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Old 03-30-2010, 07:31 PM   #1
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installing foam board on pitched roof


I have an older house with catheral ceiling over the front part of the house. There is no installation. The roofing is 2 X 6 tg cedar ( i think)
I'm ready for a new roof and want to add some foam board. I beleive its needs to be polyaocyante (sp?). Is this correct? I was planing on putting the foam board over the 2X6 roofing and covering with 1/2 OSB sheathing then reshingling. I have good pitch, not flat. This house is in Ohio
Am i on the right track?
What about mositure barriers on either bottom or top. ( I beleive the poly comes with mositure barrier onesideor maybe both
I would appreciate and help any one can provide. (I thought I had a article from this old house, but when i need it of course can't find it!)

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Old 03-31-2010, 09:43 PM   #2
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installing foam board on pitched roof


Cathedral ceilings are often framed separately from the roof. It is always best to insulate the ceiling if possible.

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Old 03-31-2010, 11:01 PM   #3
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installing foam board on pitched roof


foam on top is fine. It will act as a vapor barrier. Use 2" You buy little plates that have a spike that holds the first pieces on. After that use pl premium. Then very 24' place 1X3 strapping up the roof. Then 5/8" plywood over this. The strapping gives you the vent space needed and the foam provides insulation and a warmer surface to avoid inside condensation. Blown dense packed insulation needs to be installed in the existing ceiling.
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Old 04-01-2010, 08:04 PM   #4
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installing foam board on pitched roof


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Originally Posted by Bob Mariani View Post
foam on top is fine. It will act as a vapor barrier. Use 2" You buy little plates that have a spike that holds the first pieces on. After that use pl premium. Then very 24' place 1X3 strapping up the roof. Then 5/8" plywood over this. The strapping gives you the vent space needed and the foam provides insulation and a warmer surface to avoid inside condensation. Blown dense packed insulation needs to be installed in the existing ceiling.

Not to argue, Bob, but what good would placing the board on top of the roof do if there is "attic" space between the ceiling and roof deck?

I could see it being beneficial if, and only if, you had a ceiling/roof frame that were built the same framing members.

It depends on how it was framed, Insulation is always better inside the building whenever possible.
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Old 04-01-2010, 08:58 PM   #5
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installing foam board on pitched roof


yes, when possible. But this is a closed cathedral ceiling. And you will find that a suggested method by new building performance guidelines are adding rigid foam to the outside. Both walls and roofs. For one thing it adds a space for venting, a thermal break from the conductive losses thru rafters or studs. It is a method, not the only fix. you can find this method described in both Journal of Light Construction and Finehomebuilding magazines recently. The outside rigid insulation is the best way to provide a better overall R-Value for a wall or roof.
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Old 04-02-2010, 12:36 AM   #6
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installing foam board on pitched roof


Are they stating that it is better to put it on the outside as a matter of preventing radiant heat from the sun from heating the building components in the first place? This makes perfect sense to me, and I even use this as a sales tool when selling our seamless insulating foam roofing systems... the roof and insulation's in one, keeping the winter heat in, and the summer sum out before it gets to the building.

Now, as far as maintaining the insulation as a functioning component, i say keep it inside away from the effects of weather, or protect it. are they setting forth any ideas for exterior wall insulation?

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