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Above-grade wall insulation concerns

ksteltenpohl | Posted in Energy Efficiency and Durability on


I live in climate zone 6 and have a question on the wall assembly of the above grade exterior walls of my home. During the remodel the whole house was gutted and 2″ of closed cell spray foam installed on all exterior walls of the house. The attic had 18″ of loose fill fiberglass insulation blown in. The exterior walls are 2×4 construction 16″ on center. The original 3/4″ black sheathing was left in place with Tyvek house wrap installed over that. Then in an effort to prevent thermal bridging, a layer of 3/4″ XPS rigid foam sheathing was intalled. There is a thin plastic film over the XPS sheathing and all seams were taped. This is how the house sits now while we are waiting for the LP Smartside to be installed. A layer of 3D raindrop house wrap is going to be installed prior to the siding to provide a drainage plain.

Should I be concerned with this wall assembly and should I remove the layer of plastic film on the foam and the tape on the seams to better allow wall to dry to outside? Should I have the XPS foam removed completely before installing siding?

I am looking for advice on how to proceed

Thank you

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  1. GBA Editor
    Martin Holladay | | #1

    The R-value of your two insulation layers is about R-16 or R-17. That isn't much. In your climate zone, the code calls for at least R-20 or R-25.

    You have chosen a strange stack-up of insulation layers, for sure.

    I don't know what kind of "thin plastic film" you installed on the exterior side of the XPS, but that should be removed. If you do that, your wall will probably work, although it wasn't a great idea to sandwich your sheathing between two layers of relatively impermeable foam. You need really excellent flashing details to keep moisture out of these walls. A true ventilated rainscreen gap (3/4 inch thick) would be better than crinkled housewrap.

    If I were you, I would add some fluffy insulation on the interior side of the closed-cell spray foam. Maybe you could compress some 3.5-inch-thick batts into the remaining space.

  2. Expert Member
    Dana Dorsett | | #2

    I suspect the plastic facer on the XPS is a polyolefin vapor retarder designed to bring the permeance of the 3/4" goods down to something more comparable to 1" XPS (0.8-1.3 perms) But without a product name it's hard to look up the spec, eh?

    Typical 1.5lb density XPS at 3/4" thickness would normally run about1.5-1.7 perms without facers, but with a facer you're probably looking at about 1 perm, give or take. Find a spec sheet for the actual product, since the facer determines the drying capacity toward the exterior. Anything over 0.5 perms is probably going to be fine in your stackup, but 1-1.5 perms would be better.

    Compressing a batt designed for 3.5" deep cavities into the remaining nominal 1.5" of space (assuming it's milled 2x4s, not full dimension rough 2x4s). It's also hard to split a high density R15 batt into two 1.75" thick batts sufficiently perfectly to truly fill the 1.5" cavity without voids, but unfaced low density R19 low density batt (manufactured loft of about 6.25") can be split into two highly compressible ~3" thick batts easily enough, and would compress easily into a 1.5" nominal space.

    At the compressed density that 1.5" would be good for about R6 or a little better, the rough equivalent to compressing a 3.5" low density R11 batt down to 1.5" thickness, but easier to actually install:

    From a dew point control point of view the first condensing surface is your closed cell foam, and you would have huge margin since you have about R17 or so to the cold side of that condensing surface.

    With only the 2" foam in the cavity it means the thermal bridge is through only 2" of wood, which runs about R2.4. By filling the cavity completely to 3.5" it increases the depth of the thermal bridge to 3.5", with an R-value of about R4.2. That is the thermal performance equivalent of adding another 3/8" of XPS to the exterior for the framing fraction, which cuts the loss of the framing fraction by about 1/3. The R6-ish addition to the cavity fill cuts the loss of the non-framed fraction by more than 1/4, so the overall improvement in thermal performance is going to be better than 25%.

    That brings it closer an R20+ 5 IRC 2012 code min for zone 6 (see: ) though the thermal bridge is still too severe with 2x4 framing to actually make it, despite the ~ R3.8 foam on the exterior.

    An R20 + 5 assembly at a 25% framing fraction comes in at about R20 whole-wall. With your stackup you're looking at about R14.5- R15 whole-wall, which is comparable to a 2x6 / R20 wall with no exterior foam at a 20% framing fraction (24" on center stud spacing.)

    WITHOUT the additional ~R6 fluff you're looking at a whole wall of about R11, due to the severely shortened 2" thermal bridge. That is comparable thermal performance to a 2x4 wall with R13 batts & 3/8" fan-fold XPS siding underlayment, and woefully sub-code. Last time I checked code-min under Minnesota's statewide codes (based on IRC 2003 or 2006) was still just R19 2x6 construction, which comes in at about R13 whole-wall.

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