Clerestory Windows in Passive Solar Homes

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How clerestory windows affect solar gain and daylightingTwo alternatives to standard windows are clerestory windows – those high-up windows that sit between two levels of external roofing – and skylights. They both have their uses in passive solar design, though you have to be careful with their effects on light and heating.

Because they are placed high up compared to other windows, clerestories can deliver sunlight far deeper into a living space than normal vertical glazing. Their main function is twofold: they deliver extra daylighting and they provide a method of heating thermal mass in north walls which would normally be in shade.

To maximize the effect of clerestory windows, they should be located facing thermal mass walls, at a distance of between 1 and 1.5 times the target wall height. So for a 12-foot wall, the clerestories should be 12-18 feet away.

Clerestory windows have many benefits:

  • They allow a lot more light into the building, while reducing glare and improving privacy.
  • They increase solar gain in the living space and particularly in thermal mass placed on the north side of the home.
  • They are a great choice if the bottom-storey windows are obstructed by trees, neighboring homes or other structures.
  • They add to the overall aesthetic of the home.

However, there are also some disadvantages:

  • The extra light extends into the night, as clerestories capture street lighting, ambient light from neighbours' homes and even moonlight. This can make them a bad choice in sleeping areas.
  • They provide another exit for the home's stored heat – and since they're high up, they are closer to the warmest air in the space.
  • They are extremely difficult to shade, which means they generally remain uncovered at all times, thus increasing heat loss. You may need to install expensive powered shutters to reduce the losses.
  • They require very high ceilings compared to most homes, which means the living space contains a lot more air to heat. Vaulted ceilings always make home heating more difficult.

If you do decide to go for clerestory windows, you should choose the best possible model. The extra expense for quality will reduce your heat losses enormously, making them a more solid investment. Choose a model with low U-values, low rates of air infiltration and make sure they are double-glazed and gas-filled – even in warm climates, where the extra glazing will help keep the house cool. In very cold climates, go for triple glazing.

Clerestory windows arranged in a sawtooth pattern for very tall or wide roomsDon't forget the outside of the windows, either. South-facing windows take a serious beating year-round, so choose durable frames. If you opt for wood (which can be truly beautiful), make sure it is protected by metal cladding and anodized paint.

For very tall rooms or those which extend a long way from south to north, you may wish to install a multi-level sawtooth set of clerestories to maximize the amount of solar gain. This approach works very well in transferring a great deal of light and heat to large spaces, as well as being very aesthetically pleasing.