U-Value, R-Value, and Solar Heat Gain Coefficient (SHGC)
What is the U Value?
The U value or Uw is a measure of transmittance and allows comparison of thermal behaviour between different window units. It is a measure of conductivity of the whole window (glass and frame included) in Watts per square metre per Kelvin difference: W/(m2K).
The lower the U value the better the thermal insulation, therefore the more energy efficient the window performance. In simple terms, the Uw indicates how well a window or door will retain heat in the winter and reject heat during summer.
Aluminium conducts energy more readily than timber or uPVC and as a result, aluminium products will have a higher Uw than timber or uPVC with the same glass. The glazing also has an impact on the Uw of the product, when a frame is fitted with double glazing or low emissivity glass the Uw will fall making the product more energy efficient.
Thermal Conductivity of different frame materials (Uw value).
(calculated according to DIN EN 10077-2)
1.9 W/m2K | Hardwood Window
6.6 W/m2K | Aluminium without thermal break
2.2 W/m2K | Aluminium with thermal break (polyamide 25 mm)
1.9-1.3 W/m2K | uPVC Window
Some uPVC window systems achieve Passive House suitability as defined under the European Passive House standard, with U values of <1 W/(m²K).
The thermal conductance (the ability to allow heat to pass through it) of any material, K, is measured in Watts / m2.Kelvin, in metric units, for a given thickness of that material.
This means that if the thermal conductance of a slab of material 10 cm thick is 1 W/m2.K, then 1 Watt of heat will flow across one square meter of that slab if the difference in temperatures on the two sides of the slab is 1 degree Kelvin. One degree Kelvin is exactly the same as one degree Centigrade. If the difference in temperature of the two sides is 10°C, (for example if the temperature outside is 35°C and inside, in an air-conditioned office, it is 25°C), then 10 W of heat will flow across our slab. One watt is equal to one Joule, a unit of energy, per second.
The R-value, which is the thermal resistance of any material, is the inverse of the K-value. That is, R = 1 / K.
So if we have a material whose K value is 4 W/m2.K, its R-value will be 1/4 = 0.25 m2.K/W
We must note that so far we have been dealing with European S.I.(metric) units. However, when in the commercial world we speak of R-value, we mean in American units. Americans use BTUs per hour instead of Watts (a BTU is a British Thermal Unit), square feet instead of square metres, and degrees Fahrenheit instead of Centigrade or Kelvin. So their unit for R-value is hr-ft2-°F/Btu.
Now 1 m2.K/W = 5.678 hr-ft2-°F/Btu
So, to convert from the SI R-value to the American R-value, multiply by 5.678.
What is a Solar Heat Gain Coefficient (SHGC)?
The SHGC measures the amount of heat that can be gained through your windows at times of direct solar contact. In summer, the SHGC can add large amounts of unwanted heat to your house, whereas in winter, the SHGC can provide free heat from the direct sun entering the home. thus the SBGC is important to know as it indicates the amount of direct solar heat that will penetrate your house.
The SHGC is mainly affected by the glazing type and orientation of the building windows.
What is a thermally broken aluminium window?
Preventing thermal bridging is important to improve insulation. A Thermally Broken aluminium window frame is one which includes a thermal break. Because aluminium as a metal conducts temperature, it has poor insulation without a thermal break being added. The thermal break is a non conductive element that has been applied in the center of the framing system to stop the flow of energy (thermal bridging) through the product. Thermally breaking the frame lowers the U-value. uPVC is sometimes offered as a cladding to aluminium products to improve their insulation.