solar chimney retrofit

[rccapps]

My question isn't PV-related, so I apologize if this is the wrong forum to post it in.

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 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

 

[svetz]

Have to love attics...I too have been battling with one too (blog), but for humidity/mold rather than heat.

I doubt you need to go to such trouble to "fix" your attic, most likely the problem is clogged or insufficient air-intakes. Insulation on the attic flooring would probably do wonders too. I like your idea about the black pipe to absorb more energy, spray painting the current vents "black" to make them start pulling air sooner might be a good idea.

But the attic might not be the real problem because the attic is small - so keeping it cooler won't help much. The real issue could be heat coming through the rest of the roof. It depends on if the attic covers the entire ceiling space or if most of the house has vaulted ceilings (for example my mother-in-law's small attic sits over the hall/bathrooms, the rest of the house has vaulted ceilings, that is the roof deck is one rafter from the ceiling).

A friend here needed a new roof, turns out the code here wouldn't let them replace the metal roof with a new metal roof. So the roofer, to get the permit, removed the old metal roof and the insulation and put on a bitumen roof. The new roof has no insulation above the roof deck. When summer rolled around it was so hot he had to augment his air conditioner to double its size.

Reflective roofs aren't necessarily cooler, you want to check the Cool-Roof product guide or similar source to know for sure. Roofers might know how to install a roof, but they seem to know shockingly little about energy efficiency. When we put the new metal roof on my mother-in-law's house, we had to insist on insulation under the metal (and really glad we did). On my roof, I went with Henry's Tropico White. My roofer was unfamiliar with it and had recommended an "energy-efficient" product they recommend that was 20% lower.

If it's humid where you live in Texas, you'd probably get a lot of mileage sealing any air leaks. Every ounce of water condensed is something like 60 BTUs of cooling lost (8,729 BTU/gallon), which robs you of cooling capacity.

 

[svetz]

Probably not the answer you were looking for...but installing solar panels might be a good solution for you as they'll provide shade for your roof.

 

[HRTKD]

When my roof was replaced I had the installers put in solar powered vents that I provided. I would say that they work well, but I don't have any numbers to back that up since I don't monitor the attic. I did have to add thermostats that told the vent fan to stay off when the temperature dropped down below a certain amount.

 

[rccapps]

Have to love attics...I too have been battling with one too (blog), but for humidity/mold rather than heat.

I doubt you need to go to such trouble to "fix" your attic, most likely the problem is clogged or insufficient air-intakes. Insulation on the attic flooring would probably do wonders too. I like your idea about the black pipe to absorb more energy, spray painting the current vents "black" to make them start pulling air sooner might be a good idea.

But the attic might not be the real problem because the attic is small - so keeping it cooler won't help much. The real issue could be heat coming through the rest of the roof. It depends on if the attic covers the entire ceiling space or if most of the house has vaulted ceilings (for example my mother-in-law's small attic sits over the hall/bathrooms, the rest of the house has vaulted ceilings, that is the roof deck is one rafter from the ceiling).

A friend here needed a new roof, turns out the code here wouldn't let them replace the metal roof with a new metal roof. So the roofer, to get the permit, removed the old metal roof and the insulation and put on a bitumen roof. The new roof has no insulation above the roof deck. When summer rolled around it was so hot he had to augment his air conditioner to double its size.

Reflective roofs aren't necessarily cooler, you want to check the Cool-Roof product guide or similar source to know for sure. Roofers might know how to install a roof, but they seem to know shockingly little about energy efficiency. When we put the new metal roof on my mother-in-law's house, we had to insist on insulation under the metal (and really glad we did). On my roof, I went with Henry's Tropico White. My roofer was unfamiliar with it and had recommended an "energy-efficient" product they recommend that was 20% lower.

If it's humid where you live in Texas, you'd probably get a lot of mileage sealing any air leaks. Every ounce of water condensed is something like 60 BTUs of cooling lost (8,729 BTU/gallon), which robs you of cooling capacity.

Thanks for the feedback, svetz!
You speculated about the attic covering (or not covering) the entire ceiling space. Even if there were a hatch/door I could use to get into the attic (there isn't), I'd barely be able to squeeze thru at a crawl - that's how small it is. But it does cover the entire ceiling of the single-story rectangular-footprint home, with several-inch wide soffits all around.
Your idea about clogged or insufficient air intakes is an intriguing one. The only intakes I saw were grills/registers spaced roughly 10 ft apart all along the eave soffits. I haven't popped any of them open (saw no need until now), but I DID open all 4 roof/attic vents, and saw a bunch of loose "dust bunny" style insulation inside the pseudo-attic. Not knowing how the pseudo-attic was built, I wonder if the loose insulation could be blocking the soffit grill intakes...

 

[rccapps]

Probably not the answer you were looking for...but installing solar panels might be a good solution for you as they'll provide shade for your roof.

Another great thought, svetz. But we do have 8 x 375W PV panels rocker-mounted to the south side of the roof, shading approximately one-fifth to one-sixth of the entire roof (it's not a big house). We could always use more power, so we may expand the PV array in the near future.

 

[svetz]

The only intakes I saw were grills/registers spaced roughly 10 ft apart all along the eave soffits. The image to the right might help to determine if you've the proper ratios (click it for a bigger image). But, I think that's for northern climates. It has 1:1 soffit to vent. You might need 2:1. Lstiburek is the "man" I'd trust, he says [ref] The amount of attic cavity ventilation is specified by numerous ratios of free vent area to insulated ceiling area ranging from 1:150 to 1:600 depending on which building code is consulted, the 1:300 ratio being the most common. Your local codes might have something more appropriate for your location, but I'd say at least 1:300. Having less seems more to do about keeping warm in winter. Saw a reference that southern California required 1:150. ...saw a bunch of loose "dust bunny" style insulation inside the pseudo-attic. Not knowing how thepseudo-attic was built, I wonder if the loose insulation could be blocking the soffit grill intakes... I had blown insulation in my attic when I lived in Colorado and it was always getting over the soffit grill intakes. Used to use compressed air to "blow" them free. I'm not sure why, possibly the whole-house fan that exhausted into the attic or critters?

 

[Wellbuilt]

In Texas you really need power vents .
2 solar vents will go a long way to cooling your attic . If you look a steves pic , notice the air vents on the bottom of the roof . These need to be clear .