Robin Green

Crawl space issues: What is the best approach, sealing your crawl space or heating it?

If your house has a crawl space and you get cold winters, chances are you've experienced cold, drafty floors this last month. Is installing a crawl space heater the best way to deal with this problem? Or might there be a way to address this problem that keeps you warm for less?

We've had a number of bitterly cold days in the Midwest and Eastern Canada the last few weeks. Those of us with a crawl space below part of our home won't be surprised to feel cold floors and drafts on our ankles.

You may guess that installing a space heater in your crawl space will leave you with warmer feet. But the truth is that properly sealing and insulating your crawl space will not only address your indoor discomfort with cold, drafty floors, it will also avoid problems such as mold and rot in the crawl space itself, and will make your whole home healthier.

For starters, let's clear up a common error about the proper air flows in a crawl space. For many years, homeowners, homebuilders, and building inspectors have all believed that a crawl space needs to have exterior venting on opposite walls, so that air can flow from one vent to the other, drawing out any extra humidity from the enclosed space. But the most recent studies show that vents in a crawl space create a very different outcome, known as the stack effect.

In brief, with a good supply of outside air coming from your crawl space, with a few cracks or hair's width openings between the crawl space and the living areas, and a few drafts at the top of the house, such as old windows or cracks in the upstairs ceiling, and your house starts acting like a giant chimney stack. Hot air rises, so the warm, heated air inside your house escapes out the top openings, drawing cold air up from the crawl space.

The result is that the cold and dampness (and dust and mold spores) from the crawl space get drawn into your home, raising your heating expenses and endangering your well-being. Ironically, the better you ventilate your crawl space, the more heat escapes from your home through upstairs openings.

Even in summer time, when there is no crawl space stack effect, putting vents at both ends doesn't actually do much to solve problems of airflow or humidity. There is nothing to make the air flow through the vents, if they are both at the same level. And the ventilation approach basicaly amounts to addressing the symptoms - and not very well at that - instead of curing the illness. The illness, in this case, is excessive humidity and air entering the crawl space, and excess transfer of heat during colder months between the crawl space and the outside.

You might find that your contractor disagrees with the idea of sealing a crawl space. The idea goes against accepted wisdom - and it also contravenes many local building codes that were based on that accepted wisdom. But you'll improve your indoor air quality, cut heat loss, and resolve any problems with humidity, mold, or rotting wood down below, if you set this out-of-date belief aside and go with what recent research shows is most effective.

To properly insulate and seal your crawl space, start by getting rid of any sharp objects such as old screws, broken glass, or sharp pebbles from the floor of the crawl space, so that you don't hurt your hands or knees as you work (why did you think it was called a crawl space, anyway?). Also, you'll be installing a plastic liner on the ground and you don't want the sharp objects to pierce through the barrier and cut it as you are installing it.

Purchase a crawl space liner made specifically for the task - or buy a suitable, thick polyethylene plastic. Not the 6 mil typically used for a vapor barrier - you need to go to 15 or 20 mil thickness if you want a liner that will last. The liner should be large enough to cover the whole floor as well as the walls - preferably without cutting extra sections for the walls. The best way to figure out the size is to add double the wall height to both the floor width and the floor length, and then add 10% extra to account for any rises or dips in the floor. So if you have 2 foot walls around the crawl space and a 15 x 20 foot space, you'll need a sheet 21 by 29 feet. It's better to buy a bit too much liner than to find yourself having to cut and tape on small pieces when you find out you didn't buy enough to begin with!

Seal any ventilation openings, and for crawl space windows, either upgrade them to energy efficient windows, or at least make sure they are not a source of drafts. You may want to cut out rectangular sections of foam insulation to close off the window areas, as this will add an extra level of insulation to windows as well as cut down on drafts. Ensure any doors to the outside are also properly weatherstripped.

If you have wood framed walls, place batt insulation between the studs; if you have masonry walls, use foam board. Be sure that any large cracks in the walls are fixed first - wherever you can see outdoor light shining in from the outside.

Lay the plastic film over the floor of the crawl space, and up the walls. Trim the excess pieces off where the wall corners meet. Staple the vapor barrier to the studs, and seal all staple points and any cuts or tears in the poly with mastic tape.

Don't skip part of this job. If you seal the vents without installing the liner, or you put in the liner without insulating, you are asking for trouble down the line. And do it all within a couple of weeks - don't make this one of those projects that drags on for months or years.

Once you have well insulated and sealed your crawl space, you will find your home much more comfortable in winter. Your floors will be warmer and less drafty, and you will be safe from the ill effects of crawl space mold and mildew. In fact, so will the crawl space itself.

And remember the notion we started with, that a crawl space heater might solve the problem of cold on your floors during this chilly winter? Well, if you follow the advice above, you'll have no need for such a heater. We sealed the crawl space beneath our kitchen extension a few years back, and the room became so much more comfortable in winter, we were able to remove the baseboard heaters that had been added to the kitchen extension when it was built.

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