# 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:

H_{t}= U A dt(1)

where

H_{t}= heat loss (Btu/hr, W)

U= "U-value" (Btu/hr ft^{2}^{o}F, W/m^{2}K)

A = wall area (ft^{2}, m^{2})

dt= temperature difference (^{o}F, K)

### Common Heat Transfer Coefficients of some common Building Elements

Building Element | Heat-Transfer Coefficient | ||
---|---|---|---|

(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 - 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 | 8 inches - poured concrete 80 lb/ft^{3} | 1.5 | 8.9 |

12 inches - poured concrete 80 lb/ft^{3} | 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^{o}F/Btu)

The R-value of the single layer can be expressed as:

R = 1 / C = l_{t}/ K(3)

where

C= layer conductance (Btu/hr ft^{2}^{o}F)

K= layer conductivity (Btu in/hr ft^{2}^{o}F)

l_{t}= thickness of layer (inches)

### R-values of Some Common Building Materials

Material | R-value (hr sq.ft ^{o}F/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 ^{o}F/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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