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Rigid foam insulation Int/Ext & Condensation

chrisdc | Posted in Energy Efficiency and Durability on

I’ve heard a lot of varying opinions on sheet foam insulation and the topic of condensation. Just want to check that I’m choosing an appropriate method of insulating my home.

I will have 2″ of rigid EPS foam insulation on the exterior attached to the outside of existing shiplap through to the studding. Over the foam I’ll install building paper, then either Hardie Board or Stucco. On the inside of the home, there is some old kraft paper stapled to the shiplap. In the wall I’m installing 3″ of rigid foam, then a vapor barrier sealed with acoustic seal.

I live in central Canada where our temperature varies from +30 in the summer to -30 in the winter.

Any advice is appreciated. Thanks

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  1. wjrobinson | | #1

    Chris, I have not seen engineered plans that advocate your plan. Your idea is turning up often of late. I would love to see Joe L. or Straube blog as to how to and how not to build rigid foam assemblies as you are suggesting.

    My experience to date is that I have found lots of failed walls that had foam either inside or outside. Many. Most failed due to unforeseen water intrusion.

  2. GBA Editor
    Martin Holladay | | #2

    The best place to install rigid foam is on the exterior side of your wall sheathing. I don't think that it makes any sense to cut rigid foam into thin strips and insert it between the studs.

    If you want a higher R-value, it would make more sense to install 5 inches of exterior foam, rather than 2 inches. That makes more sense than slicing some of the rigid foam into narrow strips.

    I think that the interior polyethylene is a bad idea, but you'll probably have to fight that battle with your local building inspector.

  3. chrisdc | | #3

    I have a collected supply of smaller pieces that will fit well into my stud wall. The studs are 24" on center. I'll spray foam any gaps with Great Stuff. I was going to install with this method in my bathroom walls. Would you still say that given my foam is already cut and economical, that it is not a good idea? Should I skip on the polyethylene then? thanks for the responses.

  4. GBA Editor
    Martin Holladay | | #4

    Your plan is popular these days with homeowners, but not with professional insulation contractors. Some GBA readers call it "cut & cobble." It's fussy work to install rigid foam between studs in an airtight manner, but it can be done. Most insulation contractors would prefer to install spray foam or cellulose between wall studs.

    There is a second problem, beyond the fussiness of the work: it's usually not a good idea to sandwich wood sheathing between two layers of foam that are relatively vapor-impermeable. If the sheathing ever gets wet, it's going to have a hard time drying out unless you choose to install vapor-permeable materials on at least on side of the wall.

    My own vote: the exterior foam makes sense. Make it thicker if you can. Then install vapor-permeable insulation (for example, cellulose) between your studs.

    The polyethylene is a bad idea because it prevents the wall assembly from drying inwards. But this is a standard Canadian problem, and it has to do with the rigid mentality exhibited by some Canadian building officials. If you can't explain the building science to them, throw in the towel and install some MemBrain (a so-called "smart" retarder).

  5. Expert Member
    Dana Dorsett | | #5

    At 3" EPS that doesn't have a facer is still over 1 US perm, which is enough to provide a drying path toward the interior, but interior poly would render than moot. Under Canadian code an interior vapor barrier is required unless the exterior foam has a high enough R value that the ship-lap sheathing is above the dew point of the interior air at the 99% outside heating design temperature.

    If you can talk them into allowing (Martin's recommended) Certainteed MemBrain instead of poly it would dry more quickly, and would be far more resiliant overall, but IIRC it's doesn't carry all of the lab certifications required in some parts of Canada.

    What Martin said about cut'n'cobbled rigid vs. spray foam. Open cell spray foam would go a long way toward getting a reliable air seal, and would be preferable. Slightly better (but far more expensive) would be to use 3" of water-blown closed cell foam, which like EPS would be a bit over 1 perm, but would air-seal much more reliably.

    Note, there are houses in Saskatchewan insulated with cellulose more than 80 years ago that are still standing, and have no moisture or rot issues related to the cellulose despite the absence of an interior vapor barrier. With a 10mm rainscreen and ~1-perm exterior foam the sheathing would have quite a bit more winter drying capacity than any clapboard home with tar paper and no rainscreen and cellulose cavity-fill. The code requirement for keeping the sheathing above the dew point 99% of the time is excessive, and with 3" exterior foam + 3" cavity foam that can dry in either direction you'd do just fine in most of the Canadian midwest, code notwithstanding.

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