Insulating Concrete Slabs, Foundations and Basements
Most residential concrete-foundation systems will come into contact with the ground and face moisture-management and heat-loss issues. Protecting the foundation of the home starts with the right barrier.
Concrete slabs, basement foundation systems, and crawl-spaces all experience water-vapor issues. Moisture infiltration can lead to mold and mildew growth and cement discoloration. Heat loss through the slab can increase utility costs. Fortunately there are a number of products used by homebuilders and concrete contractors that are form effective defenses against these problems.
Vapor Retarders and Barriers When pouring a slab or foundation, a membrane is laid on a layer of crushed stone or subsoil prior to the cement pour. Some in the building industry refer to these membranes as vapor barriers, but the accurate term is vapor retarder. "Vapor retarders are used to prevent capillary action of moisture up into the slab," says Terry Collins, of the Portland Cement Association. "In many code authorities it is required." Vapor barriers are rated for water vapor permeability. By definition, any membrane rated at less than one perm is considered a vapor retarder. The lower the perm number, the more effective the retarder.
Polyethylene is the material most commonly used for vapor retarders and varies by thickness. Some consider the bare minimum for a polyethylene vapor retarder to be six millimeters. While some contractors use two 6-mil layers, a thicker vapor retarder is often desired. Retarders are also rated for tensile strength and puncture resistance—in both cases, the higher the number the better. Resistance to tearing is a definite plus when it comes to these products, as any tears or holes from the pouring process can leave the slab susceptible to moisture infiltration. Vapor barriers should be included in below-grade, on-grade, and crawl-space foundation systems.
Waterproofing In conditions where there is significant hydrostatic pressure—groundwater pushing against the foundation—waterproofing products are used to prevent water infiltration. During new construction, these products are best installed on the positive side of the slab, which is the side facing the ground. Products can vary from liquid-like bitumen-based products, to bentonite clays, to membrane sheets combining polyethylene and bentonite. "We have a variety of under-slab waterproofing options," says Chris Magnan of Nicom Coatings in Barre, Vermont. "These products are laid down on the crushed stone and cement is poured on top. They become fully adhered to the cement." Waterproofing is necessary in below-grade foundations where groundwater is an issue.
Insulation Under-slab insulation helps prevent heat loss through the slab. Slabs without insulation will lose heat through the ground, which results in heat loss from the room above the slab and higher heating costs. With insulation under the slab, heat loss is reduced, the slab heats to room temperature at a faster rate, and it maintains temperature longer. For homes with radiant heat on slab, insulation makes the radiant-heating system more efficient. Even if a basement is left unfinished, it is still advisable to insulate under the slab, as the basement is thermally connected to the rest of the house and can draw heat away from living space.
Under-slab insulation can come as polystyrene sheets or rigid boards placed over the subsurface. Under-slab insulation is measured in R-value, much like insulation found elsewhere in the home.
Two-in-One Products Hybrid products combine the insulating and vapor retarding properties of products on the market. Insul-Tarp uses a three-layer system of foam, reflective material, and poly coating to combat heat transfer and loss, as well as moisture migration. The product comes in roll form and is spread onto the subsurface prior to the cement pour. Long, light insulation rolls are often easier to install than insulation board, which must be placed board by board.
Vapor retarders and waterproofing products provide important moisture-management properties to a home's foundation. However, proper foundation construction—including proper sloping for water drainage—and an effective drainage system are equally important in the effort to seal a home's envelope. "Good water control will consist of good drainage, waterproofing, and vapor retarding to limit the water that might get in," says Collins. "But there is no true flawless envelope, because water is insidious."