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Insulating a suspended floor: Caulk the seams or not, and is cellulose okay?

michaelbluejay | Posted in Green Building Techniques on

A friend of mine is building a tiny house on a trailer in Austin, TX (Climate Zone 2A), and has questions about insulating the floor.

First, should the seams of the floor framing be sealed with acoustical sealant before installing the insulation, to prevent air infiltration and increase insulation performance, or is some air movement good to prevent moisture accumulation and mold?

Second, is loose-fill cellulose filled from above an acceptable insulation material? I’m concerned because my understanding is that the insulation must contact the bottom of the subfloor, and that might be hard to do with loose-fill. We could slightly over-fill and then the plywood subfloor panels could compress the loose-fill, but then I’d worry about settling. She could always go with rigid foamboard, but she prefers cellulose if possible because the material is greener and it’s a little cheaper.

Finally, if you approve of cellulose, is it acceptable to fluff it up in 5-gallon buckets with an beater paddle on a drill (in my test that about doubled the volume), or is it necessary to rent the blower machine?

Thank you very much for your help.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Michael,
    Q. "First, should the seams of the floor framing be sealed with acoustical sealant before installing the insulation, to prevent air infiltration and increase insulation performance, or is some air movement good to prevent moisture accumulation and mold?"

    A. I'm not sure what you mean by "the seams of the floor framing" -- is there a layer of plywood or OSB under the floor joists? -- but the basic answer to your question is that air movement through your building envelope is never a good idea. In fact, air movement is more likely to cause moisture accumulation than prevent it. So strive for airtight construction methods.

    Q. "Is loose-fill cellulose filled from above an acceptable insulation material? I'm concerned because my understanding is that the insulation must contact the bottom of the subfloor, and that might be hard to do with loose-fill. We could slightly over-fill and then the plywood subfloor panels could compress the loose-fill, but then I'd worry about settling."

    A. Loose-fill cellulose will work, sort of. It will probably work well enough for a tiny house in Texas -- well enough that you shouldn't lose any sleep over it. You're right that the cellulose will probably settle, especially if the tiny house is on a trailer and is sometimes driven down the road. This house will also have thermal bridging through the floor joists, but these issues probably won't lead to any performance problems.

    If you or your friend has an obsessive personality, a better type of insulation would be to use a continuous layer of rigid foam, sandwiched between two layers of OSB or plywood.

    Q. "If you approve of cellulose, is it acceptable to fluff it up in 5-gallon buckets with an beater paddle on a drill (in my test that about doubled the volume)?"

    A. No. The denser the insulation, the better it will perform. Fluffing up the insulation is the opposite of what you want -- it worsens the insulation performance. Cram the insulation in there tightly. You don't need a blower for a tiny house -- use your fists or small rectangles of plywood to compress the insulation.

  2. Expert Member
    MALCOLM TAYLOR | | #2

    Michael,
    When you lay plywood or OSB subfloor the seams that fall on the joists are sealed by the construction adhesive you use. If you are thinking of applying something to the T & G as you install, you don't want to use acoustical sealant. By the end of the day and for the rest of the build, the floor, you, every tool you own, and as time goes on your truck, friends, family and distant acquaintances, will be covered in a mixture of sawdust and the black sealant. I'd use something like Big Stretch instead.

  3. charlie_sullivan | | #3

    Fortunately, although acoustical sealant can spread as Malcolm describes, it does not yet spread through the internet. But in addition to what he describes, it smells terrible, continues to smell for months, and is made from "residual oil" which contains a random mix of unknown hydrocarbons, which I can only assume includes harmful ones.

    For situations where something like that is needed, I recommend Contega HF, which is low odor and its only VOC is ethanol which is reasonably safe as long as you don't swallow a whole tube before driving home. It's available from 475 building products, but is expensive. It will also get all over things, but it's green instead of black, so the result is more socially acceptable.

  4. michaelbluejay | | #4

    Thank you everyone for the replies. Martin, yes, there's a layer of plywood or OSB below the floor joists. I don't understand your suggestion of sandwiching a continuous layer rigid foam between two layers of OSB/ply. A continuous layer would mean no floor joists, and joists seemed necessary, but on further reflection, the trailer has its own metal joists, and the wood perimeter is lag-screwed to that, so maybe no wood floor joists are required. Is that what you're suggesting?

    Or maybe you're suggesting ply/OSB on top of the joists, then the foamboard, then another layer of ply/OSB, but if so then I don't know why you're seemingly suggest we ignore the floor joist cavities completely. It seems like we could get the same R-value by putting cellulose in the joist cavities, then have a wood/foamboard sandwich layer of only 1/2" of foamboard on the top of the joists, rather than the 4" or so we'd need if we didn't fill the joist cavities.

    Malcom, the joints I was talking about are the ones where the wood perimeter and joists contact the very bottom sheet of ply/OSB, which sits directly on top of the trailer.

  5. GBA Editor
    Martin Holladay | | #5

    Michael,
    You don't need to overthink this. Tiny homes are easy to heat, and don't need a lot of insulation. The idea behind continuous rigid foam is to interrupt thermal bridging through the floor joists, which is usually a good idea, but which is probably overkill for a tiny house.

    Do you need joists? It depends on the steel structure of the trailer you are building on.

  6. michaelbluejay | | #6

    Martin, I appreciate your help, but your answers invariably lead to more questions since your reply isn't clear, and when I ask for clarification, you tell me to "not overthink it". I'm not trying to "over"-think anything, I'm trying to think it the proper amount. That starts with understanding your answer, which I don't. I spelled out what I didn't understand. Your reply doesn't illuminate. So what did you mean by a continuous layer of foam? Did you envision a huge empty cavity about the metal trailer frame with no wooden floor joists/sleepers? Or did you mean something else?

  7. GBA Editor
    Martin Holladay | | #7

    Michael,
    If you have an adequate metal frame to build on, you might be able to assemble your floor directly on the metal frame, thus: OSB, rigid foam, OSB -- or, alternatively, plywood, rigid foam, plywood.

    If you need joists, you need joists. Insulating your joist bays is a good idea. If you want to reduce thermal bridging through the joists, you can install the following layers above the insulated joist bays: OSB, rigid foam, OSB -- or, alternatively, plywood, rigid foam, plywood.

    That said, your original plan to simply insulate the joist bays will work. I wouldn't recommend that approach for a floor over exterior air in Vermont -- but for a tiny house in Texas, it's fine.

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