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Can I use 1.5” x 3” rigid foam strips between exterior wall sheathing and studs?

Jhaller | Posted in Energy Efficiency and Durability on

I bought an old RV and it had so much water damage that I ended up tearing down everything and just keeping the vehicle, which is a cut off van. I am planning the rebuild of the living space now. I think that it’s going to be an RV/tiny home crossover. What I am considering doing is the following: I would like to use zip panels as my sheathing, but have a 1.5” x 3“ foam board strip in between the sheathing and the stud as a thermal break. I would then spray foam (closed cell) the cavities until it was about 3 inches to 4 inches thick. I would like to know if there any concerns with moisture being able to build up on the exterior sheathing or interior wall paneling due to condensation or vapor transfer. I’m going to make sure that all the joints of the foam are caulked and flashed as well asthe zip panels being thoroughly flashed and sealed before adding the spray foam. For some reason I have not read an article or seen a post about this type of construction yet. It has me a little worried, but seems like it could be a solid way to build.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "It seems like it could be a solid way to build."

    A. Hmm. It could be, perhaps, but there are issues that need to be considered. If you were building a house, I'd say, "Talk to an engineer." Inserting rigid foam between the wall sheathing and the studs reduces the robustness of the connection between the sheathing and the framing, undermining the ability of the sheathing to brace the walls. The usual solution (as implemented by Huber for its Zip R sheathing) is to use lots of extra-long fasteners. The fastener schedule Huber used had to be verified by a laboratory to make sure that bracing was adequate.

    In your case, the closed-cell spray foam adds some strength to the assembly, a plus that may make up for some or all of the reduced strength associated with the foam strips.

    Why not simplify your life and just use Zip R sheathing?

  2. Expert Member
    Rick Evans | | #2


    I just wanted to add that I have seen it done before but not with 3" thick rigid foam (probably for the reasons Martin stated.) There is a net-zero modular home builder in Vermont called Huntington Homes that employ this technique using 1"-1.5" foam as a thermal break.

  3. Jhaller | | #3

    Thank you for your input. I am going to take a look at the Zip R sheathing. And just to clarify I meant 1.5” thick and 3” wide. Also, am I correct to assume since it was not mentioned that there would be no moisture concerns?

  4. Jhaller | | #4

    As a follow-up, would you see any issues or that it would be better for me if I were to use regular zip panels with spray foam on their backside, but then put my foam strips between the studs in the interior wall board? The spray foam would match flush up to the foam board strips in terms of thickness.

  5. walta100 | | #5

    If this project is going to travel very much I think partial board even Zip’s high quality partial board is a poor choice. Because, 1 partial board is very heavy this weight will put needles loads on you tires, suspension and fuel mileage. 2 partial board does not stand up to vibration very well it tends to turn to back to dust around the fastener and become disconnected.

    From a structural point of view you want the sheeting in direct contact with the frame.

    High levels of insulation are only a good investment over long time periods of time. The payback time for insulation from R6 to R30 with most fuels is over 20 years. Do you think this RV has any chance avoiding the shredder for that long? Even if it does my guess is, human controlled vehicles will be less common than horse drawn vehicles are today.

    I think you should build wall similar to what you removed. It is likely the best compromise of strength, weight, space, insulation and cost.


  6. Expert Member


    I think Walter has given you good advice.

  7. Jhaller | | #7

    Thank you Walter and others. I have considered just framing out the space and directly fastening the metal siding to it. Then spray foam the inside of the metal siding like a pole barn. This would be the closest construction method to how this previously built since the original RV just had aluminum siding screwed directly to wood studs and some fiberglass bats in the cavity spaces. Are there any major concerns with fastening metal siding two 2 x 3 or 2 x 4 studs then filling cavities for spray foam? This would save weight as well as perhaps be more structurally sound, correct?

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