Photo 13: Another Foam Bathtub. Note the “twist” in the approach. The cleverness of this detail is that a granular drainage layer under the foam insulation and “yet-to-be-cast” slab has been replaced with a ¾ in. (19 mm) drainage mat that extends up the walls about a foot. The ¾ in. (19 mm) mat does what 4 in. (102 mm) of stones used to do. Figure 3: Insulating Basement Slabs. Where headroom allows, insulating the top of the existing slab works well. Note the “foam bathtub.” Photo 14: Spray Foam Layer. The foam is “structural” in that it is rigid enough to support itself if it is between 2 and 3 in. (51 and 76 mm) thick and if it adheres to the wood frame at the top of the foundation (which it does) and if it adheres to the edge of the foam bathtub at the bottom of the foundation (which it does). Photo 15: Mechanical Equipment. Very nice home for some very happy mechanical equipment. This equipment is best used when it is sealed combustion and power vented, as the old air leaky foundation is now pretty much airtight. detail is that a granular drainage layer under the foam insulation and “yet-to-be-cast” slab has been replaced with a ¾ in. (19 mm) drainage mat that extends up the walls about a foot (305 mm). The ¾ in. (19 mm) mat does what 4 in. (102 mm) of stones used to do. Also note that the inside of the foundation wall is not completely lined with a membrane lining. There is only a membrane lining at the top of the wall acting as a capillary break. We are finding that the 2 lb/ft3 (32 kg/m2) spray polyurethane foam works well as an interior drainage plane, as long as drainage is provided at the bottom of the assembly. That’s where the drainage mat comes into play. The fabric on the drainage mat has to “face up” so that when it runs up the wall the fabric prevents the drainage matrix from being clogged by the March 2010 spray foam when it is applied (Photo 14). It also helps that the spray foam does not adhere to the rubble very well, especially if you don’t clean the wall before you spray. The lack of adhesion helps drainage. The foam itself is “structural” in that it is rigid enough to support itself if it is between 2 and 3 in. (51 and 76 mm) thick and if it adheres to the wood frame at the top of the foundation (which it does) and if it adheres to the edge of the foam bathtub at the bottom of the foundation (which it does). The foam lining makes for a very nice home for some very happy mechanical equipment (Photo 15). This equipment is best used when it is sealed combustion and power vented, as the old air leaky foundation is now pretty much airtight. As nice as ASHRAE Journal 77