What is U-Value of a Building Component?

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The U-value of a building component measures the energy (heat) lost through a square meter of the material for every degree difference in temperature between the inside and the outside environment. Technically, U-value measures the thermal transmittance of material in W/m²·K (Watts-per-meter-square-kelvin).

The U-value of a building component represents how effectively it behaves as an insulator by controlling heat transfer from the inside to the outside, thereby reducing energy loss. The building component may be a homogeneous material like concrete or a combination of materials like a cavity wall. 

A better U-value of the building element demands less energy to maintain comfortable conditions inside the building. 

This article describes the features of the U-value and its calculation for building components.

Features of U-value

• The lower the U-value of a building component, the slower the heat transmittance and better its performance as an insulator. 
• The U-value of a building is lowered by using materials, building elements, and building fabric that would reduce heat transmission and overall energy consumption. Hence, they are specified in building standards and regulations to reduce the energy consumption of the building.
• The U-value of different building materials and components differs. The U-value of a solid brick wall is greater than that of a cavity wall with no insulation.
• Certain building regulations prevent building materials from being constructed by setting a maximum U-value.
• For example, the U-value of a single glass sheet used for the window pane is 6 W/m2K. This means that for every degree difference between the outside and inside temperature, a square meter of glass sheet will transmit 6 watts. If the temperature difference is 15 degrees on a typical cold day, the heat loss is 6 x 15 = 90 watts per square meter. It is a very large value and brings huge heat loss. But there are modern triple-glazing materials for windows that provide a U-value as low as 0.7W/m2K.
• The U-values of different building materials are given below:
  • Solid brick wall: 2 W/(m²K)
  • Cavity wall with no insulation: 1.5 W/(m²K)
  • Insulated wall: 0.18 W/(m²K)
  • Single glazing: 4.8 to 5.8 W/(m²K)
  • Double glazing: 1.2 to 3.7 W/(m²K) depending on the type
  • Triple glazing below: 1 W/(m²K)
• Solid timber door: 3 W/(m²K)
• Buildings are designed such that lower U-values can be achieved. It is a part of sustainability and performance measures in building construction.

Calculation of Thermal Transmittance or U-Value

The general formula for calculating U-value is: 

U = 1/Rt

Where,

U = Thermal transmittance in (W/m²·K)

Rt = Total thermal resistance of the elements composing the material in m²·K/W. 

If a building component has an interior surface, an exterior surface, and individual layers of material between them, then the thermal resistance Rt of the whole material is given by:

Rt = Ri + R1 + R2+ R3+...Rn + Re

Where, 

Rsi = Thermal resistance of the interior surface

R1, R2…Rn = Thermal resistance of individual layers in between

Rso = Thermal resistance of the exterior surface

The thermal resistance of individual layers is obtained by the formula:

R= D/λ

Where, 

D = Thickness of the material in meters (m)

λ = Thermal conductivity of the material in W/Km

Hence, from the formula, thermal transmittance is inversely proportional to thermal resistance. The greater the thermal resistance of the building material, the lower its U-value, and hence reduced heat loss.

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