Understanding U-Value for Building Envelope

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Thermal transmittance, also known as U-value, is the rate of transfer of heat energy through a structure, divided by the difference in temperature across that structure. The units of measurement are W/m²K.

The better-insulated a structure or material is, the lower the U-value will be. Thermal transmittance takes heat loss due to conduction, convection and radiation into account.

U-value plays an important role in Building heat gain. Which helps in energy simulation of buildings.

The U-value is the reciprocal of the R value, the equation is

U = 1/R in W/m2K or Watts per square meter per degree Kelvin

How U-value works ?

 If we consider a single sheet of standard glass having U-value as 2.0W/m2K –

It means that for every degree of temperature difference between the outside and the inside, a square metre of the glazing would lose 2 watts. So for example, if the temperature difference on a typical cold day was 15 degrees, then the amount of heat loss would be 15×2 = 30 watts per square metre.

Therefore, we can adopt double or triple glazed widows as they have comparatively less U -value.

The U- value of a double and triple glazed window is about 3.5-1.5 and 0.7W/m2K respectively . which lower than that of single glazed window.That means these will save more energy consumption as compare to single glazed glass.

U-value of various building materials as per ASHRAE Standards

The general formula for calculating the U-Value is

U = 1/Rt


  • U = Thermal Transmittance (W/m²·K)*
  • Rt = Total Thermal Resistance of the element composed of layers (m²·K/W), obtained according to:

Rt = Rsi + R1 + R2 + R3 + … + Rn + Rse


  • Rsi = Interior Surface Thermal Resistance (according to the norm by climatic zone)
  • Rse = Exterior Surface Thermal Resistance (according to the norm by climatic zone)
  • R1, R2, R3, Rn = Thermal Resistance of each layer, which is obtained according to:

R = D / λ


The Thermal Transmittance is inversely proportional to the Thermal Resistance: the greater the resistance of the materials that make up an envelope, the lower the amount of heat that is lost through it.

U = 1/R

R = 1/U