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ThermoMass Foundation

GBA Editor | Posted in Green Building Techniques on

The super-insulated lake cottage I’ve designed for Chipman Pond in Tinmouth VT includes the ThermoMass insulated concrete system for its walkout basement. This system places the 4″ XPS foam board centered within the wall, with 4″ of concrete on each side connected by high-tensile fiberglass rods 12″ OC. This creates the same strength as an 8″ homogeneous concrete wall, but with greater flexibility without cracking, since the moment of inertia and hence the tensile stress within each wythe is reduced. And the 12″ foundation thickness is a perfect match for my 12″ thick double-wall framing system (filled with dense-pack cellulose).

This puts the foundation insulation out of reach of insects, UV and mechanical damage, creates a capillary and thermal break within the wall, and offers almost the same dynamic mass benefit as an exterior-insulated 8″ concrete wall. It also eliminates the need to coat or otherwise protect exterior foam and eliminates compatibility issues between foundation waterproofing and foam board. So the material and installation costs are approximately the same.

The ThermoMass rep was on-site during initial set-up to explain the installation method and walk the foundation contractor through it. After a short learning curve, the forms went up relatively efficiently.

A video of the installation and formwork for this project was placed on YouTube by the client:

A couple of elevations and floor plans are on-line at her blog:

Except for a manufacturing mistake which overcompressed the XPS thimbles and made installation of the fiberglass ties impossible, which was corrected at 7 AM the next day with a new load of properly-fabricated XPS, I’m impressed with the simplicity and functionality of the system. There are simple details which allow a continuous thermal and capillary break between footings and the interior wythe of the wall, and between walk-out stem wall and slab.

Foundation section and details are available here:

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  1. wjrobinson | | #1

    Keep posting updates to this Robert.


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