Is fiber-faced polyisocyanurate OK for exterior insulation board?
Hi everyone….I am preparing to get back at the exterior of my home renovation project now that winter in New York seems to finally be at an end. I plan on installing exterior rigid foam over my sheathing with vertical furring to create drainage plane and a nailing surface for my siding.
I have access to 2″ polisocyanurate insulation with a black fiber facing. (Originally intended for a bitumen roofing application.) The material is NEW and is about 1/3 the price of XPS or foil-faced polyiso.
Is there a compelling reason I should not use this material as exterior insulation? I am unfamiliar with this facing type and I guess I am just used to only seeing foil faced polyiso used for exterior insulation.
Thanks in advance for any opinions or guidance on this project.
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Replies
Brandt,
Q. "Is fiber-faced polyisocyanurate OK for exterior insulation board?"
A. Yes.
Q. "Is there a compelling reason I should not use this material as exterior insulation?"
A. No.
I was wondering the same thing as when I get around to my residing project I will have access to the same material fairly easily. Would it be beneficial to put the WRB on the outside of the foam instead of against the original sheathing since it is a fiber facing?
Although as I sit hear and think about it, it will be easier to flash everything if the WRB is against the original structural sheathing behind the foam. Plus that is the idea of the rain screen to allow for air movement behind the siding to increase the drying potential if things were to get wet by wind driven rain.
Joshua,
Q. "Would it be beneficial to put the WRB on the outside of the foam instead of against the original sheathing since it is a fiber facing?"
A. As long as your WRB is integrated with your window flashings and penetration flashings, it can be in either location. More information here: Where Does the Housewrap Go?
I wouldn't worry about the small amount of water that gets past the siding. A ventilated rainscreen gap dries very quickly.
Roofing iso works great for this app, and is a bit more rugged that the typical foil-faced goods due to being higher density (2 or 3 lbs per cubic foot nominal, to 1.5lbs/ft^3 for most foil faced iso.)
It's R/inch is slightly lower than the foil-faced stuff- important only to those building to a tight spec on whole-wall R value or designing tight to the dew-point min. You can count on it being at least R5.5/inch, but probably never R6.0/inch or higher.