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Community and Q&A

Exterior foam as WRB

Brian Otten | Posted in GBA Pro Help on

I know this question has been asked so many times and I’ve done my research but can rigid foam be used as a trusted WRB? Specifically XPS? I am in a cold climate and have read in my research that XPS outperforms polyiso. My original plan was 4″ of XPS with house wrap over the foam as I will also be installing new windows which will be “outies”. I am just confused if the foam is sufficient with the taped joints or if the house wrap is necessary. So much research has been done but there are mixed results and I want to get right on the first try. This will be my first exterior rigid foam retrofit so naturally I am analyzing all options. Thanks in advance. I am in climate 6, north of St. Paul, MN.

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  1. Expert Member
    Malcolm Taylor | | #1

    I wouldn't be comfortable relying on taped foam joints either at the intersection of the boards or as flashing over openings.

  2. Peter L | | #2

    The only time exterior EPS can be OK as a WRB is with ICF but even so if water gets between the ICF where the forms meet, it won't cause damage since it's a 6" concrete core. When it comes to wood frame I would not feel comfortable with rigid foam being the WRB. The barriers are not that expensive and well worth the money spent especially to prevent moisture damage.

  3. GBA Editor
    Martin Holladay | | #3

    Here is a link to an article that should answer your questions: Using Rigid Foam As a Water-Resistive Barrier.

  4. Brian Otten | | #4

    also on this same topic, any guys out there disregarding the manufacturers specs on installation over foam and rainscreen? It says with over 1" of foam, the furring strips must be a 2"x4". It actually says just the dimensions but I called them and they reassured that it can be a 2x4 and not a 2x6. I like the lp product a lot but I wish I would have never read the these specific set of instructions. I am curious if there are guys out there who are disregarding this detail and using 3/4" furring. I hate to void any warranty but I feel this is such overkill considering many products weigh much more than this siding and can be installed over smaller furring. thanks in advance for any help.

  5. GBA Editor
    Martin Holladay | | #5

    Most building codes require all building materials to be installed according to manufacturers' instructions.

    If you choose to violate manufacturer's instructions, you incur a (very slight) risk that a knowledgeable code inspector will catch the violation. More importantly, you are probably voiding the manufacturer's warranty, a situation that may come back and bite you if the product fails in the future.

    That said, it's fairly common for builders to ignore manufacturer's instructions, either deliberately or ignorantly.

  6. D Dorsett | | #6

    Some advantages of 2x furring is that it's stiffer, making for a flatter exterior surface, and it's thicker, for better fastener retention for the siding. You can make up for retention issue when going with 1x furring by using ring shank nails (or using screws instead of nails), but to get comparable flatness requires tighter spacing on the timber screws that hold the furring to the studs.

    In a St. Paul MN climate with 4" of foam over a 2x6 framed wall you can get comparable performance to an all-XPS solution out of 2" polyiso next to the wood sheathing with 2" of EPS on the outside of the polyiso. This is because the exterior layer of foam keeps the polyiso in its higher-performance temperature range. The advantage of the dual-foam solution is that it uses much less damaging blowing agents, and doesn't lose performance over time as the HFC blowing agents leak out. But EPS isn't as reliably air-sealed with tapes, and thus isn't a great WRB. A housewrap WRB would work, or you could go with 2" XPS for the outer layer detailed as a WRB, cutting the blowing agent climate damage by half.

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