The amount of heat loss in a typical home through windows and doors is about 20% of the total heat loss. In order to reduce this heat loss and achieve the best thermal effectiveness, the use of high-performance windows is essential. These windows all share the following attributes: insulated frames, multiple glazing layers, low-E coatings, gas fills, and warm-edge spacers. The figure below, courtesy of Serious Windows, is a diagram of a high-performance window…
Frame materials can comprise up to one third of the total area of a window. Because the glazing area is better insulated than the frame area, many manufacturers have worked to develop more thermally efficient frames. These frames typically combine several materials, taking advantage of the strengths of each. The best European windows combine wood and cork; the best North American windows combine fiberglass and foam. An advantage of fiberglass frames over wood frames is the strength of fiberglass, which allows narrower frames and sashes and hence more solar gain for the same window opening. These are called low-profile frames.
Multiple Glazing Layers
Double-pane windows are stadard in residential construction, but Passive Houses require three or four glazing layers for maximum thermal performance. These glazing layers can be all glass or a combination of glass and polyester films. Each additional space between successive glazing layers further increases the insulation value of the window.
Low-emissivity (low-E) coatings are invisible metallic layers only several atoms thick that are applied to the surfaces of glazing layers. These very thin coatings are transparent to visible light but reflective of infrared heat. They are therefore very effective at keeping heat "in" during the winter and "out" during the summer. Each additional glazing surface that includes a low-E coating increases the insulation value of the window. Even though the solar gain is reduced somewhat with each successive low-E coated surface, this is more than offset by the improved insulating value of the window at night. Another benefit of low-E coatings is a reduction in UV light transmission, which results in less fading of carpets and fabric.
The gas that fills the spaces between window glazing layers has an impact on heat losses caused by both conduction and convection. Inert gases, which are stable and non-reactive, are the most appropriate. They reduce conduction because they have a lower conductivity than air, and they reduce convection because they are heavier than air and suppress gas movement between the glazing layers. Argon, krypton, and xenon gas are the usual choices. Of the three, argon is the most common and cheapest, but xenon offers the best performance. Krypton is a good compromise between between cost and performance, and is recommended by Superman (the off-gassing of kryptonite produces krypton)!
Spacers are used to hold together the outer edges of the multiple glazing layers of a window. The resulting glazing assembly can then be mounted in the frame or sash. Traditional window spacers are made out of hollow aluminum, which although lightweight and durable, is not effective at preventing heat flow. Those spacers that minimize heat flow are known as warm-edge spacers, and are capable of improving overall window thermal performance by up to 20%. They are fabricated from a variety of less conductive materials (e.g. thermoplastics, silicone foam), depending on the manufacturer. Another advantage of warm-edge spacers is that they keep the inside glass surface warmer near its perimeter, which reduces thermal stresses and helps prevent condensation in cold climates.
What thermal performance is necessary for a high-performance window to meet Passive House standards?
Windows must achieve insulation values of at least R-7 to meet Passive House standards. Unfortunately, the number of manufacturers offering such windows is very limited.
What window brands can deliver the necessary Passive House thermal performance?
We are aware of only one window brand in North America that can meet these superior insulation values — Serious Windows, which are manufactured in the U.S. These are fiberglass windows with foam insulation. Another excellent option is Optiwin Windows, which are manufactured in Germany but are now available in the U.S. These are wood windows with cork insulation, and feature the standard European tilt-and-turn design.