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Tuesday, February 22, 2011

Windows as Heaters

With energy modeling tools now available to architects combined with the latest in window technology new homes for cold climates can now be designed to significantly reduce the energy needed to heat the home by leveraging freely available solar heat gains. In essence, while the sun is out windows can heat your home even when temperatures are well below zero.

The most advanced windows are being manufactured in Europe with some manufactures in Canada coming close in performance. These advanced windows combine an insulated frame, glazing that allows a high percentage of solar heat to enter the home - high Solar Heat Gain Coefficient (SHGC), and air tightness.

Single panes of clear glass allow around 85% of solar heat to pass through. However the early single pane windows were little better than a hole in the wall in preventing heat from escaping from the interior of the house. As factory production became more sophisticated and glass coating technologies were invented the capacity of windows to prevent heat from escaping was greatly improved. Sacrificed in this development though was the glass’ ability to allow solar heat to pass through reducing the SHGC to around 30%. This is still true for the majority of domestically manufactured windows.

Roughly speaking the European manufactured windows from countries like Germany, Switzerland, and Austria have twice the insulation value, allow twice the amount of solar heat gain, and maintain more air tight seals. All these qualities are particularly important for energy efficient buildings in cold climates. Installing the latest in window technology in new construction in cold climates should be considered a conservation measure and given higher priority over adding renewable energy systems.


  1. Question: would these make your house really hot during the summer? I would think so?!

  2. With a good house design, the window are in shade during the summer. Or you could have shutters or awnings.


  3. Although I spoke of only windows in this blog windows need to be understood as one of many contributing components to an integrated systems design. Permaculturalists will respect it is not the properties of an individual component of the garden that is vital, it is its interaction with all the other components.

    Any design of a home that leverages direct solar energy to contribute to space heating must consider carefully overheating in the summer (and winter even.)

    Dan is correct, either appropriately designed overhangs or shutters protect the home from overheating. Passivhaus projects in Europe also employ EXTERIOR shades (they operate like horizontal venetian blinds) that can be centrally controlled by a devices that sense outside conditions and the position of the sun and adjust the shades accordingly.

    Some advancements that have developed since the pioneering passive house project's in the 70's are (1) plenty of learning from mistakes, and (2) energy modeling software that can account for solar heat gains and that can help analyze the risks of overheating. The overheating concern does need to be taken seriously and it requires good design based on experience to avoid unintended consequences.

  4. Please note also my discussion here is only relevant to new construction. Retrofitting windows in an existing building has an entirely different set of issues. Installing the advanced windows noted here in an older home rarely makes financial sense and could have the unintended consequence of overheating the home in the summer.

  5. Thanks for your responses. I would probably have to add awnings/outside shades in the summer in order for this to be a consideration.

    The reason I asked is that I am looking to replace my windows in the next few years and want to make a smart energy decision.

    I'd love to see more permaculture info about retrofitting existing houses, especially poorly sited 1950s ones!