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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
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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 Element | Heat-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 tripple 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
| Material | R-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
| Material | R-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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