Staying Warm This Winter: What's the Right Approach - a Crawl Space Heater or Proper Insulation?

We've had some very frigid days in the northeastern US and Canada the last few weeks. Those of us who have a crawl space underneath part of our home have felt plenty of chilly floors underfoot and cold air on our ankles.

You may think that putting a heater in your crawl space will leave you with warmer feet. But in fact 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 improve the health of your home overall.

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

In a nutshell, with a good supply of outside air coming from your crawl space, all you need is 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 a poorly sealed attic hatch, 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 air up from the crawl space.

The result is that the humidity and cold (and mold spores and dust) from the crawl space get drawn into your home, raising your heating costs and endangering your well-being. Ironically, the better you ventilate your crawl space, the more heat gets drawn out of your home through upstairs cracks.

Even in warmer months, when there is no stack effect from a crawl space, ventilating both ends of the crawl space doesn't actually do much for airflow or humidity. 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 basicaly amounts to treating the symptoms - poorly at that - instead of curing the disease. The disease, in this case, is excessive dampness and air entering the crawl space, and excess heat transfer during colder times of year between the crawl space and the outdoors.

You may find that your builder scoffs at the idea of insulating and sealing a crawl space. It defies conventional wisdom - and it also contravenes many local building codes that were developed from that conventional 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 do what recent research shows is most effective.

To properly seal and insulate your crawl space, start by removing any sharp objects such as old nails, glass shards, or sharp stones from the crawl space floor, so that you don't hurt your hands or knees as you work (it is a "crawl space" after all). Also, you'll be placing a plastic liner on the floor and you don't want any sharp objects to push through the barrier and cut it as you are installing it.

Buy a liner made specifically for the task - or look for 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 along with the walls - preferably without your having to cut extra pieces for the walls. The best way to compute the size is to add twice the wall height to both the width and length of the floor, and then add 10% extra to account for the slope of 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!

Close off any ventilation openings, and for crawl space windows, either replace them with energy efficient ones, or at least ensure that they are not cracked or drafty. 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. Also check that any doors to the outside are also well weatherstripped.

If part or all of the walls are wood framed, place batt insulation against the wall between the studs; if you have masonry walls, use foam board. Be sure that any large gaps in the walls are patched first - any place where you can see outdoor light shining in from the outside.

Lay the liner over the crawl space floor, 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 holes and any cuts or breaks in the poly with mastic tape.

Don't do just part of this job. If you seal the ventilation without adding the vapor barrier, or put in the liner without insulating, you will run into trouble later on. And do it all within a week or two - don't make this one of those projects thattakes months or years.

Once you have properly sealed and insulated your crawl space, you will 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 health 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 guidance above, you won't need such a heater. We sealed the crawl space below our breakfast nook a while back, and the breakfast nook became so much more comfortable, we were able to disconnect the baseboard heaters that had been installed in the kitchen extension when it was built.