How to insulate a concrete floor

The best time to insulate a slab is before it’s even poured, as the insulation can go underneath the concrete itself ensuring a warm floor. Dealing with a finished slab however limits us to placing the insulation on top of the concrete surface.

The answer as to which way is best is heavily dependent on which type of flooring you plan to use. In all cases you’ll need to ensure your basement is dry, as the application of insulation will trap the moisture underneath.

My favorite method involves laying sheets of Styrofoam SM directly on top of the slab, and then covering it with 5/8″ tongue and groove plywood, glued together at the seems. This is a floating floor in that it us not attached to the concrete in any way, and is an ideal substrate for laminate floors. Should you require a more rigid floor (to install hardwood or tile for instance) you can glue 2 x 3″ lumber flat to the floor every 14 1/2″ with PL construction adhesive, run the foam between these strips and then screw down suitable plywood to the 2 x 3’s and floor as normal.