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Residential Foundation Insulation
Introduction
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 uninsulated,
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.
Foundation types
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.
Full basements
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 wall.
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 uninsulated
basements. However, if
exterior foam insulation
is to be used, use
waterproofing products
compatible with the
foam.
Crawlspaces
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.
Slab-on-grade
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 insulation?
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
insulated.
Doesn’t placing
insulation on the
exterior improve energy
performance?
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 negligible.
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 entry?
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
dampproofing?
In can happen. Avoid ……and always follow the insulation and dampproofing manufacturer’s instructions.
What about water
proofing?
Codes often require waterproofing instead of damproofing 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 problems?
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 fire
protection?
All foams require thermal protection equal to ˝ 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 official.
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 systesm should address the same concerns outlined above for foam board.
For the best inspector
in your neighborhood
visit
www.InspectorLocator.com
or call 1-877 FIND-INS.
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George Timmerman CHI - CMIA* 280 Kemp RD * Suwanee GA 30024 * (470) 545-9824 |