Window insulation reduces heat transfer from one side of a window to the other. In the US, U-value is used to refer to the amount of heat that can pass through a window, with a lower score being better.
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Energy Efficiency Technologies
Below is a list of technologies that decrease heat transfer in windows.
Double- and triple-glazed windows
Clear single glass has a U-factor of 1 (R-1); clear double glazing has a U-factor of 0.5 (about R-2), while clear triple glazing has a U-factor of about 0.333 (R-3). Double and triple glazing are critical for energy efficiency. Single glass windows are a thing of the past.
Anyway, the insulation provided by double or triple glazing falls very short of the best. We need to further improve its performance, which can be done by selecting windows with low-e coatings, thermal breaks and argon or other similar gas between the panes; and also by selecting windows with high energy-efficient frames.
Low-e coatings
Modern double and triple-glazing windows have low-e coatings - their glass panes have thin metallic coatings, able to significantly improve their thermal insulating performance.
Another area where low-e films are used is thermal barriers. Some thermal barriers (like insulbar LEF) decreases thermal losses due to radiation by a low-e folio applied to the thermal insulation profile.
Thermal insulation profiles
To reduce heat flow and the U-factor, metal frames should have an insulating plastic profile placed between the inside and outside of the frame and sash. These profiles are called also thermal break and they are made from glass fibre reinforced polyamide 66. The glass fibre proportion is 25 %. The material used for this insulating bar stands out for its low thermal conductivity value = 0.30 W/mK and its good mechanical properties – even at higher temperatures.
A non-thermally-separated frame from an aluminium window has a thermal transmission coefficient Uf of 6.8 W/m2K. If this system is thermally separated with an insulating profile with a width of 42 mm, then the thermal transmission coefficient can be reduced to 1.3 W/m2K
A product introduced by Ensinger, called insulbar LO is a lambda (thermal conductivity) optimised solution to thermal insulation profiles. Its lambda value is only 0.18 W/m.K
Warm edge spacers
Most modern windows use glazing spacers to keep the distance between their panes and for sealing purposes. Many of these spacers are made from aluminum, which is a very conductive material. There is a better alternative for higher insulation performance: special plastic spacers, mostly named as warm-edge spacers.
Inert gas fills
Double and triple-glazed windows also use argon or krypton or other inert gas between their panes. It’s another key element for a better thermal insulation.
Most windows use argon (which is cheaper), but krypton is a better choice whenever the distance between the panes has to be smaller (the distance has to be about ½ inch for argon, and about 3/8 inch for krypton; increasing or decreasing the space will degrade performance).
Window frames
The type of window frame and the insulation of its cavity are also critical for energy-efficiency and thermal performance (or for strength, durability and maintenance).
Aluminum frames are excellent for strength, durability and maintenance but they are a poor choice from a thermal insulation point of view. Aluminum is an extremely conductive material. Therefore, thermal insulation profiles are used to create a thermal separation of the inner and outer shells of metal frames. The variety of thermal insulation profiles (insulbar) allows low Uf values (for example 1.3 W/m²K by using a 34mm insulation profile and up to 1.1 W/m²K with contemporary aluminium systems) to be reached. Polyamide insulation profiles with %25 glass fibre content are the right choice for optimum heat insulation and mechanical properties.
Wood frame windows provides a better insulation value, but they are a poor choice for durability or maintenance (unless you choose wood-clad windows).
Vinyl and fiberglass are better insulation and noise reduction materials. Certain windows have vinyl or fiberglass frames that feature chambers, to further enhance insulation and noise reduction.
High insulation-performance windows
The insulation performance of windows is measured by its U-factor/value coefficient, which should be as low as possible. To know the U-factor of a window you may look at the rating label (labels often show the U-factor of the window).
If you live in a cold climate, and you are planning on building an energy-efficient house, you should look for triple-glazing windows with a U-value between 0.19 and 0.26; in moderate and hot climates, also look for windows with a low U-value; but since, in general, quality double-glazing windows are a more cost-competitive and effective choice for mixed and hot climates, consider windows with a U-value of about 0.30, if possible.
Other ways of improving the insulation of a window
- Using glass with heat reflective properties
- Sticking on thermal reflective plastic
- Double glazing using heat-shrink plastic film and double sided tape
The frames can also be significant especially if they are made from a high thermal conductivity material such as metal. In this case plastic spacers should be used to create thermal breaks between the inner and outer sections of the frame.