Window Condensation — Why It Happens and How to Fix It

You already know how disturbing window condensation can be especially when you get up early one morning with foggy dripping windows in a chilly Toronto morning. Is it the window’s fault? Is there a leak? Is it that there is mold growing in your walls? The responses are purely conditional on the location of the condensation it is forming – and most householders do not know that the location of the moisture can tell you nearly everything you need to know.

This guide dissects all three of those variations of window condensation, and reveals how each of them occurs, and gives you the step-by-step how to fix each one- so that you can cease conjecturing about it, and instead permanently fix the issue.

What Is Window Condensation and Why Does It Form?

Condensation occurs when humid warm air comes into contact with a cold surface, cool enough to cool the air below the dew point – the temperature at which the water vapour transforms back to liquid droplets. Windows, since they are on the interface between your cozy indoors and outdoors, are the most usual place in your house where this process is executed.

What most homeowners fail to understand is the following: condensation in windows does not necessarily mean that the window is faulty. In reality, there are certain kinds of condensation that are an indication that your windows are well functioning as they need to be. Some are, though, a clear sign of danger, which must be addressed right now.

Three types exist, and they demand absolutely different answers.

Type 1: Interior Window Condensation — The Most Common Type

What it looks like

Interior window condensation is a type of cloud or mist that is seen on the inside of your glass, and is generally seen in Ontario during a winter. It can be wiped out by a cloth. It can be gathered in the form of droplets or a fine film at the lower section of the pane and can be puddled on the sill.

Why it happens

This kind of condensation in windows nearly always is a humidity problem – not a window problem. In conditions where indoor relative humidity is excessively large, and window glass is cold, room air moisture condenses upon contact with the glass surface.

Common household sources of excess humidity include:

• Not using an exhaust fan to cook
• Taking long and hot showers in stuffy bathrooms
• Hanging clothes in the house on racks or radiators
• Plants in the house transpiring
• New construction or recent renovations (drywall and lumber release moisture for months)

Counterintuitively, new energy-efficient windows can actually increase visible interior condensation at first. This is because: old, leaky, windows have always been allowing humid air out and thus maintained the humidity at artificially low levels. Installation of properly sealed replacement windows closes that means of escape and increases indoor humidity – and condensation becomes more noticeable on the glass. This is not a defect. It is testament that your new windows are closing your house.

The right fix for interior window condensation

The idea is to lower the interior relative humidity so that no condensation can be developed. Natural resources Canada recommends that indoor humidity should be just a bit above 30% to 50% in winter, when the outdoor temperatures are too cold.

Immediate steps:

1. Buy a hygrometer — a miniature and cheap appliance (sold by most hardware stores) which detects the moisture content of the air inside the home. Managing something you cannot measure is impossible.
2. Run bathroom and kitchen exhaust fans each time you prepare something, or take a shower and leave them on 15-20 minutes after you are done.
3. Increase air circulation near windows — push furniture and heavy curtains off glasses to allow circulatory air to cross the surface to cool the temperature differentials.
4. Use a dehumidifier in rooms with persistent interior window condensation, particularly bedrooms, basements, and laundry areas. A quality dehumidifier costs $200–$350 and makes a significant difference.
5. Consider an HRV (Heat Recovery Ventilator) — a whole-home mechanical ventilation system which runs the stale and humid indoor air out of the house and replaces it with fresh air outside of the house with no loss of heat. The long-term answer to airtight modern homes is HRVs, and it is gaining use in the new GTA buildings.

If interior condensation persists after addressing humidity, the issue may be related to your window installation — specifically air leaks around the frame that allow cold air to make the glass surface excessively cold.

Type 2: Condensation Between Window Panes — The Serious One

What it looks like

It is the kind that is more of a concern to homeowners and with reason. The condensation between the panes is manifested as a chronic foggy haze or visible moisture trapped within a double or a triple-pane window unit, between the two or more sheets of glass. More importantly, you can not wipe it off, on either side.

Why it happens

A contemporary example is an insulated glass unit (IGU) which is a structure that encloses an air or gas-filled chamber (usually argon or krypton) between two or three sheets of glass. It is that closed area that gives the thermal and acoustic insulation of the window. In case of failure (seal) in the perimeter of the unit, which may be as a result of old age, exposure to UV, improper installation, or heat stress, then the outside air and moisture finds its way in the space.

As soon as water is introduced to the sealed unit, it evaporates on the inner glass surfaces. It is also felt by damaging the gas fill lowering the U-factor and energy performance of the window considerably.

How to remove condensation between window panes

This is the truthful solution that most of the rivalry has shirked because you cannot permanently mend condensation of window panes using a do-it-yourself solution. Other firms offer the business of defogging, which involves making small holes in the unit to dehumidify it but that is a short-term solution. The seal is damaged permanently and the gas fill is lost so the thermal performance of the window is reduced anyway.

The right solution is the replacement of insulated glass unit (IGU). In most instances, it is just the glass unit that requires replacement and not the whole window frame. In case the frame is structurally sound, a glazier will be able to take out the failed sealed unit and replace it with a new one that would cost much less than replacing the entire window.

However, if:

• The frame is distorted, broken or has water damage
• Several units within the same window have collapsed
• The window is more than 15–20 years old
• Energy bills have been rising consistently

…then full window replacement is the less expensive long run option. The Milano Windows and Doors team can evaluate the case of IGU replacement or full replacement that suits your case better.

Worth knowing: A broken sealed unit means your window is losing insulating gas and functioning closer to a single-pane window thermally. Leaving it unfixed significantly increases your heating costs through a Toronto winter.

Type 3: Exterior Window Condensation — The One to Celebrate

What it looks like

Exterior window condensation can be seen on the outer part of your glass and normally in the spring and fall mornings when it is cool. It appears like precipitation and rubs off with ease – externally. It then generally evaporates as the sun in the morning heats the glass surface.

Why it happens

This is the most incorrect form of condensation out of windows. Most home owners panic and call their installer as they can see it – yet condensation outside windows is actually an indication that your windows are doing an extremely good job.

Here is the science: the very low-energy consuming windows (most specifically triple-pane ones with low-E finishes) are so insulated that the amount of internal heating passing through the glass to warm the exterior is minimal. During sunny and clear calm nights when the outer glass releases heat to the sky by way of the long-wave radiation, the surface temperature is lower than the outdoor dew point. The formation of the dew, the dew on the outside, is the same process which produces the dew on the grass.

Less efficient older windows lose a lot of heat through the glass so the exterior part remains warm and no dew is formed. Thus as long as you can see the exterior window condensation on a fresh installation, you can consider it confirmation that your windows are functioning as you wanted.

Does exterior condensation cause any damage?

No. It is not the case in the majority of situations and contemporary vinyl and fiberglass frames can cope with moisture. But when you observe regular exterior sill water pooling with no drainage, then it may be time to check the exterior caulking and sill gradient to ensure that it is sufficient to drain exterior water away along the frame. This is included in the normal checking we conduct on any window replacement work.

When Is Window Condensation a Sign You Need New Windows?

Condensation does not require replacement everywhere, but these two instances are heavy handed indicators of the same:

• Persistent foggy haze between panes that has been present for more than a few weeks (failed seal — replace IGU or full window)
• Ice forming on the interior surface of windows in winter, indicating extremely poor thermal performance from aging, single-pane, or damaged units
• Rot or water stain on wooden frames of conditions-prone windows – water has been seeping into the building
• Condensation on most or all windows of an older house whose windows have not been replaced in 20 or more years

When any of these is relevant to your GTA home, it will be worth the book of a free in-home assessment. Our technicians will be able to determine which of targeted repairs or full window replacement will benefit you more.