GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Does insulation under the flooring + a sealed floor = vapor sandwich?

Michael Bluejay | Posted in Green Building Techniques on

I have a pier & beam house in Austin with cold floors. I’m considering insulating the underside of the flooring, by either stapling polyiso foamboard across the floor joists and then sealing the joints with tape, or else having closed-cell foam sprayed into the joist bays. If I then seal the hardwood flooring, will that create the dreaded “vapor sandwich”, because I’d be locking in moisture from both sides? If so, can I proceed anyway if I run a dehumidifier in the living space and test the moisture content of the wood before sealing?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Your "vapor sandwich" worries are baseless, for a variety of reasons. First, hardwood flooring can still dry to the interior, even after polyurethane finish has been applied to the flooring. Second, the risk of moisture problems in your type of floor assembly are greatest when the floor is uninsulated; insulating the floor assembly makes everything safer. (The risk with an uninsulated floor assembly is highest during the summer, when exterior humidity can condense on cold subflooring if the air conditioner setting is aggressive.)

    So go ahead and install the polyiso. (By the way, in Climate Zone 2, you want a minimum of R-13 of floor insulation to meet code requirements.)

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |