In searching the web, I have gotten so much mixed advice on how to properly ventilate an attic to rid it of excess summer heat. I own a modular home and if I poke my head up in the attic, it’s crazy how hot it is. I’d bet probably 120deg or so. There are two gable vents that appear to be almost non-functional with very small slots, and the soffit vents are very small perforations in the vinyl siding. Is there any reason I don’t want to install powered fans in the gables vs. natural convection? I’m planning on replacing the soffit vents with ones that actually work.
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There's a can of worms. You won't get much consistent advice here, either. The consensus will be that gable vents aren't worth a bucket of warm spit, and that the only effective way to vent an attic is with balanced ridge and soffit vents.
I will say that attic ventilation is itself misguided and ineffective at handling what is at its root a radiant heat issue. Convective cooling as ventilation provides is nearly always overwhelmed by the amount of energy the sun provides. Powered ventilation is generally a bad idea, unless you've provided sufficient make up air from soffit vents. Good luck with this unless you use chicken wire for the vent covers. Without sufficient outside make up air, powered ventilation depressurizes attics and sucks air you've paid to condition from inside the house or inside the leaky ducts that likely exist in your attic. This is bad.
I will argue that insulation is your best answer. Not fiberglass either, which does little to combat radiation. Cellulose that covers the existing insulation and ceiling joists will help a lot. But if you've got ducts and/or air handlers in the attic, the best solution is to bring the attic into the thermal envelope by foaming the underside of the roof deck. This creates what's known as a hot roof.
For more info, go to http://www.buildingscience.com, http://www.affordablecomfort.org, http://www.pathnet.org, and http://www.homeenergy.org.
Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Anohter good source on roof color, attic insulation, radiant barrier, etc is the Florida Solar Institue. Don't have a link for them.They have published several sumamries of their work in that "other publication" <G> (JLC).
Yep, you're right.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Just discussed this with neighbor right after I fixed his AC yesterday.
He has approx 450 sq feet of south facing roof slope so that in direct sunlight @ 1 PM sunlight the math works out to about 20 kW heat absorbed that has to be convected away.
20 kW = about 68,000 BTUs/hr or 1100 BTU per minute!
1 cu ft of air with a 20 F deg rise (keeping attic under 100 F) hold only about 0.4 BTU, so airflow from a vent fan needs to be almost 3000 CFM!
So, trees are best. Son's house is in full sun, so did install a surplus furnace squirrel cage fan in his house.
I recently stumbled on a great analogy comparing the efficiency of radiation v. convection. How long does it take to warm a bowl of chili in the microwave (radiant)? How long does it take to heat that same bowl in a conventional oven (convection)? What's the energy input to a microwave? Perhaps 15 amps at 120v? And for a standard oven? Say, 25 amps at 240v?Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
microwave (radiant)?Unfortunately the analogy has a lot of 'holes' as the microwave radiation is in a resonant cavity and the chili is a slight electrical conductor with a high dissipation factor at 24.9 GHz. e.g. You can never heat a pyrex glass bowl in a microwave, but can in a hot oven. Say a 2# black glass bowl is set in the sun and is 1 sq foot in area. It will absorb about 100W (341 BTU/hr)from the sun if place perpendicular to the sun. Depending on air temp and breezes, it may warm up by 40-50F, convection = radiation at that point.
I never said it was a perfect analogy <G>
Roofs are a special case, though. They're great radiant collectors, while the convective cooling possibilities are limited.
Elk now makes shingles with granules said to reflect 25% of incoming radiation. Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
A natural convection attic cooler can be made, just make a fake chimney, the taller it is, the more air it draws.
How many of you guys ever cut a hole in the top portion of the roof when the attics hot. Hot air rushes out so fast at the top of the roof, that it makes those turbine vents furiously.
Make sure the chimney vent is at the highest portion. There should be unblocked eave vents in the near and far corners of the attic. Rig up a chained damper to close it off during the winter.
What do you make of the info and product here: http://www.savenrg.com/venting.htm? Straight up info or marketing BS?
The advice at http://www.savenrg.com/venting.htm seems correct. Its the price of the fan and its wattage rating that's in question. There never seems to be any CFM to watts rating on any of the "energy" attic fans. Since a lot of compressors are becoming more efficient, maybe you can cannibalize one of those fans.
Hi,
If you decide the venting is not going to work for you, here is another approach to getting attic temperature down:
http://www.fsec.ucf.edu/bldg/pubs/pf293/index.htm
The reflective roof coatings reduced attic temperatures by about 30F, and reduced AC energy used by about 20%
Gary
http://www.BuildItSolar.com
Good point. On new construction, another option is sheathing the roof with TechSheild. One side has a foil face, which faces inward. Lots of people think this is a reflective surface, but that's not how it works. As explained to me, it's actually a low emissivity (low-e, as in the windows) surface that radiates only about 4-5% of input heat from the sun (or whatever source - that doesn't matter). So, unlike reflective surfaces, it continues to work even when dirty, as long as there's an air space below to short circuit conductive transfer.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom