Slab perimeters should be insulated with rigid insulation at the time of construction. However, this is a somewhat new practice, meaning older homes are unlikely to have slab insulation.

Slabs lose heat at their perimeters because concrete is a fairly good conductor of heat. With no insulation, the temperature difference between the slab and the outside air can be dramatic. Heat loss beneath the slab into the earth is much less because ground temperatures are more moderate than the surrounding air temperature.

Adding slab insulation to an existing home is rarely cost effective unless done in conjunction with some other major retrofit project. It requires digging a trench to expose the foundation wall below the frost line, washing the foundation wall, spot-gluing rigid board insulation to the wall with an adhesive then covering the insulation with a protective coating.