From foam to fiberglass, insulating basements comes with options

Posted Oct. 29, 2010, at 6:28 p.m.
Last modified Jan. 29, 2011, at 12:47 p.m.

I used to live next door to someone who had a regular routine for autumn. He had bought several rolls of 2-foot-wide fiberglass insulation with shiny kraft paper on it.

Every October, the fiberglass would come out and be unrolled around the perimeter of the house. He then would install it like banking and cover the fiberglass, which was up against the foundation, with plastic. This covered the fiberglass and protected it from rain and snow.

Every spring, the fiberglass came off, a little more tattered from the moisture that it picked up from the ground and went somewhere for the summer.

I always wondered why he did not just install the fiberglass on the inside of the basement wall. I think I asked him, but cannot remember the answer. This was a matter of male pride, and I did not want to antagonize a neighbor by badgering him with my unsolicited advice. What he did worked fine, although it was a little ill-informed.

He eventually moved away, and the new owners never did banking.

For many years, Mainers have banked their homes. The practice goes back hundreds of years. We have used evergreen boughs, leaves, boards, wood chips and anything else that might stop or slow wind down while adding a little insulation to the basement walls.

The problem with basements is that they are made with massive materials like stone, brick or concrete. These materials have the insulation value of a single piece of glass. Their high mass makes them seem OK, but without some kind of thermal buffer, they siphon heat out of the basement.

There are many acceptable ways to insulate a basement.

If this was a perfect world, you could insulate your basement wall with foam insulation on the exterior from the footing right up to the sill. This keeps the entire basement wall warm. The massive wall will actually act as a heat sink that will now help store heat in the winter and cool in the summer.

There is a catch: The foam on the outside that is above grade is susceptible to damage and should be covered. It can be covered with stucco, paint or pressure-treated wood, or left alone. Weather, sunlight and lawnmowers will wear down any covering or foam over time. Another significant issue is that if the foam is running from underground to the sill of the house, a hidden path is created for bugs to get to the house. This is not a major threat here in Maine, but it can happen. This insulating practice is not allowed in southern climates due to this bug issue.

The other way to insulate basement walls is to insulate on the inside. If we run from the floor to the sill, the wall is now kept cold. There has been a concern over allowing the basement wall to get really cold. This is not a big issue with newer construction. Older basements that have cracks might have a problem with freezing. There is usually enough heat loss to prevent any major freeze damage.

I had my old basement, part of which is cement block, concrete and rock, sprayed with foam insulation on the inside from the floor to the sill with 2 inches of spray foam. After seven years there is no indication of exterior freeze damage.

You do need to know that a basement that is insulated with spray foam can be susceptible to fire. Ideally, the foam should be covered with drywall, wood or a spray coating that is a fire preventive.

Another basement insulation scheme is to install foam insulation on the exterior of the basement wall from the footing to grade and then install insulation inside from the sill down 2 feet below grade. This allows the wall to be reasonably well-insulated, and the exterior foam acts as an additional barrier against groundwater.

Fiberglass insulation can be installed in a stud wall that would be constructed in the basement. It is imperative, if using fiberglass to insulate a basement wall, that a plastic moisture barrier be installed over the concrete before any material that can absorb water is installed.

Perhaps the most economical way to insulate the basement wall is to hang 4-foot-wide plastic-faced fiberglass horizontally from the sill down on the inside of the basement. This is simple to install and cuts the heat loss in the area where it is the highest. Most lumber yards carry this material.

Regardless of what you do, seal the cracks by the sill before you cover it up. This need not be an annual ritual.

Questions for Tom Gocze should be mailed to The Home Page, Bangor Daily News, P.O. Box 1329, Bangor 04402-1329.

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