slab-on-grade foundation insulation
Properly insulating your slab-on-grade floors not only will help you save
on energy bills, but also will improve your home's comfort. Cold concrete
slabs can be a source of discomfort in a home. An insulated slab reduces
heat loss, making it easier to heat. This reduction in heat loss helps moderate
Annual energy savings and payback
Slabs lose energy primarily as a result of heat conducted outward and through
the perimeter of the slab. Therefore, in most parts of the United States,
insulating the exterior edge of the slab can reduce heating bills by 10-20%.
In climates with mild winters, slab insulation in a typical 1,800 square-foot
home would save $50-60 annually. Slab insulation with an R-value
of R-10 for an 1,800 square-foot home typically could cost $300-600 to install.
Thus, the insulation would pay for itself in 5 to 10 years.
The investment in slab insulation is also economical as part of a mortgage.
An insulation cost of $450 would add about $38 to the annual mortgage. However,
the insulation would save over $50 annually in energy bills. Therefore,
the savings exceed the additional mortgage cost from the beginning, resulting
in an immediate payback.
Determining insulation R-Value
The International Energy Conservation Code Council (IECC) specifies both
the R-value and minimum distance for the insulation from the top of the
slab downward based on a locality's heating degree days (HDDs). Consult
your local weather bureau for your area's actual heating degree days. Then
use the table below to find the IECC's recommended depth and R-value based
on your heating degree days.
| Recommended R-values and depth for
|Heating degree days
||Feet installed vertically
|0 to 2,499
|2,499 to 4,500
|4,500 to 6,000
|6,000 to 7,200
|7,200 to 8,700
|8,700 to 10,000
|10,000 to 12,400
|12,400 to 14,000
Select and install insulation using these recommendations. Be sure to
use only insulation approved for below-grade use.
Slab insulation can be installed using one of two basic techniques:
Installing rigid insulation, typically foam
board, directly against the exterior of the slab and footing
Building a "contained" or "floating" slab with interior rigid insulation,
typically foam board.
If insulation is installed on the exterior of the slab:
Install it from the top of the slab to the bottom of the frost line
unless a termite inspection gap is required.
Encapsulate or cover the exterior face of the insulation with a protective
membrane to serve as a capillary break and to protect the insulation
Cover the above-grade portion of the insulation exposed to the outside
air using a stucco coating, pressure-treated wood, brick, or aluminum
flashing. When covering insulation, be conscious of how to detect
termites in areas prone to termite infestation. Some states in termite-prone
areas address this issue by requiring a termite inspection gap near
the top of the slab insulation.
When installing a slab foundation and insulating it, it's also important
to consider moisture
and air leakage control, as well as termite control.
Termites can tunnel undetected through exterior slab insulation to gain
access to the wood framing in a home's walls. As a result, some insurance
companies won't guarantee homes with slab insulation against termites. Building
codes in several southern U.S. states prohibit installing foam insulation
in contact with the ground.
"Floating" slab foundations with interior
insulation provide more termite resistance. However, some builders in the
southeastern United States have reported termite infestations through foam
insulation on contained slabs.
To help offset termite problems, follow these guidelines:
You'll also want to regularly inspect for termites. If you use a pest control
company, obtain a good warranty for its work. Finally, you need to consider
radon resistance or control when installing slab-on-grade foundations.
Provide effective moisture control systems.
Remove all wood from around the foundation before backfilling.
Install termite shields continuously under the sill plate of the
building. The shield should project beyond the sill plate and all
other portions of the exterior wall. While not 100% effective, the
termite shield may deter or delay widespread infestation. It may also
force termites into an exposed area where they can be detected. A
continuous layer of a membrane – such as rubberized roofing
material used in commercial buildings – may be used as an alternative
to the termite shield.
Use a foam insulation treated with a termiticide. Usually a derivative
of boric acid, the termiticide
should pose no more threat to homeowners than traditional termite