INSULATION AND HEAT TRANSFER

SECTION E

ENVIRONMENTAL CONTROL

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CONTENTS Section Page E1 INSULATION AND HEAT TRANSFER E1.i

Basics of Heat Transfer E1.1 Definitions of Terms Associated with Insulation

E1.1 Calculating U-Values E1.2 Calculating Heat Loss or Gain

E1.4 Conductance and Resistance of Some Building and Insulation Materials E1.5 Resistances and U-Values for Some Typical Walls E1.9 Roofs - Heat flow upward, Winter conditions E1.17 Effectiveness of Attic Ventilation E1.19 Condensation E1.20 Predicting Condensation E1.21 Psychrometric Chart E1.21 Conditions for Visible Condensation and Frost to

Occur on Inside Surfaces E1.23 Hidden Condensation and Vapor Barriers E1.25 Permeance and Permeability of Some Construction

Materials: E1.26 Thermal Insulations E1.26 Plastic, Metal Foils and Films E1.26 Building Papers, Felts, and Roofing Papers E1.25

SECTION E1

INSULATION AND HEAT TRANSFER



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Basics of Heat Transfer The purpose of insulation is two-fold: (1) to retard the flow of heat from one place to another, and (2) to maintain temperatures such that condensation does not occur on inside surfaces. In the winter, insulation retards the flow of heat from the inside of the building to the outside, and in the summer retards the flow of heat from the outside to the inside. Also in the winter it helps keep the inside surface warm enough that condensation will not occur. Insulation should be installed in the walls and ceiling of all confinement-type buildings for better control of temperature and moisture. Heat can be transferred to or from a building by three methods: (1) conduction, (2) convection, and (3) radiation.

Conduction. This is the method by which heat is transferred through a solid material.

Convection. This is the main method by which heat is transferred within fluids (gases and liquids). The motion of the fluid carries the heat from the warm area to a cooler area. Natural convection occurs when warm air rises, cool air settles, etc. Forced-air convection is when fans move air, like in a warm air furnace where heat is transferred from the furnace to the rooms.

Radiation. Radiation is the method by which heat is transferred from hot objects to cooler objects through space by electromagnetic waves without heating the air.

Insulation acts to reduce the rate of heat transfer by all three of these methods. The insulation conducts heat at a slower rate than common building materials, thus heat transfer by conduction through the walls is reduced. By filling the open spaces and sealing the cracks in the walls, the rate of heat transfer by convection is reduced. The insulation keeps the inside surface of the walls more nearly equal to the inside air temperature and thus reduces the rate of heat transfer by radiation (hot or cold walls, ceilings, etc., "radiate" to objects or animals in the room).

Definitions of Terms Associated with Insulation

BTU - BRITISH THERMAL UNIT This is the measure of heat, and is the amount required to raise the temperature of one pound of water 1oF.

BTU/HR - The rate of heat output or heat flow for any situation being considered.

K - THERMAL CONDUCTIVITY

This is a measure of the rate of heat transfer through a material by conduction. It is the amount of heat in BTU's transferred through one square foot of material 1-inch thick 2in one hour for a 1oF. temperature difference. The lower the conductivity of a material, the less heat that will flow through it and the better the insulating value.

C - CONDUCTANCE OF A MATERIAL This is the same as the thermal conductivity except that it is for the actual thickness of the conductivity or conductance. When expressed on a per inch of thickness.



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R - RESISTIVITY OR RESISTANCE

This is a measure of the material's ability to resist heat flow and is the reciprocal of the conductivity or conductance. When expressed on a per

inch of thickness:

R = 1 K

When expressed for the actual thickness of the material under consideration:

R = 1

C

The larger the resistivity, the better the material is for use as insulation. R-values of individual materials comprising wall, ceiling, etc. can be added together to obtain an overall R-value for the composite section.

U - TOTAL OR OVERALL HEAT TRANSFER COEFFICIENT

It is the amount of heat in BTU that is transferred per square foot of air in one hour for a 1oF. temperature difference between air on one side of a wall and air on the other side. The U-value is for a complete wall, ceiling, etc.

Calculating U-Values Since the U-value is a measure of the heat transmission properties of the entire wall section when exposed to air on both sides, this is the property that should be determined when figuring the heat loss through walls, floors, ceilings, etc.

The U-value for a wall can be determined from the following relationship:

U = 1 R1 + R1 + R2 + . . .Ro

where: Ri = the resistivity of a "boundary layer" of air on the inside surface.

R1, R2 = the resistivity of each component of the walls for the actual thickness of the component used. If the resistance per inch thickness is used, the value should be multiplied by the thickness of that component.

Ro = the resistivity of the "air boundary layer" on the outside surface of the

wall. For example, to determine the U-value for the following wall section composed of three materials:

1. Hardboard, 1/4-inch thick 2. Expanded polystyrene, 2-inches thick



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3. Plywood, 3/4-inch thick

The R-values from Table E1.1 are:

Ri = 0.68 ("still" air)

R1 = 0.18

R2 = 2" X 4.00 = 8.00

R3 = 3/4 X 1.25 = 0.94

Ro = 0.17 (15 MPH wind, winter conditions)

The U-values then is:

U = 1 Ri + R1 + R2 + R3 + Roo

= 1

0.68 + 0.18 + 8.00 + 0.94 + 0.17

= 1 = 0.10 BTU 9.97 hr. - sq. ft. - oF



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Table E1.2 gives several typical wall, ceiling, floor, and roof sections and the corresponding U-values, overall R-values, and other data tabulated like the proceeding example and explanation. Calculating Heat Loss or Gain The U-value is used to determine the heat loss or gain through a wall, etc., by the following relationship. Q = (U) (A) (ÄT)

where: Q = the heat flow through the walls, etc., in BTU per hour.

U = the U-value in BTU per (hour) (square feet) (oF).

A = the area of the wall, etc., in square feet.

T = Difference in outside and inside temperatures in oF. In the previous example the heat loss for 100 square feet of wall with a 70oF temperature difference would be: Q = (.10) (100) (70) = 700 BTU/ HR For any room or building, the heat loss for each portion -- walls, ceiling, windows, doors, floors, etc., must be calculated and added for the total. When calculating the heat flow through a floor slab resting on the ground, there will not be an air boundary-layer resistance underneath (Ro = 0) and the temperature (to) will be the ground temperature (not the outside air temperature). The " deep-well" or deep-soil temperature in Alabama averages about 60oF the year around. This temperature can be used for all of the floor slab area except the outer three feet, unless perimeter insulation is used. For the outer three feet of an uninsulated slab, the outside air temperature should be used as the heat loss is to the outside foundation wall. If the sun shines on a wall or roof of a building and heats the surface much hotter than the air (as typical in the summer), the heat flow through the wall or roof would be greatly influenced by the hot surface temperature; hence, use a surface temperature rather than air to obtain a more realistic heat flow rate. Direct radiation through large glass sections is an important heat gain in summer and must be considered by procedures other than described here.



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Table E1.1: Conductance and Resistances of Some Building and Insulating Materials Conductivities and conductance are expressed in BTU per hour per square foot per degree Fahrenheit temperature difference. Conductivities (K) are per inch thickness and conductance (C) are for thickness or construction stated, not per inch thickness.

Conductivity or

Conductance Resistance

Material Description K C

Per inch

thickness 1/K

thickness

listed

AIR SPACES ¾� to 2� in width and bounded by ordinary materials Vertical �space� 1.03 0.97

Horizontal � heat flow down 0.80 1.25 Horizontal � heat flow up 1.18 0.85AIR SURFACES Still Air Vertical �surface� 1.46 0.68 Horizontal � heat flow down 1.08 0.92 Horizontal � heat flow up 1.63 0.61Outside Surface 7 ½ mph wind (summer) 4.00 0.25 15 mph wind (winter) 6.00 0.17EXTERIOR FINISHES (frame walls) Asphalt Insulating Siding ½� 0.69 1.45Brick Veneer 4� face brick 9.0 0.11 0.44Hardboard ¼ � 5.60 0.18Plywood 3/8� lapped 1.59 0.59Stucco 1� 5.0 0.20 0.20Wood Shingles 16�, 7 ½� exposure 1.15 0.87Wood Shingles and Insulation Backer Board ½� 0.71 1.40

Wood Siding, Bevel ½�, 8� lapped 1.23 0.81Conductivity or



Per inch

thickness 1/K

thickness

listed

INSULATING MATERIALS

Batts or Blankets Mineral wool, fiber-glass (2-4 lb. density) 0.27 3.70

Wood fiber (2 lb. density) 0.25 4.00 Cellular glass 0.40 2.50 Corkboard (without binder) 0.27 3.70 Plastic (foamed) 0.29 3.45 Expanded polyurethane 0.16 6.25



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Expanded polystyrene 0.25 4.00 Loose fill Mineral wool (rock, glass, slag) 0.30 3.33 Vermiculite (expanded) 0.48 2.08 Wood fiber 0.30 3.33 Roof insulation ½� 0.72 Preformed above deck, all types, 1� 0.36 Approximate thickness 1 ½� 0.24 (pressed fiberboard) 2� 0.19 2 ½� 0.15 3� 0.12 Insulating roof deck 1 ½� 0.24 Approximate thickness 2� 0.18 (pressed fiberboard) 3� 0.12 INTERIOR FINISHES Gypsum board ½� 2.25 Gypsum lath and plaster 3/8� lath, ½ � plaster 2.44 Insulation board (plain or perforated) ½� 0.35 0.70 2.86 Metal lath and plaster ¾� 7.70 Plywood 3/8� 2.12

Conductivity or



Per inch

thickness 1/K

thickness

listed

Plywood ½� 1.60 MASONRY MATERIALS Brick Common 5.0 Brick Face 9.0 Gypsum fiber concrete 87 1/2% gypsum, 12 ½% concrete 1.66 Light weight aggregate concrete Density 30 lb. per c.f. 0.90 1.11 Concrete-sand and gravel aggregate 12.00 0.80 Cinder block 8� hollow 0.58 Lightweight block 8� hollow 0.50 Concrete block 8� hollow 0.90 Stucco 5.00 0.20 ROOFING MATERIALS Asbestos cement shingles 4.76 Asphalt shingles 2.27 Built-up roofing Assumed thickness 3/8� 3.00 Asphalt roll roofing 6.50 Slate 20.00 Wood shingles 1.06 SHEATHINGS Gypsum (1/2�) 2.25 Insulation board (1/2� regular density) 0.76 Insulation board (25/32�) 0.49



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Insulation board (1/2� intermediate) 0.82 Insulation board (1/2� nail base) 0.88 Plywood (3/8�) 2.12 Wood (25/32�) Fir or Pine 1.02

Conductivity or



Per inch

thickness 1/K

thickness

listed

FLOOR COVERINGS Carpet and fibrous pad 0.48 Carpet and rubber pad 0.81 Tile (asphalt, linoleum, vinyl, rubber) 20.00 Cork tile 3.60 Terrazzo 12.50 WOODS Maple, oak and similar hardwoods 1.10 0.91 Fir, pine and similar hardwoods 0.80 1.25 Plywood Most types 0.80 1.25 Wood, subfloor 25/32� 1.02 Wood, hardwood finish floor ¾� 1.47 Fiber-board underlayment 1� 0.42

The above values are based upon data in the ASHRAE Guide and Data Book published by the American Society of Heating, Refrigerating a d Air-Conditioning Engineers.



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EFFECTIVENESS OF ATTIC VENTILATION (Summer Conditions)

The separation of the roof and ceiling to form an attic creates an added heat insulator. Its effectiveness is expressed by the data in Table E1.3 below according to whether it is non-ventilated, natural ventilated, or power ventilated. For example, if given outside air at 80oF and a ceiling resistance of R = 10, a non-ventilated attic adds an effective resistance ofR = 1.9 between the roof and ceilings whereas natural ventilation gives a better effectiveness of R = 2.8 and power ventilation of 1.0 cfm/sq. ft. gives a much larger value of R = 9.8. This latter value of R = 9.8 for power ventilation is essentially saying that forced ventilation of the attic is equal to added insulation of R = 9.8.

The data listed are averaged for various levels of solar radiation and therefore are general estimates for the hotter summer conditions. A roof deck resistance of R = 1 was used in determining these data.

The total effective resistance of the roof, attic, and ceiling of a structure can be obtained by adding the respective R-values.



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Example: An attic with power ventilation of 1.0 cfm/sq. ft. of ceiling and a ceiling R value of 10 will have an overall R value of 17.6 at 90oF outside temperature. The U-value is 1/17.6 = 0.057 for the attic including the roof and ceiling. Multiply by the ceiling area and the temperature difference (inside to outside) to obtain heat gain in BTU/HR. Table E1.3: Effectiveness of Ventilated Attics (summer conditions, non-reflective surfaces)a

Ventilation Natural Air Temp. Ventilationb Power Ventilation (outside) No (Approx. O.1 cfm/sq. ft. oF Ventilation cfm/sq. ft.) 0.5 1.0 1.5

R of Ceiling R of Ceiling R of Ceiling

10 20 10 20 10 20 10 20 10 20

80 1.9 1.9 2.8 3.5 6.5 10 9.8 17 12 21 90 1.9 1.9 2.6 3.1 5.2 79 7.6 12 86 15 100 1.9 1.9 2.6 2.7 4.2 61 5.8 87 65 10

a A reflective surface on top of the ceiling insulation will increase the attic effectiveness as an

"insulator" by two- to three-fold.



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b Natural ventilation requires a net area free of obstructions near the ridge at opposite ends of gable roof equal to 1/300 of the floor space of the building. The use of louvers and 8-mesh (1/8") screen (usually recommended) requires a gross area 2.25 times this net free area.

EXAMPLE: A 40' x 60' building would require 8 square feet of net opening (2400/300), or 18

square feet of screened louvers (8 x 2.25). Thus, 2 louvers in opposite ends having a total

of 9 square feet each are required for non-vented eaves. If the eaves are vented, the gable

louvers can be reduced to one-half this size provided the same amount is uniformly spaced

around the eaves; that is, two gable louvers at 4.5 square feet each plus 4.5 square feet of

screened eave vents (1/8" screen). CONDENSATION

Elimination of condensation on or within walls and floors is as important as reducing the heat loss through the wall or floor. In addition to the moisture damage caused to buildings by condensation, conditions which will cause condensation may make the structure uncomfortable for living. Condensation can be of two types: visible and hidden. Visible condensation occurs when water vapor condenses on a surface and the moisture droplets can be seen. Hidden condensation occurs when water vapor migrates through the wall materials and condenses within the wall section causing wet materials or insulation which may not be detected until severe wetting and damage occurs.



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Normal air is a mixture of "dry air" and water vapor. There is a maximum amount of water vapor which can be retained in a given volume of air at any given temperature. This maximum amount or "saturated" condition is defined as 100-percent relative humidity. A condition of 50-percent relative humidity has 1/2 of the vapor necessary to saturate it at that particular

temperature. The higher the temperature, the more water vapor it takes to reach saturation.

The temperature which corresponds to 100-percent saturation is called the "dew-point" temperature. When a mixture of air and water vapor is cooled, the temperature drops until saturation is reached at the dew-point temperature. (NOTE: The " dew-point" and "air"

temperature as measured by the thermometer are the same at 100-percent relative humidity or

"saturation".) If the temperature is further reduced, vapor leaves the mixture as condensation. If the surface temperature of any area is below the dew-point of the air and water vapor mixture surround it, the air in contact with that area will be cooled to the dew-point and visible condensation will form. This is what happens to car and house windows and many other objects in the winter. PREDICTING CONDENSATION

The dew-point temperature can be determined from a psychrometric chart if the temperature and relative humidity of the air are known. Figure E1.1 is a psychrometric chart illustrating these relationships for predicting the dew-point temperature.

For a building, the inside wall surface must not cool to the dew-point temperature for the existing air-vapor mixture or visible condensation will form on the wall. This inside surface temperature is dependent upon the air temperature inside and outside the building the heat transfer coefficient (U-value) of the wall. Figures E1.2a, 2b, 2c, 2d show the relationships for condensation and (and frost) on the inside surface for a given U-value, inside temperature, relative humidity and outside temperature. How To Use Figure E1.2a, 2b, 2c, and 2d:

1. Select figure for inside temperature being used. 2. To find the outdoor temperature for condensation to occur on inside surface: begin at

left margin with inside relative humidity and move horizontally right-ward to the proper U-value curve. Move downward to outside temperature. EXAMPLE: for inside temperature = 60oF, RH = 75%, and U = .60, condensation occurs at outside temperature of 40oF.

3. To find the outside temperature for frost to occur on inside surface: begin at left margin with inside relative humidity and move horizontally right-ward to the proper U-value curve. Follow U-value curve down and to the left to intersect the dashed frost line. Move down-ward to outside temperature. EXAMPLE: above conditions result in frost

forming at outside temperature of -7oF.

4. To find U-value required to prevent condensation or frost on inside surface: find intersection of horizontal relative humidity line and vertical outside temperature line, read required U-value.



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Figure E1.2a: Conditions For Visible Condensation On Inside Surface. (For instructions on how to use, see page E1.21.)



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Figure E1.2b: Conditions For Visible Condensation And Frost To Occur on Inside Surface (For instructions on how to use, see page E1.21.)



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Figure E1.2c. Conditions For Visible Condensation And Frost To Occur On Inside Surface. (For instructions on how to use, see page E1.21.)



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Figure E1.2d. Conditions For Visible Condensation And Frost To Occur On Inside Surface. (For instructions on how to use, see page E1.21.) HIDDEN CONDENSATION AND VAPOR BARRIERS

Most wall and insulation materials are porous enough that water vapor will migrate right through them. If the dew-point temperature is reached within the wall section, hidden condensation occurs and a wet wall or insulation results which reduces the insulation



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effectiveness and deteriorates these materials. A vapor barrier such as 4- to 6- mil plastic film or heavy aluminum foil must be used under the finish covering on the warm side of the wall to block this movement of vapor. Paint, in general, is not an adequate vapor barrier. Certain expanded foam or vinyl-faced insulation boards are inherently resistant to vapor flow and thus do not require the supplemental vapor barrier. The main requirement with these boards, is to seal the joints to prevent seepage of vapor and moisture through the cracks. The rate of vapor movement through a material is measured as " perms". A perm is equal to 1 grain (7000 grains = 1 pound) of moisture flow through the material per square foot) (hour) (inch of mercury vapor-pressure difference). Residences and confinement animal structures need to have a vapor barrier on the "warm" side of the insulation which transmits less than one perm. The outside surface should be more permeable (that is, have a greater permeance rating) so any moisture will escape outward and not get trapped between two barriers. This means that plastic film, or a similar material, should not be "wrapped" around the outside of the framing and insulation of a building.



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Table E1.3: Permeance and Permeability of Some Construction Materials1

Permeance Permability Material (Perm) (Perm for 1" thickness)

Concrete (1:2:4 mix) - 3.2 Brick masonry (4-inches thick) 0.8 - Concrete block (8-in. cored, limestone

aggregate) 2.4 - Tile masonry, glazed (4-inches thick) 0.12 - Plaster on metal lath 15.0 - Plaster on wood lath 11.0 - Plaster on plain gypsum lath (with studs) 20.0 - Gypsum wall board (3/8-inch plain) 50.0 - Gypsum sheathing (1/2-in. asphalt impreg.) - 2.0 Structural insulating board (interior,

uncoated, 1/2-inch) 50 - 90 - Hardboard (1/8-inch standard) 11.0 - Hardboard (1/8-inch tempered) 5.0 - Built-up roofing (hot-mopped) 0.0 - Plywood (douglas-fir, exterior glue,

1/4-inch thick) 0.7 - Plywood (douglas-fir, interior glue,

1/4-inch thick) 1.9 - Polyester, glass fiber reinforced sheet, 48 mil. 0.05 - THERMAL INSULATIONS Air (still) - 120.0 Glass - 0.0 Corkboard - 9.5 Mineral wool (unprotected) - 116.0 Expanded polyurethane (R-11 blown - 0.4 - 1.6 Expanded polystyrene-extruded - 1.2 Expanded polystyrene-bead - 2.0 - 5.8 PLASTIC AND METAL FOILS AND FILMS Aluminum foil (0.35 mil.) 0.0 - 0.05 - Polyethylene (2 mil.) 0.16 - Polyethylene (4 and 6 mil.) 0.06 - 0.08 - Polyvinylchloride, unplasticized (2 mil.) 0.68 - Polyvinylchloride, plasticized (4 mil.) 0.8 - 1.4 - BUILDING PAPERS, FELTS, ROOFING PAPERS Duplex sheet, asphalt laminated,

aluminum foil one side 0.176 -



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Saturated and coated roll roofing 0.24 - Asphalt-saturated and coated

vapor-barrier paper 0.6 - Asphalt-saturated but not

coated sheathing paper 20.2 - 15-16 asphalt felt 5.6 - Single kraft,double infused 42.0 -

1 These values are typical of the materials. Exact values must be obtained from manufacturer

data sheet or special lab reports.



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INSULATION AND HEAT TRANSFER

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