3 Ch05 Thermal En

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PROPERTIES OF THE ENVELOPE – I Thermal Proper,es of Materials – Thermal Capacity

Thermal image of building

Building Design and Thermal Comfort Wind Scoops

•  Wind scoops capture cooling breezes

Energy-‐Conserving Building Design Units of Energy

•  All forms of energy can be measured by the amount of heat that they produce: – One calorie is the amount of heat required to raise the temperature of one gram of water by 1 degree C.

– One Btu (Bri,sh Thermal Unit) is the amount of heat required to raise the temperature of one pound of water by 1 degree F. •  One Btu = 232 Calories

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Units of Energy – cont’d

– A joule is a unit of measure used for all forms of energy. It is equal to the amount of work done by a force one Newton moving through a distance of one meter. • One calorie = 4.185 Joules

Power

•  Power is the rate of energy produc,on, consump,on or conversion.

•  Power is measured over ,me –  1 Wa[ = 1 Joule per second (1 J/s) –  A 100 Wa[ Lamp produces 100 Joules of energy per minute

Energy

•  Energy is the total amount of energy consumed in ,me. – Generally measured in kilowa[s per hour

Heat Flow Between Two Objects

PROPERTIES OF THE ENVELOPE – I Thermal Proper,es of Materials – Conduc,on, Convec,on, and Radia,on

•  Conduc,on –  Heat transfer between molecules in a solid

•  Convec,on –  Heat transfer by bulk mo,on of a gas or liquid

•  Radia,on –  Heat transfer between two objects in the form of electromagne,c waves

Modes of Heat Transfer

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Modes of Transfer of Solar Heat

PROPERTIES OF THE ENVELOPE – I Thermal Proper,es of Materials – R-‐Value of a Building Component

Thermal Resistance

•  Thermal resis,vity (ρ) is a constant property of a material represen,ng the ability of the material to resist the flow of heat through it.

•  R-‐value is a measure of the ability of a material of a specified thickness to resist the flow of heat through it. –  A material with a high R-‐value resists the flow of heat, and is called a thermal insulator.

–  A material with a low R-‐value allows heat to flow through it easily, and is called a thermal conductor.

Insula,ng Materials

•  R-‐11 fiberglass insula,on in ba[ form.

Insula,ng Materials – cont’d

•  R-‐3 extruded polystyrene insula,on in board form.

Heat Transfer by Conduc,on

•  Rate of heat transfer by conduc,on (qc)

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Calcula,ng R-‐Value

•  R = ρL (Thermal resistance x Thickness) –  If the ρ value of concrete is 0.15 (see Table 5.2), what is the R-‐value of a 6-‐inch concrete wall? • R = 0.15 x 6 = .9

–  If the ρ value of fiberglass insula,on is 3.5 (see Table 5.2), what is the R-‐value of 6 inch thick blanket of fiberglass insula,on? • R = 3.5 x 6 = 21

Thermal Resistance

• Thermal resistance is influenced by: –  Density –  Moisture content –  Entrapment of air or gas

Cellular Structure of Insula,ng Materials

•  Cellular structure of various plas,c foam insula,ons.

PROPERTIES OF THE ENVELOPE – I Thermal Proper,es of Materials – R-‐Value of a Mul,layer Component

R-‐Value of a Mul,layer Component

Rt = R1 + R2 + R3 + R4 + …

Es,ma,ng the R-‐Value of an Assembly

Element R-‐value

4-‐in.-‐thick brick wall 3.625(0.2) = .0725

2-‐in.-‐thick XPS board

2.0(5.0) = 10.0

8-‐in.-‐thick brick wall 7.625(0.2) = 1.525

Rt = 0.725 + 10 + 1.525 = 12.25, round down to R-‐12

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Surface Resistances

Rt = Rsi + R1 + R2 + R3 + R4 … + Rso

R-‐Value of an Air Space in a Wall (Wall Cavity)

•  The R-‐value of a ver,cal (wall) cavity is approximately equal to R-‐1.

•  Adding an emissive layer to one side of a material in a ver,cal air cavity increases R-‐value

PROPERTIES OF THE ENVELOPE – I Thermal Proper,es of Materials – Surface Emissivity

Emissivity

•  Emissivity: –  The property of the surface of an object that refers to its poten,al to emit radia,on.

Loca,on of Low-‐Emissivity Material (Aluminum Foil) in Air Space R-‐Value of a Wall With an Air Space

Element R-‐Value

Inside Surface Resistance 0.7

½ in.-‐thick gypsum board 0.5(0.60) = 0.30

3 ½ in.-‐thick fiberglass insula,on

3.5(3.5) = 12.25

0.5 in.-‐thick plywood 0.5(0.9) = 0.45

2 in.-‐wide airspace 1.0

3 5/8-‐in. thick brick veneer

3.625(0.2) = 0.73

Outside surface resistance

0.2

Rt = 0.7 + 0.3 + 12.25 + 0.45 + 1.0 + 0.73 + 0.2 = 15.6 (round up to 16)

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PROPERTIES OF THE ENVELOPE – I Thermal Proper,es of Materials – U-‐Value of an Assembly

U-‐Value

•  Measure property of thermal transmi[ance •  Inverse of R-‐value •  Used in heat transfer calcula,ons

1 Rsi + R1 + R2 + R3+ … + Rso

Overall U-‐Value of an Assembly

U0 =A1U1 + A2U2 + A3U3 + …

A1 + A2 + A3 + …

Example

U0 =160(0.064) + 20(1.0) +25(0.7) + 35(0.3)

= .24 240

R0 = 1

= 4.2 0.24

Thermal Bridging in Building Assemblies

Effect of exterior insula,ng sheathing in metal stud assemblies.

Thermal Bridging in Building Assemblies – cont’d

Thermal bridges (shown by red arrows) in an insulated metal stud wall.

Thermal bridges (shown by red arrows) in a concrete block.

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PROPERTIES OF THE ENVELOPE – I Thermal Proper,es of Materials – Where and How Much to Insulate

Ques,ons about Insula,on

•  Where to insulate? –  Building envelope

•  How much insula,on to use? –  Varies with

•  Climate (hea,ng degree days, cooling degree days) •  Economics (return on investment) •  Energy code

Loca,on of Insula,on Climate Zone Map of the United States

Climate Zones of the United States Energy Use in the United States

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Minimum Insula,on Requirements (Walls)

Minimum insula,on requirements for the walls and roof of residen,al buildings (single-‐family and mul,family dwellings of up to three stories) constructed of wood light frame.

Insula,on in Floors

Insula,on in a floor over a vented crawl space.

Minimum insula,on requirements for the floors of residen,al buildings (single-‐family and mul,family dwellings of up to three stories) constructed of wood light frame.

Minimum Insula,on Requirements (Floors) Example

Minimum insula,on requirements for condi,oned basement and condi,oned crawl space walls of residen,al buildings (single-‐family and mul,-‐family dwellings of up to three stories) constructed of wood light frame.

Minimum Insula,on Requirements (Condi,oned Basement/Crawl Space)

Heat Flow Paths (S.O.G.)

•  A simplified visualiza,on of heat loss from a slab-‐on-‐ground shows that there are three possible paths along which heat may flow from the slab: –  Ver,cal path through the

thickness of the slab and into the ground

–  Horizontal path through the slab

–  Through-‐the-‐ground paths (shown by the curved lines in the illustra,on )

Various heat flow paths through a slab-‐on-‐ground

(also called slab-‐on-‐grade ).

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Insula,on of S.O.G. (Alterna,ves)

PROPERTIES OF THE ENVELOPE – I Thermal Proper,es of Materials – Building Design and Thermal Comfort

Thermal Capacity (Thermal Mass) of a Component

•  The ability of a component to store heat is referred to as its thermal capacity, which is defined as the amount of heat needed to raise the temperature of 1 q2 of the component by 10F.

•  The greater the thermal capacity of the component, the greater the amount of heat it will absorb for a given rise in its temperature.

Envelope proper,es that govern heat flow under steady and dynamic states.

Thermal Capacity vs. Insula,on

•  Under unidirec,onal heat flow, only the U-‐value of the envelope (i.e., the insula,on) determines the energy consump,on of a building.

•  The envelope’s thermal capacity plays an insignificant role under unidirec,onal heat-‐flow condi,ons, even when unidirec,onal heat flow is dynamic. Envelope proper,es that govern

unidirec,onal and bidirec,onal heat flows.

PROPERTIES OF THE ENVELOPE – I

Thermal Proper,es of Materials – The Most Effec,ve Face of the Envelope for Insula,on

The Most Effec,ve Face of the Envelope for Insula,on

•  The placement of insula,on on the outside of the assembly (compared with its loca,on in the middle of the envelope sec,on or toward the interior of the building) is more effec,ve for the following reasons: –  It reduces energy consump,on because it eliminates (or reduces) thermal bridging in the envelope.

–  It moderates the temperature of structural components, reducing their expansion and contrac,on, thereby increasing their life span.

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The Most Effec,ve Face of the Envelope for Insula,on – cont’d

•  Only when a building is intermi[ently heated or cooled, such as a church hall, the interior placement of insula,on may be considered.

•  The interior placement of insula,on allows faster hea,ng or cooling of interior air because it reduces energy loss to heat or cool the structural components of the building. PROPERTIES OF THE ENVELOPE – I

PRINCIPLES IN PRACTICE: Insula,ng Materials

Principles in Prac,ce – Insula,ng Materials

•  A classifica,on that is more suitable for describing various insula,ons is based on the insula,on’s physical structure.

•  Insula,ng materials may be divided into the following three categories: –  Fibrous insula8on –  Granular insula8on –  Foamed insula8on Graphical nota,ons for rigid and flexible

insula,ons for use in cross-‐sec,ons of building assemblies.

Fibrous Insula,on

•  Ba[s •  Blankets •  Semi-‐rigid Boards

Unfaced Fiberglass Insula,on

Unfaced fiberglass insula,on installed between studs. The insula,on will be covered with a vapor retarder before gypsum board is installed on the studs.

Unfaced Fiberglass Insula,on With Vapor Retarder

A clear polyethylene sheet vapor retarder being installed over unfaced fiberglass insula,on between wood studs.

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Kraq Paper Insula,on Granular Insula,on

•  In materials with a granular structure, the air voids are contained inside ,ny hollow beads or granules.

•  Three types of granules are in common use: – Expanded perlite granules – Expanded vermiculite granules – Expanded polystyrene (EPS) granules

Blown-‐in insula,on in an asc.

Granular Insula,on – cont’d

Three stages of perlite produc,on show the great increase in volume that takes place on expansion by hea,ng the perlite rock.

Granular Insula,on – cont’d

Manufacturing process of EPS boards.

EPS and Insula,ng Concrete Forms (ICF)


– cont’d

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– cont’d

Insula,ng concrete form (ICF) walls are used as exterior walls for residen,al and commercial buildings. The wall’s high R-‐value makes ICF construc,on an a[rac,ve alterna,ve where energy conserva,on is cri,cal.

Insula,ng Concrete

Cross sec,on through an insula,ng concrete roof deck with sandwiched EPS boards.

Insula,ng Concrete – cont’d

Insula,ng concrete being poured over a roof deck.

Insula,ng Concrete – cont’d

Because EPS boards are sandwiched between two layers of concrete, the holes in the EPS boards structurally integrate the layers.

Foamed Insula,on

Manufacturing process of XPS boards. Manufacturing process of ISO boards. Spraying of polyicynene insula,on in stud cavi,es. The insula,on expands and cures (hardens) quickly when it is shaved flush with the studs.

Foamed Insula,on – cont’d

3 Ch05 Thermal En

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