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Old 05-20-2013, 10:31 PM   #1
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Fiberglass vs open cell foam in attic under roof


Hi. I have a 20 yo home in New Hampshire. The unfinished attic has insulation in the floor of the attic but none at all between the joists. There is a ridge vent and 3 whirlybird bird vents in the roof. I have a whole house fan that we don't use. My issue is that I have multiple Unico high velocity AC ducts running along several joists which get absolutely baked in the summer from the adjacent hot roof-- they all had to be recently replaced due to saturation from condensation at quite a cost.

My question: would you use R38 fiberglass batts with foam vents between the FG and roof between the joists, or go for the r37 icynene open cell foam at roughly double the price to protect these unico AC ducts (and to save a little $ on heating). Icynene cost is double the FG.

Thanks.

Last edited by Nhfixit; 05-20-2013 at 11:48 PM.
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Old 05-20-2013, 11:00 PM   #2
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Fiberglass vs open cell foam in attic under roof


All you needed was the ridge vents.
The other vents defete the whole reason for the ridge vent.
The air just getting sucked in from one vent to the other instead of drawing from the soffit area, pulling out the hot moist air.
How much insulation is up in the attic, in your area there should be R50 which would be about 12" of insulation.
Just insulated the ducts, not the roof.
Only time you would insulate the roof is if the attic was going to be converted to conditioned space.
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Old 05-20-2013, 11:48 PM   #3
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Fiberglass vs open cell foam in attic under roof


Thanks for the reply. Neglected to mention the house has a whole house fan in the attic which we never use (prob why the whirlybirds are installed)--didn't think it made a difference. Guess I was wrong. The ac vents are insulated, but I guess not enough, since it was the insulation around them that became saturated. FG insulation in the attic floor is exposed where the roof converges on the soffit area and the FG is packed in right over the soffit vents--the have stuck in a few foam roof vents into that area, presumable to let air flow around the FG. Is that a normal setup?
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Old 05-21-2013, 08:06 AM   #4
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Fiberglass vs open cell foam in attic under roof


a few questions:

do you have soffit vents?

what is the depth for your rafter bays?

how much insulation is on the ducts?

have you had a leak test performed on your a/c system?
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Old 05-21-2013, 09:48 AM   #5
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Fiberglass vs open cell foam in attic under roof


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FG is packed in right over the soffit vents
Above the soffit vents need to be clear so air can flow into your attic. You need to clear that out. You won't have air flow. That's makes your attic hotter which transfers more heat into your AC runs, and decreases the life of your shingles.
But a hot attic does not make AC vents wet. That happens from warm moist air hitting a cold surface (the duct). Or possibly cold air escaping the duct and hitting the moist warm air. Sounds more like you have (or had) a duct insulation problem. Did you discuss your concern with the outfit that replaced the duct insulation? Will they warranty that their insulation will not have to be replaced due to getting wet?
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Old 05-21-2013, 10:01 AM   #6
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Fiberglass vs open cell foam in attic under roof


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Originally Posted by Nhfixit View Post
Hi. I have a 20 yo home in New Hampshire. The unfinished attic has insulation in the floor of the attic but none at all between the joists. There is a ridge vent and 3 whirlybird bird vents in the roof. I have a whole house fan that we don't use. My issue is that I have multiple Unico high velocity AC ducts running along several joists which get absolutely baked in the summer from the adjacent hot roof-- they all had to be recently replaced due to saturation from condensation at quite a cost.

My question: would you use R38 fiberglass batts with foam vents between the FG and roof between the joists, or go for the r37 icynene open cell foam at roughly double the price to protect these unico AC ducts (and to save a little $ on heating). Icynene cost is double the FG.

Thanks.
OK a few things just FYI for you you want under stand. 1 you will recoop the cost of the foam in about 10 years. you spray that on the sheathing of your roof. OK now say your roof fails some how were does the water go? how does your roof breath? how does water damage from a leak dry out on the sheathing? Spray foam for attics were I live is not code it causes way to much problems then it would solve. I would do two things 1 blow in insulation 2 make sure the duct work is insulated.
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Old 05-21-2013, 10:11 AM   #7
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Fiberglass vs open cell foam in attic under roof


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Originally Posted by Nhfixit View Post
the have stuck in a few foam roof vents into that area, presumable to let air flow around the FG. Is that a normal setup?
You mean baffles? Yeah the traditional way to do that is to install baffles, which are channels that allow air to move from the soffit to the ridge vent and prevent blockage. You should have them at every available spot.

And definitely plug every other vent in your attic. They are destroying the suction effect you get from soffit to ridge vent.
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