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

How can I turn my root cellar into good foundation insulation?

JoePan | Posted in General Questions on

We’re in detail design phase of our new house in northeast CT. The frame will be a 2,000sq ft Saltbox, set into a slope. General scheme is superinsulated & airtight shell, and decision tree is in progress – on the HP window, active solar, and back-up choices.

The foundation will be conventionally formed concrete walls and slab. But, based on natural grade will have full-height walls only in the northwest corner: approx. 1/2 the length of the north wall (16-20′) and 1/3 the west wall (8′), with one half-height step wall making up half the remaining length, down to a slab-on-grade making up the remaining length of north and west (east and south foundation will be all slab-on-grade).

Reading here and elsewhere, I’ve seen lots of good info, but no ideal choices, for insulating those full-height foundation walls. So, I’m considering just using root cellar space (with some added garden storage space) as the ‘insulation’. That is, make an insulated interior wall buffering the full-height foundation walls, with an (approx.) 8’x16′ air space behind it.

Closing the envelope and breaking the thermal bridges around that root cellar/airspace seems like a bit of a puzzle, but I’m guessing you all can suggest some straightforward solutions – at least for the ends/bottom/top of the interior basement wall, the root-cellar ceiling/overhead floor, and rim joist-to-exterior wall junction. Any/all ideas would be welcome.

Much more perplexing, however, is a thermal break of some kind between the root cellar floor and the basement floor on the other side of that interior buffer wall. Would some kind of break like a channel of porous blocks (set right into the universal slab) under the buffer wall work, and be worth the bother? If my whole slab is insulated underneath anyhow, do I even need to worry about this?

Looking forward to all your good insights on this, thanks!

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

    As you have surmised, you can draw your thermal boundary wherever you want. If you want a room in your basement to be outside the thermal envelope of your house, so that the room can operate as a root cellar, you can do that. Just draw the thermal boundary carefully, and make sure that the thermal boundary is insulated and air sealed.

    A root cellar should have a dirt floor. The heated basement should have a concrete floor. The perimeter of the concrete slab should be insulated with vertical insulation. It's also a good idea to have a continuous layer of horizontal insulation under the basement slab.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |