Below Grade Insulation

Experts agree that thoroughly insulating your house is one of the best investments you can make when building a new home. Most building codes require full insulation inside all exterior walls. But for even better comfort and energy efficiency, it’s a good idea to install  insulating foam boards on the outside of the walls in places that are difficult or impossible to insulate from the inside.

Many homeowners take great pains to ensure that their homes are well insulated in the exterior walls and attic spaces. And as we say over and over, that’s a great way to be sure that you live in a comfortable, energy-efficient house.

But industry experts claim that many people drop the ball when it comes to making sure that the home’s foundation is properly insulated. Doug Bibee, a technical specialist with Dow Building Materials says that up to 40 percent of the entire heat loss from the house can occur through uninsulated basement walls.

Insulating Existing Basements
There are several methods of insulating concrete basements after construction has been completed. One common method of insulating an existing basement wall is to build a wood stud and fill it in with batt insulation. One problem with this method is that condensation can form on the surface where the warmer interior air meets the concrete surface.

Foam products, like Dow’s Styrofoam Wallmate® are made expressly for the purpose of insulating interior basement walls while reducing the possibility that condensation will occur. The Wallmate® panels have channels cut into the foam so that a wood furring strip can be placed in the channel. Then nails are driven through the furring strip, through the foam and into the concrete. Drywall is then applied over the wood furring strips.

Insulating During Construction
It’s even a good idea to insulate concrete basement floors. When insulating with foam boards, the foam is applied over compacted gravel, then the concrete floor slab can be installed directly over foam. (For existing basements, the foam is installed over the concrete slab then covered with a ¾ inch OSB or plywood sub-floor, then flooring is installed over the sub-floor.)

In a raised floor foundation, crawl spaces should definitely be insulated. Crawl spaces can be huge energy-wasters. The best way to insulate crawl spaces is to place a polyethylene film (6 or 10 mil) over the dirt to reduce ground moisture entering the crawl space. Then, insulate the inside walls with a foam plastic insulation. In the story on the sow, Michael showed how Dow Thermax® polyisocyanurate foam is applied to the inside of the foundation stem wall, adding up to R-7 insulating value.

Polyisocyanurate foam is made of millions of tiny bubbles that contain a low-conductivity gas in the cells. Through a process known as “thermal drift”, over time, the R-value of the foam drops as the gas slowly escapes and is replaced by air. The Thermax® boards resist this tendency because they have a foil sheathing that inhibits the escape of this gas, maintaining a consistent R-value over a period that testing shows exceeds 10 years.*

Using foam boards to insulate the exterior wall of a basement during construction has several advantages over other methods. Typically a waterproofing or damp proofing agent is sprayed on the outside of concrete basement walls. Placing an insulating foam board over the waterproofing layer helps protect the waterproofing from damage after the basement perimeter is backfilled with dirt. In addition, Dow makes an insulating foam board that contains channels that help drain water away from the foundation. In areas that have poor soil drainage conditions, it may also be necessary to construct a perimeter drainage system.

In some areas, it’s common practice to cut off the insulating foam at grade because there’s a concern about how to cover the foam in the area that’s exposed above grade. According to Dow’s Doug Bibee, cutting the foam off above grade is a mistake because about half the heat loss that takes place in the basement can occur in the top foot and a half of concrete that’s exposed above grade. Doug Bibee recommends using one of the recently-developed sprayed-on concrete, or brush or trowel-on products that are aesthetically pleasing, firmly adhere to the foam and are resistant to damage from weed-eaters, lawn mowers nd other sources of external damage.