Is It Best To heat your crawl space or insulate it?

We’ve had a number of extremely frigid nights in the northeastern US and Canada the last few weeks. Those of us who have a crawl space vapor under our home won’t be surprised to feel cold floors underfoot and cold air on our ankles.

You might guess that placing a space heater under your living areas will cut the drafts and cold. But it turns out that properly sealing and insulating your crawl space will not only address your indoor problems with cold, drafty floors, it will also prevent problems such as mildew and rot in the crawl space itself, and will improve the health of your home overall.

First, let’s clear up a common error about the air flows in a crawl space. For many years, homeowners, builders, and building inspectors have believed that a crawl space needs to have exterior openings on opposite walls, so that air can flow from one vent to the other, drawing out any excess humidity from the enclosed crawl space. But the most recent research shows 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, all it takes is a few cracks or hair’s width openings between the crawl space and the upstairs, 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 works its way out the top openings, drawing cold outdoor air up from the crawl space.

As a result, the humidity and cold crawl space mold and dust from the crawl space get sucked into your home, increasing your heating costs and risking your well being. Ironically, the better you ventilate your crawl space, the more heat gets drawn out of your home through upstairs cracks. What You will need is vapor barrier crawl space.

Even in summer time, when there is no stack effect from a crawl space, ventilating both ends of the crawl space doesn’t actually do much to solve problems of airflow or dampness. There is no effect of rising heat to draw the air through the vents, if they are both at the same level. And this approach basically amounts to addressing the symptoms and not very well at that instead of curing the illness. The illness, in this case, is excessive moisture and air entering the crawl space, and excess transfer of heat during colder months 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 breaks many municipal 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 conventional wisdom aside and do what the research shows is most effective.

To properly seal and insulate your crawl space, begin by getting rid of any sharp objects such as old nails, broken glass, or sharp pebbles from the floor of the crawl space, so 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 vapor barrier on the floor and you don’t want any sharp objects to pierce through the liner and cut it as you are installing it.

Purchase a liner made specifically for the task – or buy a suitable, thick polyethylene plastic. You’ll need thicker than the 6 mil usually 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 big enough to cover the entire floor as well as the walls – preferably without cutting extra pieces for the walls. The best way to figure out 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 slopes. 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 waste a little extra liner than to find yourself having to cut and tape on small pieces when you find out you didn’t buy enough!

Seal 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 pieces of foam insulation to fill in 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 between the studs; for masonry walls, use foam board. Be sure that any large openings in the walls are patched first wherever you can see sunlight shining in from the outside.

Place the poly over the floor of the crawl space, and up the walls. Trim the excess folded triangular pieces off where the wall corners meet. Staple the vapor barrier to the studs, and seal all staple holes and any cuts or tears in the barrier 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 will run into trouble later on. And do it all within a couple of weeks – don’t make this one of those home renovations that drags on for months or years. Just get it done!

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 and less drafty, 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’ll have no need for such a heater. We sealed the crawl space beneath our kitchen extension a while 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.

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