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Residential Foundation Insulation
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.
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.
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.
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 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.
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, 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.
In can happen. Avoid ……and always follow the insulation and dampproofing manufacturer’s instructions.
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.
Properly installed foundation insulation, interior or exterior, should last as long as insulation installed any where else in the building.
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.
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.
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.
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
ICFs can be competitive but costs are project specific. Foam used in these systesm should address the same concerns outlined above for foam board.