07-19-12, 08:37 AM | #11 |
Lex Parsimoniae
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There are tar-like stains on the outside of my house, around the vents. That's caused by out-gassing from the (1956) wood, and maybe roofing tar paper. I have climb a ladder with pressure-washer sprayer to clean those stains. The hot air in my attic accumulates in the peak, and had little effect on the ceilings of the living space. But, as the day wears on and the peak hits 120f, the air down at the insulation becomes very warm. I think the hot air just soaks into the insulation, warming the wood and the sheet rock under it. Radian heat from the roof bombards the insulation directly. The insulation and the ceiling it covers, warms to the room temperature, of the attic.. Before long, the living space starts to warm up. I can reach up and feel the warmth of the ceiling. Radian heat is raining down. When possible, we pull down the attic stairs and let cooler air updraft into the attic. But, when we have the AC running and the house is sealed, there is very little natural outflow of air. Picture a can with two holes in the top, sitting in the sun. At first the air expands with the heat and some air flows out. But once that's over, stagnate hot air will just sit in the can. So, installing a fan in the east side vent (see pic above), sucks hot air out, and pulls cooler outdoor air into the attic from the west side vent. That air flow keeps the 120f air off the attic floor, and helps keep my living room ceiling cooler. So, my Sanyo ASHPs don't have to work as hard..
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07-19-12, 09:07 AM | #12 | |
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This is probably all be good logic, but Soutface.org, which is generally a pretty good outfit has some reservations about powered attic vent fans: http://www.southface.org/factsheets/...s%2000-771.pdf They mention two things The power used by an AC powered vent fan might be more than the saving. Obviouosly this would not apply to a solar powered one. If the living space ceiling is not sealed well, the powered vent fan might lower the pressure in the attic enough to pull conditioned air from the living space up into the attic. This seems like a valid concern to me as a lot of homes don't have well sealed ceilings. Gary |
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07-19-12, 09:55 AM | #13 | |
Home-Wrecker
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Air flow takes the path of least resistance. Houses like mine will have vents on the underside of the roof overhang. That's how air gets into the attic. Air going out the ridge vent, end vent, or powered attic vent, comes IN through these vents on the bottom of the overhang. Obviously, start with the "low-hanging fruit". As a general disclaimer on all home projects - start with the easy and cheap stuff - caulking, weatherstripping, and insulation. If your powered attic vent is working so hard that it's sucking air out of conditioned space, you have worse problems!
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07-19-12, 10:21 AM | #14 | |
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What type of fan are you using at your vent, how is it powered, and switched on/off? |
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07-19-12, 10:50 AM | #15 | |
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Not sure that's true. I've read elsewhere that air movement from the living space to the attic space is the single largest source of infiltration/ex filtration in a lot of homes. I'll try to find that reference. I know when I added insulation to my attic I first pushed the old insulation out of the way and sealed every penetration for wires, plumbing, vents, can lights etc. -- there were lots of them All those little holes add up, and even if most of the flow out the fan is coming from the attic vents it seems like it would not take a lot of flow of conditioned living space air to offset the benefits of the attic fan? Maybe someone can find some measurements on real attics? Gary |
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07-19-12, 11:22 AM | #16 | |
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FSEC-GP-171-00 Home Energy Magazine - Ventilation :: Drawbacks Of Powered Attic Ventilators The FSEC study found a good reduction in attic temp (20F) and about a 6% saving in AC costs. But, the AC ducts in this house went through the attic and that may have been a big contributor to the AC saving. They found a payback period of 20 years, but they paid a lot for their PV powered attic fans. The HomeEnergy on looked at 8 homes and found that powered attic vents did depressurize the attics and caused significant flow from the living space to the attic. Not sure what kind of ventilators they were using in this one, and they may have been more powerful than the typical solar ones. They also found some other problems associated with the power ventilators. There are a bunch of refs out there saying that AC powered attic vents don't payoff. Gary |
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07-19-12, 11:24 AM | #17 | |
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Just wondering what an "attic tent" is -- is that a radiant barrier? Gary |
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07-19-12, 11:53 AM | #18 | |
Lex Parsimoniae
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Looks a bit like this one. When I first installed it, I was standing on the ground about 18 feet below it, and I could feel the blast of very hot air coming down on my head!! It was installed during the 1980s and I'm amazed that it's still working after all these years.. One thing I found out when my old roofing shingles were replaced, the tar paper underneath was so brittle, it crumbled like saltine crackers. My Roofer told me that excess heat ages shingles a lot faster. He won't install new shingles without installing a ridge vent, because of heat damage.
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07-19-12, 04:50 PM | #19 |
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I just measure the temperature in my attic, midway between the roof and the rafters it's 138 degrees F. The outside temperature is 110 degrees, in the shade. Does that warrant additional attic ventilation?
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07-19-12, 06:28 PM | #20 | |
Lex Parsimoniae
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Yeah, I would consider a ventilated attic and maybe a reflective barrier to bounce back the radiated heat from the roof.. Weather Station History | Weather Underground
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