Concrete under SLAB INSULATION
Uninsulated slabs are a cause of energy losses and uncomfortable floors. Insulating the under slabs (usually concrete slabs) will improve your home's energy-efficiency and comfort.
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. Concrete under slab insulation can reduce heating bills by 10%–20% in cold and moderate climates.
Concrete Under Slab energy loss
Insulating the foundation wall under the slab isn't enough. That will not prevent energy loss: see picture at right (from DOE's Office of Energy Efficiency and Renewable Energy), showing the mechanisms of that loss.
Slab insulation
A correct installation involves insulating the under-face of the slab, and also the slab edge (perimeter insulation). See images showing these methods:
Slab Insulation Materials
Concrete under slab insulation uses a rigid insulation material, typically foam boards. Rigid insulation materials as Extruded polystyrene foam or Polyurethane foam are excellent in slab insulation.
See: Rigid foam insulation products and Insulation Materials
Slab insulation Depth and R-values
The depth and the R-value required for concrete under slab insulation vary with climate zones: extreme temperatures demand higher insulation levels.
A technical method to determine such values uses the Heating Degree Days (HDD), that is, an index reflecting the energy necessary to heat or to cool a home in a specific climate zone.
In a climate zone with a HDD close to zero (between zero and 2,500) there is no need to insulate. And that because the close to zero value means that during most of the year the outside temperature in that zone is close to an ideal situation of 65ºF/18ºC, that is, the temperature considered ideal to get a comfortable indoor temperature without resorting to insulation and to heating and cooling systems.
The higher the difference (and the number of days) between the outside temperature and the 65ºF/18ºC reference, the higher the HDD and the required insulation levels for a climate zone.
An example: New York City has a HDD of 5,000, while the HDD of Barrow, in Alaska, is over 20,000.
To get your particular HDD, contact your local weather bureau.
The table below shows the R-values and the insulation depth recommended by the official Energy Conservation Code Council (IECC), taking into account the HDD.
Recommended R-Values and Depth for Slab Insulation |
||
Heating Degree Days (HDD) |
Feet/cm Installed Vertically |
|
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 |
outside or inside foundation wall Insulation
Since heat loss happens through the space of foundation wall that is close to the slab, you may want to insulate just that space, without insulating the whole under-slab surface. In that case, apply a rigid insulation panel as showed in the figures below.
The insulation can be applied outside or inside the foundation wall… In the first case you must also apply a metal flashing or other good finish as well as a protective termite shield (also advisable if the insulation involves the inside foundation wall).
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.
An alternative, in new slabs, is to build a "contained”/"floating" slab, that is, a slab with insulation in its own interior. The material, in that case, is also board rigid insulation.
Slabs and termites
A protective termite shield is crucial in areas prone to termite infestation.
See also:
Installing Under floor Insulation
Home Insulation Solutions
Insulation materials
Insulation value
Professional quality insulation
Insulation and climate