solar chimney retrofit
Our manufactured home has a small pseudo-attic space with four squat vents thru the roof and no access from the inside. Despite installation of reflective roof panels (as well as the aforementioned vents), this space still traps a lot of heat in the east Texas summer, much of which seeps into the living space below.
Replacing at least one or two of the short and ineffective vents with solar chimneys appeals to me as a relatively simple fix. However, I have no experience with them, merely an understanding of the general principles involved. Would wedging a screened and capped-off length of black stove-pipe thru the vent hole(s) work well enough? If so, how tall should they be for a reasonable amount of draw? Any other design considerations I should be aware of? Also, any “RTFM” replies with relevant resource links/references (books, websites, etc) would be welcome.
Thanks in advance,
Rob
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Replies
Generally what you want is an equal area of vent on the highest part of the roof and the lowest part of the roof. Cool air comes in at the bottom and goes out at the top. Without some way for air to enter you don't get much air flow.
The other aspect to look at is insulating and air sealing your ceiling to keep as much heat out as you can. If the ceiling is drywall that's a pretty good air barrier, sealing can be as simple as caulking around everything that goes through the drywall. Attics are generally easy to insulate because you can just pour loose insulation in on top of the ceilling.
Depending on construction you may be able to fill that space with insulation.
DCContrarian:
Thanks for the quick feedback!
I haven't made careful measurements, but there are soffit vents every 10 feet or so, whose combined opening area seems roughly equivalent to the combined openings of the four larger roof vents. So there appears to be a path for airflow through the pseudo-attic already. Could it be that our addition of reflective roofing is hampering or outright preventing any existing chimney effect (the original roofing was tarry shingles, by the way)?
In any case, even if the ceiling is not well-sealed - maybe ESPECIALLY if it isn't - if the draw thru the roof vents is sufficient, shouldn't it also suck the hot air up out of the living space? Hence my interest in improving the draw with one or more solar chimneys. In fact, instead of sealing the ceiling better, wouldn't that suggest adding adjustable ceiling vents might improve the draw from the living space?
Thanks again for the feedback!
Once your attic is reasonably well vented, increasing the venting won't reduce the attic temperature much. A typical vented attic would see somewhere around 5ACH of flow, doubling this to 10ACH would lower your attic temperature by around 5F. A roof absorbs way too much heat from the sun for you to be able to cool it with reasonable airflow rates.
Best way to deal with heat from the roof is to add extra insulation.