Residential Foundation Insulation
Homes being constructed today are more energy
efficient than those built even just a few years ago, primarily
due to significant improvements in building products and
techniques as well as development of high-performance heating
and cooling systems and other appliances. However, the benefits
of foundation insulation are often overlooked. Heat loss from an
un-insulated, conditioned basement may represent up to 50
percent of a home's total heat loss in a tightly sealed,
well-insulated home. Foundation insulation is used primarily to
reduce heating costs and has little or no benefit in lowering
cooling costs. In addition to reducing heating costs, foundation
insulation increases comfort, reduces the potential for
condensation and corresponding growth of mold, and increases the
livability of below-grade rooms.
Foundations are either full basement, slab-on-grade, or
crawlspace. Deep frost lines and low water tables often make a
full basement the primary foundation of choice. However,
slab-on-grade with walkout basement construction is common and
home additions often have crawlspace foundations.
Basements can be insulated either on the interior
or exterior. Interior insulation can use conventional 2´4
framing with batt or wet-spray insulation. Unless the vapor
retarder covering on the batt insulation is fire rated, it
should be covered with drywall. Rigid foam is also used on
basement interiors. Furring strips are used to hold the foam
insulation in place. Extruded polystyrene expanded polystyrene,
or polyisocyanurate insulation boards can also be used. Fire
codes require most foam insulation board to be covered with dry
Exterior foundation insulation uses extruded or expanded
polystyrene directly on the outside of exterior basement walls.
Insulation exposed above grade must be covered to protect it
from physical abuse and damaging effects of the sun. Typical
cover materials include roll metal stock to match the siding,
cementous board attached to the sill plate, or application of a
stucco like finish.
A third option is to use a foam-form foundation
system. Polystyrene foundation forms are set on conventional
footings, much like building a Lego's® wall. Concrete is placed
into the forms where it cures to form both the structural and
thermal components of the basement wall. Exterior foam, either
foam boards placed on the exterior of a conventional foundation
or in a foam form wall system, may provide a concealed entry
path for subterranean termites. Termites can tunnel through and
behind many foam products. If exterior foam insulation is used,
a continuous metal termite shield must be used between the top
of the foundation and the sill plate to force termites out of
the foam and into view. Even then, treatment with conventional
termiticides to stop the infestation may be difficult.
Foundation waterproofing, site and footing drainage, and termite
treatments are similar for insulated and un-insulated basements.
However, if exterior foam insulation is to be used, use
waterproofing products compatible with the foam.
In many respects, crawlspace walls are just short basement
walls. Exterior foam and foam-form insulation systems can be
used. However, interior crawlspace wall insulation is usually
either foam board or draped insulation. If foam insulation is
used, it extends from the top of the foundation to the top of
the footing. The cavity formed by the rim joist should be filled
with fiberglass batts or a foam-in-place product. Most fire
codes allow up to two inches of polystyrene exposed on the
interior of a crawlspace before covering is required.
If crawlspaces are insulated with fiberglass or mineral wool
batts, the batts are usually tacked to the sill plate and draped
down and onto the floor. Four foot- wide batts incased in a
plastic cover work well when installed horizontally.
Conventional 16- or 24-inch-wide batts leave voids between the
batts and do not perform as well.
Some jurisdictions require a ventilated crawlspace to help
control moisture. Vent requirements are significantly reduced if
the floor of the crawlspace is covered with plastic sheeting
with joints overlapped and taped to reduce crawlspace moisture.
If required, install operable vents so they can be closed. Don’t
forget to fill the rim joist space with fiberglass batt or
foamed-in-place foam to complete the insulation treatment.
The floor over the crawlspace can also be
insulated. This raises the thermal envelope from the crawlspace
walls to the space floor. While this technique offers many
advantages, piping must be freeze proofed and heating and
cooling ducts must also be insulated.
Heat loss is greatest at or near the exterior grade. To reduce
heating costs and reduce the cold-floor syndrome common to
slab-on-grade construction, insulation is critical. Exterior
foam insulation, similar to exterior basement insulation, works
well. Insulation should extend from the top of the slab to the
top of the footing. Foam insulation inside the footing is also
common. It is necessary to provide a thermal break to prevent
thermal wicking from the slab to the outside. Installing a
pressure-treated nailer or beveled slab edge provide the thermal
break while still allowing floor-covering attachment. Climate,
cost of fuel, efficiency of heating equipment, and type of
foundation determine the cost effective level of insulation.
Savings from insulated foundations vary with fuel
price, heating equipment performance, and climate. The cost of
full-basement foundation insulation will vary but builders have
reported prices between $800 and $1,200. If the mortgage of a
new home were increased by $1,200, the increase in home payment
would be $106 annually for a 30- year, 8% loan. The combined
heating and mortgage costs would be similar and the home would
be more comfortable and provide a healthier indoor environment.
Frequently Asked Questions
If a basement is unfinished does it still need foundation
Yes, unless the floor above is insulated. Even if
used only for storage and heating and cooling equipment the
basement is thermally connected to the rest of the house.
Is floor insulation above a basement or a crawl space an
alternative to foundation insulation?
Yes, but keep in mind that pipes, ducts and HVAC
equipment located in the basement would then need to be
insulated to meet the MEC and to protect pipes from freezing.
Sometimes these can be grouped in a small area with insulated
walls while the floor above the rest of the basement is
Doesn't placing insulation on the exterior improve energy
If the basement incorporates passive solar design
with a significant amount of south facing windows, exterior
insulation will be beneficial, provided the walls are exposed to
solar gain. In a typical basement the energy savings are
Should the interior of foundation walls have vapor barriers?
If interior insulation is used, YES. The concrete
must be allowed to dry, but moist basement air typical of
Midwest summers should not be allowed to reach the cool wall
where in can condense. Batt insulation specifically designed for
the interior of foundation walls has a perforated poly facing
that prevents air from circulating through the batt, but allows
water vapor from the wall to escape.
Will foundation insulation increase the risk of termite
Foundation insulation does not increase the risk of termine
entry. If termites are present in the soil and wood is used in
the building, the risk of infestation exists. Exterior
insulation may reduce the probability of early discovery and
inhibit treatment when discovered.
Is an inspection band where foundation insulation is omitted
to permit inspection for termites a good idea?
In some southern states with a high incidence of
termite infestation, including, Florida, South and North
Carolina, Georgia, Alabama, Mississippi, Louisiana, eastern
Texas, southern and central California, Georgia, Tennessee, and
Hawaii, rigid foam insulation is not allowed in contact with the
soil. In other areas a six inch gap between the top of
foundation insulation and any wood framing member is required to
permit visual inspection for termites.
Will exterior foundation insulation materials be chemically
attacked by damp-proofing?
In can happen. Avoid ……and always follow the
insulation and damp-proofing manufacturer’s instructions.
What about water proofing?
Codes often require waterproofing instead of
dam-proofing if the wall is adjacent to habitable space.
Manufactures of some foam products offer specific
recommendations for waterproofing of their foam systems.
How long will exterior foundation insulation last?
Properly installed foundation insulation,
interior or exterior, should last as long as insulation
installed any where else in the building.
Should foam insulation above grade be protected?
Foam above grade must be protected from both sun
and physical damage. Ultraviolet light degrades or destroys most
foams. In addition, damage from lawnmowers, balls, and other
incidental contact can degrade the appearance and performance of
the foam. Common materials used to protect the foam above grade
include two- or three-layer stucco finishes, brush-on
elastomeric or cementitious finishes, vertical vinyl siding,
cement board, aluminum coil stock, and fiberglass panels.
Will insulating the foundation increase the risk of radon
Radon entry into a home is through cracks and
other opening below grade. The use of foundation insulation
should minimize thermal stresses on the foundation and help
minimize cracking, thus reducing of radon entry.
Should crawl space be ventilated?
The CABO One and Two Family Code requires one
square foot of crawl space ventilation for each 150 square feet
of “floor” area. Operable vents 1/10 as large can be used if a
vapor barrier is installed. Warm damp summer air can condense on
the cool earth, even when covered with a poly vapor diffusion
retarder, increasing the risk of crawl space moisture problems.
Installing a vapor barrier and closing the operable vents is
preferred. If local code interpretation requires crawl space
ventilation, insulating the floor and incorporating a vapor
barrier is preferred.
Do foam insulation boards installed on the interior require
All foams require thermal protection equal to ½
inch of gypsum wall board when installed on the interior of a
building, including a crawl space. The only exception is Celotex
Thermax Polyisocyanurate which may be installed without a
thermal barrier where approved by the local building code
Are insulating concrete form (ICF) systems less expensive
than an insulated poured in place concrete wall?
ICFs can be competitive but costs are project
specific. Foam used in these systems should address the same
concerns outlined above for foam board.