The passive solar home: fed up with high energy bills? Make your house into a solar collector! Architect Chris Larson tells how

New Life Journal, April-May, 2002 by Chris Larson

What attracts me to passive solar design is its simplicity. Except for sunshine and the change of the seasons, there are no moving parts. In the winter, I'm lying on the couch, and the house is heating up. In the summer, I'm lying on the couch, and the house is keeping cool. The energy performance and comfort of any house, (or commercial or institutional space, for that matter) can be greatly enhanced by the common sense principles employed in passive solar design.

Remember all those whacked-out solar houses of the Seventies when solar house design was first getting going? They looked like solar collection machines that somehow people lived in. With the help of technology and experience, passive solar houses today can enjoy the warmth and comfort of solar heat and still look and feel just like home. In the our region, passive solar design usually means using the sun to help heat the house, but it can also refer to passive cooling strategies. For the purpose of this article, I have limited what follows to what I consider to be the easiest and most cost-effective means of passively heating a house: using living space as a solar collector. Other Strategies such as greenhouses, atriums or Trombe walls are related but can be part of another discussion. Also, much of this article relates to new construction, but many possibilities exist for renovation of or additions to existing houses as well.

The first consideration in building a passive solar house is its orientation to the sun. Obviously, access to the sun is critical. A sloped site that is south-facing is ideal. If significant amounts of winter sun reach an east, west, or even north facing slope, don't despair, there still may be great potential, but special considerations may be in order. Ideally, the desired views from the house would be in the same direction as the sun, but again, this is not required. Also, keep in mind, not every room in the house needs to face south. It may be helpful to have a prospective house site reviewed by someone with expertise to determine if the site has the potential for what you intend to accomplish.

The concepts of passive solar design are simple: capture the heat of the sun and then hang onto it. To capture the heat of the sun, make the east-west axis of the house as long as possible, and open up the south side with windows. A good rule of thumb would be to have 10-15% of the floor area of what you are trying to heat in south facing windows. Exactly how this works will depend somewhat on the specific layout of rooms. So, if you are are trying to heat a 2000 square foot area, this might be 200-300 square feet of south-facing windows. This also means being careful with the location and extent of covered porches on the south side because they will block the sun.

An ideal orientation for the house would be to have its long dimension face ten degrees east of south. This will 'allow the sun into the house a little earlier in the day in winter and help block out the low afternoon sun in summer. Siting the house as much as thirty degrees either way of south is possible with only a ten percent reduction in efficiency, but much beyond this thirty degrees will adversely effect performance.

So, now that the heat is streaming inside all those south facing windows, there are two components to keeping it in. It doesn't make much sense to let the heat in if it's going to be going out just as quickly. The first component of this involves examining the performance of the exterior shell of the house. There are two parts to this as well. The first looks to reduce the paths that heated air has of escaping to the outside--i.e. tightening up the exterior shell--but don't make it so tight that indoor air quality becomes a problem. A consideration may be the type of windows used: casement windows are less leaky than double hung windows. Another consideration will be the building and insulation systems--some are inherently less leaky than others. The second part of examining the performance of the exterior shell of the house involves optimizing the resistance of exterior building components to the flow of heat. For windows, this may mean double glazing with low-E coatings, or for some projects, triple glazing. For walls, this may mean looking at ways to reduce thermal bridging or increase insulation R-values. Whatever building system you are considering--straw bale, timber frame, steel frame, aerated concrete block, insulated structural panels, or conventional framing, each will have its own built in advantages and disadvantages in the area of building physics. The important aspect of this is to consider everything together as a whole.

The second component of keeping the heat in is adding mass to the house. This added mass is tuned to the amount of south facing windows and serves to absorb the heat of the sun to be released after the sun has gone. Absorbing this extra heat controls overheating and keeps indoor temperatures stable. To qualify for the passive solar North Carolina tax credits, this mass must be at least three inches thick and have the sun shining directly on it. A concrete slab, either suspended on framing or placed on grade with a tile or stone surface are easy ways to do this. There are other ways, as well. The whole house does not have to be this type of hard surface, and placement of rugs and furniture over these areas can be accommodated in the design. An energy efficient house with south facing windows but without the added mass is not technically a passive solar house, but is referred to as a sun tempered house.