# polyisocyanurate



## cc-rider (Jul 29, 2003)

I'm FINALLY going to build that cottage! Over the studs/under the outside sheeting, I'd like to put some sort of thermal break. I found foil-faced polyisocyanurate sheets. Any good??? I read that they absorb water, so I'd need to get the sheeting on pretty quickly, but are they worth the money? Will the foil help deflect outside heat, and/or keep the heat and cold inside (whichever I need)? I believe they are 1/2" sheets and that should have an R-factor of about 3, I think. 

Any advice is GREATLY appreciated!
CC


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## wy_white_wolf (Oct 14, 2004)

Can you supply a link to the ones you want?


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## cc-rider (Jul 29, 2003)

I can't find anything on it on the website.  I picked up a flyer on it at Menards. My biggest concern is that it might be toxic?? Here is information from the website....but I can't find a general page for it.

http://www.jm.com/search.asp?CiRestriction=ap+foil+faced

I want something for a thermal barrier, but to also add to the insulation factor. I believe this product will do both, plus is a vapor barrier? I'm using 2X6's 2'OC if that matters.

CC


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## barn-apart (Feb 10, 2005)

Perhaps the rolls of bubble wrap with foil may work , easy to install ,less seams to deal with. I used it on the outside of a travel trailer, 18', roof , walls and windows for winter and weathered -20 in Wisconsin using the propane heater that came with the trailer ,1973 model. Did not do the floor and that was the cold part of the experiment. The foil reflects the radiant heat back very well. 500 sq. ft. was $125. delivered to the door. It was about 1/4" thick ,called it double bubble I believe. insulation for less . is one site that handles it, but should be availible at Menards or Home Depot also.


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## SolarGary (Sep 8, 2005)

cc-rider said:


> I'm FINALLY going to build that cottage! Over the studs/under the outside sheeting, I'd like to put some sort of thermal break. I found foil-faced polyisocyanurate sheets. Any good??? I read that they absorb water, so I'd need to get the sheeting on pretty quickly, but are they worth the money? Will the foil help deflect outside heat, and/or keep the heat and cold inside (whichever I need)? I believe they are 1/2" sheets and that should have an R-factor of about 3, I think.
> 
> Any advice is GREATLY appreciated!
> CC



Hi,
Polyiso is good stuff. It has the highest R per inch of any of the foam board insulation products. Its R6.7 per inch vs about R5 per inch for the "blue" extruded polystyrene insulation board, and R4 per inche for the polystyrene white bead board stuff.

It does absorb water a little more than the extruded polystyrene -- thats the reason it always comes with foil or fiber face sheets -- they act as a water vapor barrier. You should put up an inside poly sheet vapor barrier anyway.

If you can leave a small air gap between the foil and the wall, the R value will go up a worthwhile amount. This goes for both the inside and outside foil.
My book says that the reflective foil facing an air gap is worth about R1.5 for vertical insulation and even more for horizontal insulation on the ceiling.

If you can tape the seams, it will help to stop air infiltration.

Still, a half inch is not much insulation. 12 inches is better these days 

Two layers with an air space between?


Gary


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## cc-rider (Jul 29, 2003)

SolarGary said:


> Still, a half inch is not much insulation. 12 inches is better these days
> Two layers with an air space between?
> Gary


Actually, this foil-faced iso will be on the outside of the studs, under the sheathing. Then I'll have another 5-1/2 of either foam (blown in), or batts, in the stud wall. I assume I put the plastic wrap on the INSIDE walls between the insulation and the drywall (or whatever I finish the inside walls with)?

I'm only using the foil-faced stuff as a wind-barrier and the 1/2" of insulation is a bonus. 

CC


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## SolarGary (Sep 8, 2005)

cc-rider said:


> Actually, this foil-faced iso will be on the outside of the studs, under the sheathing. Then I'll have another 5-1/2 of either foam (blown in), or batts, in the stud wall. I assume I put the plastic wrap on the INSIDE walls between the insulation and the drywall (or whatever I finish the inside walls with)?
> 
> I'm only using the foil-faced stuff as a wind-barrier and the 1/2" of insulation is a bonus.
> 
> CC


Hi,
I think thats a good way to go.
The layer of polyiso will keep the studs from thermally bridging heat out of the cabin. 

This is a nice "whole wall" insulation calculator that gives the real R value for various setups -- for example a 2X6 stud wall with "R19" insulation actually comes out about R15 with the thermal bridging of the studs:
http://www.ornl.gov/sci/roofs+walls/AWT/InteractiveCalculators/rvalueinfo.htm

Just be sure the walls still have enough shear strength without the plywood sheathing.

Gary


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## greg273 (Aug 5, 2003)

You're using 2x6 studs? That in itself will give you plenty of insulation, so the poly-iso would be good, but not entirely neccesary. I am just going by my own experience, using 2x6 walls in my house, I am very impressed with the thermal performance this past winter. I could let the fire go out, and even with outside temps in the low teens, the house would only lose about a degree per hour.
Before using poly-iso on the walls, I would consider using it in the roof, under the rafters, or possibly IN the rafter cavity, (with a small air gap, for radiant reflection AND maintaining ventilation under your sheeting), foil side up. THis will be beneficial in repelling summer heat.
Just remember, when you lay out a wall for framing, set your first stud at 15 and 1/4th inch, and pull 16" centers off of THAT. That way, your 4x8 sheets will break perfectly in the middle of a stud.


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## WisJim (Jan 14, 2004)

If the poly-iso is foil faced, it is a good vapor barrier, especially if the seams are taped. Therefore, it should be on the warm side (inside) of the wall for best effectiveness. If you use it on the outside, under the siding, make sure that you have an excellent properly installed vapor barrier on the inside of the wall. Otherwise you are asking for moisture accumulation within the wall space.


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## SolarGary (Sep 8, 2005)

WisJim said:


> If the poly-iso is foil faced, it is a good vapor barrier, especially if the seams are taped. Therefore, it should be on the warm side (inside) of the wall for best effectiveness. If you use it on the outside, under the siding, make sure that you have an excellent properly installed vapor barrier on the inside of the wall. Otherwise you are asking for moisture accumulation within the wall space.


Hi,
That's a good point, and also applies to the "pink" and "blue" polystyrene foam board, which are very good vapor barriers. You need the good inside vapor barrier -- it will help all your insulation perform better.

Gary


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## fishhead (Jul 19, 2006)

Is there any sheet insulation that allows moisture to migrate through it so it can be used under siding?


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