All materials at the Savoy house are being tested for toxicity. Commonly used rigid foam boards such as extruded polystyrene (XPS) and expanded polystyrene (EPS) are made with flame-retardants; XPS owes its light weight to a greenhouse gas blowing agent. Now there is a petroleum free solution. Foamglas is a blend of sand, limestone, and recycled glass fused into millions of sealed glass cells. Water, fire, and bug proof, it has a compressive strength of 90 psi – stronger than soil – so it is suitable for both above and below ground purposes. At Savoy, the team used Foamglas to insulate beneath the foundation and around the the outside of the walkout wall.
For an in-depth discussion about insulating with Foamglas, see Brian Wilson’s article, Using Foamglas Instead of Polystyrene to Insulate Beneath Our Basement Slab and on the Foundation Falls.