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Mold Problem, New Subfloor being built, Icynene/Spray Foam question

Michael_Kalman | Posted in General Questions on

Hey there,

I am building a tiny house and somehow a lot of water ended up in my subfloor and rotted all the subfloor,  the joists are in decent condition but some are in bad condition so I am gutting them and sistering new joists into place. Long story short, I have metal flashing at the bottom of my tiny house trailer and I am wondering if it is acceptable to spray foam 4″ of foam ontop of the metal flashing, will this cause other problems in the future?

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  1. Expert Member
    Akos | | #1

    There is nothing wrong with the SPF there, but it won't prevent your current issue. A better solution is to add a bit of drainage to your metal pan, search for duckbill drain. This will prevent the inevitable water leak down the road from rotting your joist again. Much less than the SPF.

    You can also put a square of a sturdy housewrap over a perforated section to provide additional drying if needed.

    1. Michael_Kalman | | #2

      Sorry Akos! I don’t think I clarified. I drilled holes into the bottom of the flashing to let the water out already so that’s done. There area few grill vent registers I put in the bottom of the trailer in the flashing to let air through, about half a sq ft per 100 sq ft of trailer. I have no idea how to insulate the trailer if I am going to have to remove the 3/8 pieces of plywood nailed to the bottom Of the joists, this will just leave joists that have no ply on the bottom, do I spray right on top of the metal flashing and have an air cavity above the foam which is my joists?. What should I do? How can I remove the ply from the bottom? Angle grinder or a saws all?

      1. Michael_Kalman | | #3

        Oh! I think I understand. If I were to put a duckbill drain in the spray foam layer it would have a route to drain down to the bottom of the metal pan? Is that the idea?

      2. Bto | | #10

        Hi Michael, where is or was this tiny home located and why do you think you had that problem of the subfloor getting rot? We are building a tiny home and i would like avoid these kind of problems. thanks!

  2. Michael_Kalman | | #4


  3. Expert Member
    Akos | | #5

    Maybe a more detailed description, or even better a sketch of your floor would help. Also how did the water get in there in the first place?

    Typical trailer has a metal frame on with sheet metal on the bottom. Insulating inside this metal framing does almost nothing because of the steel's thermal bridging, so it is best left open.

    All your insulation should be above this. Water can collect in these cavities, so you need to make sure there is a way out for it. Duckbill is something you want under areas that can see a big leak like washrooms/utility room. The rest can be holes, I like the idea of a perf panel covered with house wrap as it will keep liquid water out but still allow a bit of drying.

    The insulation above the framing can be anything, SPF is fine but it doesn't get you that much more R value VS batts. The wood framing should never get wet enough that you need foam insulation there.

    1. Michael_Kalman | | #6

      I had installed insulation before the roof and the walls and rain had got into the floor. Totally screwed that up. I have no way to put plywood on the bottom of the existing joists. I’m not sure how I would insulate the joist system if I have nothing to have The SPF to sit on.

      1. Expert Member
        Akos | | #7

        Staple some house wrap on the side of your floor joists and rest the insulation on that.

        1. Michael_Kalman | | #8

          Do you think it would hold spray foam? Would that itself cause any condensation problems in the future?

          1. Expert Member
            Akos | | #9

            I've sprayed SPF against tyvek without issue but that was over sheathing. You can try a test area, if it doesn't work, cobble some foam pieces between the floor joists and spray against that. Thinking about it, rigid foam in this case might be better as you need less SPF to fill the bays.

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