Heat Loss through Building Elements due to Transmission

Heat loss through common building elements due to transmission, R-values and U-values - imperial and SI units

heat transmission through walls

The heat transmission through a building wall or similar construction can be expressed as:

Ht = U A dt          (1)

where

Ht = heat loss (Btu/hr, W)

U = "U-value" (Btu/hr ft2 oF, W/m2K)

A = wall area (ft2, m2)

dt = temperature difference (oF, K)

Common Heat Transfer Coefficients of some common Building Elements

Building ElementHeat-Transfer Coefficient
(Btu/hr ft2 oF)(W/m2K)
Doors Single sheet - metal 1.2 6.8
1 inch - wood 0.65 3.7
2 inches - wood 0.45 2.6
Roofing Corrugated metal - uninsulated 1.5 8.5
1 inch wood - uninsulated 0.5 2.8
2 inches wood - un-insulated 0.3 1.7
1 inch wood - 1 inch insulation 0.2 1.1
2 inch wood - 1 inch insulation 0.15 0.9
2 inches - concrete slab 0.3 1.7
2 inches - concrete slab - 1 inch insulation 0.15 0.9
Windows Vertical single glazed window in metal frame 5.8
Vertical single glazed window in wooden frame 4.7
Vertical double glazed window, distance between glasses 30 - 60 mm 2.8
Vertical triple glazed window, distance between glasses 30 - 60 mm 1.85
Vertical sealed double glazed window, distance between glasses 20 mm 3.0
Vertical sealed triple glazed window, distance between glasses 20 mm 1.9
Vertical sealed double glazed window with "Low-E" coatings 0.32 1.8
Vertical double glazed window with "Low-E" coatings and heavy gas filling 0.27 1.5
Vertical double glazed window with 3 plastic films ("Low-E" coated) and heavy gas filling 0.06 0.35
Horizontal single glass 1.4 7.9
Walls 8 inches - poured concrete 80 lb/ft3 1.5 8.9
12 inches - poured concrete 80 lb/ft3 1.1 5.9

U and R-values

U-value (or U-factor) is a measure of the rate of heat loss or gain through a construction of materials. The lower the U-factor, the greater the material's resistance to heat flow and the better is the insulating value. U-value is the inverse of R-value.

The U-value of a construction consisting of several layers can be expressed as

U = 1 / ∑ R         (2)

where

R = "R-value" - the resistance to heat flow in each layer (hr sq.ft oF/Btu)

The R-value of the single layer can be expressed as:

R = 1 / C = lt / K         (3)

where

C = layer conductance (Btu/hr ft2 oF)

K = layer conductivity (Btu in/hr ft2 oF)

lt = thickness of layer (inches)

R-values of Some Common Building Materials

MaterialR-value (hr sq.ft oF/Btu)
Wood bevel siding  1/2" x 8", lapped 0.81
Wood bevel siding  3/4" x 10", lapped 1.05
Stucco (per inch) 0.20
Building paper 0.06
Plywood 1/4" 0.31
Plywood 3/8" 0.47
Plywood 1/2" 0.62
Hardboard 1/4" 0.18
Softboard, pine or similar 3/4" 0.94
Softboard, pine or similar 1 1/2" 1.89
Softboard, pine or similar 2 1/2" 3.12
Gypsum board 1/2" 0.45
Gypsum board 5/8" 0.56
Fiberglass 2" 7
Fiberglass 6" 19
Common brick per inch 0.20

R-values of Some Common Wall Constructions

MaterialR-value (hr sq.ft oF/Btu)
2 x 4 stud wall, uninsulated 5
2 x 4 stud wall with 3 1/2" batt insulation 15
2 x 4 stud wall with 1" polystyrene rigid board, 3 1/2" insulation blanket 18
2 x 4 stud wall with 3/4" insulation board, 3 1/2" batt insulation, 5/8" polyurethane insulation 22
2 x 6 stud wall with 5 1/2" insulation blanket 23
2 x 6 stud wall with 3/4" insulation board, 5 1/2" batt insulation, 5/8" polyurethane insulation 28

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