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

Raised four-season room — insulation

KelliSean | Posted in General Questions on

We are remodeling a sunroom that had one supply vent. We are finishing our basement also and had hvac install another supply run and a return to sunroom. we live in St. Louis Mo.  my plan 2” foam board against subfloor can foamed in place between joists. Then mineral wool insulation for a total or r30
can I also put some Tyvek on before the plywood and trim? Basically to hold everything up while I trim it.
Thank you kelli

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

    If you are talking about insulating a floor assembly, you should read this article: "How to Insulate a Cold Floor."

  2. KelliSean | | #2

    Thank you Mr Holladay,
    Can eps or XP’s be used in place of the spray foam to cut down on cost?
    Thank you

  3. Jon_R | | #3

    As discussed in the link, a 1" air gap between insulation and subfloor slightly reduces cold spots on the floor (ie, will be warmer above the joists) . The effect continues (but less) even with continuous foam underneath the joists. I verified this with THERM software.

    1. KelliSean | | #4

      Thank you
      Can I use Tyvek under the insulation before plywood I have some lying around.

      1. Jon_R | | #5

        Sure. Keep in mind that air sealing is critical - multiple air barriers (eg the foam and taped plywood) are probably worthwhile.

  4. Expert Member
    Dana Dorsett | | #6

    If going with the 1" air gap the air sealing of the band joists becomes more critical lest the air gap become a thermal bypass channel. It's also important to insulate the band joist to at least R15, even for that 1 air gap.

    Using foil-faced polyiso or foil faced EPS as the top side air barrier for the fluff is preferred, since a shiny foil facing an gap adds another R1 or so to the stackup.

    Using 1" polyiso for the air barrier and 1" polyiso for the spacers to establish the air gap works. On the exterior walls make the spacer at least 3.5" wide to deliver the R20-ish R-value rather than a strip of cold band joist in direct contact with the gap air.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |