# Polyisocyanurate insulation



## n2omichael (Aug 5, 2012)

I need to sheath my exterior walls with insulation, 1 1/2" would be a perfect thickness and I found some Polysio 4 X 8 sheets that are reasonable. Has anyone used this insulation? I have heard mostly good things so far. Any thoughts?


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## joecaption (Nov 30, 2011)

Going to need way more info.
Where are you?
Adding that much insulation on the outside as you suggest is going to effect anything on the outside.
Window and door trim, where the wall meets the soffits,
Would require all new siding, ECT.


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## ddsrph (Nov 23, 2013)

n2omichael said:


> I need to sheath my exterior walls with insulation, 1 1/2" would be a perfect thickness and I found some Polysio 4 X 8 sheets that are reasonable. Has anyone used this insulation? I have heard mostly good things so far. Any thoughts?


I am using polyiso in my new construction house on the inside of a steel and concrete filled block construction. I am putting XPS Blueboard against block and polyiso on top of that. The issue you will have to be concerned with is retaining moisture in the walls and getting a good answer will be difficult as experts seem to have differing opinions. Important issues will be what type of siding will go over the polyiso and what kind of covering the polyiso boards have. Some are aluminum foil covered and some has a reinforced paper covering. 

JM


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## n2omichael (Aug 5, 2012)

I'm in Ontario, Canada.Right on the St Lawrence, weather would be the same as upstate NY.
Building foundation is 4ft concrete with 4ft knee wall on top. We moved the existing cottage on top of this foundation. The existing cottage was wooden siding, then vinyl siding was added. Since that was 40 years ago we where unable to match the siding, so, we either had to reside the entire building or use vertical siding on the lower half. The bottom of the building sits 1 1/2" away from the top level . The building has plywood sheets over the studs wrapped in blueskin then we planned on using the Polysio under the vertical siding.


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## ddsrph (Nov 23, 2013)

N2O
Unless you can get advice from some expert I would go for it. It would be safer to use the paper backed polyiso as the foil backed is more of a vapor barrier. I would make sure that the only vapor barrier material was on inside wall directly behind Sheetrock. I like poly ISO and have bought up a lot for my house project. I got it from a commercial roofing company. They took it off a large building and I bought it for pennies on the dollar. I got some 1 1/2 inch but most is 2 1/2 inch. I am going to have R 35 walls and R 60 ceiling. The two side walls I turned the two by fours flat and the front and rear walls normal framing to better accommodate wiring. The front wall is not block but standard 2 by 6.
Thanks
JM


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## n2omichael (Aug 5, 2012)

Great! I appreciate the info, I will use it, like yourself I got it for less than 1/2 the price of new. It IS the foil backed style. I wonder if I should install the stuff with the foil side against the Henry Blue, or against the back of the siding? Your thoughts?


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## ddsrph (Nov 23, 2013)

n2omichael said:


> Great! I appreciate the info, I will use it, like yourself I got it for less than 1/2 the price of new. It IS the foil backed style. I wonder if I should install the stuff with the foil side against the Henry Blue, or against the back of the siding? Your thoughts?


IS the Henry blue a vapor permeable material like DuPont house wrap? The only thing I have read about the foil backed is the foil side should not be directly against vinyl siding. True or not I don't know. There is a organization called Building Science that has published a lot of articles on insulating structures. If you could find any articles that relates to your situation they could be of great help. When I was planning my house design with the block walls and being partial underground I got advice all over the map with a lot of people saying I should leave a dead air space between framing and block. A good article by building science showed five different ways to insulate my block walls with science backing up each method. None of them recommended a dead air space.
JM


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## Gary in WA (Mar 11, 2009)

Compare your annual HDD with location for Zone/application; http://www.buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-design

Gary


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