Finishing basements requires special care

Fall is the time of year when many Canadians remember how much they'd like to have a finished basement, then gear up for that perennial do-it-yourself construction project that never ends. But besides perseverance, a successful basement reno also depends on understanding technical details that aren't immediately obvious.

I suspect most Canadian homeowners live over a basement because of our northern agricultural heritage. Once upon a time basements were a matter of survival. They were essential for keeping food -- especially root crops -- safe from winter's icy tentacles. Nowadays, most of us still regard a home as less-than-complete without a basement, even though the only root crops stored in most are teenage couch potatoes growing bug-eyed in front of the TV.

Finishing a basement presents unique technical challenges because of the natural tendency for holes in the earth to fill up with water. The most important prerequisite for a successful finished basement is that it be absolutely dry all year round. Water doesn't mix well with carpet, drywall or furniture.

Two kinds of water

There are two sources of water you've got to keep out of your basement -- liquid water seeping through floors and walls, and water vapour wafting in through open windows on humid summer days.

Besides all the usual advice about keeping your eavestroughs in good order and downspouts spouting, there's something else you should do to keep out water: Treat the inside of your basement walls and floors with a waterproof coating.

There are lots of products around, but the best I've seen work by crystallization. Look for something that comes as a powder and is meant to be mixed with water into a slurry. This is then sloshed onto moistened concrete, bricks, blocks or stone with a long-bristled brush, then kept moist for a week. During this time it seeps into masonry pores and expands microscopically, forming a permanent waterproof barrier that can't peel off. Xypex is one brand name.

Even if your basement appears dry, a waterproof coating is wise insurance before you start putting up finished walls and floors. The main drawback with crystallizing compounds is their cost (about 50 cents a square foot for materials), and the need to apply them only over bare, clean masonry. They can't be used over old paint.

Water vapour is less visible than a basement flood, but it causes things to rot at least as effectively as liquid water. Here's how: On humid summer days the air that seeps into your basement is fully loaded with moisture. As it travels down it cools, reducing its ability to hold moisture, and forcing out small droplets on walls and carpets. Microbes love moisture, and condensation causes their populations to explode, eating up everything that's even remotely organic.

Ventilation isn't the cure for summertime basement humidity because the source of moisture is the outside air itself. More ventilation in the summer just means more moisture. Unfortunately, the only solutions to summertime dampness are both energy intensive -- either a stand-alone dehumidifier, or an air conditioning system. Both remove moisture from the air much like a fridge does. Of course, basement windows must seal tightly if either of these are to work.

Thermal insulation strategies

Considerations surrounding basement moisture enter into the picture again when you decide how you'll insulate the masonry walls down there. The standard approach for building a warm wall is to assemble wooden studs into a wall frame, fill the hollow spaces with fiberglass, then seal the wall under a sheet of polyethylene vapour barrier. Although this system works well above ground, it's less than ideal in the basement because moisture can creep through walls from the outside, despite your best efforts. Moisture, sealed under plastic, within several inches of warm fiberglass, may provide great conditions for growing mushrooms, but that's not what you want in your rec room walls.

There's a less moisture-sensitive approach to insulating basement walls that's also easier to install than stud frames. At least one extruded polystyrene foam product -- Styrofoam Wallmate -- is designed to mount directly onto basement walls. It's held in place with wood strapping that fits into pre-moulded slots along the edges of the foam's inside surface. This strapping spreads out the pressure of the nails or screws used to anchor the system to the masonry walls. The strapping also provides a place to fasten drywall or paneling.

Keeping the noise down

Finished basements need to do more than just look good. They also have to keep sounds in or out. Electrified teenage couch potatoes can get pretty loud sometimes.

An excellent strategy for increasing the sound resistance of partition walls involves the staggered stud approach. Take a 2x6 horizontal base and top plate, then spike alternating, vertical 2x4 studs to them. You want to start with a 2x4 flush with one side of the plates, then add another stud 8 inches away, flush with the other side. Repeat along the length of the wall.

This results in staggered studs spaced 16 inches apart on each side of the wall's wide top and bottom plates. Since no studs extend through the full width of the wall, sound transmission is greatly reduced, even if drywall is the only wall covering you use. You can increase sound resistance even more by installing a layer of soundproofing board against the staggered stud frame, with a layer of drywall on top of that.

The best way to increase the sound resistance of basement ceilings is with resilient channel. These are strips of pre-formed sheet metal that fasten to the underside of the joists with screws. Drywall is then fastened to this channel. By keeping the drywall away from the ceiling joists, workshop noise, stereo tunes or blaring TV dramas are kept below, where they belong.