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.