It's Hot Upstairs More Insulation or Attic Fan?

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On Sat, 05 Jul 2008 23:38:49 -0400, snipped-for-privacy@mucks.net wrote:

If that were true, why did putting the roof fan lower the temperature of my second floor by more than 10 degrees? When the attic fan is out of service, and the motor only lasts between 3 years and 10 years**, so it's out of service for a couple days when it breaks, and I stand on a chair and put my hands near the ceiling, it's terribly hot. If I were to do that when the fan is working, it would only be a little hotter than at the 4 foot level. (I have 8 foot ceilings.)
Now maybe I could benefit from more insulation in the attic (i have one or two inches of cellulose plus one batt) but it's still not ture that because heat risess, the heat in the attic will have little effect on the floor below.
**It's amazing to me that some motors lasted only 3 years and others 7 to 10. I first sent away to the fan maker, but now I just buy new ones at the motor shop in town.

My comparisons are drawn with no AC in both situations. The only difference is whether the fan in the roof is running or not.
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mm wrote:

You are correct, especially after dark. That big mass of superheated air and building materials called an attic, radiates heat back into the 2nd floor until well after dark. Keeping attic vented, and adding insulation to provide a thermal barrier, does a lot to keep the 2nd floor cooler. Even in this one-story house, I noticed a big difference after adding more attic insulation and improving the attic venting. I don't have to immediately set the a/c on full when I get home from work anymore, and ceiling is no longer warmer than walls to the touch.
-- aem sends...
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Likely because the vent system is poorly designed or executed and you are not getting enough natural air flow. Too little vent area or partly blocked vents or poorly placed vents causing short cuts.
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Tom, the fact is that fiberglass is useless as an insulation as, the heat/air move through it too easily. What is needed is a closed cell polystyrene insulation sprayed/poured between the joists in the attic A five inch thick layer of polystyrene or similar poured between the joist will spread into every gap and seal all the holes that are letting the heat in. Perry.
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PerryOne wrote:

Hmmm. Not useless! All insulating material is per it's R value. Nothing in this world is 100% insulator, whether it's heat or eletricity. Also you have to consider cost per R value. I still say first thing is to have proper attic ventilation. Soffit, roof vents, proper baffles under the eave between roof trusses. Insulation material shouldn't block air flow for proper venitilation. I often see covered baffles by blown in insulation deafeating the purpose of baffles.
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Not at all useless. Just compare R values. That is the measure of their value. Good well applied foam can be better per inch, but it is the total R value that counts. The best for the money is usually fiberglass. Blown in has some additional advantages but mostly in the winter, not summer.
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Sounds like more insulation or a radiant barrier under the roof would help.
Warm air rises. Whatever a fan can do, more attic ventilation can do... For instance, 250 Btu/h-ft^2 of sun shining on a 1200 ft^2 roof with 60% solar reflectance over an attic with a 1000 cfm fan and a roof with a US U2 (2 Btu/h-F-ft^2) conductance to 100 F outdoor air makes an equivalent electrical circuit like this, viewed in a fixed font:
T = 100+I/1000 = 135.3 F attic temp | 0.5/1200 | 1/1000 ----------www----------www------ 100 F outdoor air | -------------------> | I = (150-100)/(0.5/1200+1/1000) = 35.3K Btu/h | | 100+250(1-0.6)0.5 = 150 F sol-air temp --- - | -
We can also make 35.3K Btu/h flow with 2 A ft^2 vents with a 35.3 F temp difference and an 8' height difference if 16.6Asqrt(8)(35.3)^1.5 = 35.3K, ie A = 3.58 ft^2, eg 2'x2' high and low gable vents with doors to close them up in wintertime.
Nick in PA
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Thanks, everyone, for all of the advice.
We do have some blocked vents (but not a lot) so I'm clearing those out. Also plan to add insulation as a first step, then maybe do the attic fan (gable vent fan) as well.
Here's another question. The only 24" width fiberglass insulation (which is the size I need) available at the local home stores has the kraft paper backing. I'm putting this on top of existing insulation so if I use it I guess I'd have an additional vapor barrier between the original and the new insulation. Is that a problem?
Thanks again!
Tom
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You dont want another vapor barrier, use unfaced only on top, what zone are you, what R value do you have now. To see what is optimal manufacurers have optimal tables on R values.
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On Wed, 9 Jul 2008 13:31:31 -0700 (PDT), ransley

I've been known to pull the facing off of insulation. If the OP is limited in supply, it is a option. (don't know cost difference in faced and un-faced).
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Or just slit it a few times with a knife.
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