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How much spray foam in wall cavities? If any.

drewfridley7 | Posted in Energy Efficiency and Durability on

200 sq ft tiny home. 2×4 framing. 2×6 rafters. One inch of exterior insulation with zip r. Two inches of rigid foam board in the roof already. I had a gentleman give me a quote on spray foaming the inside. He recommended three inches in the roof and only batts in the wall cavities. He said he would recommend this to save me money on foam and labor. And because I’ve already sealed the sheathing joints and between the framing members. Is this true? Would it not be worth the money to have spray foam versus batts in the walls?

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Replies

  1. bearcreekbuilder | | #1

    Please DO NOT spray foam your tiny home!

    A tiny home is already so small that you risk creating an envelope that is far too tight for its size not to mention the potential for foam off gassing continuously while living in it and possible structural damage from incorrect detailing and water intrusion.

    There are a litany of reasons NOT to do this.

    If you want to spend a bit extra get some recycled blue jean insulation batts or wool batts but fiberglass and/or mineral wool batts would be perfectly fine for your application.

    Look for spray foam horror stories. There’s no shortage…

    I won’t even GC on homes with spray foam anymore.

    Edit: I forgot to take into account your exterior insulation.

    You need NOTHING more on a 200sq/ft tiny home with exterior wall and roof insulation than regular batts.

    By spray foaming you would be encapsulating your wood framing and any moisture intrusion at all would spell an early demise.

    1. Expert Member
      MALCOLM TAYLOR | | #2

      bearcreekbuilder,

      I'm with you except for this: "A tiny home is already so small that you risk creating an envelope that is far too tight for its size" which I don't understand.

      1. bearcreekbuilder | | #3

        My thought here is that without an HRV/ ERV spray foaming could seal a small 200 sq/ft structure so tight that carbon dioxide and/ or monoxide could become a serious threat in a winter living situation. Not to mention shower steam, cooking, occupant residual moisture.

        Could be done, it just would require the necessary ventilation system and I just assumed that was not in the plans. Lol

        Of course a larger structure that was this tight could “buffer” some of these concerns with the air volume. Though of course this would still be bad!

        I think my over arching thought here was that for the size of structure going that tight with all the additional details needed to make it work safely/ correctly it’s just not necessary given the potential for a simpler path. IMHO

        1. drewfridley7 | | #4

          I have a Panasonic ERV installed. My aim is to make the structure as tight as possible. If I spray foam it, I still will be months from even thinking about moving into it. Major off-gassing will be long since finished by then. Plus, it will trapped behind drywall. My question remains unanswered though. Does a high r-value in the roof and air sealing everywhere warrant a much smaller r-value in the walls?

          1. Expert Member
            DCcontrarian | | #7

            >Does a high r-value in the roof and air sealing everywhere warrant a much smaller r-value in the walls?

            Assuming that as a tiny house you're not bound by codes, the answer is "it depends."

            The basic game with insulation is you add it until it no longer makes sense to add any more. That's a surprisingly hard decision to make, because you pay for insulation today and receive benefit from it over the lifetime of the building, so it's hard to make direct comparisons. The answer depends on what your climate is like, what your source of heat is going to be, and what you expect the energy required for it to cost in the future. It also depends on how much you discount future spending compared to current.

            Let me throw out some back-of-the-envelope numbers. Let's assume your 200 square foot building is 20x10, with 8' walls. A 2x4 wall with fiberglass batt is R13, with 1" exterior it's probably R17. Your walls would be 480 square feet, the floor and ceiling are 200 each so 880 square feet total. If the whole thing is R17, when it's 0F outside and 70F inside your heating load would be (480*70)/17= under 2000 BTU/hr. That's less than 600 watts, you probably won't need much heating at all. But ventilation of 50 CFM at 0F is 3800 BTU/hr, even with an ERV that's going to be a big chunk of your heating bill. And that's always going to be there regardless of the insulation level.

            To really answer the question you should do a full model where you include windows and doors and actual insulation levels for ceiling and floor. Then it's easy to change values and see what happens to the total. But my guess is you're already past the point of diminishing returns.

        2. Expert Member
          MALCOLM TAYLOR | | #5

          bearcreekbuilder,

          Ah, I've got it. I'm not sure you should ever rely on poor air-sealing to provide safe indoor air quality though.

  2. Expert Member
    Michael Maines | | #6

    What is your climate zone, or the coldest zone you will travel to? Spray foam is definitely overkill for such a small space, and if you have taped sheathing it will do nothing to improve airtightness. There are specific safe ratios of foam-to-fill insulation, depending on the climate zone, and different for walls and ceilings/roofs.

    1. drewfridley7 | | #8

      Thanks Michael. I am in Richmond, Virginia. The home will stay here, 15 minutes east of the city. I do have taped zip-r sheathing. I am aiming for a long-term energy efficient home. How would you do it if it were your home?

    2. drewfridley7 | | #9

      ?

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