Transmission Heat Loss through Building Elements
The heat transmission through a building wall or similar construction can be expressed as:
Ht = U A dt (1)
where
Ht = heat flow (Btu/hr, W, J/s)
U = overall heat transfer coefficient, "U-value" (Btu/hr ft2 oF, W/m2K)
A = wall area (ft2, m2)
dt = temperature difference (oF, K)
The overall heat transfer coefficient - the U-value - describes how well a building element conducts heat or the rate of transfer of heat (in watts or Btu/hr) through one unit area (m2 or ft2) of a structure divided by the difference in temperature across the structure.
Online Heat Loss Calculator
Common Heat Transfer Coefficients of some common Building Elements
| Building Element | Heat-Transfer Coefficient U-value | ||
|---|---|---|---|
| (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 | 6 inches (150 mm) - poured concrete 80 lb/ft3 | 0.7 | 3.9 |
| 10 inches (250 mm) - brick | 0.36 | 2.0 | |
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 overall U-value of a construction consisting of several layers can be expressed as
U = 1 / ∑ R (2)
where
U = heat transfer coefficient (Btu/hr ft2 oF, W/m2K)
R = "R-value" - the resistance to heat flow in each layer (hr ft2 oF/Btu, m2K/W)
The R-value of the single layer can be expressed as:
R = 1 / C = s / k (3)
where
C = layer conductance (Btu/hr ft2 oF, W/m2K)
k = thermal conductivity of layer material (Btu in/hr ft2 oF, W/mK)
s = thickness of layer (inches, m)
Note! - in addition to resistance in each construction layer - there is a resistance from the inner and outer surface to the surroundings. If you want to add the surface resistance to the U calculator below - use one - 1- for thickness - lt - and the surface resistance for the conductivity - K.
Online U value Calculator
This calculator can be used to calculate the overall U-value for a construction with four layers. Add the thickness - lt - and the layer conductivity - K - for each layer. For fewer than four layers, replace the thickness of one or more layers with zero.
Example - U value Concrete Wall
A concrete wall with thickness 0.25 (m) and conductivity 1.7 (W/mK) is used for the default values in the calculator above. The inside and outside surface resistance is estimated to 5.8 (m2 K/W).
The U value can be calculated as
U = 1 / (1 / (5.8 m2K/W) + (0.25 m) / (1.7 W/mK))
= 3.13 W/m2K
R-values of Some Common Building Materials
| Material | Resistance R-value | |
|---|---|---|
| (hr ft2 oF/Btu) | (m2K/W) | |
| Wood bevel siding 1/2" x 8", lapped | 0.81 | 0.14 |
| Wood bevel siding 3/4" x 10", lapped | 1.05 | 0.18 |
| Stucco (per inch) | 0.20 | 0.035 |
| Building paper | 0.06 | 0.01 |
| Plywood 1/4" | 0.31 | 0.05 |
| Plywood 3/8" | 0.47 | 0.08 |
| Plywood 1/2" | 0.62 | 0.11 |
| Hardboard 1/4" | 0.18 | 0.03 |
| Softboard, pine or similar 3/4" | 0.94 | 0.17 |
| Softboard, pine or similar 1 1/2" | 1.89 | 0.33 |
| Softboard, pine or similar 2 1/2" | 3.12 | 0.55 |
| Gypsum board 1/2" | 0.45 | 0.08 |
| Gypsum board 5/8" | 0.56 | 0.1 |
| Fiberglass 2" | 7 | 1.2 |
| Fiberglass 6" | 19 | 3.3 |
| Common brick per inch | 0.20 | 0.04 |
R-values of Some Common Wall Constructions
| Material | Resistance R-value | |
|---|---|---|
| (hr ft2 oF/Btu) | (m2K/W) | |
| 2 x 4 stud wall, uninsulated | 5 | 0.88 |
| 2 x 4 stud wall with 3 1/2" batt insulation | 15 | 2.6 |
| 2 x 4 stud wall with 1" polystyrene rigid board, 3 1/2" insulation blanket | 18 | 3.2 |
| 2 x 4 stud wall with 3/4" insulation board, 3 1/2" batt insulation, 5/8" polyurethane insulation | 22 | 3.9 |
| 2 x 6 stud wall with 5 1/2" insulation blanket | 23 | 4 |
| 2 x 6 stud wall with 3/4" insulation board, 5 1/2" batt insulation, 5/8" polyurethane insulation | 28 | 4.9 |
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