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Miscellaneous Insulation Questions

Hi all,

I posted my first question last night after months and months of reading. Now that I'm over that hurdle, I thought I'd ask a few more about a house I'm building for my family in CZ 4A.

1. 2x6 construction with headers built up (1 ply, 2 ply, 3 ply) depending on load requirements (I'm a structural engineer and designed the house). There was a thread that I understood to say that filling the remaining portion of the cavity with foam (I'd be using the pink, Owens Corning Foamular board) was not really worth the effort. Is there any harm in doing it? (ie, 1" or 2" is too thin and cause moisture problems? As I understand it. Foamular is a vapor retarder and since I don't have any other vapor barriers in the wall, I should be ok.

2. The HVAC contractor installed a 3"x12" x 2' tall duct in the wall for the range hood. I'd like to push Foamular behind this as well. Any issues?

3. My stairs are on an exterior wall. The rim joist/band board was framed out as a box beam for the length of the stairs. How do I insulate the box beam? Foam-in-a-can isn't recommended by the manufacturer because it's an enclosed space. Dense pack cellulose isn't ideal because it's enclosed on all 6 sides. The only options I see are dense pack fiberglass, foamed-in-place foam ("shaving cream" style similar to Polymaster R 501 or Applegate R-Foam), or just air seal it and leave it uninsulated.

4. Despite my best efforts to detail "insulatable corners", they didn't all get built that way (and I only have 4 corners x 2 levels!). Some corners are completely boxed off, some have a 1" gap on the inside face. Similar question to #4...how would you insulate these areas?

5. I have a steel beam supported on an exterior wall which supports a portion of the second floor. The beam was was supposed to be 2" from the outside face of sheathing. Instead, it is 1/2" from the outside face of sheathing, and no sheathing was installed at the beam end (The Tyvke and fiber cement board cover the end of the beam. At this point, I think the best thing I can do is foam the end of the beam in place to minimize air movement at this area. In reality, this condition occurs all of the time in commercial construction (and sometimes residential where I've seen 5.5x5.5 steel tubes used as bearing posts). Any other thoughts?

Thanks to all who might endeavor a response...these are just the questions that have been bothering me that I haven't been able to resolve on my own yet.


Asked by David Baca
Posted Jan 1, 2013 11:47 AM ET


1 Answer

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I don't really have time to answer all of your questions in detail. but here is my first reaction: the questions you ask -- about window headers, stud bays filled with ductwork, rim-joist beams, and uninsulated corner pockets -- are problems with a similar solution. All of these problems could be solved by a thick layer of exterior foam insulation. These areas provide an excellent argument in favor of using exterior rigid foam.

Window headers: definitely include rigid foam in your sandwich.

Rim joist / box beam: I'm not sure of the construction, but you definitely want insulation, not air voids.

"Uninsulatable corners" -- get a hole saw and pump in some dense-packed cellulose.

Ducts in exterior walls: Just say no! If you have to run a vertical duct near an exterior wall, build a chase on the interior side of your wall insulation and your interior air barrier.

Answered by Martin Holladay
Posted Jan 1, 2013 5:20 PM ET
Edited Jan 2, 2013 9:41 AM ET.

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