Rigid insulation in the attic.


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Old 07-08-14, 08:57 PM
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Question Rigid insulation in the attic.

I just received quite a bit of Polyiso mp-h tapered boards for free (overstock from a commercial construction job) and I was hoping you guys could give me an idea on what is required for using it in my attic space.

I have a 1970s Ranch style home with a low roof and little roof insulation. I had planned to go the loose fill option but then received a couple thousand square feet of this material free of charge.

I don't plan on making the attic space liveable so I'm mostly thinking of use it on the floor of the attic to add some more insulation to the house.

I'm an amateur woodworker and diyer so outside of handwork it shouldn't be a problem but I'd love some professional insight into the subject.
 
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Old 07-09-14, 06:00 AM
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I izpep and welcome to the forum.
AZ has several climate zones, so do you ac, heat, both, or predominately one? Do you have and ducts from ac or heat in that attic?

Tapered foam is intended for flat roof applications where the taper results in a bit of a slope. How thick is your foam and how much does it taper?

What is up there now for insulation?

Bud
 
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Old 07-09-14, 07:27 AM
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Foam is about 1 3/4" thick says "C" taper and it's made by Versico and rated for 20psi if that helps

The house has very little insulation in the attic. Just some 40 year old pink batting in the ceiling. Nothing else in the way of insulation for the attic. I'm in North Phoenix so we use A/C & Heat but mostly it's A/C we have an issue with.

We have ductwork in the attic as well. My first thought was to set it down flat and cut it to fit the attic floor then tape the seams.

Any advice?
 
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Old 07-09-14, 08:07 AM
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What you suggest has some delicate issues, mainly creating a condensation plane. Assuming no effective vapor barrier below the attic floor, moisture will migrate up until it meets some resistance. If the foam is not sufficiently thick (50% of total r-value) then the bottom of the foam may drop below the dew point and liquid water may form. A lot of if's and maybe's as much cannot be determined from 2,000 miles away.

If you cut to fit and foam sealed what you have into each joist cavity and then re-installed the fiberglass over the top then the high resistance layer would be first.

Perhaps some others can suggest more.

Bud
 
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Old 07-09-14, 03:30 PM
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I figured i might need to install a moisture barrier. Anyone have any suggestions on what materials I'd need?

Be nice if a sheet of plastic on top would do but I'm pretty sure it'll be more complicated than that.
 
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Old 07-09-14, 04:52 PM
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Any air leakage from below would allow warm humid air to seep up and condense on a plastic barrier on top. Winter conditions, the VB goes to the warm side, next to the drywall. But then comes the summer conditions when the ac is running and the warm side is the top. The paradox is somewhat resolved by not using plastic.

The number one mode for moisture transport is air leakage. Air seal the ceiling plane, link below, and don't worry about a vapor barrier. Also air seal and insulate and ducts up there.

http://www.efficiencyvermont.com/ste...ide_062507.pdf

Bud
 
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Old 07-16-14, 07:58 AM
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Sorry took a bit to reply. Work was catching up on me. Next paycheck I should have enough to cover the cost of installation.

Do you have any advice of the type of foam adhesive and tape I should be using for this project?

Also should I bother screwing each piece down or just glue and lay them flat on the joists?

I apologize for all the questions. Just want to make sure I get it done properly since I'm using some rather odd material for inside the attic insulation.

Was also thinking of replacing all the ac duct work while up there since it's also a relic of when the house was made.
 
 

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