Insulation for a Floating Slab Foundation

Slope the external soil at least 5 percent outside of the structure. This means that the structure will be built on a small hill, which is important for proper drainage. The amounts to roughly 1.25 inches slope per 10 feet of ground. Floating slab foundations require excellent drainage to work, and a slight slope away from the exterior walls is very important.

Pour about 4 inches of gravel under the location of the foundation. The insulation will go on top of that. The gravel serves as stable “soil” that will bear the brunt of the weight. If the soil underneath is unstable or mixed with clay, your gravel layer should be increased by an inch or so. A floating slab is a good choice of an inexpensive foundation, but only so long as the ground has been properly prepared. Unstable or heavily moist soil is not the best base for a slab. If this is what you are facing, install your drain and use a liberal amount of gravel. This should compensate for the soil condition.

Install a drain pipe in the middle of your gravel layer. Before you build, install a below-ground drain that will exit on the external side of the wall and down the slope. This is for relieving hydrostatic pressure from underneath the slab. The drain will relieve some pressure and keep the floor stable. The pipe should be brass for durability, but PVC can work as well.

Place a thick, stiff layer of fiberglass insulation over the gravel. Remember, the drain is under the gravel; the gravel thickness is based on the nature of the soil; and on top of the gravel sits the layer of insulation. This should be roughly 2 inches in thickness and made of the stiffest fiberglass insulation you can find. If your environment is damp, then it makes sense to coat this fiberglass layer with about 10 millimeters of polyethylene. This will protect the insulation, increase its life and serve as another layer against moisture damage. Other materials that can work are stucco, treated plywood or rubberized, flexible roofing material. All of these will protect the fiberglass and add protection to the slab above it.

Place this same type of insulation vertically on the interior wall of the structure. This would refer to the bottom of the wall in a garage or house. This is ideal to seal the sides of the slab against the soil underneath. The U.S. Department of Energy, in its brochure on insulating floating slabs, strongly recommends adding vertical insulation to the sides, not just the top. This should stretch from the top of the slab down as far as your gravel goes into the earth. Treated plywood could be used to “damp proof” this vertical insulation. It is also a good idea to include some kind of damp-proofing on the external wall as well.

Seal the borders between the slab and the foundation walls. There are many materials that can be used for this. Polyethylene and polyurethane are common substances to seal any potential cracks. If you use concrete or a grouting type of sealer, technically, it will no longer be a “floating” slab. It might however, add to your overall protection.