Understanding U-Value for Building Envelope
U-value
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
Where:
- 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
Where:
- 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
Where:
- D = Material Thickness (m)
- λ = Thermal Conductivity of the Material (W/K·m) (according to each material)
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.