Tubing is attached to the underside of the existing subfloor. Aluminum plates can be used to spread the heat evenly under the subfloor. Insulation is placed in the joist space beneath the tubing. A 2 inch air space is usually left between the insulation and the bottom of the subfloor. If aluminum plates are used which significantly cover the underside of the subfloor, the insulation may be pushed up tight against the plates. Obviously, care must be taken when nailing any floor covering from above.