An uninsulated foundation can result
in a large heat loss from an otherwise tightly sealed, well-insulated house. It
can also make below grade rooms uncomfortable. Foundation insulation can result
in lower heating requirements, and also helps in avoiding water vapor
condensation problems, which is often caused by the difference in temperature
between the basement interior and the earth around the foundation. However, a
poorly designed foundation insulation system can cause many problems such as
radon infiltration, moisture problems, and insect infestation.
The economic benefits of insulating
a foundation and doing the job correctly depend on factors such as: Is it new
construction or retrofit? Does the house have a full basement, crawl space or
slab-on-grade foundation? Are you planning to heat the lower level? Will you
have radiant floor heating? What heating climate is the house in? Does the soil
drain well? Each factor narrows the choice of insulation type and installation
Installing insulation on the
exterior or "dirt side" of a basement wall has the following
advantages and disadvantages:
* it minimizes thermal bridging and reduces heat loss through the foundation,
* it protects the damp proofing coating from damage during back-filling,
* it can serve as a capillary break to moisture intrusion,
* it protects the foundation from the effects of the freeze-thaw cycle in
* it reduces the potential for condensation on surfaces in the basement, and
* it reduces the loss interior space when foundation insulation is desired, and
* it conserves room area, relative to installing insulation on the interior.
* installation is expensive for an existing building unless a perimeter drainage
system is also being installed,
* many exterior insulation materials are susceptible to insect infestation, and
* many contractors are unfamiliar with proper detailing procedures.
Building scientists feel that the
best way to have a dry basement is to insulate the outside of exterior walls
with a rigid fiberglass type "mat." Under the mat is a damp-proofing
coating over the entire foundation, from footing to just below the finished
grade. A carefully designed perimeter drainage system consisting of washed
gravel, perforated plastic pipe, and filter fabric is also strongly recommended
for locations with poor soil drainage. A waterproof paint over the room side of
the foundation wall is also often recommended.
Some foam insulations are
impregnated with boric acid to discourage termite infestation. However, the
borate chemical often slowly leaches out of most materials when exposed to
Adding insulation to the interior of
the foundation is often more cost effective for an existing building. This has
the following advantages and disadvantages:
* much less expensive to install than exterior insulation for existing
* there is a wider choice in materials since you can use almost any insulation
* the threat of insect infestation is eliminated, and
* the space is isolated from the colder earth more effectively than when using
New Methods in Foundation
* many insulation types require a fire-rated covering since they release toxic
gases when ignited,
* it reduces usable interior space by a few inches,
* it does not protect the damp proofing coating like the exterior insulation,
* if the perimeter drainage is poor it may become saturated by moisture weeping
through the foundation walls, and
* superior air sealing details and vapor diffusion retarders are important for
Some new construction systems lend
themselves to both structure and foundation insulation at the same time. For
example, an Insulating Concrete Form (ICF) system uses a rigid foamboard applied
in the middle of a cast-in-place concrete wall, or, serves as both inner and
outer concrete forms in place of steel or plywood forms.
When using masonry blocks for a
foundation wall, filling the block cores with high-pressure foam works better
than most other block filling methods, such as poured-in insulations like
polystyrene beads and vermiculite.
There are also foam inserts for the
block cores. These are installed as the blocks are mortared into place. Some
concrete block manufacturers attempt to increase the thermal resistance of their
product by adding materials such as polystyrene or wood chips to the concrete
mix. However, even though filling the block cavities and special block designs
improve a block wall's thermal characteristics, it doesn't reduce heat movement
very much when compared to insulation installed over the surface of the blocks
either on the exterior or interior of the foundation walls. Field studies and
computer simulations have shown that core-filling of any type offers little fuel
savings since the majority of heat is conducted through the solid parts of the
walls such as block webs and mortar joints.
Slab-on-grade foundations are often
insulated in one of the following ways: Over the exterior of the footing/ slab
edge, or, between the interior of the footing and slab. Often the bottom of the
slab is insulated from the earth to some extent as well. Each approach has its
advantages and disadvantages.
On the exterior of the footing/slab
edge it reduces heat loss from both the foundation and the slab. Sometimes a
foamboard insulation is extended outwards beyond the foundation for several
feet. This offers more protection to the footing from freezing. It also allows
the builder to dig a more shallow footing without the risking of damage due to
frost-heaving. All exposed parts of the insulation must be covered with metal,
cement, or other type of membrane to protect it from damage.
When installing insulation on the
interior of the footing/slab, it must be vertical between the footing and slab.
This protects the insulation from insects and damage better than an exterior
application while it isolates the slab from the colder footing.
Insulating under an existing slab is
usually impractical. However, insulating under a slab in new construction
consists of the following cross-section (from top to bottom):
* floor slab,
* 2 to 3 inches (51 to 76 millimeters [mm]) of sand,
* 1 to 2 inches (25-51 mm) thick rigid insulation,
* a layer of 6 mil [0.006 inch (0.15 mm)] polyethylene plastic as a moisture
* 4 inches (102 mm) of washed gravel and under-slab drainage and plumbing pipes.
Insulation may be applied on top of
an existing slab in this way (from top to bottom):
* finish flooring,
* rosin paper,
* rigid foam insulation laid between moisture resistant furring strips that are
attached to the concrete,
* a layer of 6 mil [0.006 inch (0.15 mm) ] polyethylene plastic as a moisture
An alternative is to make a
"floating floor." This consists of (from top to bottom):
* finish wooden flooring,
* rosin paper,
* 2 layers of half inch OSB or plywood screwed together, overlap all seams by
several feet, hold the edge of the wood back from the walls by half an inch to
be the subflooring,
* rigid foam board insulation without the furring strips (as in the last
The above methods have the following
advantages and disadvantages:
* a relatively simple installation for retrofit work,
* thermally isolates the floor from the earth below, and
* floor surface is approximately the ambient interior air temperature and more
comfortable to stand on than concrete.
* foamboard requires a fire rated covering,
* it may increase frost depth around the slab edge in extreme climates,
* in the summer, it separates the space above from the cooler earth, and
* there is a loss of about 2 inches (51 mm) of head room.
How to insulate a crawl space
depends on whether you vent it. Traditionally, crawl spaces have been vented to
prevent problems with moisture. However, this often did not always work well.
Today, building researchers are moving towards treating the crawl space the same
as any other basement. This section will address both options.
If the crawl space is to be vented,
carefully seal any and all holes in the floor above ("ceiling" of the
crawl space) to prevent air from blowing up into the house. Insulate between the
floor joists with rolled fiberglass. Install it tight against the subfloor.
Cover the insulation with a house-wrap or face the fiberglass vapor barrier
down. Seal all of the seams carefully to keep wind from blowing into the
insulation. Also, adequately support the insulation with mechanical fasteners so
that it will not fall out of the joist spaces in the years to come. DO NOT just
rely on the friction between the fiberglass and wood joists to secure it in
Install a polyethylene vapor
retarder, or equivalent material, over the dirt floor. Tape and seal all seams
carefully. You may also cover the polyethylene with a thin layer of sand or a
thin slab of concrete to protect it from damage. Do not cover the plastic with
anything that could make holes in it, such as crushed gravel. Be sure the
head-room of the crawl space meets local code regulations if you are considering
pouring a concrete slab.
If the crawl space is to be unvented,
seal all holes in the foundation where outside air can enter. Install the
plastic ground cover as described above for an unvented crawl space. Run the
plastic up the walls and attach it to the first piece of wood (the mudsill).
Install rigid insulation foamboard against the foundation from the subfloor to
the plastic (or concrete slab) on the floor of the crawl space. Do this all the
way around the perimeter of the foundation. An alternative to foamboard is to
drape fiberglass roll insulation down the foundation walls with the edges butted
tightly together. This is an acceptable alternative to foamboard insulation as
long as the crawl space stays dry.
Installation Cost and
Although you can achieve
considerable savings in space conditioning costs by insulating the foundation,
the installation costs can become relatively high, especially for retrofit
projects. The type of materials used, the application method, and the extent of
work all affect the overall cost. Simple payback is typically in the range of 6
months for a simple do-it-yourself installation to 20 years for
"professionally" installed and more involved work. Adding foundation
insulation during new construction is usually less expensive.