Before you buy, look at other options

A crawl space heater may seem like the right solution to cold floors above a crawl space in winter. Heat the crawl space, and that will make the living room warmer too. Heat rises, so you’ll have a toasty warm floor and the cost of heating the living space itself will go down. Right?

Yes, up to a point. Heat from the crawl space will rise into the home. But heat will also escape out poorly insulated crawl space walls, or through any gaps between the walls and the ground, through wall cracks, and through drafts on any windows and doors.

So while a crawl space heater may solve the problem of cold living area floors, it probably shouldn’t be the first thing you try. You could save a whole lot of money – and find a better solution to your problem – by looking at other options first.

What if the problem is high craw space humidity or crawl space condensation in winter? Heating the crawl space will reduce moisture there, but once again you’ll be better off to solve the source of the moisture than to try to band-aid the problem with a crawl space heater.

In this article I’ll try to give as many reasons as possible for why you shouldn’t buy a crawl space heater – at least not until you’ve tried everything else. A crawl space heater is a big financial commitment if you don’t do your homework on the crawl space heat envelope first, because you’ll be running that crawl space heater for many hours through cold weather, and electric heaters are very expensive to operate. (I strongly advise against a combustion-based crawl space heater because of the fire risks.)

Crawl space heater

Not the best use of your hard-earned money

But if you only want to see what crawl space heaters are available, you can just jump to the last section where I look at some of the available heaters.

Keeping the cold out

Before you try warming up your crawl space with a crawl space heater, you should do what you can to keep the cold out. That cold air is what’s making the upstairs floors cold in winter. And air circulating from outside into the crawl space, which you might think would help dry out the crawl space and keep moisture from entering the house, may well be having the opposite effect. (More on this below.)

Are there air leaks to the outside? This is the first thing to try fixing. Putting a crawl space heater into a drafty crawl space just means you’re paying to warm up the out of doors. Close off any major air gaps by repairing the exterior walls or shoring up crumbled masonry under the above-ground walls. If the crawl space has exterior openings (doors or windows), make sure these are properly weather stripped and that they close tightly and stay closed. Cracked window panes should be replaced with energy saving thermal windows. Better yet, if the crawl space doesn’t need natural lighting, you should just close off any leaky or broken windows and insulate and seal the openings.

If your crawl space walls are wood framing over masonry, check that there are no gaps between the framing and masonry. Remember that any bond point between the framing and masonry needs to have a sealed barrier such as 6 mil plastic to stop moisture and termites. You can inject expanding insulation foam between the foundation masonry and wood framing from the inside of the crawl space (but not from the outside, as this insulation degrades quickly in sunlight).

Many crawl spaces built to older building codes were built with outside ventilation on opposite walls. The idea was that air would flow in one vent, draw moisture out of the crawl space, and then flow out the vent in the opposite wall.

Unfortunately, things don’t really work this way. If there is any way for air to rise from the crawl space into your home – which there surely is – the cold air entering from outside is warmed in the crawl space, and since heat rises, this warmed air rises through the floor into your home, drawing in more cold air from both crawl space vents. While this stack effect is now well studied and well understood and some building codes have been revised to take it into account, there are still many areas in North America where the building codes still require venting on opposite walls of a crawl space.

Instead of using vents, you need to properly seal your crawl space against air leaks from the walls and humidity from the ground. See the next section for help on sealing your crawl space. Don’t close the vents without also sealing against humidity, or you’ll be inviting humidity in without giving it a way out.

Are the walls of your crawl space insulated? In the crawl space under my kitchen extension, the exterior walls consisted of two-by-fours 24 inches apart, with wood siding nailed onto the outside. So of course the kitchen extension was freezing cold in the winter. But sticking a crawl space heater in there would just have meant sending most of that heat through the wood paneling to the outside. So first I insulated.

Insulation will not only help your crawl space stay warmer, reducing the need for a crawl space heater; it will reduce the problem of humidity condensing on cold outside walls, since the walls won’t be a direct contact point between the cold outside air and the warmer inside air.

Ensure that the exterior facing has no cracks or holes. Then add fiberglass insulation or a mineral wool such as Roxul between the joists. Add a vapor barrier, tape it, and cover with 3/8″ plywood.

Between sealing for air leaks and insulating the walls of your crawl space, you’ll be well on the way to keeping the outdoor cold from leaking into the crawl space and cooling the living areas above it. And while a crawl space heater uses up energy – and your hard-earned money – every time it starts up, a well-insulated, well-sealed crawl space doesn’t cost you anything to keep warm, if the crawl space stays warm all on its own.

That’s how things turned out for me – once I sealed and insulated, the breakfast nook floors stayed toasty warm and there was no need to install a crawl space heater. In fact, we were even able to get rid of the baseboard heaters in the breakfast nook.

Note that crawl spaces typically involve a wood frame sitting atop a masonry foundation, in which case you should probably try to insulate this juncture well.

Getting the moisture out

Condensation can be a big problem in a crawl space

Condensation can be a big problem in a crawl space if it is sealed but not insulated

Another popular reason people have for buying a crawl space heater is that their crawl space is too moist. There may be signs of excess humidity such as black mold or a mildew smell in the crawl space, moisture damage to floors upstairs, or damage to baseboards or the base upstairs walls. Particularly if the edges of the floor or the upstairs walls are uninsulated, cold air can draw heat out from the bottom of the upstairs walls, leading to condensation which in turn can cause the drywall to grow mildew or mold. Finally, you may have actual signs of water in the basement: water dripping from joists or condensing on walls, damp crawlspace floor or soil, heavy moisture build-up on exposed pipes.

Before you consider installing a crawl space heater, make sure you’ve got this moisture under control. Follow these steps to control crawl space humidity:

1. Find any water sources and deal with them. For example, handle leaks coming from pooled rainwater on pavement or soil outside the foundation by providing proper drainage outside the foundation. If you find water leaking in from walls or from the bond between masonry and framing, first check for leaking gutters or overflowing downspouts on the roof above, then seal or patch where the water is entering.

If humidity is coming up through the soil because you have poor drainage around the crawl space foundation, you may want to dig around the foundation and install drainage tile. (You won’t have to dig that deep since it’s just a crawl space.) If there’s a lot of water in the below-ground soil or you can’t get good drainage around the foundation, install a sump pump.

2. Seal the crawl space from the earth. You need to install a high-quality vapor barrier over the soil underneath your crawl space to keep moisture from the soil – and drafts, believe it or not – from entering the crawl space from below. Use a plastic crawl space liner. These liners are made of a thicker plastic than the standard 6 mil poly film used in vapor barriers, and are stronger and last longer. It’s just too easy to damage 6 mil film when crawling around in there, or to tear its edges away from the walls. A 15 or 20 mil crawl space liner will last longer and be less prone to damage.

Before installing the liner make sure you remove any sharp pebbles, discarded nails or screws, or any other objects that may cause the liner to be punctured from beneath when someone crawls around on top of the liner. The liner should seal off the ground completely and connect to the vapor barrier for your walls. In fact, you can insulate the walls and cover with plywood, then place the liner on the floor of the crawl space with enough material to spare so that you can run the liner all the way up the walls. Seal all seams with mastic tape.

3. Seal the vents. You should already have plugged any leaks in the walls or masonry / wall bond, and in any windows. Now that the floor is properly lined, seal the vents to keep outdoor air from being drawn into the crawl space. You will see conflicting advice on this on some other web sites – and from contractors in your area – because the notion persists that the right way to deal with crawl space humidity is to use vents to increase air flow through the crawl space and let the moisture out. But as explained above, what actually happens is that the stack effect pulls cold air into the vents and then up into the house, and the cold air, crawl space moisture, and airborne dust and mold spores come into your living area.

Caution: If you have a combustion appliance in your crawl space – a gas water heater or space heater for example – don’t seal the vents without first checking that there is an adequate air supply for the combustion. A combustion air supply directly to the appliance will provide it with the air it needs without causing the entire crawl space to cool down. Also make sure you have a carbon monoxide detector with battery backup for any crawl space that has a combustion appliance.

Once you have sealed your crawl space from outside air and moisture, and insulated the crawl space walls, it’s time to look at the next problem: cold in the floors above.

Staying warm upstairs

If you have properly insulated the walls of your crawl space and have encapsulated the crawl space so that no air can enter from walls, the ground, or any windows or doors, the crawl space should stay considerably warmer in winter. Still, you may find that the floors above the crawl space continue to feel cold. If cold feet is your only concern, I suggest you start by insulating the floor above the crawl space. But do not use a vapor barrier – you already have one around the walls and floor of the crawl space, so you definitely don’t want one above. Add batt insulation between the joists, and cover with 3/8″ plywood.

If you have heating ducts running through your crawl space to warm the living areas upstairs, it’s a good idea to seal and insulate the ducts (see Heating duct insulation). There’s no point in the heating ducts heating the crawl space if you can just insulate the ceiling to keep cool crawl space air from getting into the living areas.

Keeping the crawl space from freezing

The next concern is freezing pipes within the crawl space. If the pipes run between the joists, first insulating the pipes with foam pipe wrap such as Armaflex insulation, and then insulating the joists as described above (along with the encapsulation and sealing of the crawl space itself) may be enough to keep the pipes from freezing. During cold spells, letting a tap drip to keep water moving through the pipes will reduce the risk of freezing as well.

But if you do think freezing pipes are a real concern, you can install electrical pipe tape around the pipes. This tape contains a heating element that keeps the pipes above freezing. Here are a few pipe heating products you can buy directly from

Depending on your climate, you may be able to boost the temperature in the crawl space to keep it above freezing, simply by installing incandescent lights that are controlled by a thermostat. Remember that incandescent lights are only about 10% efficient at converting electricity into light – because the other 90% gets converted to heat. So using incandescent bulbs to heat your crawl space is not substantially less efficient than installing a crawl space heater, and is probably a lot safer.

You may also be able to use low-wattage infrared lights, provided you have at least 18 inches of space between their location and the nearest solid object the main beam of the infrared light reaches. One advantage of infrared lights is that their energy is only converted to heat when it strikes a solid object. So if you have a drafty crawl space, you can shine lights on the ceiling and the pipes to keep those warm, without sending a lot of heat out the openings that cause the draft. But be aware that infrared lights do pose a greater fire risk than standard incandescent bulbs.

One inexpensive possibility for a home-built crawl space heater is to buy a thermo electric switch such as the Thermo Cube TC-2, which provides power to attached devices when the temperature is below 30F / -1C. You can install plain light fixtures with 100 watt incandescent bulbs in the ceiling of the crawl space (close to the pipes), wire them through a plug to the TC-2, and plug the TC-2 into a wall socket. This way you will only be heating the crawl space when the temperature dips enough below freezing to risk bursting the pipes.

Just don’t use compact fluorescent bulbs for this job – you’ll save energy for sure – but you won’t be heating the crawl space at all, and it’s the crawl space heater effect, not the light, that you’re after. And remember that in many areas, standard incandescent bulbs are being phased out to force people to switch to CFLs. While I agree that getting people to switch to CFLs where there is a real energy savings, it’s unfortunate that this heavy-handed approach is necessary, as there are situations, such as building your own crawl space heater, or lighting a crawl space where you will only be in there a few minutes each year, where an incandescent bulb makes more sense.

Another option to keep the crawl space from freezing – without installing a crawl space heater – is to circulate air between the house and the crawl space. If you have properly encapsulated the crawl space to remove air flow and moisture from outside and from the soil, there should not be a problem with circulating air between the living areas and the crawl space, provided you properly filter it and deal with pressure differences. (Remember that in an unencapsulated crawl space, this air flow is happening constantly anyway, and is usually going one way – from the crawl space into the house.)

A crawl space ventilator is not the right tool for this job. Crawl space ventilators are designed to circulate air between the crawl space and the outside, which is exactly what you are trying to avoid by encapsulating your crawl space. A crawl space fan that draws unconditioned air into the crawl space from the rooms above may do the trick, provided you can install an air filter on it to keep dust and other airborne particles from the crawl space from entering the home.

Remember that if you use a crawl space fan to inject air into the crawl space from your living area, or to suck air out of the crawl space into your living area, you need to provide a return air flow elsewhere in the crawl space, or you will create a high-pressure crawl space or a vacuum (if the crawl space is properly encapsulated). Either way, this will create problems. If there are combustion appliances in the crawl space, a vacuum can be extremely dangerous, and either high pressure or a vacuum can increase the likelihood of air leakage to the outside even in a well sealed crawl space.

A crawl space fan is also not appropriate in summer if the house has high humidity levels (for instance, you live in a humid climate and don’t use air conditioning). You do not want to be circulating humid air into the crawl space. Instead, use a crawl space dehumidifier such as the SaniDry crawl space dehumidifier.

Many basement and crawl space repair companies recommend the Crawl-O-Sphere air system over a standard crawl space fan. This fan draws only 18 watts (that’s 157 kilowatt hours per year if running 24x7x365, or about $16 if your electricity cost is $0.10 per kwh). The Crawl-O-Sphere is designed to be installed in an encapsulated crawl space. However in reviewing information on the Crawl-O-Sphere, while I found they consistently stated that the Crawl-O-Sphere conditions the air flowing between the crawl space and living areas, they didn’t give any indication of how the air was conditioned or why a regular crawl space fan could not do the job. And of course every site recommending the Crawl-O-Sphere was also selling them.

So you really want a crawl space heater?

If you read the sections above, you will know that most of the reasons that lead people to look for a crawl space heater can be addressed by other methods – methods that won’t result in a decades-long commitment to higher energy bills. Electric heaters are very costly to operate, combustion heaters are a risky proposition for an airspace you seldom visit, and everything you can do to reduce airflow and heat loss in your crawlspace will help you avoid needing a crawl space heater at all.

To reduce the risk of fires in your heated crawl space, you may want to install a remote thermometer probe in the crawl space near the crawl space heater, and the wireless thermometer itself in your living area, so you can periodically check the crawl space temperature close to the heater.

It turns out that there really is no such thing as a crawl space heater – a heater specifically designed for a crawl space. Instead, choose an electric space heater that is suitable for the volume of the crawl space in question. You can install any convection-based electric heater in a crawl space provided you run it on a circuit that has adequate amperage, and that any extension cord you use is high enough amperage to carry the load. In terms of sizing, remember that a crawl space can have a far smaller volume of air than a room with the same square footage in your house, and that with proper encapsulation and insulation you should not need much additional heat.

Because heat rises, putting a convection heater in a crawl space with a very low ceiling does present a fire risk. For this reason you may want to install a fan-blower heater and build ductwork to carry the heat through the middle of the crawl space and distribute through various openings in the ductwork.

For more information on choosing the right space heater, see my most popular article, Energy efficient electric heaters.

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