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February 23, 2008

A Cathedralized Attic in a Hot Humid Climate - Is it Worth Conditioning?

This was the title from a colleague's paper recently presented at the Buildings X Conference. Yes, it's a little dry sounding, as it was written for a building scientist crowd, but I want to discuss some aspects of the type of construction used in the high performance house where this study took place.

The house was built near Orlando, Florida, and utilized a lot of best practices to make the house use much less energy than a conventional house its size. While it's bigger and more opulent than I'd personally choose to live in (let alone afford as a building scientist), this type of house is being built and showcased as a desirable product. It is nice to see attention to performance, despite the fact I wouldn't call it a “green” home. We're trying to make any difference we can. Modeling shows that it will use 39% less energy than a code-built house with the same architecture. The HERS score is 91, using the HERS 1999 methodology. The house is very airtight and has great windows, upgraded wall insulation, and efficient HVAC equipment.

So, that's a little of the general background on the house. Let's get back to the cathedralized attic and the issue of conditioning it.

A cathedralized attic (more or less) means that the insulation is located at the roof instead of the attic floor. This brings the attic volume into the thermal envelope of the house. A good way to accomplish this is to use some sort of sprayed-in foam insulation on the underside of the roof deck. Done correctly, this can provide a well-insulated space for location of the heating and cooling equipment and ductwork. Location in conditioned space is really the only way to optimize performance of the heating and cooling system. Note, that a conditioned basement, sealed crawlspace, or indoor mechanical room are other possible ways of installing the equipment in conditioned space. If the system and the ductwork are inside the thermal envelope, they will not be affected by the outside temperature and can provide better comfort with less energy loss.

The paper looked at whether there was any advantage to directly conditioning the attic space with supply and return registers. The alternative is to only provide ductwork to the living space and allow the attic to be indirectly conditioned by the spaces below it. His results show that it's better to allow the attic to be indirectly heated and cooled. He found that the system ended up overcooling and overheating some areas of the living space more frequently when the attic was directly conditioned. The energy use was also higher when the attic was conditioned directly. Note that this house is located in a hot, humid climate, and these conclusions may not hold true in different climates.

-e

Posted by Eric Helton at 5:16 PM | Comments (2) | TrackBack