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Insulation question for new house build

JAC3232 | Posted in GBA Pro Help on

I’m building a brick bungalow in zone 5, please help critique my insulation. The sheathing is 1” Polyiso (foil faced) wrapped in Typar building wrap. I realize the polyiso should be thicker, but I’m told it’s too late. The walls are up and the windows are purchased.

Attic – R50 loosefill cellulose

Main Floor – R22 CC spray foam in 2×6 wall with no VB, drywall and latex paint
– The other option is R10 CC spray foam with R14 Roxul and no VB.

Rim Joist – R12 CC spray foam and R14 Roxul (thermal barrier required)
– I understand it’s a double VB, and from what I read, the 2” CC spray foam has a permeability of 1 perm. My concern is whether it’s enough to dry, as the exterior rigid foam is not thick enough to keep the dew point out of the wood. I’m concerned about this design, but I’m not sure of a better solution.

Basement – 2” R12 continuous CC spray foam, 2×4 stud with R14 Roxul, no VB, drywall and latex paint

Under slab – R10 rigid insulation


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  1. GBA Editor
    Martin Holladay | | #1

    Your situation can serve as a warning to other GBA readers who are planning to build a house. Here's the moral of the story: if you wait until halfway through the construction process to research your insulation options, you may end up making specification errors that are very hard to reverse.

    It's best to research these issues before construction begins.

    As you evidently now realize, if you want to install rigid foam on the exterior side of 2x6 walls in Climate Zone 5, the rigid foam needs to have a minimum R-value of R-7.5.

    If this were my house, I wouldn't install spray foam insulation between the studs. I would install dense-packed cellulose insulation. If you install a "smart" vapor retarder like MemBrain on the interior, and if you keep your eye on the indoor relative humidity during the winter -- don't let it get too high -- you'll be fine.

  2. GBA Editor
    Martin Holladay | | #2

    If you install 2 inches of closed-cell spray foam on the interior side of your rim joists, you'll probably be OK -- especially if the lumber is dry on the day that the spray foam is installed.

    Your plan for insulating the interior side of your basement walls will work.

  3. JAC3232 | | #3

    Thank you for your help.
    I’ve been unable to locate a company in SW Ontario that will do cellulose in new house walls. What is the reasoning for selecting dense-packed cellulose over spray foam in this specific case? Although not the cheapest, I thought spray foam would be the best choice given that the exterior foam was insufficient at keeping the dew point out of the wall.
    Also, is Latex paint not sufficient as a vapor retarder?


  4. GBA Editor
    Martin Holladay | | #4

    Here's the theory: once you have installed foil-faced polyiso on the exterior of your wall, you have made outward drying impossible. So this type of wall depends on inward drying. You want the insulation between your studs to be as vapor-permeable as possible. So open-cell foam or cellulose is better than closed-cell foam, because of the aluminum foil layer you have already installed.

    The use of MemBrain is an attempt to give you a variable-permeance layer to try to keep your wall out of trouble during the winter, while still allowing some drying during the summer. It's a tricky balancing act.

  5. Expert Member
    Dana Dorsett | | #5

    Cellulose will also buffer some of the wintertime moisture burden without damage or loss of fuction, as long as you have MemBrain or similar on the interior.

    When the proximate air on both sides of MemBrain is below ~35% it's vapor permeance is about 1 perm, and at 30% RH it is reliably below 1-perm. So, if you keep the interior at 30-35% RH during cold weather the moisture transfer rate from the house into the cold sheathing & cellulose is very slow. But in the early spring when the sheathing & cellulose warm up during the day the RH inside the cavity rises to over 50% during the warmer hours, which raises the permeance of MemBrain to 10+ perms, releasing some of that moisture into the house,a rate limited by the permeance of the finish paint (3-5 perms typ.) Then at night when it cools off the sheathing re-adsorbs some of the cavity moisture, which then lowers the permeance of the MemBrain.

    So it becomes something of a moisture-diode during the shoulder seasons it dries at something like 3x the speed that it takes on moisture until the average temp at the sheathing is reliably above the interior air's dew point. During the winter season the rate of moisture build up is very low, and during the summer it's almost always fairly vapor-open, able to release any moisture that finds it's way in.

    You could use dense-packed fiberglass as well, which would deliver a slightly higher R-value, but provides no moisture buffering, which means you are relying more on the smart vapor retarder.

    BTW: In a zone 5 location you will get better mid-winter performance out of 1" of EPS than 1" of polyiso in a 2x6 wall. This is because the mean temperature through the foam layer will be about 35F, which is beyond the knee at which polyiso falls off a performance cliff, and a point at which the performance of EPS has risen about 8-10%. (Instead of R4.2, at a mid-foam temp of 40F most Type-II EPS tests at R4.5 or better.) At 35F mid-foam temp polyiso would only be delivering R3-3.5.

    AND, at only 1" the permenace of unfaced Type-II EPS is about 2.8-3 perms, comparable to the latex-paint on the interior. This is a 100x improvement in drying rate over foil-faced polyiso. The polyiso would thermally outperform the EPS during the shoulder seasons, and would probably average about R5 to EPS at R4.3 or so over the heating season, which is a small performance hit to take for the advantages of better dew-point management and having at least some drying toward the exterior.

    If it's not to late to re-spec the foam, you're better off with 1" EPS than 1" polyiso.

  6. JAC3232 | | #6

    Thank you for your posts Martin and Dana.
    From what I’ve been reading on the web, dense-pack cellulose hasn’t really taken off in Canada. That’s possibly why I can only locate companies who install it in existing walls. I will keep looking, and MemBrain is certainly an option. We will have an ERV.
    Unfortunately this is common practice around here. The plans we selected had 1” polyiso with Fiberglas batts and VB. The walls are now up with the polyiso (not taped) and housewrap. I feel sick that I’ve missed this detail, and my walls have a greater chance to rot. I almost feel like I should rip the foil-faced Polyiso down and select a different product as Dana suggested. I’m not sure my builder would agree.
    As I mentioned, the sheathing is the Polyiso, not OSB or plywood. My building inspector wants an interior VB unless 2” of CC spray foam is used. MemBrain should qualify. My thought to use 3.5” CC spray foam with no VB was that 1/3 of the stud bay would remain exposed in the wall cavity, allowing the studs to dry to the inside. This is not sufficient? I was also considering 2” of CC spray foam + R14 Roxul and no VB. Should these wall designs be completely ruled out as they won’t allow sufficient drying?
    I will keep searching for a dense-pack cellulose installer for a new build. I guess I could put the drywall up, and cut holes for the cellulose install. I’d much rather find an installer who doesn’t need the drywall up to install.

  7. GBA Editor
    Martin Holladay | | #7

    If it's not too late, why not add a layer of EPS rigid foam insulation on the exterior side of the existing polyiso?

  8. wjrobinson | | #8

    Jeff, I think Martin and Dana missed seeing that you do not have wood sheathing.

    I think your build options you posted are fine.

  9. wjrobinson | | #9

    Jeff, you don't put holes in the drywall if someone does cellulose.

    This is what is done mostly today or wet spray....

  10. GBA Editor
    Martin Holladay | | #10

    AJ is correct: I didn't realize that your house has no OSB or plywood sheathing. Now that I realize my misunderstanding, my advice changes.

    Either closed-cell spray foam or open-cell stray foam will work. You should still avoid interior polyethylene, however, so that the studs will be able to dry to the interior if necessary.

  11. JAC3232 | | #11

    Okay, I wondered if that was the case. Thanks everyone for your input. I’ll stick with the R22 CC spray foam for the main walls, no Roxul, VB, or MemBrain.
    The humidity in the house will be controlled with an ERV. The attached garage exterior walls are also 2x6 with 1” Polyiso sheathing. It will have a natural gas heater that will be used once and a while when doing work out there. Those walls are to get 2-3” of CC spray foam (no VB or MemBrain). If you recommend MemBrain out there, please let me know. Otherwise I’ll leave it out.

  12. Expert Member
    Dana Dorsett | | #12

    Sticking with R22 ccSPF with empty cavity space will deliver LOWER thermal performance than a full cavity fill of oc foam or the R10 foam/R14 Roxul solution due to the dramatically higher thermal bridging. That's because instead of 5.5" of stud (~R6.5) you would only have 3.5-4" ( R4.2- R5) of wood thermally bridging the foam.

    Also note, almost all closed cell foam is blown with HFC245fa, a potent greenhouse gas (about 1000x CO2), and using any more than the minimum needed for dew point control, vapor retardency control, or air-sealing isn't exactly very green.

    Since there are no moisture sources in the garage, it's fine to use all Roxul or open cell foam solution there. Open cell foam will air-seal slightly better than 2-3" of closed cell foam (not that you'd ever get the garage door to seal super-tight in the first place.

    If you're going to use closed cell foam anyway, please seek out Icynene's water-blown 2lb foam (MD-R-200), which runs R5.1/inch but is still an class-II vapor retarder at R22. It's more inches of foam, but it's a lot greener due to the far more benign blowing agent.

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