Replies

1. GBA Editor
   Martin Holladay | Feb 04, 2012 05:30am | #1

   Greg,
   The answer to your question can be found here: Calculating the Minimum Thickness of Rigid Foam Sheathing. In your climate zone, you want at least R-7.5 of foam insulation on the outside of the fiberglass batts to be safe. You insulation contractor's proposal provides that.

   However, there are a couple of things you should know:

   1. It is extremely hard -- some might say impossible -- to consistently apply closed-cell spray foam in such a way as to achieve a uniform thickness of 3/4 inch. It is far more likely that the actual thickness will vary -- for example, from 1/4 inch in thin spots to 1 inch in thick spots.

   2. You didn't tell us whether or not your house has OSB or plywood sheathing. If the 1-inch Dow Styrofoam is your only sheathing, then a flash-and-batt job might make sense. But if you have OSB or plywood sheathing, I would advise you that it usually isn't a good idea to sandwich wood fibers between two layers of closed-cell foam.

   Anyway, here's the bottom line: if you don't have OSB or plywood sheathing, the flash-and-batt system will work fine. Just be sure that the spray foam installer really gives you a minimum of 3/4 inch. And since that is a relatively thin layer -- too thin to be sure that it provides an effective air barrier -- I would definitely tape the seams of the XPS sheathing.

2. 
   RRzLxF6ZNL | Feb 04, 2012 09:21am | #2

   Thanks for the response, Martin.

   I assume the spray closed-cell foam and the rigid foam board can be counted together when calculating the R value outside the batts. So where I read "a minimum of 2 inches" other places online, I can think of my installation as 1.75" (1" rigid foam board + 0.75" sprayed closed-cell foam). Is that correct?

   1. Would it be better to request that the spray foam be 1"?

   2. There is some OSB (or plywood) on parts of the walls where it looked like extra structural support may have been needed, such as in corners and by windows. So in those cases, there's OSB, then Dow styrofoam, then brick. You're saying it would be bad to spray foam overtop of the OSB?

   Thanks!

3. GBA Editor
   Martin Holladay | Feb 04, 2012 11:27am | #3

   Greg,
   As long as the OSB or plywood is dry when it is covered with spray foam, you'll probably be OK. One inch of spray foam will provide a better air seal than 3/4 inch.

4. Expert Member
   ARMANDO COBO | Feb 04, 2012 11:29am | #4

   Greg,
   I would consider using DOW’s 1” SIS panels for sheathing and structural, one installation and instead of OSB or plywood, then CC and finishing with cellulose (no batts). Make sure your brick ledge is wide enough to install sheathing and have 1” air space clearance from the brick.

5. 
   RRzLxF6ZNL | Feb 04, 2012 12:46pm | #5

   Thanks, again.

   @Armando, they're already past the point where that would be an option—the brick is going up. But thank you for the suggestion.

   So it sounds to me like the flash&batt installation that I described in the original post should be OK. But does it sound like it's a *good* idea? I mean, is it worth the extra $3100 to do this flash&batt rather than a normal fiberglass insulation?

   Thanks in advance.

6. 
   wjrobinson | Feb 04, 2012 12:59pm | #6

   Sprayfoam will make your walls more airtight. Much more can be done to improve the airtightness of the rest of your structure.

   Do not spray in too cold weather. Use the most experienced contractor in your area.

7. 
   wjrobinson | Feb 04, 2012 01:02pm | #7

   You value GEO, so it makes sense to value insulation. Write the check.

8. 
   RRzLxF6ZNL | Feb 04, 2012 01:05pm | #8

   I'm happy to write the check as long as it's actually better. I was just worried that I'd be installing something too thin that would potentially create a moisture/condensation/mold problem in my walls. But it sounds like it shouldn't be a problem, and that it is better than plain fiberglass.

   Thanks for all the help everyone.

9. 
   dankolbert | Feb 04, 2012 02:53pm | #9

   This once more points to the critical need for the energy and mechanical design to be part of the design process and not crammed in as an afterthought. The detailing necessary for building an energy efficient home includes the foundation, the framing, etc - lots of things need to be thought through BEFORE the last minute decision to call in the spray foam dudes and assume that a layer of goo will solve all your problems.

10. 
    snfh | Feb 05, 2012 11:07am | #10

    Martin,
    Does any thickness of closed cell spray foam on the interior change the calculation for minimum thickness of rigid foam sheathing on the exterior? This is more of a theoretical question since closed cell foam is an air barrier. I'm not advocating not using rigid foam sheathing for thermal bridging. Also, can you elaborate on the potential ramifications/issues of sandwiching wood fibers between layers of closed cell foam? I suspect the walls of most custom homes in the midwest that use spray foam are built with plywood sheathing (corners, windows, doors, etc.) and covered with a min. of 1/2" rigid foam sheathing. So the sandwhich you refer to is created multiple times every working day of the week.I realize encapsulating wet plywood with closed cell foam will not allow the plywood to dry, but if the plywood is dry, are there any other considerations other than a future leak?

    Thanks,
    Bruce

11. GBA Editor
    Martin Holladay | Feb 05, 2012 01:25pm | #11

    Bruce,
    I generally advise builders to choose one place to put their foam: either on the exterior side of the wall sheathing, or the interior side of their wall sheathing, but not both.

    If one side of foam is thin -- like your hypothetical 1/2-inch of rigid foam -- there isn't too much concern, I suppose, because some vapor diffusion can occur through such a thin layer of foam. Needless to say, however, there isn't much R-value in 1/2 inch of foam, so it's hardly worth installing.