Q.: We have a job building a parking deck above habitable space at a hospital. The deck will slope to roof drains and must have a U-factor of 0.1. We have been advised by a manufacturer of vermiculite that vermiculite concrete would not be suitable because the thickness required would be too great and also because it is too compressible. What else could be used?

A.: We are not sure the manufacturer is correct in saying that vermiculite concrete is too compressible for this application. The structural slab above it should distribute any excessively concentrated loads that would indent the vermiculite concrete if they were imposed on it directly. We suggest you investigate expanded polystyrene concrete which has been used as an insulating barrier under pavements. The Portland Cement Association has done research on this kind of concrete in highway applications and could perhaps make recommendations about the grade and thickness of material to use.