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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!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Joe,
    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.

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