Transmission Heat Loss through Building Elements

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

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

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

Wall Area (ft2, m2)

Temperature Difference (oF, oC, K)

Common Heat Transfer Coefficients of some common Building Elements

Building ElementHeat-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 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 = lt / K                                                (3)

where

C = layer conductance (Btu/hr ft2 oF, W/m2K)

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

lt = 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.

1. lt (in, m) K (Btu in/hr ft2 oF, W/mK)

2. lt (in, m) K (Btu in/hr ft2 oF, W/mK)

3. lt (in, m) K (Btu in/hr ft2 oF, W/mK)

4. lt (in, m) K (Btu in/hr ft2 oF, W/mK)

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

MaterialResistance
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

MaterialResistance
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

Related Topics

• Heating - Heating systems - capacity and design of boilers, pipelines, heat exchangers, expansion systems and more

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