Analysis of the Hygrothermal Behaviour of Residential High-Rise Building Components (Canada Mortgage...

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Technical Series 96-217

Analysis of the Hygrothermal Behaviour of Residential High-Rise BuildingComponents

Introduction

Moisture is the leading cause of damage in high-rise residential buildings. The use of advanced computer simulationtechniques to examine the hygrothermal performance (combined heat, air and moisture transfer) in high-rise buildingshas the potential to lead to improved building design. The main objective of this joint IRC-CMHC research projectwas to examine the use of modelling to predict the hygrothermal performance of a few selected high-rise wall androof systems and to predict the long-term performance of various retrofit strategies from which these wall systemscould benefit. The prediction of the long term hygrothermal performance of residential building components is

important in assessing the durability, the energy efficiency and the effects of rehabilitation of high-rise structures.

Research Program

The heat, air and moisture transport in high-rise residential buildings is a complex hygrothermal process. Some of thetransport mechanisms that must be considered in a computer simulation include: water vapour transport; liquid watertransport; moisture capacity in the materials; condensation and evaporation processes; freezing and thawingprocesses; incident solar radiation and sky radiation; rain penetration; and air flow through and around the buildingenvelope.

Much of this research project involved adapting a computer simulation model and determining the sensitivity of thesimulation of hygrothermal processes in high-rise residential buildings to various factors, including the accuracy ofmaterial properties, the initial conditions and the boundary conditions. A number of existing simulation programsavailable world-wide were examined but no single model had all of the features needed to fully address the transportof moisture in high-rise residential buildings. The decision was made to further expand and improve a programpreviously developed by IRC called LATENITE, which incorporated all features needed other than that of air flow.

Once satisfied with the computer simulation program, 1-dimensional analyses were carried out. This wassupplemented with more sophisticated 2-dimensional analyses on an additional, limited number of buildingassemblies.

1-D Simulations

For the 1-D simulations, three exterior walls were considered: brick veneer and steel stud back-up; brick veneer andconcrete block back-up; and exterior insulated finish system (Figures 1 to 3). Each of these wall systems was thenretrofit under two different scenarios (exterior and interior) and the simulations were repeated (Figures 4 to 9). Inaddition, three roof systems were simulated: built-up roof; single membrane roof; inverted roof (Figures 10 to 12).

lysis of the Hygrothermal Behaviour of Residential High-Rise Build... http://www.cmhc-schl.gc.ca/publications/en/rh-pr/tech/96217.ht

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Figure 1. Wall 1 - Base Case Brick Veneer/Steel Stud

Figure 2. Wall 2 - Base Case Brick Veneer/Concrete Block Back-up

Figure 3. Wall 3 - Base Case Exterior Insulating Finish System


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Figure 4. Wall 1 - Exterior Retrofit




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Figure 7. Wall 1 - Interior Retrofit




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Figure 10. Roof 1 - Built-up Roof

Figure 11. Roof 2 - Single Membrane Roof

Figure 12. Roof 3 - Inverted Roof

For each high-rise wall, four different wall orientations (N, S, E, W) were used because of the strong effects of solar

radiation and wind-driven rain on hygrothermal performance. One orientation was used for roof simulations (i.e., flatroof only).

A separate simulation model was used to calculate the exterior pressure and rain mass flow rates for each surfaceof the high-rise for use in the hygrothermal simulation. This was the first project world-wide to include the effect ofrain in the moisture modelling, an important issue because wind-driven rain can be the single most important sourceof moisture entering a high-rise structure. Since wind-driven rain is strongly influenced by the height of the high-risebuilding, the effect of elevation above ground level was investigated for three different wall heights: 1st floor, 5th floorand 10th floor (top).

The effect of exterior boundary conditions (i.e., atmospheric weather conditions) on the long-term performance ofvarious wall and roof systems was investigated by using weather data from seven different cities within Canadachosen to represent a wide variation of climatic conditions: Ottawa; Vancouver, Winnipeg, Fredericton, Montreal,

Resolute Bay, and Toronto. Interior boundary conditions (i.e., interior temperature and relative humidity) wereassumed to be constant at 20C and 40% rh, which corresponds to typical conditions in Canadian residentialbuildings.

Long-term (3 year) hygrothermal simulations were performed. While very little change was observed in moisture andthermal behaviours from year 1 to year 2, only the 3rd year results were analysed to ensure the results were


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independent of initial conditions.

2-D Simulations

The 2-D analysis included the effects of air exfiltration and infiltration by the introduction of cracks in the buildingassembly - two interior and one exterior. The cracks allowed air leakage to occur directly across the bottom orindirectly through the bottom crack, the insulation and the air space and the top crack (Figure 1). Only the brickveneer/steel stud wall was simulated in the 2-D analysis, for a 2 year period, using Ottawa as the climatic location.

To investigate the influence of stack, the bottom of the wall was simulated to be at different heights with respect tothe neutral pressure plane: 1 m above, 1 m below, 20 m above and 20 m below. Mechanical overpressurization of 10Pa was assumed. For comparison purposes, the simulation was also run with the cracks sealed. The simulationincluded rain, but all walls were given the same amount of rain (i.e., no height effects).

Results

LATENITE was experimentally benchmarked against several other simulation programs and in round-robin testing aspart of the International Energy Agency (IEA) Annex 24 and very good agreement was observed, providingconfidence in the simulation results.

In general, the simulations revealed that climatic conditions have a strong effect on the moisture performance of eachwall system. The performance of high-rise systems can be completely different depending on the particularorientation, climatic location and height of the building. For example, the appearance of moisture peaks occurs atdifferent times of the year for the four orientations. Vancouver, Ottawa, Toronto and Montreal walls are subject tolarge moisture loads that vary considerably throughout the year. (No hourly precipitation data was available forWinnipeg, Resolute and Fredericton.) These effects can be closely related to the amount of rain precipitationavailable. Most of the moisture accumulation occurs in the bricks. The effect of location above ground level affectedthe relative humidity levels in the wall materials. Maximum relative humidity differences of 20% to 30% were founddue to the height of the walls. Thermal performance was not significantly affected by the presence of moisture,although it did deteriorate by approximately 1.7% from the first floor to the fifth floor and an additional 2% from thefifth floor to the tenth floor (top). The most prominent accumulation and drying occurred for Resolute. Orientationeffects were noted for Resolute, but they were relatively small.

Brick Veneer/Steel Stud Wall

The effect of orientation was found to be very important with respect to the hygrothermal behaviour of a brickveneer/steel stud wall, largely due to the influence of rain. When retrofit on the exterior with stucco, the influence ofwind-driven rain was removed; however, that effect will only obtained as long as perfect, impermeable conditions aremaintained on both sides (stucco and polyethylene vapour retarder), as was simulated.

The brick in the base case wall in Vancouver, which has the highest rainfall rate, maintained a relative humidity above95% during the fall and winter, but temporarily dried out during the summer periods, with the exception of wettingduring rainy periods. The relative humidities of materials on either side of the vapour barrier were indicative of theeffectiveness of the vapour barrier.

With the exception of Resolute, the exterior retrofit performed better than the base case and the interior retrofit withrespect to moisture transport. There was no beneficial hygrothermal response observed when insulation was addedto the interior of the brick veneer/steel stud wall in the colder climates of either Ottawa and Winnipeg; in fact, whilethere was still no net yearly accumulation, the condensation period was prolonged in each case. However, when thewalls in Winnipeg and Ottawa were retrofit on the exterior, the condensation plane was eliminated in both cases.

In the cold Resolute climate, a net yearly accumulation of moisture was found in the brick veneer/steel stud wall basecase, even without including rain in the simulation. There was also a net yearly accumulation of moisture with boththe interior and exterior retrofits. However, the exterior retrofit reduced the amount of moisture accumulation byapproximately half that of the base case.

Brick Veneer/Concrete Block Backup

For all locations other than Resolute, the brick veneer/concrete block backup wall, and both the interior and exteriorretrofits, did not have any net moisture accumulation. In Resolute, the base case did not have a net moistureaccumulation, but with both the interior and the exterior insulation strategies, moisture accumulated in the concreteblock. A strong dependency on wall orientation was noted for both the base case and the interior retrofit for


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Vancouver, Ottawa, Toronto, and Montreal. In general, the brick veneer/concrete block backup wall was found to bemore accommodating with respect to retrofitting than the brick veneer/steel stud wall.

EIFS Wall

The EIFS wall, while having no vapour retarder, was the driest of the three walls studied. In general, it performedwell without the type of problems typically associated with colder climates. The addition of cellulose insulation(interior retrofit) reduced the magnitude and the fluctuations of the relative humidities in the stucco and polystyrene

layer while the exterior retrofit slightly increased the moisture levels in the stucco and polystyrene and actuallyallowed the development of a condensation layer. The condensation was found to occur between the interface of themineral fibre insulation and the polystyrene insulation for the colder climates, such as Resolute.

Roofs

The results of the roof simulations showed that without the addition of moisture sources, the roof structures modelledwill perform satisfactorily independent of climatic location. The effect of climate is more apparent in the thermalperformance of the roofs than in the moisture performance analysis.

2-D Simulations

The wall 20 m above the neutral pressure plane had the highest overall moisture accumulation, as expected, due tothat being the location of the most exfiltration. However, the moisture accumulation in the insulation layer in that wallsection was less than that for the wall 1 m above the neutral pressure plane, even though much more air wasexfiltrating through the higher wall. This was primarily due to the fact that as more air flows through the insulation, itwarms up the insulation layer and thus the potential for moisture accumulation is substantially reduced.

The wall 20 m below the neutral pressure plane was the driest during late fall and winter and accumulated less waterduring those seasons than did the base case wall (no openings). An investigation of the spatial velocity distributionsrevealed that infiltration of cold, low moisture content air caused the wall to dry out. After the cold season was over,however, the -20m wall system behaved similarly to the other wall cases, especially with regards to rain. During thelate summer period, large spikes in the moisture content of the building materials occurred due solely to precipitation.

Implications for the Housing Industry

To date (world-wide) almost all of the hygrothermal models and simulations of building envelope systems have beencarried out using only vapour transport mechanisms. In the LATENITE hygrothermal model, both vapour and liquidtransport are accounted for at all boundary surfaces and also within the building assembly. Also unique is that rainpenetration is considered.

The simulations demonstrated the strong influence rain penetration has on the moisture regime of a buildingassembly. In fact, in wet conditions, liquid diffusivity is the dominant moisture transport mechanism and theimportance of vapour resistance is of less significance. This issue may prove to be of particular importance, sincethe concept of vapour resistance is widely used by "moisture experts" as the only concept for comparing building

envelope moisture performance.

The simulations accentuated the fact that retrofit strategies must be evaluated for the climatic location beingconsidered and should be based on the combined heat, air and moisture transport performance. For example, whileall three walls simulated in the research study performed satisfactorily with the climatic conditions in Fredericton, thesame walls presented serious service life and durability concerns for the climatic conditions of Resolute.

While LATENITE 1.2, the computer simulation program developed, is still too complex and onerous to be used byindividual designers, it is expected to be of great benefit to the housing industry at large. Results show that suchstate-of-the-art hygrothermal modelling, which accounts for the effects of air flow, surface rain, and vapour and liquidtransport, can provide a useful design assessment. The program is ideally suited to researching industry-wideconcerns, such as the expected performance of brick veneer/steel stud wall systems or exterior insulating finish

systems and options for retrofitting older high-rise buildings.

Project Manager: Duncan Hill

Research Report:Analysis of the Hygrothermal Behaviour of Residential High-Rise Building Components


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Research Consultant: Institute for Research in Construction, National Research Council

A full report on this research project is available

from the Canadian Housing Information Centre.

The information in this publication represents the latest knowledge available to CMHC at the time of publication, and has been thoroughly reviewedby experts in the housing field. CMHC, however, assumes no liability for any damage, injury, expense or loss that may result from use of thisinformation.

1999 CMHC-SCHL.All rights reserved.


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