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Community and Q&A

Thermal Bridging and Moisture Concerns with Spray Foam Insulation in a 2×4 Wall Cavity

UmURgU4xtc | Posted in Energy Efficiency and Durability on

I am close to the borders of climate zone 2A and 3A in Texas. I’m more in 2A territory than 3A, though. The weather is hot and very humid during the summer (with the exception of our drought this summer!), and cold and soggy in the winter.

I am building an energy-efficient movable cabin as an experimental project. Because of the high cooling loads during the summer, I want to make as efficient a building envelope as possible. I have been all over the place with different designs for the walls and roof of the structure.

Currently, I’m planning to use 2×4 walls @ 24″ O.C. due to weight concerns. This means that I don’t have a very deep cavity for insulation. On the outside of this frame, I will use ZIP system wall sheathing. My original plan was to use spray polyurethane foam to a depth of about 3 inches in this cavity, but it did not account for the thermal bridging of studs, headers, and other wooden structural elements. I’ve looked at a number of different ways to address the thermal bridging, and the two most promising ones are to place rigid foam insulation between the frame and the ZIP sheathing, or to place it on the exterior of the ZIP system sheathing. From a thermal standpoint, both would work, but I’m very concerned about moisture flow through the wall system. Closed cell polyurethane spray foam is pretty much vapor-impermeable, so installing rigid foam on the outside of the ZIP system creates a sandwich of ZIP system between 2 vapor-impermeable layers. Installing rigid foam inside of the ZIP system would consolidate the impermeable layers, but I’m concerned about how much it would affect the use of the ZIP panels as structural panels to resist shear. Because this building is meant to be movable on a trailer, it will be subjected to higher shear loads than usual, so I don’t want to overlook shear strength in favor of energy efficiency.

I’m also a little on the fence about the roof. To minimize weight, I’m looking at 2×4 rafters, which should span the 8 foot width (4 feet on each side of the ridge beam) of the cabin without trouble according to span tables. However, this narrow space doesn’t allow for much insulation space if I ventilate it. I was looking at an unventilated design similar to the wall design above, with ventilated roofing, so it can dry to the exterior. This area has little to no snow loading or icing in the winter.

I have a few questions:

1. What is the impact on shear strength of sandwiching, say, 1 or 1.5 inch rigid foam under ZIP system OSB sheathing?

2. Am I worrying too much about the vapor sandwich of rigid foam and closed-cell polyurethane spray insulation around ZIP system sheathing?

3. If I place rigid foam on the outside of the structure, would it address the moisture issue to back-ventilate the space between the ZIP panels and rigid foam panels? It seems to me that this would be detrimental to the insulating performance of the rigid foam.

4. What other ways might I use to maximize insulation in a limited 2×4 cavity space, while minimizing thermal bridging? I thought about using rigid foam strips on the inside edges of the 2×4, but that seems like a detailing nightmare, not to mention the non-standard use of the rigid foam. I also considered lining the inside of the structure with a cellulose blanket or similar vapor-permeable insulation under the finish wall. Again, it sounds like a bit of a detailing nightmare.

I am open to feedback on any of my determinations thus far; as I said, this is an experimental project.

Thank you all for your time.


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  1. GBA Editor
    Martin Holladay | | #1

    I agree with you concerns about sandwiching Zip sheathing between two layers of vapor-impermeable foam. One possible solution to this problem is to use open-cell spray foam rather than closed-cell spray foam in your stud bays -- that way the wall assembly could dry to the interior. However, open-cell foam will provide a lower R-value per inch than closed-cell foam.

    If you put rigid foam between your Zip sheathing and the studs, the Zip sheathing will no longer provide any shear bracing. One option to consider: provide shear bracing with L-profile metal strapping, and omit the Zip sheathing on your walls. Instead, use rigid foam sheathing without the Zip sheathing.

  2. Expert Member
    ARMANDO COBO | | #2

    You could use Dow SIS panel, structural foam board, with an R5.5 at 1" and an R13 of 3.5" of open cell, sealed and taped, plus a WRB on top, I would think that's pretty good wall assembly. You can always use additional lateral bracing for extra strength.
    Moisture management detailing is a priority with any type of insulation, but you are right not to install OSB between 2 CC foam layers.
    High on you list of priorities should be ventilation and I would look into installing a small Panasonic Whisper Comfort ERV.

  3. Billy | | #3

    Another possibility, in addition to or instead of foam boards on the exterior, is to cross-strap the interior of your 2x4 wall with 2x material (such as 2x4 ripped in half or other strips of scrap framing material that would otherwise be thrown out). The strapping is laid horizontally to eliminate most thermal bridging through the studs. Tack up Insulmesh and then blow in cellulose or Spider chopped fiberglass, making sure you get a dense-pack installation. This combined with foam on the exterior provides superior insulation and eliminates the vapor barrier sandwich.

    Google "Mooney wall" for details about this approach.


  4. homedesign | | #4

    Jonathan, what is the purpose of the experiment?
    Do you plan to build more 8 ft wide portable cabins?

  5. UmURgU4xtc | | #5

    Thanks for your response! I like the idea of using an open-cell foam on the inside of the structure, but if I'm going to go with an open-cell material, I'll probably just go with an open material like cellulose, since the R-Value of open-cell is not much better than other, much cheaper materials. I don't want to lose the rigidity of sheathing panels, though I'll admit it stems mostly from the fact that I don't have experience with structures that are braced separately, then sheathed with rigid foam. I'm concerned that when the structure is moved, that the foam will not be durable enough to withstand the stresses of being transported. If the foam is placed directly on top of Advantech, ZIP, or other sheathing, does there need to be a space to allow water to drain between structural and insulating panels?

    The SIS panels are interesting. Where would I acquire such things? I'm not a contractor, just an electrical engineering student working on a tiny cabin with a focus on energy efficiency.

    The Mooney wall was one of the designs that I seriously considered. I'm still considering it, but I am limited by the width and weight that I can transport with standard trailers (8.5 feet wide) without a special permit. The strapping reduces the available interior dimensions just a little bit more. I like the advantages, though.

    The purpose of the experiment is to see about how effective a tiny mobile cabin can be, and for what price point. I may build a few if I find the first one reasonably functional, but the first one is a test case and guinea pig.


  6. GBA Editor
    Martin Holladay | | #6

    You wrote, "I am limited by the width and weight that I can transport with standard trailers (8.5 feet wide)."

    I assume that these regulations are based on the exterior dimensions of your trailer. I fail to see the difference between adding insulation on the exterior of your studs or on the interior of your studs. In either case, your wall becomes thicker.

  7. UmURgU4xtc | | #7

    You are correct; it is in reference to the exterior dimensions of the load. And yes, a thicker wall is a thicker wall. I wasn't terribly precise in my response to William. It was my intention to just communicate information about a limitation, not represent a relative difference between a Mooney wall and other methods of addressing thermal bridging. Sorry about that.


  8. wjrobinson | | #8

    Transport strength=closed cell sprayfoam

    Thinnest wall= closed cell sprayfoam

    What else?

  9. user-659915 | | #9

    You haven't mentioned your interior wall finish. In such a small structure radiant effects are going to be significant - your body is always going to be close to a wall surface. With a transportable structure you'd also be wise to use a movement tolerant material. For both reasons I'd avoid sheetrock. 1 x 10 pine board would be pretty nice - with foamed stud cavities you'd not have to worry about air-sealing.

    The SIS referred to above is a Dow product, it may be a special order for your local lumber yard as addressing thermal bridging is generally more of a cold-climate concern. They may take some persuading as a minimum order would be enough for a couple of large houses.

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