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

Elevated porch- exposed door rafters

Kevin J | Posted in General Questions on

I have a 90sf porch that is 8’ above grade with uninsulated 2×6 floor joists. What R value should I try to hit outside of Boston? Can I do an inch of DIY spray foam and then fill the rest with foam board?

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.

Replies

  1. Jason S. | | #1

    John,
    If you're looking for cost-effectiveness, fill the full cavity depth with cellulose. The extra R value from "cut-and-cobble" foam yields meager additional energy savings for the cost, unless you can install it continuously underneath the joists to stop thermal bridging.

    1. Kevin J | | #2

      Not sure I want to deal with blowing in that small of a space to be honest. What’s next best? Don’t I need a layer of foam?

      1. Jason S. | | #3

        If IECC 2018 is mandated where you're building, zone 5 calls for R30 or full cavity insulation fill not less than R19. If you're installing from the exterior then yes, R10 rigid foam covering the joists filled with cellulose or batts would be great, but if you're pulling up subfloor and installing from the inside, loose fill cellulose I would think is as cheap as it gets.

        I'm sidestepping the airtightness issue, but I've always been skeptical that cut-and-cobble foam airsealing can be done well and actually holds up over time when wood framing deflects and recovers and goes through its thermal/moisture cycling year after year. I have no data on this, just an inkling.

        1. Kevin J | | #4

          I’d insulating from the underside of the exposed joists. Headroom might be an issue with the foam boards covering the joists which was why I was leaning towards a CC spray foam Product between the joists.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |