Question from Evelyn, Spruce Grove, Alberta, Canada

We have a very well insulated house and are struggling with high humidity levels. We live in north-central Alberta, Canada and have had high humidity in our house for a very long time. We had our house inspected about 5 years ago for energy efficiency and it was found that our house was “too air-tight” so we put in a fresh air vent but this still has not eliminated the problem.

We also installed new windows throughout our house 5 years ago so they are not leaking. When the weather changes, moisture starts to accumulate around the bottom of the windows, especially our living room window, it is the worst. We just had some extremely cold weather and the frost buildup was awful and now that it has warmed up there is lots of water on the windowsills. We have a dehumidifier going 24/7 (for the past couple weeks) and it has not helped the situation. What could be causing this problem and what is the solution? Please help!

Answer from Green Energy Efficient Homes

It sounds like the problem is a combination of poor air exchange and too much humidity being generated in your house. The humidity can’t escape and condensation from the cold windows causes frost buildup, and water when the weather warms.

You mention your house was air-tight so you installed a fresh air vent. How much circulation does it provide? Is it a heat exchanger or just an air vent? An air vent will either pull in a lot of very cold air, or do little to help. A heat exchanger would be better – it makes a huge difference for any R-2000 home or very well sealed home, and improves indoor air quality, since VOC’s build up in a well-sealed house from paints, carpets, fabrics, and other manufactured goods. The other thing to look at is where the humidity is coming from. If your home is well insulated you should have good vapor barriers. (In any event, if you find the humidity in your house very high even on cold nights, it’s unlikely much humidity is leaking in from outside since it’s probably a dry cold.) But a crawl space is one possible source of high humidity in your house; the lack of a vapor barrier and insulation to keep the crawlspace dry could lead some of that humidity into the house. And it would be good to confirm whether your well insulated, well sealed house also has effective vapor barriers throughout.

Sources of humidity from within the house include breathing, cooking, laundry, house plants, basement humidity, bathing, and central heating. Let’s look at each source of humidity in a house.

Breathing is something people and many of their pets do. As we breathe air we release water. The more you breathe the more water vapor you release, so if you’ve got a jogging treadmill or other home fitness equipment and you use them regularly, that may be one big source of water vapor. Dogs, cats and other mammals produce vapor too but again unless they are running around like mad and panting feverishly (or unless you keep dozens of dogs or hundreds of cats) it’s unlikely they’re a major source of humidity in your house. Fish aren’t a source of water vapor – unless they jump out of the tank and are gasping on the carpet – but their tanks or aquariums can raise house humidity, especially if the water is heated (for tropical fish) and/or has a bubbler. Keeping your aquarium covered can help here.

Cooking is a big source of humidity in houses which is wh an over the range vent is recommended for use any time you’re boiling water, simmering, sauteeing, or otherwise cooking someting in water or cooking the water out of a food. Note that some cheaper vent hoods vent the air back into the room, and merely filter out a bit of the grease vapor but do nothing for water vapor. If you’re cooking, use lids wherever possible, and run the range vent to draw the extra moisture out. Natural gas cooking produces more water vapor than electric range cooking, because natural gas produces heat by combining methane (CH4) with oxygen (O2) to release heat and produce carbon dioxide (CO2) and water (H2O).

Laundry is only a source of humidity in your house if you dry your clothes on an indoor line or laundry tressle, or if the exhaust from your electric or gas dryer is venting into the home. For an electric dryer, it is okay to vent some of the exhaust into the home (for example, using a dryer vent heat recovery kit such as the one shown at left), but if you are suffering from high humidity in your house you would probably stop using such a kit. For a gas or electric dryer it occasionally happens that the vent comes loose from the dryer (especially if a front-loading washer and dryer are stacked, since the vibrations from the washer spin cycle can move the units away from the vent location). In such cases you could be inadvertently venting air, humidity, and lint from your dryer into the house air. If the dryer is a gas dryer you’ll also get the combustion gases from the burning natural gas, as pointed out for the gas stove above. But especially for dryers be aware that the combustion gases may include carbon monoxide, a deadly and silent killer.

House plants breathe, just as people and animals do, so if you have a lot of house plants you may be releasing a lot of water vapor through them. An obvious way to gauge this is by how much you water your house plants. If you have a couple of cactuses and feed them a tablespoon of water a week that won’t contribute to humidity in your house, but if you have a large collection of indoor tropical plants that require frequent watering your green thumb may be contributing to the humidity problem.

Basement humidity is a frequent contributor to high winter humidity levels, since even in subzero weather the ground can still be wet below the frost line and a poorly sealed basement will absorb humidity through the walls. An energy efficient dehumidifier in your basement can help here as a stopgap measure – although you say you are already using a dehumidifier and it is running constantly, so this is not likely the problem unless your soil is very moist. It may be worth looking at waterproofing your basement next spring if you determine that the basement is a major source of home humidity.

Bathing is an obvious source of humidity in a house – including showers (especially when the bathroom is not vented to the outdoors with a proper fan), baths left full, indoor hot-tubs left full, frequent steam baths, and so on. And if you have a gently trickling fountain on a wall, that may have a small impact, but again you would already know whether an electric pump powered fountain is a source of humidity in your house by how often you have to replenish the water.

Finally, home heating systems sometimes come with a built-in humidifier designed to add humidity to the indoor air, since traditionally when the temperature drops below freezing the air gets very dry. It’s possible you have a malfunctioning humidistat on a furnace humidifier, that is injecting far too much moisture into the heated air.

I recommend you buy a hygrometer to measure indoor humidity levels, and place it at various places around your house. Water vapor is a gas just like air and naturally travels from areas of high concentration to areas of lower concentration, but especially if you can close doors between rooms, you can still see slight differences in house humidity levels if you take repeated readings across several rooms. Your current dehumidifier may not have enough capacity to deal with the humidity that needs to be extracted from your house, but the dehumidifier is a stopgap measure anyhow; for your situation there are more effective and permanent measures you can take to reduce the humidity of your house. Hopefully one or more of the suggestions above will help you determine where your moisture is coming from and how to stop it!

4 replies
  1. Lillian
    Lillian says:

    I hear what Evelyn is saying… we too live in Alberta (South of Calgary) and experience this humidity issue / icing up windows.

    1760Sq. Ft bungalow (built in 2000), NO humidifier even installed, run furnace fan continuously for circulation, humidity level constant 45%-50% (neighbors in area require a humidifier to maintain 35%!)

    Note – When the furnace ignites and goes into a heating cycle, just prior to feeling heat come out of register, I get a scent of steam… like the furnace is burning off water therefore contributing humidity into the house… is this possible?

  2. Chris
    Chris says:

    We purchased a home heated with propane. The first winter when it was in the -20’s C I noticed a lot of water on the windows and sills. Thought for sure my windows were junk! Than along comes my propane guy and I said look these windows are shot. He looked at them and said no, you have no fresh air circulation. Do to check my humidity I went to Home Hardware and picked-up a temp-humidity gauge. Wow my humidity was like 80%. To correct the problem I had an air circulation system installed. What a difference, clean air, humidity now 38%, windows are dry and no potential for mold. Best investment I have made so far.

  3. Suzanne
    Suzanne says:

    I just moved into a 1200 sq ft townhouse in northern Iowa that has no basement. My humidity runs 55-60 continually. Haven’t had the heat or air conditioner on for 2 months. Is this an issue? I did see a couple of snow bugs but my concern is the high humidity. The house is very well insulated and built in 1998. What should I do as I’m afraid it may breed mold? How high humidity is too high?

    • Robin
      Robin says:

      The EPA recommends humidity of 50% or less to reduce the risk of dust mites, mildew, and mold. In winter in cold climates you need the relative humidity even lower – around 30-40% – to reduce condensation on windows. You say your house is very well insulated, so I suspect part of the problem may be that there is not adequate air exchange with the outdoors, and therefore the humidity building up in your house has nowhere to escape to. If you start with a perfectly sealed house and 0% relative humidity, the humidity will eventually climb higher and higher because the moisture has nowhere to go.

      Your options could include reducing sources of humidity (see Energy efficient dehumidifiers for tips on sources of humidity and how to reduce them), using a dehumidifier (the same article gives tips on choosing the right one), or improving air exchange with the outdoors, for example by periodically opening windows. (On the other hand, if your outdoor humidity is higher than 50%, increasing air exchange could increase indoor humidity.)


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