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Old 01-09-2012, 07:37 AM   #1
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Need Help with Vapor Barrier


I just purchased my first house. Its a 1830's farm house in need of some TLC. I live in upstate NY so my first concern has been getting heat in the place. I have an Outdoor Wood Boiler providing plenty of heat to the place. My problem is the heat is all escaping. I bought some re-used Poly Iso foam. It was a great deal for 4 inch thick sheets. I have enough to do 8 inches in the roof and 4 inches thick from the sills down the floor of the drafty laid rock foundation. My question is how should I lay this stuff in to have an appropriate vapor barrier. My understanding is that because it is foil faced it is its self a vapor barrier. I was going to cut the 4 ft sheets in half fold them on top of each other and lay them between the rafters in the attic. i would then seal around the edges with great stuff. This will create multiple vapor barriers. Should I lay a poly vapor barrier down, lay strips of lath to raise the foam off the poly vapor barrier, perforate all foil faces, lay 4 inches in, lay another layer of lath , and finally lay the last 4 inches of foam. This will give me R-56 in the roof and R-28 in the basement. Please let me know that the best options are for this application. I really appreciate the help!

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Old 01-09-2012, 08:49 AM   #2
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I just purchased my first house. Its a 1830's farm house in need of some TLC. I live in upstate NY so my first concern has been getting heat in the place. I have an Outdoor Wood Boiler providing plenty of heat to the place. My problem is the heat is all escaping. I bought some re-used Poly Iso foam. It was a great deal for 4 inch thick sheets. I have enough to do 8 inches in the roof and 4 inches thick from the sills down the floor of the drafty laid rock foundation. My question is how should I lay this stuff in to have an appropriate vapor barrier. My understanding is that because it is foil faced it is its self a vapor barrier. I was going to cut the 4 ft sheets in half fold them on top of each other and lay them between the rafters in the attic. i would then seal around the edges with great stuff. This will create multiple vapor barriers. Should I lay a poly vapor barrier down, lay strips of lath to raise the foam off the poly vapor barrier, perforate all foil faces, lay 4 inches in, lay another layer of lath , and finally lay the last 4 inches of foam. This will give me R-56 in the roof and R-28 in the basement. Please let me know that the best options are for this application. I really appreciate the help!
Foil faced iso is a Class 1 vapor retarder (vapor barrier.

Are you treating your attic space as conditioned space? If not, why the foam in the rafters?

Perforating the foil layer does not change the fact that it is still a Class 1 vapor retarder.

Please detail your basement plans a bit more clearly and tell us more about the construction of the home.

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Old 01-09-2012, 09:02 AM   #3
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I haven't nailed down my plans for the basement yet. I'm concentrating on the attic first. The attic will ne used for storage but it will not be heated. It's post and beam with 2x8 rafters on 24 inch centers. The foam will fill the cavities and then I can put plywood floor over the foam so I don't loose use of the attic. It was also cheaper to insulate with the polyiso than with fiberglass or cellulous. Will the multiple layers of vapor retarded trap moisture between the panels of foam and eventually soak the foam?
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Old 01-09-2012, 09:07 AM   #4
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I got 2304 sq ft of 4 inch thick polyiso for $700
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Old 01-09-2012, 10:02 AM   #5
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I haven't nailed down my plans for the basement yet. I'm concentrating on the attic first. The attic will ne used for storage but it will not be heated. It's post and beam with 2x8 rafters on 24 inch centers. The foam will fill the cavities and then I can put plywood floor over the foam so I don't loose use of the attic. It was also cheaper to insulate with the polyiso than with fiberglass or cellulous. Will the multiple layers of vapor retarded trap moisture between the panels of foam and eventually soak the foam?
Not if the furthermost interior vapor barrier is sealed up tight. This is not easily done to say the least.

If the attic is currently vented, how do you plan to close that up? You should be supplying conditioned air to the space if you move your insulation and envelope layer to the roof deck.
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Old 01-09-2012, 10:07 AM   #6
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I'm sorry I wrote rafters but they are the ceiling joints that the foam will ne going into. Shouldn't the space above the insulation be vented. The current attic is vented and I planned on leaving that alone so the air will move through the attic above the foam.
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Old 01-09-2012, 10:13 AM   #7
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How long is the unsupported span on those 2 X 8"s.
There not wide enough for any real "storage".
If the spans to long your going to end up with cracked plaster in the rooms below.
I've worked on dozens of 100 plus year old houses and all of them tended to use way to long a span below the floors and in the attic.
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Old 01-09-2012, 10:16 AM   #8
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I haven't measured but im sure they are too long. It will just be for light stuff like summer/winter clothes, a tent, small light stuff like that.
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Old 01-09-2012, 11:23 AM   #9
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I'm sorry I wrote rafters but they are the ceiling joints that the foam will ne going into. Shouldn't the space above the insulation be vented. The current attic is vented and I planned on leaving that alone so the air will move through the attic above the foam.
You could do that and that does somewhat give you the best of both worlds.

Depending on the pitch, a 1.5"-2" air space between the roof decking and the insulation is sufficient. That will cut down your overall available depth to about 6" which is not ideal and sufficient in my opinion.

I would prefer to see some rigid foam over the rafters to thermally break them from the rest of the home as well.

Be sure to observe code when it comes to leaving exposed foams in the attic.
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Old 01-09-2012, 01:12 PM   #10
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Its a full attic that I can stand in and I'm 6 ft 5. so there should be plenty of air above the foam. Here is what the layering starting from the inside out will be.

-Drywall (Bedroom Ceiling)
-4 inches of foil faced polyiso between joists, great stuffed in the seams
-4 more inches of foil faced polyiso, tacked in place with great stuff
-plywood (only in some areas as floor to store some small light boxes)
-Open air from 7ft air the peak down to 2 inches depending on where in the attic you are measuring due to the pitch of the roof.
-sheathing
-slate


Would this combo work? Will the multiple layers of vapor retarders trap moisture or will the foam absorb water and not be able to breathe and release it?

The original foil facing is marked up a bit on a lot of the foil. Should I lay a vapor barrier down that I know is continuous.

Sorry about the repeat of questions i just want to make sure I am doing this right. I would like to do this only once so I want to make sure I have my facts straight.
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Old 01-09-2012, 01:18 PM   #11
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Old 01-10-2012, 09:33 AM   #12
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Air movement carrier 100X the amount of moisture that diffusion will and probably even more when you compare a largely intact vapor barrier (i.e. just a few tears in the foil facing).

Just make sure you air barrier/sealing measures are spot on and you will be fine.

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