On Sat, 05 Jul 2008 23:38:49 -0400, email@example.com 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.
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.
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.
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.
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.
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
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?
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