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Does the insulation on this wall make sense?

user-6832947 | Posted in Green Building Techniques on

I live in Madison WI and am renovating my 1927 house. I have stripped the outer walls to the studs and shiplap and have been trying to figure out the best insulation scheme. The constraints are 1. in Madison they really want plastic on the inside, and 2. because of architectural details I can’t put more than 1 inch rigid insulation on the outside.

So what I’m thinking is dense pack cellulose between the studs (with plastic as a vapor barrier) and then 1″ Roxul comfort board 110 covered with tyvek (because I think the inspector will insist plus it aids in the window flashing). I want Roxul so the walls can dry to the outside. Then I guess I was going to put up .75 furring strips and then cement board.

Here are my questions:

1. DOes this insulation scheme make sense? Is there something better ?

2. Is the idea that walls need to dry a myth, or a minor issue that I don’t need to worry about?

3. Roxul comfortboard 110 has a compressive strength of 1220 psf. That would be 101 pounds per square inch, right? If so, do I really need furring strips? Could I put up vinyl without furring strips?

Thanks

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    User-6832947 (It would be nice if you told us your name, by the way),

    Q. "Does this insulation scheme make sense?"

    A. If I were you, I would substitute a variable-permeance membrane (a "smart retarder") like MemBrain for the interior polyethylene. Your local building inspector is likely to approve of the substitution.

    Q. "Is there something better?"

    A. That's a vague question. You could certainly add more R-value if that was your goal. But your details depend on your goal and your budget.

    Q. "Is the idea that walls need to dry a myth?"

    A. No.

    Q. "Or a minor issue that I don't need to worry about?"

    A. No. Thinking about drying paths is good.

    Q. "Roxul Comfortboard 110 has a compressive strength of 1220 psf. That would be 101 pounds per square inch, right? If so, do I really need furring strips? Could I put up vinyl without furring strips?"

    A. Good question. To answer the question, you could (a) call up the vinyl siding manufacturer and ask if this approach violates the company's warranty requirements, or (b) build a small mock-up to test the idea and see how it feels.

  2. user-6832947 | | #2

    Thanks for your reply Martin.

    If I use Membrane then the walls theoretically breath both ways. Isn't it better to just breath in one direction. And isn't better to seal off the inside for air leaks and humid air in the winter?

    And about better solutions, I'm just wondering if anyone has some better ideas. I can't put thicker insulation on the outside but other than that I am open to ideas. People here seem to have lots of good ideas.

  3. GBA Editor
    Martin Holladay | | #3

    User (I still don't know your name),
    Q. If I use MemBrain then the walls theoretically breathe both ways. Isn't it better to just breathe in one direction?"

    A. I don't like to use the term "breathe," because I'm not sure whether you are talking about air leakage or vapor diffusion. In most cases, there are no problems arising from a wall that has vapor-permeable skins on both sides, although you always have to evaluate every assembly depending on its location and function. (A vapor-permeable basement slab assembly is usually a disaster, for example.) Air leakage through a wall assembly is always bad.

    Q. "Isn't better to seal off the inside for air leaks and humid air in the winter?"

    A. Sealing air leaks is always good. MemBrain is both an air barrier and a smart vapor retarder. During the winter, it limits outward vapor diffusion, limiting the amount of interior moisture entering the wall assembly.

    Q. "I'm just wondering if anyone has some better ideas. I can't put thicker insulation on the outside but other than that I am open to ideas. People here seem to have lots of good ideas."

    A. Dense-packed cellulose between the studs will work fine. If you don't want to use MemBrain, another code-compliant vapor retarder you might consider is vapor-retarder paint.

  4. user-6832947 | | #4

    Thanks,

    I have been looking at Hardie siding info just now and it seems like furring strips is the way to go. Do you think 3/8" treated plywood furring strips over the 1" insulation would be thick enough?

  5. GBA Editor
    Martin Holladay | | #5

    User,
    In most cases, the minimum thickness of furring strips that are used to hold siding fasteners is 3/4 inch. But you should check with Hardie concerning the fastening requirements for their different types of siding, especially if you intend to use very long fasteners that reach through the mineral wool to the underlying studs.

  6. pjmeg | | #6

    If you can add wall thickness to the interior, one idea would be to add foam-backed furring strips to the interior. After installation of cellulose, you can install the variable-permeance membrane. See FHB article link below for more detail on this idea.
    http://www.finehomebuilding.com/membership/pdf/9750/021250059.pdf

  7. user-6832947 | | #7

    Peter,

    That's a really interesting idea. I'm going to have to think about that. You lose 2.25 inches on each wall inside. But it would cost way less and be way easier. I could combine that with more dense pack cellulose. I'm really going to have to consider that.

    Thanks. That's what I was talking about when I said there were lots of good ideas on this site.

  8. GBA Editor
    Martin Holladay | | #8

    User,
    In my first answer, I mentioned that you could always increase the R-value of the wall. I assumed that you would understand that R-value can be added either on the interior side of the wall or the exterior side of the wall.

    For more ideas to get you thinking, see this article: Walls With Interior Rigid Foam.

  9. user-6832947 | | #9

    Thanks Martin,

    I think I am going to do like Pete Marthaler suggested above. 1" thick foam strips nailed to the studs and then 1/2" plywood furring strip over that. That will give me an 1 1/2" of extra depth for the dense pack cellulose. I'll cover that with the Membrane like you suggested. Then I'll 3/4" furring strips over the sheathing on the outside and install fiber cement board. That will work really well with my house's particular architectural issues. Do you see any reason not use tar paper instead of Tyvek over the exterior sheathing? When I tore off the clap board on my 90 year old house I found the tarpaper underneath was in great shape as was the sheathing underneath. Maybe tarpaper is more environmentally friendly.

    Thanks again for your help.

  10. user-6832947 | | #10

    Or actually, now that I think about it, why wouldn't I just put up sheets of foam like the article says. Easier than strips.

  11. GBA Editor
    Martin Holladay | | #11

    Asphalt felt (sometimes called "tar paper") is a perfectly acceptable water-resistive barrier. However, it isn't an air barrier.

    If I were you, I would install plastic housewrap with taped seams, because the housewrap will do a better job of reducing air leakage than asphalt felt.

  12. user-6832947 | | #12

    great, thanks.

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