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I am looking at putting rigid foam on an existing house prior to re-siding

talusscree | Posted in Green Building Techniques on

I am on the eastern shore of Maryland (Marine Zone 4, I assume) and am looking at putting rigid foam on an existing house prior to re-siding. The walls are 2×4 and have paper-faced fiberglass batts installed in them that will not be removed. Will I have any issues?

The windows are being replaced as well so would it be better to have the foam face the water management plane or a WRB behind?

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  1. Dana1 | | #1

    Paper facers don't trap moisture, so exterior foam of any type will not create a problem

    If the glass is going to be co-planar with the sheathing, the flashing and housewrap go between the foam and the sheathing (crinkly type housewrap preferred). If the glass is going to be co-planar with the siding , the flashing & housewrap goes between the siding and foam (flat housewrap is fine.)

    Current IRC code min for zone 4 would be at least R5 on the exterior of 2x4 framing. An inch of foil faced polyisocyanurate would get you there, which is cheap & easy to air seal with tapes, and is greener than XPS which is manufactures using VERY high global warming blowing agents. More than R5 is of course better, and 2" of polyiso would not be insane, which would cut the wall losses a bit more than half compared to 2x4 /R13 without foam.

  2. GBA Editor
    Martin Holladay | | #2

    All of your questions (and a few questions that you haven't thought of yet) are answered in the following three articles:

    How to Install Rigid Foam Sheathing

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    Where Does the Housewrap Go?

  3. RD3Sunworks | | #3

    I also live in Maryland, and I did just what you are planning. Look at my article on this GBA site,
    under Green Homes, Making an Old Tract House Sunnier and More Efficient.

    There need not be any harmful issues for you. It's a great idea; you can really cut energy use. In addition to the other articles listed earlier, read and learn as much as you can before starting.

    You might want to research and consider passive solar energy techniques, if applicable to your floor plan/house orientation. As noted in my article, it works if done correctly---and Maryland gets plenty of good winter sun. It may be almost as simple as adding more window area to the south side of your house--this is the best time to do it, with window and siding replacement.

  4. GBA Editor
    Martin Holladay | | #4

    Here is a link to the article that Rick Miller is talking about: Making an Old Tract House Sunnier and More Efficient.

  5. talusscree | | #5

    I have read extensively on this subject as this is not the first one we have done. The other was done with 2" foam and was a little more complicated because the home was very old and the windows were not being replaced. This made detailing around the windows somewhat tedious. This current project is a newer(mid 70's) home and and we will be putting new flanged windows in and installing LP Smartside lap siding. I intend to set the windows at the foam plane to simplify flashing details. Would polyiso with seams taped be the best water management plane or should housewrap be applied over? I keep reading to find answers and never seam to find anything definitive. My client wants systems that have a proven track record so any personal long-term experience or studies would be appreciated. thanks

  6. GBA Editor
    Martin Holladay | | #6

    Every wall needs a water-resistive barrier (WRB). You can use housewrap as your WRB, or you can use rigid foam as your WRB -- it's your choice.

    To my way of thinking, there are a lot of disadvantages to using rigid foam as a WRB. For one thing, it's much harder to integrate the window flashing and the door flashing with the WRB if you use rigid foam, because you have to depend on tapes rather than laps (gravity).

    For more information on this issue, see these two articles:

    Using Rigid Foam As a Water-Resistive Barrier

    All About Water-Resistive Barriers

  7. talusscree | | #7

    Thanks Martin. I read the articles and am working out installation details for windows and trim.

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