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(U) Heat Transfer Coefficient

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Coefficient of Heat Transmission (U)

The U-value, also known as the heat transfer coefficient, measures the rate at which heat transfers through a material or assembly of materials (like walls, windows, roofs, etc.). It is a critical factor in determining the thermal performance of building elements.

The U-value is expressed in units of watts per square meter per degree Celsius (W/m²°C) or in Imperial units, BTU per hour per square foot per degree Fahrenheit (BTU/h·ft²·°F). Essentially, it indicates how much heat energy passes through a specific material or assembly per unit area and per degree of temperature difference between the interior and exterior environments.

A lower U-value indicates better insulation properties because it means the material or assembly has lower heat conductivity and reduces heat transfer more effectively. Conversely, a higher U-value signifies higher heat transfer and poorer insulation properties.

U-values are used in building design and construction to ensure energy efficiency, as they help determine the insulation needs for various components like walls, windows, roofs, and doors. Lower U-values are desirable in colder climates to minimize heat loss from the interior, while in warmer climates, they can help reduce heat gain from the exterior.

Construction Material Heat Transfer Coefficients (U) in W/(m2⋅°C)
      Roof Material       Floors              Windows           Walls
Aerated Concrete 0.16 Concrete 1.35 Metal Frame (single glaze) 5.7 Solid Brick 2.1
Clay Tiles 1 Pine 0.13 Metal Frame (double glaze) 3.4 Brick + Insulation 0.28
Felt / Bitumen 0.3 Steel 5.0 Wood Frame (single glaze) 4.8 Solid Stone 2.25
Concrete Tile 1.5 Hardwood 0.18 Wood Frame (double glaze) 2.8 Stone + Insulation 0.32
Shingles (asphalt) 0.5   Wood Frame (triple glaze) 2.1 Concrete 3
Wood Shingles 0.1 Vinyl Frame (single glaze) 4.8 Concrete + Insulation 0.31
  Vinyl Frame (double glaze) 2.7 Hollow Wall (air gap) 1.3
Vinyl Frame (triple glaze) 1.9 Hardwoods 0.18
Doors   Pine 0.14
Solid Wood 3.0 Drywall 0.16
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