How can we evaluate how well built and effective an existing passive solar house is?
GBA Editor
| Posted in Green Building Techniques on
We’re considering purchasing a passive solar house and wondering how to evaluate how well built and effective it is. The house was built in 1983 by a local contractor and is located in the Portland, Oregon metro area on a south facing hill. We’re also interested in learning what might be done to further utilize and exploit the house’s passive solar strengths. In addition to written responses, we’d welcome suggestions of other source materials (books and other print sources), as well as individuals or companies who we might utilize in evaluating the house and, in the event we purchase it, improving it.
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Replies
Travis,
1. Hire an experienced HERS rater to inspect the house. The inspection should include a blower door test and an evaluation of existing insulation levels. Ideally, you want to have R-40 walls and R-60 ceilings, but these levels are unlikely for a 1983 house.
2. Ask the HERS rater to evaluate the windows. Triple glazed is obviously better than double-glazed. An experienced inspector should be able to tell you whether the windows are low-e.
3. Interview as many previous occupants as possible. Ask them whether the house ever overheated in February and March.
4. Calculate the ratio between the south glazing and the floor area of the house. If it's more than 7% or 8%, ask questions about the thermal mass of the house.
Try and get previous energy bills for the house. Tie this data in with heating degree days for the corresponding period and you will have a good idea of the efficiency and quality of the house.
In addition to the HERS rater, who will give you a good idea of the thermal efficiency and air tightness of the house, I would also hire a house inspector to evaluate the integrity of the structural and mechanical systems.
In a passive solar house, which means 7-12% of the floor area in south glazing and some additional thermal mass to store diurnal solar gains for evening release, triple-glazed windows are not necessarily better on the south facade. All my energy analyses for the northern New England area (7000-8500 DD with 50% solar availability) have shown that double-glazed lowE windows with high solar heat gain coefficient (SHGC) are the optimal south windows.
Properly designed south overhangs for summer solar shading are also essential to prevent overheating in the warmer months when the sun is high and to allow full winter sun when the heat is needed. The landscaping and house siting also have to minimize the shading of low winter sun.
Beyond those two professional appraisals, I would also contact other homeowners who live in houses built by the same builder to get a sense of quality, livability and maintenance issues.
Robert...why do you think it is that your energy analyses favor double glazed over triple glazed?? In any comparisons that I have done using Canada's Energy Rating system for windows, triples with high SHGC's almost always rate the best...just curious.
Canadian windows are significantly better for passive solar applications than anything easily available in the States. Even double-glazed lowE windows here have relatively poor SHGC (often less than 0.5, sometimes as little as 0.4) and triple glazed lowE (as little as 0.25) are useful only on non-solar orientations.
Even the current US federal tax credits, aimed at increasing the thermal efficiency of homes, require windows to have a SHGC of 0.30 or less!!!!!!!!!!!!!!!!!!!!!!!!!!!!