why does condensation form on the inside of windows

Some homes have a problem with windows sweating on the inside during the winter, whenever the temperature drops to about 40 degrees F. Even the frames and the sills become wet. This even happens on newer homes that are well insulated. If you have this problem, the humidity level in your home is probably a bit too high. With double-pane windows, you should be able to sustain somewhere around 50 to 55 percent relative humidity indoors on a 40-degree night and not get condensation on your windows. That means that if the relative humidity in your home is higher, say 60 or 70 percent, your windows will become dehumidifiers and condense water from the air until the relative humidity level inside drops to the 55 percent range. Newer homes are typically built much more tightly than older ones. This is good for many reasons, but one bad side effect is that moisture generated indoors doesn t escape as easily. There are a number of potential solutions, but before you try one, I d recommend that you buy an inexpensive hygrometer to monitor your indoor relative humidity. This will allow you to track the problem and watch the effects of changes. Gas and propane release a lot of moisture when they burn, so the fireplace will increase the moisture level in your home if you use it extensively. Watch your hygrometer to see if running the fireplace causes a significant increase in indoor relative humidity. It s also possible that your new home is still drying out: New concrete, wood and other materials usually take 12 to 18 months to dry, depending on drying conditions.


You can t control this, but on cold evenings you can open your doors and flush the warm humid air out of your home for 10 to 15 minutes. Again, once the indoor temperature returns to normal, your hygrometer should tell you that the relative humidity has dropped, and the problem may disappear after another heating season. Sometimes this problem occurs only at the beginning of the heating season, when materials are drying out after a humid summer, and disappears after a few weeks. If neither of these seems to be the problem, look for other sources of excess moisture. Some common sources, like lots of green plants, can be easily controlled. But other common ones can t, like ground moisture moving up through the concrete slab. If you can t diagnose the problem, talk with your local building inspector about the moisture problems common to your area. Or call in a home inspector who understands condensation (and mold) issues for a home inspection and diagnosis. The only tool you ll need is a hygrometer.
Does condensation build up on the inside of your homeвs windows during the heating season? If it does, youвre not alone. Winter window condensation is a growing problem in Canada and its root has a surprising origin. As homes are sealed better against air leakage, natural ventilation to the outdoors is reduced. As a result, indoor air becomes much more likely to contain damaging levels of moisture during winter. If your windows sweat enough during the heating season to require periodic wiping with a towel, then you have a problem.


And this problem goes beyond ruined window-frame finishes and mould growth on windowsills. It includes the very real potential for decay within wall cavities and attics, too. Window condensation can also be a sign of low indoor-air quality which affects your health. When warm, moist indoor air meets the cooler surfaces of windows during winter, condensation develops on the glass. Itвs the same thing that happens on the outside of a drinking glass filled with a cold beverage on a hot summer day. Flaws in your homeвs vapour barrier (and there are bound to be some in every home) can allow warm moist air to seep into internal wall cavities, condensing there as it did on your windows, and creating a perfect breeding ground for hidden moulds, fungus and other nasties. Breathing, cooking, showering and drying clothes all release huge amounts of moisture into the air. In the good old days, this moisture would make its way outside through all the cracks that were once common around windows and doors. Thatвs why old, leaky houses are often so dry during winter with no window condensation at all. While todayвs homes mean lower energy bills, they also demand that we consciously provide some sort of fresh air to vent off all that water vapour. Boosting home ventilation is the key to solving the window condensation problem. This approach is about as easy as they come. Yes, opening windows will cost you a bit more in heating, but it still may be the cheapest way to solve your moisture problem.


Bathroom exhaust fans, in particular, should be used during every shower or bath and for at least 15 minutes afterwards. Installing an exhaust fan in high-moisture areas of your home can help if you continue having minor condensation problems even with your windows opened. Dryers that vent indoors spew massive amounts of moisture into your home. Proper outdoor venting of your dryer could solve the whole problem. Although this option will cost $2,000 to $2,500 installed, it will fix the problem once and for all. It will also retain most of the heat that youвd normally lose through open windows and out of exhaust fans. In fact, HRVs are so effective and energy efficient that theyвre now required by code for new houses in some jurisdictions. HRVs incorporate fan ventilation with a built-in heat exchanger that typically extracts 75 to 85 per cent of the heat out of stale indoor air before exhausting it outdoors. This saved heat is then transferred to a fresh stream of air coming into your home from outside. The higher the R-value of a window, the better it can handle humidity and keep condensation from forming. Triple pane windows, for instance, are much less likely to form condensation than double-pane, all else being equal. Replacing your windows with ones that have better sealing, but the same insulation value as the original ones, can actually increase window condensation because the new windows reduce air leakage and natural ventilation.

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