white styrofoam vs pink styrofoam as a thermal barrier

I am wanting to use a thin layer of styrofoam outside of the sheathing of my house as a thermal barrier... what i see out there (here in canada) is the standard pink (extruded) stuff and the white 'duro-foam' covered in a green vapor barrier which is quite a bit cheaper. I can tell that the durofoam is more likely to break, but assuming that i can install it in one piece, is there any difference in the lifespan/performance of these products? (mold? rot? disintegration?) the r-values are very close.
also would i still need to use housewrap outside of the styrofoam?
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Polyisocuanurate usualy a foil covered boad is R7.2, the blue and pink R 5, some white boards are R4 or so, You could have moisture locked in and get mold, the foamboard is a vapor barrier, it should be on the inside, it depends on how your house was made.
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the house is not yet built (excavating wednesday), so i still have the luxury of choice... anyhow, isn't osb sheathing a vapor barrier as well? is there any way to up the r-value beyond the r-22 of roxul and a reflective vapor barrier inside? something to break the thermal bridge that the timberframe creates with the osb? i need to figure this out so i can know the thickness of the walls and order the windows...
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wrote:

Depends what you're using for the external wall covering.
I live in a warm area in central TX. My use of the foil covered poly stuff was on a detached garage. I did not use any sheathing on the walls per one framing method for Hardie lap siding using integral wind bracing. I did put housewrap over the framing before applying the poly foil covered stuff. Foil taped all the joints. Applied felt paper over all that, then the siding I mentioned. All the siding was nailed by hand as the poly stuff is compressible. A nail gun would be iffy and show waves on the exterior wall had I used that. The inside has fiberglass batts between the studs. Garage is cozy temperature-wise almost all the time.
--
Dave

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It's not true that you should only put foam on the inside. In fact, I think it is safer on the outside. That is because it will raise the temperature of the back of the stud cavity above the dew point, so if any moisture does leak into your stud cavities, condensation will not occur. If the foam is on the inside, the opposite happens--the stud cavity is cooler and any moisture that gets in will condense. I didn't just make this up. Exterior foam is widely used in my climate (Northern MN). The trick is to figure out how much foam is required. Google "building science" and you will get hours of reading on the subject.
As to the different types of foam, the pink extruded polystyrene and white expanded polystyrene will both last (they are both the same material, but manufactured in a different way). Expanded polystyrene has a higher perm rating (water vapor will go through it to a greater extent than other foams).
If I was building a new house, I would use exterior foam and blown in dense pack cellulose in the walls.
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If its not built and you want the highest R value then look a SIPS construction R 40 walls, and R80 ceiling, vs R19 on your idea, but the Vapor barrier should be inside. Or have the open area foamed instead of fiberglass batt. 2x6 walls will give you what you need with foamed walls.
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