# Foil-Faced Foam with Housewrap



## e247 (Dec 18, 2017)

I'm building a new house and I'm installing rigid foam (foil faced polyisocyanurate) over the OSB sheathing and then the Tyvek housewrap over the rigid foam. There will be 3/4" furring strips installed over the foam and the siding will be attached to the furring strips. I was looking at the specs for the foam and it gives an R value for the foam and then a R value with air space. For example, the 3/4" foam has an R value of 5, but the R value of the foam with 3/4" reflective air space is 7.5. If the Tyvek is installed directly over the foam, do I have a reflective air space?


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## chandler48 (Jun 5, 2017)

I don't see the advantage of the low R value foam over the OSB. Your insulation value should be concentrated in the stud bays on the other side of the OSB. If you do plan on this extra thickness, make sure your windows and doors are set out enough to compensate. You will also have to install jamb extensions on the inside once you set the windows and doors out.


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## Bud9051 (Nov 11, 2015)

Hi e247, you have a few problems with your proposal in addition to what Chandler pointed out.

First I don't see your location so I will base this on a cold climate for now.

When adding a layer of rigid foam to the outside it needs to be sufficiently thick such that its inside surface remains above the dew point. I can provide a chart once we know where you are.

Yes, the foil face needs to be exposed to an air gap to get the added advantage of the radiant barrier. Properly taped the foil surface would act as your drain plane.

Since the foil surface will be a full vapor barrier, not just a vapor retarder, you will want to be sure there is no interior vapor barrier.

Not sure where your new home build progress stands but air sealing is among the highest priority and can be best accomplished if done during construction.

Fill in some details and we can proceed.

Bud


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## e247 (Dec 18, 2017)

I'm in climate zone 2 (Houston, TX area) so the 3/4" foam is sufficient for my area. There will not be a vapor barrier inside to allow drying to the interior. I want to use a dedicated WRB for the drainage rather than using the foil faced foam since I've heard that the foam can move and I don't want to risk the tape on the seams failing.

Which is better for my climate, XPS or Foil-Faced Polyiso?

Air sealing is one of my top priorities.


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## Bud9051 (Nov 11, 2015)

Sounds like you are getting good advice for a southern climate, my feet are stuck here in the north. I would just be guessing so will let others advise.

Bud


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