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Need Help insulating Attic with Poly ISO


Biglurr54's Avatar
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Join Date: Jan 2012
Posts: 57
NY

01-13-12, 07:44 AM   #1  
Need Help insulating Attic with Poly ISO

I recently purchased my first house a 1830 post and beam fixer upper. Right now i am struggling to keep heat in the house. I purchased a ton of Poly Iso foam from a recycling company. The foam was used as a roof for commercial buildings. I got a great deal on 4 inch thick 4 foot by 4 foot sheets. I would like to put these in my attic where there is no insulation. I need to do a vapor barrier because the foil faces are in rough shape and I don't trust them to be a good continuous barrier. How can I go about adding this insulation to the attic with an effective vapor barrier? My thoughts were to put 6 mill poly sheeting down as a vapor barrier so it goes over the floor joists and then down into the bays. Then i will cut the foam to fit in the bays between the joists and spray foam them in place and then lay another layer over the top of them so i have 8 inches of foam. My only concern with this is that it will trap moisture around the joists. Will this be an issue?

 
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Biglurr54's Avatar
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01-13-12, 07:47 AM   #2  
another idea i had was to cut 6 mill poly sheeting to fit in the bays and tape it an inch or so up the joists so the joists can breath but the foam is protected.

 
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01-13-12, 10:36 AM   #3  
I like number two better, but before installing the rigid foam, address as much air leakage from below as possible. Even a small crack can allow moist air to circulate and continually dump moisture that can cause problems. If moisture gets through and into the wood, leaving it exposed will at least allow it to dry to the attic side. That would be a very small amount of moisture. The devil here will be in the details and cutting 4" foam is a PITA. The best tool I have for foam has been my battery powered circular saw as it is slow enough not to melt the foam, but I would have to cut from both sides for that thickness. Poly (PIC) may not melt as bad as the pink and blue I'm using. I'll add a link on air sealing from Vermont to give you some tips.

Being a very old home, is the construction what we call "balloon", where air can flow up through the wall cavities from basement to attic. If so, it is extremely important to properly block this path. Plus the method needs to meet fire codes as this is a chimney in case of a fire. Fiberglass insulation does not do a good job of blocking air flow, think mineral wool like Roxul if you are going to stuff something into those cavities and cap with drywall. You might also check code requirements to be sure what you use is approved. Same goes for that rigid foam.

Bud

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

 
Biglurr54's Avatar
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01-13-12, 10:59 AM   #4  
I am lucky enough to have original post and beam construction so the wall cavities can only move the air to the 8 by 8 beams. New sheet rock was put up for the ceilings about 10 years ago but they never put up a moisture barrier so there shouldn't be too many cracks to fill. I have further developed idea number 2. the new improved idea is to cut the vapor barrier to fit in the bays with 1 inch extra on each side. I will lay the foam in and then seal around each piece of foam with great stuff.

I have been cutting it with a circular saw hooked to my shop vac. I cut on one side then snap the piece. it snaps fairly evenly and has been working well with closing up other spaces. I use a sheet rock knife to do the odd shapes and that cuts great but it is messy and leaves a rough finish.

 
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01-13-12, 01:24 PM   #5  
They make those utility knives with extra long blades where you snap off a small section and have a new sharp edge to work with. Well those blades are rather tough and you can use the full length of the blade for some deeper cuts. I haven't broken one as yet, but I'm careful.

When leaving a gap to seal with can foam, I like to leave a slightly larger gap so I can get the foam down inside. I also use a piece of plastic tubing over the existing straw to extend it further down to where I want it. Use some good tape to secure the tubing to the straw so it won't blow off. And once some foam hardens inside the tube you must clean it before you apply any pressure.

Bud

 
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