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Moisture control in a tiny house trailer

Jayne_Torres | Posted in General Questions on

I am preparing to insulate and sub-floor my 10′ x 28′ tiny house trailer.  The trailer was built specifically for tiny house construction and arrived with a seamless steel belly pan that is welded solid to the trailer frame, joists and runners.  I plan to use rigid foam insulation, topped with rockwool, then advantech subflooring, then hardwood.  My concern is moisture control.  Should I drill holes in the belly pan to allow moisture to escape?  Is there some material that I could cover holes with that allows moisture OUT but not IN?

Help.  I’m stumped!!


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  1. Jon_R | | #1

    There is little value to insulation put between steel framing (even more so with a steel pan attached to the underside). So build everything above the frame.

    Here is some info about exterior perms (if you don't ventilate the pan) and how it effects a design. But trapped moisture (eg, between floor and pan) isn't good for wood or steel.

    If you want to apply rigid foam directly to a steel frame, you may be able to use round squash blocks to avoid foam compression without adding much thermal bridging.

    From recent experience: the time to take additional action against a trailer frame rusting is when it is new and open.

  2. GBA Editor
    Martin Holladay | | #2

    You don't need to drill holes in the steel pan. Adding a continuous layer of rigid foam above the steel pan is a good idea. Skip the mineral wool, however -- just add whatever thickness of rigid foam you need to achieve your R-value goal, followed by a layer of OSB subflooring. You can attach the OSB to the steel pan with long screws.

  3. Jayne_Torres | | #3

    Thanks to you both. Jon, that info about exterior perms is invaluable!

    I'll go with the rigid foam. Martin...why OSB instead of Advantech?

    Thank you!!

    1. GBA Editor
      Martin Holladay | | #5

      As Tim explains below, Advantech is a brand of OSB -- so the OSB was your suggestion, not mine. Many builders (including me) might prefer to use plywood. Either will work.

  4. user-6184358 | | #4

    Advantech is a brand name OSB

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