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Engineering ToolBox > Heat Loss through Building Elements due to Transmission
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)
| 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 | 1.1 | 6 |
| Vertical double glazed window | 0.5 | 3 | |
| Vertical 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 | 0.25 | 1.4 |
| 12 inches - poured concrete | 0.2 | 1.1 | |
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)