You may think that installing a heater under your living areas will leave you with warmer feet. But the truth is that properly insulating and sealing your crawl space will not only address your indoor discomfort with cold floors and drafts, it will also avoid problems such as mildew and rot in the crawl space itself, and will improve the health of your home overall.



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

Basically, with a good supply of outside air coming from your crawl space, you only need 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 heated air inside your house works its way out the top openings, pulling cold outdoor air up from the crawl space.

As a result, the cold and dampness (and dust and mold spores) from the crawl space get pulled into your home, increasing your heating costs and risking your well-being. Ironically, the better the ventilation in your crawl space, the more heat gets drawn out of your home through upstairs cracks.

Even in warm weather, when there is no vertical air flow drawing cold air up into your house and hot air out the upstairs, ventilating both ends doesn't actually do much to solve problems of airflow or dampness. There is no effect of rising heat to make the air flow through the vents, if they are both at the same level. And this approach really amounts to treating the symptoms - and not very well at that - instead of curing the disease. The disease, in this case, is excessive air and humidity entering the crawl space, and excess transfer of heat during colder times of year between the crawl space and the outside.

You may discover that your builder objects to the idea of sealing and insulating a crawl space. The idea defies accepted wisdom - and it also contravenes many local building codes that were based on that accepted wisdom. But you'll improve the quality of your inside air, reduce heat loss, and solve any problems with dampness, mold, or rotting wood down below, if you set this out-of-date belief aside and go with what the research shows is most effective.

To seal and insulate your crawl space, start by getting rid of any sharp objects such as old screws, glass shards, or sharp stones from the floor of the crawl space, so you don't injure your hands or knees as you work (why did you think it was called a crawl space, anyway?). Also, you'll be placing a plastic liner on the ground and you don't want any sharp objects to pierce through the liner and wreck it as you are putting it in.

Purchase a liner made specifically for the task - or buy a suitable, thick polyethylene plastic. Not the 6 mil typically used for a vapor barrier - you should get 15 or 20 mil thickness if you want a liner that lasts years. The liner should be large enough to cover the whole floor along with the walls - preferably without your having to cut extra sections for the walls. The best way to compute the size is to add twice 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 plus any measurement errors. So if you have walls two feet high around the crawl space and a 15 x 20 foot space, you'll need a 21 x 29 foot liner. It's better to buy a bit too much liner than to find yourself having to cut and tape on small pieces when you discover you didn't buy enough to begin with!

Seal any vents, and for crawl space windows, either replace them with energy efficient ones, or at least make sure they are not cracked or drafty. You may want to cut out rectangular pieces of foam insulation to close off the window areas, as this will add an extra layer of insulation to windows as well as cut down on drafts. Ensure any doors to the outside are also properly weatherstripped.

If part or all of the walls are wood framed, place batt insulation against the wall between the studs; for masonry walls, use foam board. Make sure any large cracks in the walls are fixed first - any place where you can see sunlight shining in from the outside.

Place the plastic film over the crawl space floor, and up the walls. Trim the excess pieces off where the wall corners meet. Attach the vapor barrier to the studs with a staple gun, and seal all staple holes and any cuts or breaks in the liner with mastic tape.

Don't do just part of this job. If you close off the vents without installing the liner, or you add 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 sealed and insulated your crawl space, you should find your home much more comfortable in winter. Your floors will be warmer, you'll have fewer drafts, and your home will be safe from the ill impacts of crawl space mold and mildew. In fact, so will the crawl space itself.

And remember the idea we started with, that a crawl space heater might cut the cold on your floors during this cold spell? Well, if you follow the advice above, you won't need such a heater. We sealed the crawl space beneath our kitchen extension a few years back, and the room became so much more comfortable, we were able to remove the baseboard heaters that had been added to the kitchen extension when it was built.