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Community and Q&A

Is one inch of XPS foam on the exterior of my walls enough?

Mike O'Keefe | Posted in GBA Pro Help on

I work in Zone 4a and would like to know if it would be a problem to put Tyvek over OSB sheathing and then cover the Tyvek with one inch XPS foam.

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Replies

  1. John Klingel | | #1

    Do a search here. I believe Martin has said "either over or under, but seal the seams in both materials." And, make sure 1" is enough to keep the OSB warm in your climate.

  2. GBA Editor
    Martin Holladay | | #2

    Mike,
    Yes, one inch of XPS (R-5) is enough in Climate Zone 4. For more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  3. Expert Member
    Armando Cobo | | #3

    Mike,
    As you can see in a Dew Point Analysis I did for Wichita, KS (CZ4), if your relative indoor humidity is higher than 45% AND your indoor temperature is above 68ºF, you need to install more than 1” rigid foam. So the key is, how are you or your client plan to control the winter temperature and RH?
    I follow ASHRAE Fundamentals protocol, which is conservative but good building science and liability insurance. I hope this helps.

  4. Kevin Dickson, MSME | | #4

    Armando,

    In the winter, 45% RH would be unlikely, even in a very tight house, without a humidifier running.

    Now that's just my opinion based on observations from living in a fairly tight house. But even if they had a humidifier running, it should be set no higher than 35% RH. (source needed)

    What did you assume for the infiltration air changes per hour in your table?

  5. Expert Member
    Armando Cobo | | #5

    Kevin,
    You are right in most cases, but what happens when it doesn’t. It’s not often you see RH more than 45% in winter, but it does happen depending on occupant behavior and/or numbers, wet winters, crawl spaces or really bad infiltration and thermal bridging. Also, in most cases folks keep their winter temps over 70ºF, and that makes the issue much worse.

    One of the downsides of using ASHRAE Fundamentals formula is that it is simplistic and assumes other issues, like infiltration and craftsmanship, to be perfect. However, as Dr. Joe and Dr. Straube have said to me, if you need to know dew-point limits and do a good job on your designs, ASHRE Fundamentals will give you the right answers, but if you need to impress an Engineer, perform a WUFI Analysis and give them hundreds of reports.

    Here is the same analysis with 74ºF Temperature, which is most common, specially as we get older or have older clients...

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