U Value and Windows


Also known as the U-factor or coefficient of heat transmission, a measure of the rate of non-solar heat loss or gain through a material or assembly. U-values gauge how well a material allows heat to pass through. U-value ratings generally fall between 0.20 and 1.20. The lower the U-value, the greater a product’s resistance to heat flow and the better its insulating value. The inverse of (one divided by) the U-value is the R-value.

The U-value is a measure of how many watts (representing the rate of flow of energy) pass through one square metre (m2) of construction for every degree difference in temperature between the inside and the outside. Temperature is measured in Kelvin, where 1K = 1°C.

As an example, a U-value of 6.0W/m2K (that of a single glazed window) means that six watts will be escaping through each square metre of glass when the temperature difference is one degree. If it is 20°C in the house and 0°C outside, then the heat loss is 20 x 6 = 120 watts per square metre.

To reduce U-factors, some manufacturers apply a low-E(low-emittance) coating to glazing surfaces. These low-E coatings reduce heat loss, improving both heating and cooling performance. Windows can also be assembled to improve thermal performance. Some assembly strategies include using two or more layers of panes or films, low-conductance gas fills between the layers, and thermally improved edge spacers, which are placed between the panes.

The sash and frame of a window represent 10% to 30% of a window’s total area, depending on the window size and design. The material used to manufacture the frame can thus impact heat loss and related condensation resistance. In colder climates, in non-residential buildings, where aluminum frames are used, thermal breaks should be specified in order to minimize heat transfer and condensation on the frames. In colder climates, with residential buildings, most products use wood, vinyl, or other non-metallic frames.

Some door frames will also conduct heat readily. For solid doors, insulated metal or fiberglass doors are usually the best choice.

Window coverings, such as shades, shutters, and insulating or storm panels, can help reduce heat loss too.


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