Basements: The New Living Space

Paul Eldrenkamp

For a long time in the history of houses, the basement was primarily a place to put stuff that could get cold and damp without suffering much damage or a space to relegate a noxious piece of heating equipment and its accompanying piles of coal. The basement as a space to put people was considered like something out of Dickens—a prelude to a long-lingering case of consumption.

Flash-forward a couple of centuries. Instead of low-level servants and a coal-fired furnace lurking in the cellar, we have cola-fired teenagers and large-screen TVs down there. These days, a lot of families spend more time in the basement than in the living room. What happened?

First, heating equipment got smaller and cleaner, mostly by converting from coal to natural gas. Also, dehumidifiers and air conditioning became more prevalent—and the electricity required to run them got pretty cheap—meaning we had easy ways to deal with the humidity. Finally, real estate (especially in metropolitan Boston) got really expensive, and all that square footage in the basement that traditionally had been left inhospitable became too valuable not to try to civilize in some way.

The challenges in "civilizing" basements all originate from the fact that a basement is a hole in the ground. Holes in the ground, at least in New England, are wet and dirty and have lots of bugs in them. Not surprisingly, then, basements are often wet and dirty and have lots of bugs in them. Interestingly, houses in the winter are very effective devices for moving this dirty, damp, buggy air from the basement to upper reaches of the house. This is known as the "stack effect"—a phenomenon that stems from the fact that warmer air rises.

Your boiler or furnace warms the air in the basement—sometimes inadvertently, sometimes on purpose. The basement, as a result, becomes depressurized—as warm air rises and exits the basement through various holes in the ceiling, replacement air is drawn in from the outside. Some actually comes from the ground around the house; more comes in near the top of the basement walls, where the foundation is above grade. A steady flow of air develops, moving from low to high: warm air rises, and exits through leaks in the upper parts of the house (the attic, for instance); outdoor air enters into the basement (and the lower parts of the house in general) to take the place of the air that leaked out the attic. Bottom line: much of the air that circulates through your house in the winter—when the stack effect is at its most pronounced—originates in the basement.

But back to the basement renovation. The most thoughtful, comprehensive basement renovations involve insulating the walls and floor of the basement with a material that reduces heat flow (as all insulations are designed to do) but that also resists moisture flow and does not deteriorate if it gets damp (as only a few insulations are able to do). Rigid foam insulation and spray-foam insulation are two good products for this purpose. They warm the surfaces they're applied to, which not only reduces heat flow outward in the winter but minimizes condensation in the summer; they help head off moisture that may be migrating through the porous masonry walls via capillarity; and they do not lose performance if they get damp. On the other hand, fiberglass batt insulation is one of the worst materials to insulate a basement with—notwithstanding the fact that it's the most commonly used material for that purpose—because its performance deteriorates considerably when it's wet, the paper facing and dust caught in the fibers can be food for mold, and because it has few if any air-sealing capabilities.

As part of our basement renovations, we use foam insulation to create a sort of "boat hull" to manage the water and to warm the walls and floor; and then apply the finishes (water-resistant sheetrock on the walls; resilient flooring, for instance, on the floors) to the inside of that "boat hull." The net effect is a dry, warm basement; in fact, when we monitor temperature and relative humidity over time, we find that a well-remodeled basement has the most stable conditions of any floor of the house. And not only is the basement healthier, the indoor air quality throughout the house is often significantly improved because the basement is no longer the source of damp, dirty air throughout the house in the winter. The basement—no longer just a "hole in the ground"—becomes eminently suitable for living, and the house takes a giant step forward.

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