Transmission Heat Loss through Building Elements
Heat loss through common building elements due to transmission, Rvalues and Uvalues  imperial and SI units.
The heat transmission through a building wall or similar construction can be expressed as:
H_{t} = U A dt (1)
where
H_{t} = heat flow (Btu/hr, W, J/s)
U = overall heat transfer coefficient, "Uvalue" (Btu/hr ft^{2} ^{o}F, W/m^{2}K)
A = wall area (ft^{2}, m^{2})
dt = temperature difference (^{o}F, K)
The overall heat transfer coefficient  the Uvalue  describes how well a building element conducts heat or the rate of transfer of heat (in watts or Btu/hr) through one unit area (m^{2} or ft^{2}) 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  HeatTransfer Coefficient Uvalue  

(Btu/(hr ft^{2} ^{o}F))  (W/(m^{2}K))  
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  uninsulated  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 "LowE" coatings  0.32  1.8  
Vertical double glazed window with "LowE" coatings and heavy gas filling  0.27  1.5  
Vertical double glazed window with 3 plastic films ("LowE" 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/ft^{3}  0.7  3.9 
10 inches (250 mm)  brick  0.36  2.0 
U and Rvalues
Uvalue (or Ufactor) is a measure of the rate of heat loss or gain through a construction of materials. The lower the Ufactor, the greater the material's resistance to heat flow and the better is the insulating value. Uvalue is the inverse of Rvalue.
The overall Uvalue of a construction consisting of several layers can be expressed as
U = 1 / ∑ R (2)
where
U = heat transfer coefficient (Btu/hr ft^{2} ^{o}F, W/m^{2}K)
R = "Rvalue"  the resistance to heat flow in each layer (hr ft^{2} ^{o}F/Btu, m^{2}K/W)
The Rvalue of the single layer can be expressed as:
R = 1 / C = s / k (3)
where
C = layer conductance (Btu/hr ft^{2} ^{o}F, W/m^{2}K)
k = thermal conductivity of layer material (Btu in/hr ft^{2} ^{o}F, 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  l_{t}  and the surface resistance for the conductivity  K.
Online U value Calculator
This calculator can be used to calculate the overall Uvalue for a construction with four layers. Add the thickness  l_{t}  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 (m^{2 }K/W).
The U value can be calculated as
U = 1 / (1 / (5.8 m^{2}K/W) + (0.25 m) / (1.7 W/mK))
= 3.13 W/m^{2}K
Rvalues of Some Common Building Materials
Material  Resistance Rvalue  

(hr ft^{2} ^{o}F/Btu)  (m^{2}K/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 
Rvalues of Some Common Wall Constructions
Material  Resistance Rvalue  

(hr ft^{2} ^{o}F/Btu)  (m^{2}K/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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