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Housewrap, foam board, vapor retarder

Ian Anderson | Posted in Green Building Techniques on

I know this topic may have been beaten to death, but I’m struggling finding a definitive answer to my questions.

I live in zone 5b (Colorado east of Rockies). I am replacing my old cedar siding and windows at the same time with Hardiplank. This house was built in the early 70’s. 2×4 construction, kraft-faced insulation. The exterior sheathing is about 3/4 fiberboard, in decent shape. It appears an R-5 is minimum for my area regarding rigid foam board. I had planned on using tyvek as well. I’ve read many papers at BuildingScience.com as well regarding this topic.

According to building science, the paper suggested that foil-faced boards weren’t recommended in cold climates, however I find many people using it in cold climates. I am keen of foil faced because of the reflectivity of the foil regarding radiant heat, higher R value (6.5) and higher compression strength. If I can use foil-faced, should I tape the seams if I use tyvek over it as the water plane? Should I even use tyvek? Should I use Tyvek with a xps board instead? I find a split preference of placing the tyvek on top of or beneath the foam board. The Tuff-R has a low permeability, and wonder how that would work in conjunction with housewrap.

At this point my wall would be: siding, Tyvek, foam board, fiberboard, 2×4, drywall.

We get very little rain, other than flash storms for 5 or so minutes at a time. Not sure if a rain screen wall is necessary?

So in summary, I’m stuck at order of wrap and foam, and type of foam.

Thanks for any guidance.

Ian

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Ian,
    Q. "According to Building Science, the paper suggested that foil-faced boards weren't recommended in cold climates; however I find many people using it in cold climates. I am keen of foil faced because of the reflectivity of the foil regarding radiant heat, higher R value (6.5) and higher compression strength."

    A. I think it's fine to use foil-faced foam, as long as the R-value is high enough, and especially if you don't have interior poly.

    Q. "If I can use foil-faced, should I tape the seams if I use tyvek over it as the water plane?"

    A. Yes, I would tape the seams -- both for air tightness and water resistance.

    Q. "Should I even use Tyvek?"

    A. That's up to you. Some builders use rigid foam as a WRB. Here's more information: Using Rigid Foam As a Water-Resistive Barrier.

    Q. "Should I use Tyvek with a XPS board instead?"

    A. Again, it's up to you. Polyiso is more environmentally benign than XPS. It's easier to integrate housewrap with flashings than it is to integrate rigid foam with flashings.

    Q. "I find a split preference of placing the Tyvek on top of or beneath the foamboard."

    A. You're right. More information here: Where Does the Housewrap Go?

    Q. "The Tuff-R has a low permeability, and wonder how that would work in conjunction with housewrap?"

    A. There is no conflict between the two products.

    Q. "We get very little rain, other than flash storms for 5 or so minutes at a time. Not sure if a rain screen wall is necessary?"

    A. If you are installing almost any type of siding over foam, you really need to install vertical furring strips. With the furring strips, your siding nails won't be so long, and the paint on the siding won't be stressed by differences in temperature and humidity between the front of the siding and the back of the siding.

  2. Ian Anderson | | #2

    Thanks Martin. Exactly what I was looking for.

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