Watts Radiant's UL-listed HeatWeave UnderFloor mats are designed for electric radiant heat under existing wood subfloors. The mats are constructed of high tech heating elements woven together to be easily installed between 16" joist spaces. The installer secures the mats between the joints and within an inch of the subfloor's underside. Foil-faced insulation fills the joist space to improve heat transfer to the finished floor above. The mats are available in many lengths, and the heating elements bridge over nail-protected channels, so that one mat can fill multiple joist spaces with a single connection back to the FloorStat. Watts Radiant.