Will two-sided foil-faced polyiso cause an issue?
Aledoux13
| Posted in Energy Efficiency and Durability on
Well my house is nearing completion and we are about to add the foam and siding. I’ve been shopping reclaimed foam and was able to get a nice deal on 3″ polyiso in perfect condition the only issue is it has the foil face on both sides. I was wondering if I would have any issues using this foam on the exterior. My buildout from inside to out is as follows:
1/2 Gypsum
2×6 16″ OC with dense packed cellulose
1/2 Zip Sheathing
Tyvek Drainwrap?
Foam either poly or xps (taped)
1×4 rainscreen
LP Smartside
Also is the tyvek drainwrap a necessity or just a backup to zip and taped foam? Thanks!
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Replies
Arthur,
If you are installing 3 inches of polyiso on the exterior side of your wall, you can't expect any diffusion drying to happen through the polyiso. The foil facing won't cause any problems, because this type of wall is designed to dry to the interior. (That means that you shouldn't install any interior polyethylene.)
For more information on detailing this type of wall, see How to Install Rigid Foam Sheathing.
Tyvek DrainWrap is a perfectly acceptable water-resistive barrier (WRB). Every wall is required by code to have a WRB. Building scientists also advise builders that a WRB is essential.
If you don't want to use Tyvek DrainWrap, there are other options. One possibility is to use rigid foam as your WRB. (Personally, I don't like this approach, but it's possible to do it.)
You can also use taped Zip sheathing as a WRB. Whatever type of WRB you decide to use, it is essential that you come up with details to integrate your window flashing with your WRB. The main disadvantage of rigid foam or Zip sheathing as your WRB is that you can't lap these materials, shingle-fashion, over your other types of flashing.
For more information on these issues, see:
All About Water-Resistive Barriers
Using Rigid Foam As a Water-Resistive Barrier
Where Does the Housewrap Go?