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Condensation Explained

Does condensation build up on the inside of your home’s windows during the heating season? If it does, you’re 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.

Whether it’s old or brand new windows, there is one problem that comes up in conversations with homeowners planning a replacement: condensation on windows.

If you have older windows that aren’t as efficient as modern vinyl windows, they are more likely to fog-up, accumulate moisture around the sill, or even freeze. But does this necessarily mean the window has to be replaced? What if the window has just been replaced and the condensation seems to be even worse than with the old windows?

We will show you how to understand and reduce or prevent condensation on your windows in this blog. In order to get there, we need to start at the beginning with the most basic question:

                                        What is condensation?

Condensation is the process which turns water vapour that is present in the air into liquid water. One of the biggest misconceptions that homeowners have about condensation is that it is a result of a problem with the surface on which it appears, usually a window or a wall. While problems in these areas can arise as a result of prolonged moisture exposure, condensation actually comes from the air, or rather moisture in the air.

                            Why is there condensation on windows?

As the air in a room warms up, it expands allowing it to hold more moisture. As it cools down again, it contracts. When the air reaches a cooling saturation point, the excessive moisture turns into a liquid. This is exactly what happens when the warm air from the inside of your home comes in contact with the cold glass pane in your window. The air cools rapidly against the surface of the window pane and becomes water.

Although this is most noticeable on the glass, your windows aren’t the only place condensation can occur. Breaks in your house vapour barrier can allow warm moist air to leak into wall cavities, condensing there the same way it does on your windows. Because this problem is not evident to the naked eye, it can go untreated for some time and result in rotting or mold growth over a period of time. This problem is especially prevalent in the older with an abundance of older homes.

Condensation on windows is most noticeable when the temperature outside drops quickly, creating a greater difference in temperature between the inside and outside air. It is often most noticeable in the early fall when the days are warm and nights are cold. During a humid summer, the home’s structure can absorb a fair amount of moisture. When the temperature drops outside, this moisture becomes trapped inside the home, resulting in higher humidity levels and a higher chance of condensation on glass panes.

                            Where does excessive moisture in the air come from?

One of the biggest contributing factors to the excessive moisture in our homes is us. According to Natural Resources Canada:

A family of four can add moisture equivalent to 30 or 40 litres of water per week to the atmosphere in your home.

Showering, cooking, bathing, and washing can add 15 to 20 litres per week.

Drying clothes indoors can add 10 to 15 litres per week.

Of course, another significant contributor to high levels of humidity is your household appliances and even your ventilation system. That’s why kitchens and bathrooms are areas with a higher chance of condensation.
During the wintertime, an appropriate level of humidity indoors is considered to be between 25 and 40%

                        How to prevent condensation from forming on windows?

Exactly how to battle condensation in your home depends on just how much condensation there is, the condition of your existing windows, your vents, etc. In short, it is about balancing the airflow system in your entire home. As such, the right fix for your condensation problems may be in a combination of fixes.

In essence, preventing condensation is about three main factors: reduce the level of humidity in your home, vent the humid air out of your home, and circulate air inside your home properly to lower the and maintain appropriate humidity levels.

  • Purchase a hygrometer. 
    • A hygrometer is a small and relatively cheap device that measures the humidity levels in your home. It can not only help you keep track of the humidity levels, but also help you assess whether different methods of battling condensation are working. Many modern thermostats come equipped with a humidity meter.
  • Lower your thermostat.
    • As we mentioned above, warmer air contains more moisture. If you are like most Canadian household, you can probably afford to turn the thermostat down a degree or two and still be comfortable. As a result, you may see a decrease in humidity and condensation.
  • Use your fans.
    • Your appliances, like the stove and dryer, can create a lot of humid air. Same goes for your shower. Make sure that these areas and appliances are vented to the outside of the house. Run a vent fan in your bathroom when you shower and turn on your range hood while cooking.
  • Unblock your vents. 
    • A lot of times the biggest problem with airflow is a man-made one. Placing furniture above or directly in front of the vents prevents air from distributing properly, and affects the circulation in your rooms. Same goes for the return vent that takes the air back into the system. Ultimately it is all about making the air move through and around the whole house for best results.
  • Clean your vents and change your filters.
    • Dust on your vents or return grill can be a sign that the air moving around your home is not as clean as it should be. Physically cleaning the dust off can lead to increased airflow. Replacing or cleaning the air filter not only provides your family with cleaner breathing air, but it also makes your furnace run more efficiently. Some companies recommend changing the filter as often as every two to three weeks, so you may want to add it to the list of your regular chores. At the very least try to check on the health of your air filters monthly. This will be a good sign of their health and overall furnace performance.
  • Seal leaky ducts.
    • According to Energy Star, about 20% of the air that moves through the duct system is lost due to leaks, holes, and poor connections. That’s one-fifth of your heating bill disappearing into the ether. By redirecting, sealing off, or fixing the leaky ducts you can improve the flow of air and increase circulation.
  • Circulate the air inside your home.
    • As we said earlier, colder air means less humidity. One of the quickest ways to cool the air in your home is by simply opening a door or a window. This may not always be ideal because of the weather, but you should consider ventilating the air in your home daily, even if for a few minutes.
  • Get a dehumidifier. 
    • One solution that seems to work well for everyone who experiences above normal condensation levels in their house is the use of dehumidifiers. These units are fairly cheap and you can find one that will do the job for 200-250$. This is also a good solution for homeowners with new windows that experience condensation, where other areas in the house may not be as significant in contributing to moisture on windows.

Preventing condensation on windows you might be asking yourself ‘How can I make my home more efficient?‘ There are many small and relatively simple home improvements you can make to have a positive impact on your personal comfort, the longevity of your home and the security of your budget.

What is an energy recovery ventilator?

Energy recovery ventilators (ERVs) are a type of HRV that can exchange both heat and moisture.

Moisture control:
An ERV can give you more control over moisture levels in your home during warm and humid weather, by keeping excess moisture out of your home. Because less energy is required to lower the temperature of dry air compared to moist air, an ERV can reduce the work your air conditioner needs to do and save you money.

Moisture recovery:
If your winter climate is extremely dry, ERVs recover some of the moisture that would leave your house through a regular HRV. This helps you maintain a comfortable humidity level within your home, avoiding static electricity, sore throats and other discomforts caused by air that is too dry.

Recently, Ontario updated its series of ventilation code requirements to increase the energy performance of a building by 15 percent. The updates center around heat recovery on ventilation systems with incentives for making buildings more airtight.

While much of the Ontario Building Code 2020 (OBC) Section 9 remains the same, a heat-recovery ventilator (HRV) or an energy-recovery ventilator (ERV) will be mandatory for all commercial and residential buildings.

Frequently Asked Questions

What is condensation?
Fog and water droplets on windows are forms of condensation. So is the water that appears on the outside of a glass of iced tea in the summer. It all comes from water vapor in the air.

What causes condensation on windows and sliding glass doors?
Cold air holds less moisture than warm air. When temperatures start to drop, warm air within your house comes into contact with cool glass surfaces. Water vapor that can no longer be held by the cooled air is deposited on the glass. During the first weeks of Winter, it can take several days for your home's interior water vapor levels to drop enough to avoid condensation. The process can repeat itself if moisture is added to the air in your home, or if there is a quick drop in temperature during a cold snap.

Why does condensation appear on windows and sliding glass doors first?
Condensation is generally seen first on windows and sliding glass doors because they tend to have the lowest temperature of any of the visible surfaces in the house.

Do windows cause condensation?
Windows do not cause condensation. They provide relatively cool surfaces where water vapor can condense.

Do drapes and window shades cause window condensation?
No, but drapes and other window coverings can restrict the flow of warm room air over glass surfaces. Therefore, condensation is more likely to occur when drapes are closed and shades are pulled down.

What causes condensation on the inner surfaces of storm windows?
This indicates that air is leaking outward past the inner window, and is being trapped by a tight-fitting storm window. The moisture in that trapped air condenses onto the interior glass surface of the storm window. Many storm windows have one or more vents to the outside to relieve this problem.

Is there anything I can do to my windows to eliminate condensation?
If you have windows with single-pane glass, consider replacing them with windows that have double-paned glass with a low-e coating and argon gas filling. This is not guaranteed to eliminate condensation, but at the least, it should significantly reduce it.

Is window condensation really reduced that much with double-paned glass?
Laboratory testing shows that modern double-paned windows with low-e glass and argon gas allow about 37% relative indoor humidity without condensation (at 70 F inside, 0 F outside). Old single-pane windows only allow about 12% relative indoor humidity.

What is humidity?
Humidity is water vapor, or moisture, in the air. Usually it is visible, but sometimes, such as with steam or ground fog, it's concentrated enough to be seen. Visible or not, all air contains some moisture.

Where does the moisture come from?
There are many things that generate indoor moisture. Perspiration and breathing of the occupants of a home adds moisture to the air. So does cooking, baths and showers, doing the laundry, etc. In fact, every activity that uses water adds moisture to the air. The normal daily activities of a family of four can add more than 18 gallons of water a week into the air in their home, greatly increasing interior relative humidity.

What is relative humidity?
Air can hold only a limited amount of water vapor, and that amount depends on the air temperature. When air at a certain temperature contains all the vapor it can hold, it's said to be "saturated", which means a relative humidity of 100%. When it holds only half the water vapor it can hold, the relative humidity is 50%. Cooler air cannot hold as much water vapor as warmer air.

What are some other symptoms of excess humidity?
Problems like peeling paint, rotting wood, buckling floors, insulation deterioration, mildew, and even moisture spots on ceilings and walls.

How do I know if I have excess indoor humidity?
Check for damp spots on ceilings and room-side surfaces of exterior walls, particularly closets. Look for water and ice on the interior surfaces of windows and doors.

Excessive interior humidity can be annoying to a homeowner and destructive to a home. It can damage sheetrock, paneling and window sills. It can also penetrate walls, deteriorating wood framing and reducing the effectiveness of insulation.

What does excess humidity do to my home?
Excess humidity contributes to the deterioration of a home. Excessive humidity can pass through walls and freeze in the insulation. In spring it melts, damaging your ceiling and walls. Humidity has been known to force its way out through siding to form blisters under exterior paint.

Can moisture actually go through walls?
Yes, through a force called "vapor pressure". Moisture in wet air tries to flow toward drier air to equalize itself. This flow acts independently of air currents. In winter, inside air is much more humid than colder outside air. So, the vapor pressure actually pushes the inside moisture through wood, plaster, concrete and brick, toward the outside.

What happens then?
Paint and varnish can block the flow of moisture, causing condensation to occur between the inside and outside walls, or under exterior paint surfaces. It can rot a home's wood frame and blister the paint.

Is condensation more prevalent in any geographical region?
Yes. Condensation is more apt to occur in climates where the average January temperature is 35 F or colder.

Does condensation occur only in winter?
Usually, but it can occur during cold weather anytime, and occasionally it will form on the outside of windows on hot, humid summer days, when your air conditioner has cooled the glass.

Does condensation depend on whether my home is new or old?
Generally, yes. Before the late 1970's, houses were not built as weather-tight as later ones. With the recent emphasis on energy-efficiency and ongoing improvements in construction techniques and materials, newer houses are much "tighter." An unfortunate by-product of these advances has been the tendency to lock moisture inside. Without adequate provisions for ventilation, excessive moisture can build up in the home, revealing itself as condensation.

Controlling Indoor Humidity

How do I measure indoor relative humidity?
To get an accurate reading, you can buy a humidity-measuring instrument such as a hygrometer or a sling psychrometer. Otherwise, watch your windows for symptoms of excess humidity. When excessive moisture collects on the inside glass in a living room or bedroom, you're approaching the humidity danger level.

Isn't high indoor humidity healthy in winter?
That's a common belief, but there is little evidence to support it. High or low humidity in a heated house has not been shown to be an important health factor to a normal healthy person.

What are the recommended indoor relative humidity levels for winter?
The University of Minnesota Engineering Laboratories performed a series of long and careful experiments on that subject. The following table shows the maximum safe humidity for your home, paint, insulation, and structural members:

Recommended interior relative humidity

-30 F or below -- not over 15%
-20 F to -10 F -- not over 20%
-10 F to 0 F -- not over 25%
0 F to 10 F -- not over 30%
10 F to 20 F -- not over 35%
20 F to 40 F -- not over 40%
(Assumes 70 F indoor air temperature)

Recommended interior humidity levels

When safe humidity levels are maintained, condensation is very unlikely, and a healthy interior environment exists for the home and its occupants.

What are some low-cost ways I can reduce or eliminate condensation?

  • If you have a hot air furnace, install an outside fresh air intake so that high-humidity interior air is not being pumped back into the house.
  • Use exhaust fans in the kitchen, bathrooms, and laundry. They not only help reduce excessive moisture, but will clear away stale air as well.
  • If you have a basement, take the necessary steps to prevent leakage of soil moisture into the basement. These steps will vary with soil and drainage conditions on your lot.

Ventilate your home. Because outside air usually contains less water vapor, it will "dilute" humidity of inside air. This takes place automatically in older homes through constant infiltration of outside air. But again, in newer "tighter" homes, the only way outside air can get in is by ventilation.

How can I ventilate my home?
There are basically two types of ventilation: interior and structural ventilation.

As a temporary solution, open a window in each room for just a few minutes. Remember that inside air continually gains humidity through daily living activities. Opening windows allows the stale, humid air to escape, and fresh, dry air to enter.

After a shower, for example, open the bathroom window or turn on the exhaust fan, so steam can go outside instead of remaining in the home.

Structural ventilation is slightly more complex, but will save you costly repair bills in the long run. Miniature louvers in exterior walls can be installed to prevent moisture from condensing between the outside and inside walls. This will keep paint from peeling as a result of indoor vapor pressure.

Does structural ventilation include attics?
Yes. Many homeowners cover all attic louvers in winter in hopes of saving fuel. If the attic is properly insulated, this practice can cause harm. Because the indoor moisture penetrates ceilings, then condenses on the cool underside of the roof and can even form frost. If the attic were ventilated, moisture would be transferred to the outside air.

What harm can attic condensation do?
A lot. Moisture condensing in attics produces mildew, or rotting conditions. Or it drips down to the ceiling below to damage plaster or paint. Thermal insulation also becomes wet and provides less resistance to heat loss.

Are some kinds of attic ventilation better than others?
Yes. A combination of vents at the eaves and at the gable ends is better than gable vents alone. And, a combination of continuous eaves and ridge venting is best of all. However, regardless of the type you have, there should always be at least two vent openings, located so that air can flow in one and out the other.

How much attic ventilation should I have?
That's a difficult question to answer, because the size and number of vents depends on the location of the home, wind direction, physical size of the building, quality of workmanship and kinds of building materials used. A heating and ventilating contractor should be able to tell you how much ventilation your attic should have.

What about the crawlspace? Should it be ventilated, too?
Yes. The crawlspace beneath a house is another place where ventilation is important. The crawlspace can evaporate gallons of water each day. When you seal the crawlspace, that water penetrates the floor above and causes more humidity problems in the home.

Providing foundation vents in the crawlspace reduces the humidity, and a vapor barrier (like polyethylene film) on the ground prevents moisture leakage into the house above.

Can excessive humidity do any damage?
Excessive interior humidity can be annoying to a homeowner and destructive to a home. It can damage sheetrock, paneling and window sills. It can also penetrate walls, deteriorating wood framing and reducing the effectiveness of insulation. It can cause the paint to peel from the sash of wood windows. Water can run down into window frames, causing dampness in the adjacent walls.

Are there any cases where window condensation is only temporary?
Yes, there are primarily three: new construction or remodeling; the beginning of each heating season; and after quick changes in temperature.

At the beginning of the heating season there may be a certain amount of temporary condensation. During the humid summer your house absorbs some moisture. After the first few weeks of heating, your house will dry out, and there should be less condensation.

Can windows help control moisture in my home?
Only in the sense that they can be opened for ventilation. Otherwise, windows are only indicators of excessive moisture in the air.


The best way to avoid condensation is to reduce excess humidity inside your home.

While it can certainly be a problem, in the vast majority of cases, it can be controlled or eliminated.

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