FLOOR SLAB INSULATION

Uninsulated slabs are a cause of heating losses and uncomfortable floors and energy-inefficiency.

Energy Savings and Payback

Slab insulation in cold climates allows significant annual energy savings. EERE data shows that the investment will pay itself in 5 to 10 years, depending on the climate. Slab insulation can reduce heating bills by 10%–20% in cold and moderate climates.

Slab energy loss

Even if the foundation walls under the slab are insulated, that will not help much if the slab edge close to the outside air isn’t also insulated. See the figure below (this and other figures in this page credit: DOE's Office of Energy Efficiency and Renewable Energy):

Slab and Heat Losses

Slab insulation

Slab insulation uses a rigid insulation material: typically foam boards. The installation is easy: just apply the right product with the right r-value under the face of the slab. See the images above:

Slab Insulation

As the figures show, a well done slab insulation also comprises perimeter insulation, that is, insulation of the slab edge closer to the outside air.

Slab Insulation Materials

Rigid insulation materials as Extruded polystyrene foam or Polyurethane foam are common in slab insulation.

See: Insulation Materials

Slab insulation Depth and R-values

The depth and the R-value recommended when insulating a slab varies with climate zones. More exactly they vary with the Heating Degree Days (HDD) of your particular climate zone. The HDD is an index reflecting the energy needs to heat or cool a home.

If your climate zone has a HDD of zero that means that you do not need to insulate your slab. That happens whenever the difference between 65ºF/18ºC and the average outside temperature is very close to zero during the whole year (outside 65ºF/18ºC - «the base temperature» - is the temperature that technicians consider the ideal to get a comfortable indoor temperature; with that outside temperature, the occupants and the home appliances will add more heat to indoor home temperatures, making them comfortable).

The higher the difference between the outside temperature and the 65ºF/18ºC, the higher the HDD and the needs of insulation; the higher the number of days with a high HDD, the higher the needs of insulation.

An example: New York City as a HDD of 5,000, while in Barrow (in Alaska) the HDD is over 20,000.

To get your particular HDD, contact your local weather bureau.

The table below shows recommended R-values and slab insulation depth by the the official Energy Conservation Code Council (IECC) .

Recommended R-Values and Depth for Slab Insulation
Heating Degree Days (HDD)	Feet/cm Installed Vertically	R-Value
0 to 2,499	none required	none required
2,499 to 4,500	2 feet/6 cm	R-4
4,500 to 6,000	4 feet/12 cm	R-5
6,000 to 7,200	4 feet/12 cm	R-6
7,200 to 8,700	4 feet/12 cm	R-7
8,700 to 10,000	4 feet/12 cm	R-8
10,000 to 12,400	4 feet/12 cm	R-9
12,400 to 14,000	4 feet/12 cm	R-10

Insulation of the outside or the inside foundation wall

Heat losses often happen through the foundation wall space close to the slab, and you may want to insulate just that wall, without insulating the under-slab face. In that case, you should apply the rigid insulation panel as showed in the figures below.

The insulation may be applied outside or inside the foundation wall… In the first case you should also apply a good finish protection (metal flashing, is a common option) and a protective termite shield (also advisable in the insulation involving the inside foundation wall).

Slabe Perimeter Insulation

Insulation Slab Techniques

Slab insulation can be installed using foam board rigid insulation directly against the footing and the exterior of the slab, as shown in the images above.

Alternatively, when building the slab you can also build "contained”/"floating" slab with interior insulation. The material is typically the same: board rigid insulation.

Slabs and termites

In areas prone to termite infestation, take them into account. A protective termite shield is crucial, in that case. Some states require a termite inspection gap installed near the top of the slab insulation.