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cold climate + interior poly vapor barrier = ?exterior insulation

Kevin Manley | Posted in Green Building Techniques on

We live in Northern Vermont (5A) We have a midcentury prefab home we are re-siding. We’ve peeled off two layers of siding (shingle and the original clapboard siding underneath). Unfortunately when a previous homeowner installed new windows they nailed the flange over the clapboard siding and then shingled over that. Now that we’ve taken down both layers of siding we have a window that sticks out 3/4 further than the sheathing because of the clapboard. We need to pad out the wall by 3/4 before we can reshingle. Considering the current cost of plywood we were wondering if we can use a vapor impermeable product like rigid foam instead? I realize 3/4 wouldn’t provide much insulation but it’s better than plywood and about the same cost right now.

The biggest consideration for us here is the interior side has poly barrier between the drywall and the fiberglass insulation. Also we would be installing this rigid foam on exterior walls of the kitchen and bathroom (so very humid indoor rooms)

Other considerations:

-we are leaving old tar paper on house and we are adding new tyvek rain screen over that and then new cedar shake siding. The builder we are using to help feels like the tar paper was vapor impermeable and he doesn’t see an issue adding rigid foam on top of it before he puts up the rain screen.

-house is on slab. lots of poor craftsmanship and detailing through the years = many breaks in interior vapor barrier and external sheathing. 

-this is a heated home but not cooled. so internal humidity in summer is similar to outdoors.

I saw this post which makes me thing EPS would be the way to go if adding insulation board https://www.greenbuildingadvisor.com/question/renovation-with-interior-poly-vapor-barrier

So external insulation:

Good idea? EPS? 

Terrible idea? Just suck it up and use plywood?

 

Any other suggested products or detailing would be GREATLY appreciated.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    >"We live in Northern Vermont (5A)"

    All of VT is IECC zone 6A.

    >" Considering the current cost of plywood we were wondering if we can use a vapor impermeable product like rigid foam instead? I realize 3/4 wouldn’t provide much insulation but it’s better than plywood and about the same cost right now.'

    Using 3/4" board foam (any type) would pretty much double the R-value of the framing fraction at that point (it's not nothing!), and would be a fine way to pad out behind the nailing fin and around the windows, assuming the fin is still structurally supported by the rough framing. Be sure to attend the window flashing details no matter what it's padded out with.

    >"-we are leaving old tar paper on house and we are adding new tyvek rain screen over that and then new cedar shake siding. The builder we are using to help feels like the tar paper was vapor impermeable and he doesn’t see an issue adding rigid foam on top of it before he puts up the rain screen."

    The #14 felt/tarpaper is variable permeance- becoming more vapor open as it's moisture content rises, making it a "smart" vapor retarder. At 3/4" most foam products are still somewhat vapor open as long as it doesn't have foil or vinyl facers. At 3/4" Type-II EPS runs about 3.5-4 perms, which is comparable to the vapor permeance of interior latex paint. The tarpaper is about 0.5-1 perm when bone dry, 5 perms when fairly damp.

    1. Kevin Manley | | #2

      Thanks Dana. Two small things... I don't think it would change your response but I just wanted to be clear. We did not remove the clapboard siding behind the window flange but we did remove it all around the window. I'm not sure we could remove the small small pieces under the actual fin without taking out the windows. Will give it a shot though. In terms of flashing the only thing we can do at this point is to tape the fin off to the new layer of EPS and of course layer our rain screen over that.

      Would two products that are somewhat vapor permeable when layered create a impermeable surface? Our total exterior is ply/solid wood sheathing + #14lb felt + 3/4 eps + tyvek rain screen plus + painted shingle. Still sound all good as long as we avoid foiled foam board.

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