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Community and Q&A

Soffit for EPDM non-vented roof

Sealik | Posted in General Questions on

Hello…..the tiny home has a 60 mil EPDM roof, 4.5″ of closed cell insulation with R22 bat below that. The ‘issue’ I’m having is some condensation on the underside of the OSB eaves. Wasn’t sure if it would work but I installed metal soffit assuming any warm air would condense on the cold metal. It’s better but not 100%. So I’m thinking I could attach some tar paper or Tyvek to the underside of the rafters before I put the soffit on. and basically seal it up. This should work…correct? Didn’t really want to use plywood and or….the black metal looks much better.  Spaces between rafters (overhang) would be uninsulated as it is now? The polystyrene between the rafters is there for the needed R20 above the exterior walls. I left the job and my helper had already started blocking between the 2×10 rafters which were flush with the inside…not the outside. Had the spray insulation guy showing up in an hour….so we just carried on. Thoughts….? Thanks.
Climate Zone 6/ North Thompson, B.C.
Pic of my tiny with the twisted roof….

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  1. Jon_R | | #1

    Spray foam on the exposed OSB should eliminate condensation. If there isn't spray foam on the other side of that rigid foam, it would also improve air sealing.. Otherwise, paint on the OSB will help with underside condensation mold.

    1. Sealik | | #2

      Thanks for the reply.
      The other side (behind/interior) the rigid foam is closed cell. Isn't it possible to stop any vapour before it hits the underside of the eaves? Not sure how hot those areas would get in the summer time though....possibly creating humidity/moisture with nowhere to go. The 2 fans for bath and stove go through the walls....which compounds the issue. Didn't want to have too many holes in EPDM. Pic shows sealed soffit.

      1. Expert Member
        MALCOLM TAYLOR | | #3


        The air in the soffits has the same moisture content as all the exterior air around it, so there is no way to stop it getting to the OSB. It isn't that the air is moist, it's that the OSB is cold enough to cause the moisture to condense on it. Trying to air-seal the soffit while there is still an air space on top won't make any difference. The solutions Jon gave you will work.

  2. Expert Member
    Akos | | #4

    Un-insulated soffits with metal roofs are very common. You can get some condensation there from night time radiation but it evaporates quickly during the day (you don't want the soffits sealed). Unless I'm misunderstanding the problem, to me this is a non issue.

    P.S. Really like the twisted roof. How did you deal with the plywood seams, did you have to cut each one to fit?

    EDIT. I just realized it is an EPDM roof. That works the same way, flat roof with uninsulated soffits work.

    1. Deleted | | #5


  3. Sealik | | #6

    Guess I'll have to slather the underside with some quality Thanks and...Yes...the sheets have to be trimmed to fit the twisted roof. All interior framing and gable walls are compound angles. . Even the 2x10 nailer on the ends of the rafters sit at a slightly different height relative to the rafter itself..... because of the transition from a leading and trailing edge roof line.

    1. Expert Member
      MALCOLM TAYLOR | | #8

      A lot of work I bet, but a fun roof. I like the model too!

  4. Sealik | | #7

    Did a little test. Dried out the underside of 3 rafter bays with a heat gun and sealed it up with Tyvek. Waited 3-4 days with varying degrees of outside temp while running the propane stove exhaust fan occasionally, which exhausts directly below it. Just had a peak......and.....all were dry. Whereas the rest of the overhang(s) is damp. Shall seal it back up and check it again in a week or 2.

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