Heated Frost Protected Shallow Foundation Systems
What the heck is a Heated Frost Protected Shallow Foundation System you ask? ( or maybe not, but I’m going to tell you anyway..!)
HFPFS are pretty simple: a concrete slab on grade, reinforced with steel to make it a structural load bearing surface upon which you build most commonly a house, and it sits on several inches of expanded polystyrene to insulate it from the ground, and 6 inches or so of compacted granular. This type of system is also known as a Raft Slab. ( as in a raft, that floats!)
Now, you can heat these slabs as well. Why? The thermal mass of the concrete acts like a heat sink, or a sponge. It absorbs the energy used to heat it and then slowly and releases it, providing comfortable and evenly heated spaces throughout the home with out any cold spots. While it initially takes time to heat the slab to the desired temperature, once its there, it takes very little energy to maintain this temperature.
So who wants a raft slab? With the changes to the building code requiring better building envelopes, and the rising cost of gas and electricity, plus the issue of radon gasses leaking into basements and the concerns for the effects on our general health, apparently quite a few people are opting for this type of construction.
There are a couple of methods for providing that heat into the slab, with hydronics, using a boiler and heated water pumped through pipes embedded in the concrete, or using an air heated system. One type of system that offers all the benefits of air heated raft slabs is the Legalett system. The following article written by Mike Reynolds of EcoHome compares the different methods of heating these slabs. Take a read and leave your comments!