Staying warm this winter: What's the right approach - heating your crawl space or insulating it?
We've had some extremely frigid nights in the Midwest and Eastern Canada this January. Those of us who have a crawl space below part of our home have had enough of chilly floors and drafts on our ankles.
You might guess that running a space heater under your living areas will leave you with warmer feet. But it turns out that properly insulating and sealing your crawl space will not only resolve your indoor discomfort with cold floors and drafts, 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 belief about the proper air flows in a crawl space. For many years, homeowners, builders, and building inspectors have believed that a crawl space needs exterior vents on opposite walls, so that air will flow from one vent to the other, drawing out any excess humidity from the enclosed crawl space. But recent research shows that vents in a crawl space create a very different result, known as the stack effect.
In brief, with a good supply of outside air 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 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 escapes out the top openings, pulling cold air up from the crawl space.
As a result, the dampness and cold (and dust and mold spores) from the crawl space get pulled into your home, increasing your heating costs and endangering your health. Ironically, the better the ventilation in your crawl space, the more heat gets drawn out of your home through upstairs drafts.
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 moisture. There is no effect of rising heat 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 - poorly at that - instead of curing the illness. The disease, in this case, is excessive air and dampness entering the crawl space, and excess heat transfer during colder months between the crawl space and the outside.
You might discover that your contractor objects to the idea of sealing and insulating a crawl space. It goes against accepted wisdom - and it also contravenes many local building codes that were written from that accepted wisdom. But you'll improve your indoor air quality, reduce heat loss, and solve any problems with moisture, 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, begin by removing any sharp objects such as old screws, glass fragments, or sharp pebbles from the crawl space floor, so that 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 barrier and cut it as you are installing it.
Purchase a crawl space 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 stands the test of time. The liner should be big enough to cover the entire floor along with the walls - preferably without your having to cut extra sections for the walls. The best way to calculate the size is to add double the wall height to both the width and length of the floor, 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 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 get enough!
Seal any vents, and for crawl space windows, either replace them with energy efficient ones, or at least make sure they are not a source of drafts. You may want to cut out rectangular sections 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. Ensure any doors to the outside are also well weatherstripped.
If you have wood framed walls, place batt insulation against the wall between the studs; for masonry walls, use foam board. Be sure that any large openings in the walls are fixed first - wherever you can see sunlight shining in from outside.
Lay the poly over the floor of the crawl space, 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 points and any cuts or breaks in the poly with mastic tape.
Don't skip part of this job. If you seal the vents without putting in the vapor barrier, or add the liner without insulating, you are asking for 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.
Once you have well insulated and sealed 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 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 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 closed off the crawl space below 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 installed in the kitchen extension when it was built.
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