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

Crawlspace interior walls, above and below grade

WCBosch | Posted in General Questions on

Hello, zone 4a east TN. I’m in the process of encapsulating my crawl space which starts with a double door walk-in 7′ high that quickly narrows down and gradually gets to 3′ high on the back wall. Construction is the typical concrete block wall with piers and dirt floor. The house is built on a slope so about two thirds of the house is mostly below grade while the other third is above grade. Fiberglass batt has been removed from floor joists. So no insulation at all at this point.

I plan on using either DOW Thermax or JM CI Max 2″ thick foamboard on interior walls, thick poly sheeting on the dirt floors, air sealing, insulating rim joists and sealing off vents.

Questions regarding the process:

1 – How far down from sill plate to I need to insulate below grade, the entire height or just a portion or none at all?

2 – Do I first install floor poly sheeting taped to wall and adhere foam board over or install foam board first and tape poly over?

Hope that’s clear, thanks for any input. William

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
    Brian Pontolilo | | #1

    Hi William.

    I recommend you read this: Building an Unvented Crawlspace

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |