professional Roofing, August 2012.

Recent searches of FM Global’s RoofNav online database6 found that there are 2.5 to 3 times

fewer tested systems with a vapor retarder as the number of systems without a vapor retarder.

These searches included both adhered and mechanically-attached insulation systems; in the

authors’ experience, roofing system designs with adhered insulation and a vapor retarder have

even fewer options.

If the vapor retarder is going to be adhered, moisture in the concrete deck can affect the adhesion,

so the question of acceptable moisture content in the concrete deck still needs to be addressed.

Similar to adhering a plaza waterproofing membrane or deck coating to concrete, it is advisable to

contact the manufacturer for recommendations and use a mockup as the final criteria for

evaluating whether good adhesion can be achieved.

Fortunately, designers have other options for securing roof insulation besides adhesive. Roof

insulation (or membranes) can be mechanically attached through the vapor retarder into the

concrete deck, which avoids the difficulty in finding a tested system that relies on adhesion of

(and to) the vapor retarder. Alternatively, roofing systems in some regions can be ballasted;

however, building codes prohibit stone ballast in some high-wind regions.

2.2 Product Selection

Another challenge is determining the appropriate vapor permeance for the vapor retarder. Some

designers rely on past experience and rules of thumb or the minimum requirements of the

building code, while others use moisture vapor transmission calculations to predict the in-service

moisture contents of any moisture-sensitive materials in the roofing system. Rules of thumb and

calculations are discussed in more detail below.

2.2.1 Rules of Thumb

One rule of thumb7 that is sometimes used is that every layer in the system should be 10 times

more permeable than the vapor retarder to avoid creating a vapor trap. Table 1 lists the typical

range of permeance of some common roof membranes and vapor retarders. Because of the low

6 www.roofnav.com searched September 2012 with the following search terms: single-ply roof system,

new roof application, adhered cover, structural concrete deck, board stock insulation, and wind uplift

ratings ranging from 60 to 300. 7 Like any rule of thumb, this is a generalization, and there are some conditions where it may not be

suitable.

http://www.roofnav.com/

permeance of most roof membranes, using a vapor retarder often violates the rule of thumb for

avoiding a vapor trap. This means many roofing systems have very limited ability to self-dry any

water that leaks through the membrane. Even a small membrane defect, which may not produce a

large enough volume of leakage to appear on the inside of the building, can cause water to

accumulate in the roofing system over time and cause concealed damage. Desjarlais explains the

disadvantages of compact roofs in several of his publications8

9

10 and recommends avoiding

vapor retarders where they can be shown to be unnecessary. However, in many cases including

new construction with concrete decks, vapor retarders have been shown to be necessary11

.

Table 1 – Permeance of Selected Roof Membranes and Vapor Retarders

Material Thickness

(mils = 0.001

inch)

Permeance (US perms)

- ASTM E96

Procedure B12

Sources13

Roof Membranes

BUR Not reported 0.0 ASHRAE14

EPDM 60 mils 0.03 – 0.04 2 manufacturers

PVC 60 mils 0.05 - 0.22 4 manufacturers

TPO 60 mils 0.01 – 0.05 3 manufacturers

Vapor Retarders – Loose Laid

Polyethylene or

“polyolefin” sheet –

various grades

6 mils 0.059 - 0.13 2 manufacturers

7-8 mils 0.038 1 manufacturer

10 mils 0.019 – 0.039 3 manufacturers

15 mils 0.009 – 0.021 2 manufacturers

Polyolefin – aluminum

composite

14-15 mils total 0.000 1 manufacturer

8 Kyle, D; Desjarlais, A. Assessment of Technologies for Constructing Self Drying Low Slope Roofs,

CON 380, Oak Ridge National Lab, p. 82, May 1994. 9 Desjarlais, A; Byars, N. A New Look at Moisture Control in Low Slope Roofing, 4

th International

Syposium on Roofing Technology, pp. 341-346, NRCA, September 1997. 10

Desjarlais, A; Karagiozis, A. Review of existing criteria and proposed calculations for determining the

need for vapor retarders. North American Conference on Roofing Technology, pp. 79-83, NRCA,

September 1999. 11

Condren et al, 2012. 12

The conditions at which permeance is measured can significantly affect the result, but most

manufacturers only report data for standard conditions. Some manufacturers do not report the test

procedure, but most do. Those that do report a procedure report Procedure B. 13

Manufacturer’s data is from a combination of published data sheets and telephone conversations with

technical representatives, September 2012. 14

ASHRAE Fundamentals, American Society of Heating, Refrigerating and Air Conditioning Engineers,

2009.

Material Thickness

(mils = 0.001

inch)

Permeance (US perms)

- ASTM E96

Procedure B12

Sources13

Vapor Retarders – Self Adhered

Polyolefin -

bituminous composite

32 mils total

.017 2 manufacturers

2.2.2 Moisture Vapor Transmission Calculations

Calculations provide a more sophisticated analysis than simple rules of thumb, but have their own

challenges.

Moisture moves between components of the roofing system by diffusion over time. The moisture

content of a particular layer may vary with daily or seasonal weather variations, and there may be

a net wetting or drying over the long term (multiple years). The construction industry has been

developing and publishing methods for evaluating moisture vapor flow for many years. Hand-

calculation methods developed specifically for the roofing industry include two published in

198015

and 198916

; other criteria exist in ASHRAE and NRCA publications. In the past two

decades, exponential increases in available computing power have made state-of-the-art computer

programs for predicting moisture vapor flow (e.g., MOIST and WUFI) readily available. These

programs have made moisture vapor transport easier to quantify and are more accurate than

previous hand calculations. These state-of-the-art computer programs are now in widespread use

by building envelope consultants.

WUFI17

, a computer program by the Fraunhofer Institute for Building Physics (Germany),

calculates transient one-dimensional heat and moisture transport, and can be used to

quantitatively predict how moisture levels within a building envelope assembly vary over time.

WUFI uses historical, hourly weather data for user-selected project locations, to simulate time-

15

Condren, S.J., Vapor Retarders in Roofing Systems: When Are They Necessary?, Moisture Migration in

Buildings, ASTM STP 779, M . Lieff and H.R. Trenschell, Eds., American Society for Testing and

Materials, 1982, pp. 5-27. 16

Tobiasson, W. Vapor Retarders for Membrane Roofing Systems. 9th

Conference on Roofing

Technology, NRCA, pp. 31-37, May 1989. 17

A free educational version is available online at http://www.ornl.gov/sci/btc/apps/moisture/ ; the

professional version is for purchase.

http://www.ornl.gov/sci/btc/apps/moisture/

varying exterior conditions (temperature, relative humidity (RH), solar exposure, etc.) during the

course of the simulation. An example output screenshot is shown in Figure 1.

Figure 1 – Example WUFI simulation screenshot for a roofing system, showing the results for a typical year in

International Falls, MN. The physical layers of the roofing system from top (exterior) to bottom (interior) are

listed from left to right. The red shading indicates the range of service temperatures, the green indicates the

range of relative humidity, and the blue indicates the water content.

WUFI can also simulate several years of moisture migration to analyze seasonal variations and

year-to-year cumulative wetting or drying trends. The output data can be easily reviewed to

determine, for each layer in the roofing system, moisture-related data such as (1) maximum

annual moisture content, (2) quantities of condensate (if any), (3) number of occurrences of

moisture content exceeding established thresholds, and (4) number of freeze-thaw cycles.

A recent study18

used WUFI to analyze a variety of roofing systems in a variety of U.S. climates

and found that in new construction, all roofing systems constructed over cast-in-place concrete

decks accumulated problematic levels of moisture within the roofing system unless a vapor

retarder was included. The study further found that selection of an effective vapor retarder

depends on the roof membrane; on typical buildings, the vapor retarder should have a lower

permeance than the roof membrane. The selection of an effective vapor retarder also depends on

18

Condren et al, 2012.

the local climate; the study “did not find systems that perform equally in all climates… each

geographical location required some fine-tuning of the roof system to make it work”.

One limitation of WUFI analysis is that it focuses on vapor diffusion and generally does not

account for bulk air or water leakage; therefore, the accuracy of the results depends on the roof

membrane and air barrier to be functioning properly. Also, interpretation of WUFI results

requires knowledge or assumptions regarding the acceptable moisture limits of the materials

being considered; the study recommends more research “to determine the maximum amount of

moisture that roofing materials can safely tolerate.” Quantitative knowledge of moisture content

and acceptable moisture limits would also be valuable when one is asked to evaluate existing

roofs to determine whether problematic levels of moisture are present.

3. ACCEPTABLE MOISTURE CONTENT OF ROOFING MATERIALS

Several industry sources have recognized the need for more research to determine acceptable

moisture limits for roofing materials. The recommendation in the concrete moisture study

discussed in the previous section echoes an earlier recommendation by Kyle and Desjarlais19

that

“researchers must establish a set of moisture limits with a reasonable safety factor by means of

well-controlled experiments.” This need was discussed even earlier by Tobiasson20

, who stated:

“For most roofs in most locations, the objective is to limit seasonal wetting to an acceptable level.

This level will vary with the moisture-sensitivity of the materials present… However, developing

moisture limit states for each of the myriad roofing systems on the market is a sizable task that

has not yet been accomplished.” Kirby21

similarly concluded that “the roofing industry does not

have a consensus evaluation method for determining whether insulation is wet.”

This section discusses information on acceptable moisture limits for roofing materials based on

three sources: (1) limits proposed in industry publications, (2) manufacturers’ recommendations,

and (3) recent laboratory testing conducted in support of this paper.

19

Kyle and Desjarlais, 1994. 20

Tobiasson, W. Condensation Control in Low-Slope Roofs, Proceedings of Workshop on Moisture

Control in Buildings, Building Thermal Envelope Coordinating Council (BTECC), Washington, D.C.,

September 1984. Also available as CRREL Misc. paper 2039. 21

Kirby, J. Determining When Insulation is Wet, Professional Roofing, February 1999.

3.1 Industry Publications

A wide variety of information and opinions on acceptable moisture content are available,

illustrating the lack of consensus. We reviewed technical literature spanning the past 35 yrs and

found several conflicting theories regarding acceptable moisture limits in roofing materials. The

theories for acceptable moisture limits discussed below are generally listed in order of least

stringent to most stringent, although there is ambiguity in some of the criteria.

Thermal Resistance Ratio (TRR) 80% maximum: The thermal resistivity (R value) of

insulation is reduced when it becomes wet. In 1991, researchers at the U.S. Army

Corps of Engineers Cold Regions Research and Engineering Laboratory (CRREL)22

,

proposed the following criterion: “The ratio of a material’s wet thermal resistivity to its

dry thermal resistivity is termed its TRR... Insulation with a TRR of 80 percent or less

is, by our definition, ‘wet’ and unacceptable.” The paper acknowledges that “For some

insulations, less moisture than that required to reduce the TRR below 80 percent can be

detrimental for other reasons (e.g., delamination, rot and corrosion of fasteners). It is

not yet known what those moisture “limit states” should be… As additional information

on other moisture “limit states” becomes available, it is expected that maximum

acceptable moisture contents for some materials will decrease below the 80 percent

TRR values.” The paper further acknowledges that EMC (discussed below) is a more

appropriate pass-fail criterion for new materials to be installed; TRR is proposed

primarily for deciding when to replace existing insulation. Table 2 summarizes some

the EMC [equilibrium moisture content] and TRR data for some other insulation

products still in use today.

Seasonal Wetting of 1-2% by volume23

. For foam insulation with 2 pcf density, this

equates to 31-62% moisture content when calculated as a % of dry weight.

No Visible Liquid Water: Kirby24

states, “one factor can never be ignored – if liquid

moisture is present in existing insulation, the insulation is too wet to be left in place or

re-covered.”

Only Small Amounts of Condensation - Tobiasson25

stated that “A small amount of

moisture may condense then without doing any real harm.” He also stated that “A little

condensation on the coldest day of the winter will do no harm, but when condensation

occurs for many days, weeks, or months, the amount of moisture deposited can create

major problems.”26

Condren took a similar approach but quantified his acceptable

22

Tobiasson, W.; Greatorex, A; VanPelt, D. New Wetting Curves for Common Roof Insulations,

International Symposium on Roofing Technology, pp. 383-390, 1991. 23

Tobiasson, 1984, cites the following source: Hoher, K. “Environmental and Climatic Factors in the

Specification of Roofing Membranes” 1982, Sarnafil, Canton, MA. 24

Kirby, 1999. 25

Tobiasson, 1989 26

Tobiasson, 1984.

“small amount” of condensation, proposing a limit of no condensation deeper than 1/16

in. or 1/32 in. below the membrane, depending on the substrate material27

. Desjarlais

and Karagiozis28

base their analysis on the criteria that the insulation should not have a

relative humidity of 100% (i.e., condensation occurring) for more than 24 hrs, which is

consistent with one of the three criteria in ASHRAE 160 (discussed below).

No Condensation – Desjarlais and Byars29

contend that “moisture accumulation in a

roof system must not be large enough to cause condensation within the roof, since this

can damage the insulation and reduce its effectiveness.” They also state that “moisture

accumulation should not be great enough to cause degradation of the insulation material

or membrane. To pass this requirement, there must be no condensation under the roof

membrane.”

Equilibrium Moisture Content (EMC) 40%, 45%, or 90% maximum – The concept of

EMC as a metric for determining acceptable moisture levels in roofing materials was

first proposed in a 1977 paper30

regarding roofing felts, but later expanded to include

insulation and other materials. The 1977 paper proposed 40% EMC as the limit for

roofing felts that were sufficiently dry to avoid the appearance of blisters when hot

asphalt is applied to the felts in construction of a built-up roofing membrane. In

addition to the blistering concerns, the 1977 paper found that conditioning organic and

coated organic roofing felts in a moist environment reduced their tensile strength

(compared to oven-dried samples) by 6-18% when conditioned at 40% RH for 6 weeks,

and by 11-38% when conditioned at 90% RH for 9 weeks. The effects of moisture on

roofing felts were also studied by several other researchers around the same time31

. The

1977 paper also examined the EMC of insulation materials, and notes that “The test

results do not allow definite conclusions as to the “tolerable” moisture level of the

insulations. However, we expect any “excess” moisture in the insulation to be available

to influence the roofing membrane… in view of our field experience and pending

further research results, it seems prudent to assume that insulation moisture will not

damage the system, if the moisture does not exceed the equilibrium moisture content

attained by 40% RH storage.”

A 1985 article32

provides equilibrium moisture content (EMC) for various roofing

materials conditioned at 20° C (68° F) and both 45% and 90% RH. The article notes

that “When the material contains more water than its EMC, it is wet and may donate

water to surrounding air or materials”. The EMCs for unfaced polyisocyanurate

27

Condren, 1982 28

Desjarlais and Karagiozis, 1999 29

Desjarlais and Byars, 1997. 30

Schwartz, Thomas A., and Cash, Carl G. Equilibrium Moisture Content of Roofing and Roof Insulation

Materials, and the Effect of Moisture on the Tensile Strength of Roofing Felts, Symposium on Roofing

Technology, National Bureau of Standards and National Roofing Contractors Association n 28 p 238-243,

September 1977. 31

Busching, H; Mathey, R; Rossiter, W; Cullen, W. Effects of moisture in BUR: A state of the art

literature survey. Tech Note 965, National Bureau of Standards, July 1978. 32

Cash, Carl G. Moisture and Built-Up Roofing, published in “A decade of change and future trends in

roofing - proceedings of the 1985 International Symposium on Roofing Technology”, September 1985.

insulation were updated in 200333

. EMCs for selected products that are still in use

today are summarized in Table 2, along with TRR data.

Table 2 – EMC and TRR of selected roofing materials

Material EMC, Mass% @ 20° C34

Moisture content,

Mass % @ 80%

TRR35

45% RH 90% RH

Faced Polyisocyanurate 1.1 2.9

Unfaced Polyisocyanurate 1.7 5 262

Expanded polystyrene (1pcf) 1.9 2 383

Extruded polystyrene 0.5 0.8 185

Perlite board 1.7 5 17

Wood fiberboard 5.4 15 15

Gypsum 0.4 0.6 8

D226 Asphalt-Organic Felt 4.1 - 4.3 7.9 - 8.2

D2178 Glass Felt 0.5 - 0.9 0.6 - 1.1

A material that contains less than the 45% EMC is considered dry; between the 45%

EMC and 90% EMC is considered moist; and over the 90% EMC is considered wet.

Avoid Three Conditions Favorable to Mold Growth - There is broad consensus that

mold growth should be avoided in buildings due to potential health impacts and because

mold growth implies at least some level of decay. International Energy Agency (IEA)36

states that susceptible surfaces are in danger of developing mold growth if the relative

humidity at the surface rises above 80% RH for a sustained period of several days.

Temperature also plays a role; below 32°F, fungal cells may survive but rarely grow,

and above 104°F, most cells stop growing and soon die. ASHRAE 16037

is generally

consistent with IEA but provides more specific recommendations. ASHRAE

recommends avoiding the following humidity conditions in order to minimize problems

associated with mold growth on surfaces of components of building envelope

assemblies; these criteria apply when the running average surface temperature (for the

duration of interest) is between 41°F and 104°F:

30-day running average surface RH >/= 80%

7-day running average surface RH >/= 98%

24-hr running average surface RH >/= 100%

33

Cash, Carl G. Roofing Failures, Spon 2003, p. 46. 34

Extracted from Cash, 1985 and Cash, 2003. 35

Extracted from Tobiasson et al, 1991. 36

International Energy Agency (IEA) Condensation and Energy Sourceboook, Report Annex XIV, Volume

1, 1991 37

ASHRAE Standard 160, Criteria for Moisture-Control Design Analysis in Buildings, 2009.

In summary, a wide range of theories have been proposed regarding acceptable moisture limits in

roofing materials. Most of past physical testing on the effects of moisture on roofing materials

has focused on two issues: (1) loss of thermal resistance of insulation, and (2) weakening, decay,

and dimensional instability of built-up roofing. In our experience, loss of strength of the

insulation (or its facers) and cover board are also significant concerns, because they affect the

ability of the roofing system to resist common loads such as foot traffic, hail, or wind uplift.

Moisture-induced degradation of water-based membrane bonding adhesive has also been

reported38.

However, we are not aware of any industry publications containing data on how

moisture affects the strength of insulation, cover boards, or bonding adhesive, with the exception

of some limited data on gypsum cover boards discussed in the next section.

3.2 Product-Specific Manufacturer Recommendations

We contacted manufacturers of isocyanurate insulation and gypsum cover boards to ask for

recommendations on acceptable in-service moisture limits for their products. This section

summarizes the information provided by manufacturers.

3.2.1 Isocyanurate Insulation

We contacted four manufacturers of isocyanurate roof insulation and inquired about acceptable

in-service moisture limits. One manufacturer was unresponsive, and two said they do not have

any data or recommendations. The fourth manufacturer cited 3% moisture content as a rule of

thumb, but did not provide any supporting data39

.

3.2.2 Gypsum Cover Boards

A 2001 article40

provides data on the water absorption of a gypsum cover board product after

24 hrs conditioning at 95% RH, 2 hrs exposure to surface moisture, 2 hrs submersion, and 24 hrs

submersion. The article also provides peel adhesion data for hot asphalt application to boards at

ambient conditions and after 7 days at 95% RH. The article does not provide any

recommendations for acceptable moisture limits; its main focus is on avoiding heat damage to the

gypsum during installation of roofing membranes set in hot asphalt or torch application.

38

MRCA T&R Advisory Bulletin, Noteworthy Limitations of Water Based Bonding Adhesives, 2/2011. 39

Telephone conversations with manufacturers, September 2012. 40

Murphy, C.; Mills, R. Dens-Deck Roof Board. Interface, March 2001

In addition, we contacted two manufacturers of gypsum cover boards and inquired about

acceptable in-service moisture limits. One manufacturer stated that it manufactures its gypsum

board to less than 2%, and recommends that its product does not become “wet”, but could not

define what moisture content it considers to be “wet” or provide any recommendations for in-

service moisture limits 41

. The other manufacturer stated that its product often has less than 1%

free water as delivered from the factory, and cited a variety of thresholds of concern for moisture

content in service including 2%, 4%, and 5%, but without clear recommendations for an

acceptable level for long-term performance42

. Also, the manufacturer did not provide any

supporting data on the strength of their material at those moisture levels.

In summary, very little information is available from manufacturers of roofing materials

regarding their products’ resistance to moisture degradation.

3.3 Laboratory Testing

To collect some initial data on the moisture resistance of roofing materials, we selected three

insulation cover board products for testing. Two of the products are gypsum-based, the third is

high-density isocyanurate, and all are nominally 1/2 in. thick. Insulation and bonding adhesive

are also of interest, but are excluded from this initial testing.

3.3.1 Description of Testing Program

We exposed the samples to a variety of moisture conditions, and tested their flexural strength.

Flexural strength is relevant to the wind uplift resistance of intermittently-attached cover boards.

Tensile strength and compressive strength perpendicular to the plane of the board are also of

interest; these properties are excluded from this initial testing, but should be considered in future

testing.

Three samples of each of the three products were exposed to each of the following ten different

moisture conditions (90 samples total):

Oven dried at 100° F (below the 110 to 115° F maximum recommended by one of the

gypsum manufacturers).

41

Telephone conversation with manufacturer, September 2012. 42

Private correspondence from manufacturer, 2010 through 2012.

Standard laboratory conditions (50% RH and 74°F).

High humidity (95% RH and 74°F).

High humidity (95% RH and 74°F) followed by oven drying.

Water immersion for 30 minutes.

Water immersion for 30 minutes followed by oven drying.

Water immersion for 1 hr.

Water immersion for 1 hr followed by oven drying.

Water immersion for 24 hrs.

Water immersion for 24 hrs followed by oven drying.

Following conditioning, we tested the flexural strength of the samples according to a modified

ASTM C47343

method B. The test procedure generally followed C473 but the sampling

procedures and the definition of breaking load were modified to suit the goals and scope of our

test program. Our test method is briefly summarized as follows, and is depicted in Photos 4 and

5:

Samples were cut to 12 in. x 16 in., with the 16 in. dimension parallel to the long

dimension of the board. Some samples included the factory edge of the board. All

samples were tested face up.

The flexure fixture was set up with supports at a 14 in. span and a center loading nose.

Load was applied at a constant crosshead rate of 1 in./min. until the load-resistance of

the sample decreased; the maximum load was reported.

43

ASTM C473-10, Standard Test Methods for Physical Testing of Gypsum Panel Products. ASTM

International, 2010.

Photo 4 – Flexural test setup

Photo 5 – Flexural test in progress.

The isocyanurate board has anomalies in the foam structure, known as “knit lines”, where the

ribbons of foam came together during the manufacturing process. The knit lines are parallel to

the long dimension of the board. By testing only samples spanning parallel to the knit lines, we

avoided testing the weaker orientation.

3.3.2 Results

The results of our testing are summarized as follows. We refer to the two gypsum products as

“GYP #1” and GYP #2”, and the isocyanurate product as ISO. Our test results are not suitable for

design strength values; our testing was intended only to explore the trends of strength-loss caused

by exposure to moisture.

Moisture conditioning had the following effects on the flexural strength:

50% RH - None of the three products was significantly weakened by conditioning to

standard laboratory conditions (50% RH) compared to oven-dry conditions.

95% RH - The ISO was unaffected at 95% RH, but the two gypsum products lost some

strength. GYP #1 was reduced to 70% of its standard-laboratory strength, and GYP #2

was reduced to 80%.

Water Immersion - The three products showed differing rates of water absorption when

immersed. After 24 hrs, GYP #1 had gained 4% weight, GYP #2 had gained 24%

weight, and ISO had gained 11% weight. (Figure 2) All three products rapidly lost

strength when immersed in water (Figure 3); most of the strength loss occurred in the

first hour of immersion. In the first hour, the two gypsum products were reduced to

approximately 60% of their standard-laboratory strength, and ISO was reduced to

approximately 70% of its standard-laboratory strength. The rate of loss slowed

significantly after 1 hr of immersion; at 24 hrs, the two gypsum products retained

approximately 50% of their standard-laboratory strength, and ISO remained around

70%.

Figure 2 – Moisture content (% of original mass) vs duration of water immersion (hrs) for three cover board

products. Each data point indicates an average of three or more samples.

Figure 3 - Flexural strength (lb) vs duration of water immersion (hrs) for three cover board products. Each data

point indicates an average of three or more samples.

0.000

5.000

10.000

15.000

20.000

25.000

0 5 10 15 20 25

mo

istu

re, %

of

ori

gin

al m

ass

Soaking Time (hrs)

Moisture Content vs Duration of Water Immersion

GYP 1

GYP 2

ISO

0.000

20.000

40.000

60.000

80.000

100.000

120.000

140.000

160.000

0 5 10 15 20 25

avg

load

(lb

f)

Soaking Time (hrs)

Flexural Strength vs Duration of Water Immersion

GYP 1

GYP 2

ISO

Moisture Content - When we plot strength versus moisture content rather than wetting

time (Figure 4), we see that the most dramatic strength loss occurs in the range of 0 to

3% moisture content by weight. We also see more of a difference between the three

products. At 6% moisture content, GYP #1 is reduced to approximately 45% of its

standard-laboratory strength, GYP #2 is reduced to approximately 63%, and ISO to

73%.

Figure 4 – Percent of original flexural strength retained vs moisture content (% of original mass) for three cover

board products. Each data point indicates one sample.

Oven Drying after Wetting - All three products regained all of their original strength (to

within the level of accuracy of the test method) when oven dried after one wetting

cycle.

3.3.3 Discussion of Test Results

Our testing showed that some common cover board products lose strength quickly and at

relatively low moisture contents (less than 5% by mass) when wetted. This result is consistent

with our field observations on existing roofs with wet cover boards, and is also consistent with

statements from one gypsum board manufacturer that moisture content over 2-5% is a concern.

The isocyanurate product was less affected by moisture than the two gypsum products.

0%

20%

40%

60%

80%

100%

120%

0 3 6 9 12 15 18 21 24 27

% o

f st

ren

gth

ret

ain

ed

moisture content (% of original mass)

% Strength Retained vs Moisture Content

GYP 1

GYP 2

ISO

Our testing further showed that some common cover board products exposed to one relatively-

short wetting cycle will regain virtually all of their original strength after oven-drying. While this

result supports the concept that a small amount of short-term condensation may be acceptable in

some circumstances, it does not provide sufficient data to define those acceptable circumstances.

On many roofs, the wetting is longer term and the drying is less complete than in our tests. In

addition to longer-duration wetting, other factors that could result in unacceptable strength loss in

cover boards include: (1) a greater number of wetting and drying cycles, (2) freeze-thaw cycles,

which are common because the coldest weather coincides with the peak moisture levels directly

under the roofing membrane, and (3) loading from wind or foot traffic during one of the wetting

cycles when the materials are temporarily weakened may result in permanent strength loss or

even immediate failure. There is no guarantee that loading will occur only when the materials are

dry and at their maximum strength.

4. CONCLUSIONS AND RECOMMENDATIONS

We conclude the following regarding roofing system design for moisture in concrete decks:

Most roofs on new concrete decks need vapor retarders; this poses some challenges

with wind uplift rating and product selection, but these challenges can be addressed

through careful design. As additional roofing systems that include an adhered vapor

retarder are developed and tested for wind uplift, designers will gain more flexibility.

State-of-the-art analysis tools are available to predict time-varying moisture levels due

to vapor migration through the roofing system and assist selecting an appropriate vapor

retarder. Interpreting the results of these analyses requires knowledge of the acceptable

in-service moisture limits of roofing materials.

We conclude the following regarding acceptable in-service moisture limits:

Past publications have proposed various different acceptance criteria for moisture

content of roofing materials. An industry consensus does not exist, and product-

specific data and recommendations from manufacturers are lacking.

Our test data shows that some common cover board products lose strength quickly and

at relatively low moisture contents (less than 5% by mass) when wetted. However, our

data is insufficient to establish acceptable moisture levels.

We recommend additional research into the acceptable in-service moisture limits of

roofing materials. In the interim until additional data become available, roofing

professionals will have to continue to rely on their experience and judgment.

Acknowledgments

The authors would like to thank the management and shareholders of Simpson Gumpertz &

Heger, Inc. for their financial support of the laboratory testing described in this paper.

The authors would also like to acknowledge the significant contribution made by Stephen

Condren, Joseph Piñon, and Paul Scheiner, in their August 2012 work referenced in this paper.

Their work has lent much-needed clarity to the issue of moisture in concrete roof decks, and

provided a clear direction for future research needs.

MEMO - Prism Marketing Group

Documents

Transcript of MEMO - Prism Marketing Group