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Proper vapor barrier for back porch remodel / insulation

tkasha | Posted in Building Code Questions on

We are in the process of remodeling a back porch and trying to turn it from a 3-season porch into part of the living area. I currently reside in the US in a Zone 5 climate and our city follows the 2009 International Building Code.

Walls. 2×4 that we have furred out with 2×2 and are planning on doing a flash and batt technique with 2” of closed cell foam and then fiberglass insulation. We are debating on if we should use kraft paper for the fiberglass. Then ½” drywall.

Ceiling. 2×4 that we have furred down with 2×2 and plan to fill entire 5” with closed cell foam. Then we are planning on using 1” of XPS on the interior side in order to meet R38 which is needed for energy rebate. Then ½” drywall.

Are additional vapor barriers needed for either the wall or ceiling? Are there any concerns about condensation between the 5” closed=cell insulation and 1” XPS insulation for the ceiling? Would it be better if we furred down the ceiling further and did the entire thing with closed cell? We liked the idea that the XPS would serve as a thermal break for the studs and additionally it was significantly cheaper than the spray closed cell insulation.

Any suggestions are greatly appreciated.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Ted,
    Q. "Are additional vapor barriers needed for either the wall or ceiling?"

    A. No. For more information, see Do I Need a Vapor Retarder?

    Q. "Are there any concerns about condensation between the 5 inches of closed-cell insulation and 1 inch of XPS insulation for the ceiling?"

    A. No.

    Q. "Would it be better if we furred down the ceiling further and did the entire thing with closed-cell?"

    A. No. But you might consider substituting polyiso for the XPS. Polyiso is more environmentally friendly than XPS and has a higher R-value per inch.

  2. Expert Member
    Dana Dorsett | | #2

    On the ceilng end...

    Putting 5" of closed cell foam between rafters thermally-bridging that layer is a bit of a waste, since it only adds ~R1.5 to the "whole assembly" R over a full 5.5" of cellulose or open cell foam, after factoring in the thermal bridging of the joists. To meet IRC on R-ratios between foam fiber with the closed cell foam on the underside of the roof deck takes R20 (3" of the R7/inch good ), saving a bit on foam, but also saving on environment, and you could fill the rest of the space with R4/inch high-density rock wool or high density fiberglass batting (or compressing some cheaper R13s in there) and hit pretty much the same performance point, independent of what the center-bay R value is.

    The blowing agents used for most closed cell foam are nearly as bad as those used for XPS, and either has more than 1000x the global warming potential (GWP) of CO2. The blowing agents used for polyiso and EPS only runs ~7x CO2. Until/unless the closed cell polyurethane & XPS vendors switch over to much lower GWP blowing agents (a very few have), they're on the not-so-green list. Icynene has a water-blown 2lb foam that is very low GWP, but it's only about R5.1/inch.

    With an unvented assembly with 3-5" of closed ell on one side and foil faced iso between the gypsum & framing could conceivably create a moisture trap, but if there's a vented air space between the top of the ceiling joists and the roof deck that risk is low. If this is an unvented cathedral ceiling, putting R20 roofing iso above the roof deck and R23 rock wool in the 5.5" space (or even R20 open cell spray foam, if it's not a standard batt width) would be a significant performance uptick, at a lower cost than what you've described, since the R20 foam over the top side would have very low thermal bridging (only a few screws, 24" o.c. ) undercutting it's performance.

  3. Expert Member
    Dana Dorsett | | #3

    BTW: On the wall assembly, a 1" flash-foam would be adequate dew-point protection & air sealing, the cavity, and you could then compress unfaced R23 rock wool designed for 5.5" cavities into the remaining 4" nominal space. A the higher compressed density it'll be performing at about a true R19 (R4.5/inch) rather than the R23 (R4.2/inch) it delivers in a standard 2x6 cavity. You'd still need to caulk the seams of the top plates of the framing and the seams between doubled-up studs, the bottom plate & subfloor, etc.)

    If you stick with the 2" foam plan for the walls, unfaced R13s will compress nicely into the remaining 3" space, and still be delivering better than R11 at that thickness & density.

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