By Bud Coburn
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 2 inches of polystyrene exposed on the interior of a crawlspace before covering is required.
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 help 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.
If a basement is unfinished, does it still need foundation insulation?
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.
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 it 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.
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.
In some southern states with a high incidence of termite infestation, including Florida, North and South Carolina, Georgia, Alabama, Mississippi, Louisiana, eastern Texas, southern and central California, Georgia, Tennessee, and Hawaii, rigid foam insulation is not allowed to come in contact with the soil. In other areas, a 6-inch gap between the top of the foundation insulation and any wood framing member is required to permit visual inspection for termites. An InterNACHI inspector can be hired to perform required pest inspections.
In can happen. Always follow the insulation manufacturer’s instructions for damp-proofing.
Codes often require waterproofing instead of damp-proofing if the wall is adjacent to habitable space. Manufacturers of some foam products offer specific recommendations for waterproofing their foam systems.
Properly installed foundation insulation should last as long as insulation installed anywhere else in the building.
Foam above grade must be protected from both sun and physical damage. Ultraviolet light degrades and 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.
Radon entry into a home occurs through cracks and other openings below grade. The use of foundation insulation should minimize thermal stresses on the foundation and help minimize cracking, thus reducing radon entry.
The CABO One and Two Family Code requires 1 square foot of crawlspace ventilation for each 150 square feet of floor area. Operable vents 1/10th 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 crawlspace moisture problems. Installing a vapor barrier and closing the operable vents is preferred. If local code interpretation requires crawlspace ventilation, insulating the floor and incorporating a vapor barrier is preferred.
All foams require thermal protection equal to ½-inch of gypsum wall board when installed on the interior of a building, including a crawlspace. The only exception is Celotex ThermaxÒ polyisocyanurate, which may be installed without a thermal barrier where approved by the local building code official.
ICFs can be competitive, but costs are project-specific. Foam used in these systems should address the same concerns outlined above for foam board.
In summary, taking time to plan the best insulation system for you new home, and taking stock of the insulation currently installed in your home, can lead to energy savings in the long run.