Everybody in Alberta understands winter window condensation. It is January, it is −28°C, the glass is freezing, the house is humid, and the windows run with water. Fine. But it is the middle of July, the air conditioner is on, and there is moisture on your windows — and that one is genuinely worth paying attention to. Summer condensation is not a nuisance, it is a diagnostic. And the single question that decodes it takes two seconds to answer: which side of the glass is wet?
First, The Physics — In One Sentence
Condensation happens when air touches a surface that is colder than the dew point of that air. That is the whole mechanism. It is why a can of pop sweats on the picnic table, and it is why the coldest surface in any room — almost always the glass — is where moisture shows up first. Windows are not causing condensation. Windows are just the first place your house is cold enough to reveal it.
Which means the location of the water is information. There are three possibilities, and they have almost nothing to do with each other.
1. Moisture On The Inside Surface (You Can Wipe It Off)
Reach out and touch the glass on the room side. If your finger comes away wet, this is interior condensation, and in a Canadian summer it is the one that should get your attention.
Here is why it is significant. Health Canada’s guidance is explicit that in summer, high relative humidity usually will not cause condensation problems on above-grade surfaces — because the building structure itself is warm. Warm walls and warm glass do not condense. So if your above-grade windows are wet on the inside in July, something has changed that equation. Almost always it is a combination of two things: air conditioning making the glass considerably colder than the structure around it, and a real, ongoing moisture source pushing indoor humidity up.
Health Canada lists the usual suspects for indoor moisture: showering or bathing and cooking without working exhaust fans; water infiltrating through cracks or leaks in the foundation, floor, walls or roof; plumbing leaks; flooding from heavy rain or snowmelt; and high indoor moisture sources such as aquariums or numerous plants. To that list add a clothes dryer that is not actually venting outdoors — a disconnected or badly blocked dryer duct dumps several litres of water vapour into your house with every load, and it is one of the most common hidden humidity sources we find in Edmonton-area homes.
The fix is not to wipe the window. The fix is to find the source and ventilate. Buy a hygrometer — Health Canada notes they are inexpensive and available at any hardware store — and find out what your house is actually running at. Target below 50% in summer.
Don’t let the sills stay wet while you investigate. Health Canada’s advice is to wipe window frames and sills daily if you notice condensation, to keep water from dripping and causing mould to grow. Dampness on a wooden sill and the drywall under it is a textbook mould site — and Health Canada is clear that because some people are far more sensitive than others, there is no “safe” limit for mould. Deal with the water while you hunt the cause.
2. Moisture On The Outside Surface (Usually Good News)
Now touch the outside. If the dew is on the exterior of the glass — typically an early-morning thing, often on windows facing the open sky, and it burns off by mid-morning — relax. That is not a defect. It is the same dew that formed on the lawn and on the roof of your car, and it formed on your window for the same reason: the outer pane got cold overnight and the humid morning air condensed on it.
Counterintuitively, this often means your windows are doing their job. A modern insulated window keeps indoor heat inside so effectively that the outer pane is barely warmed from within — so it cools down at night like any other outdoor surface. An old, leaky single-pane window bleeds enough heat outward to keep its own exterior glass too warm for dew to form. So the drafty window stays clear and the efficient window gets dewy. Nothing to fix.
3. Moisture Between The Panes (The Real Failure)
This is the one that is never normal. You wipe the inside — still foggy. You wipe the outside — still foggy. The haze is sealed inside the glass, and often it looks like a permanent smear or a cloudy film that comes and goes with the sun.
Your window is an insulated glass unit: two or three panes bonded around a spacer, with a desiccant inside the spacer to keep the cavity bone dry. When the perimeter seal fails, humid outdoor air works into that cavity, the desiccant eventually saturates, and moisture condenses on interior glass surfaces you cannot reach.
Two practical consequences. First, there is no home remedy — you cannot wipe it, you cannot dry it, and drilling it does not end well. Second, the fog is a symptom of a bigger loss: many sealed units are filled with argon or another inert gas, and once the seal is gone, the gas is gone, and so is a chunk of the window’s insulating value. You are paying for that in cooling now and heating in January.
Summer is when people notice it, for a reason worth knowing: winter air is dry, so there is often not enough moisture in that cavity to make visible fog. Come July, humid air infiltrates a warm cavity and hits glass chilled from inside by your air conditioner — and the failure that has been there for months finally becomes visible.
The good news is that the repair is usually cheaper than people fear. In most cases the sealed glass unit can be replaced on its own, leaving the existing frame and hardware in place. Get a glass company to price replacing the IGU before anyone talks you into replacing whole windows.
The Alberta Wrinkle: Basements, Pipes And Toilet Tanks
If interior condensation upstairs is unusual in summer, the basement is a different story. Health Canada specifically notes that while summer humidity usually will not condense on above-grade floors, basements may be affected, along with uninsulated cold water pipes and toilet tanks. Those below-grade surfaces stay cool all summer while the air around them is warm and humid — the classic recipe.
So if you find condensation on basement windows, a sweating toilet tank, or damp cold-water lines in July, that is not the same problem as fog on your living room glass. That is the basement telling you it needs dehumidification and better air movement.
What To Actually Do About High Indoor Humidity
Health Canada’s recommendations for preventing high relative humidity break into three jobs:
- Find and fix the moisture source. Repair plumbing leaks promptly. Repair cracks or leaks in the foundation, roof, walls and window frames. Slope the grade so rain, irrigation water and snowmelt drain away from the house. Clear debris from eavestroughs and downspouts and make sure the outflow runs to the street.
- Keep surfaces warm and dry. Wipe frames and sills daily while condensation is happening. Insulate poorly insulated exterior or foundation walls. Let air and heat circulate, especially behind furniture and curtains. Run a dehumidifier in damp areas such as the basement.
- Ventilate. Always use bathroom exhaust fans while showering and the range hood while cooking — and confirm they actually vent outdoors, not into the attic. Always vent the clothes dryer to the outside. Use your home’s mechanical ventilation, or consider installing one, ideally with an HRV or ERV.
Notice how many of those are HVAC and ductwork items rather than window items. That is the point of this whole article: the window is the messenger. The message is about the air in your house.
Where Home Pros Group Fits
We don’t install windows or replace sealed units — if the fog is between your panes, call a glass company. What we do handle is the part of the chain that is quietly making your indoor air wetter and dirtier than it should be: dryer vents that are blocked or disconnected and dumping moisture into the house, exhaust runs that are not moving what they should, ducts and A/C coils carrying the dust and dampness around, and the general question of what your home’s air is actually doing. If the windows started this conversation, we are happy to finish it.
Summer Window Condensation: Quick Answers
Is condensation on windows in summer normal?
It depends entirely on which side of the glass is wet. Moisture on the outside surface in the early morning is normal and usually harmless — it often means your windows are insulating well. Moisture on the inside surface in summer is unusual in a Canadian home and is worth investigating, because it means indoor humidity is high enough to condense on air-conditioned glass. Moisture sealed between the two panes is never normal.
What humidity should my house be at in the summer?
Health Canada recommends keeping relative humidity in your home below 50% in the summer, and between 30% and 35% in winter. For mould prevention specifically, Health Canada advises maintaining relative humidity between 30% and 50% year-round. You can check yours with a hygrometer, an inexpensive tool sold at most hardware stores.
Why is there condensation inside my windows when the air conditioning is on?
Air conditioning cools the interior surface of the glass. If the indoor air holding a lot of moisture then touches that cooler glass, the moisture condenses out — the same way a cold drink sweats on a hot day. Health Canada notes that in summer, high humidity usually will not cause condensation on above-grade surfaces because the building structure is warm, so if you are seeing it, you likely have both aggressive cooling and a real moisture source: showers or cooking without exhaust fans, a plumbing leak, water getting in from outside, a dryer venting indoors, or a lot of plants or aquariums.
What does it mean when there is fog between the two panes of glass?
It means the sealed unit has failed. A modern window is an insulated glass unit — two or three panes bonded around a spacer with a desiccant inside to keep the cavity dry. When that perimeter seal fails, humid air gets into the cavity, the desiccant eventually saturates, and moisture condenses on surfaces you cannot reach. You cannot wipe it off and you cannot dry it out. The window has also lost some of its insulating value. The fix is replacing the sealed glass unit — often just the glass, not the whole window or frame.
Can window condensation cause mould?
Yes, if it is left alone. Health Canada advises wiping window frames and sills daily if you notice condensation, to keep water from dripping and causing mould to grow. Persistent dampness on sills, frames and the surrounding drywall is exactly the condition mould needs. Health Canada also notes that people living in homes with mould and damp conditions are more likely to have eye, nose and throat irritation, coughing, wheezing and worsening asthma symptoms.
How do I lower the humidity in my house?
Find the source first, then ventilate. Health Canada's guidance includes repairing plumbing leaks promptly, fixing cracks or leaks in the foundation, roof, walls and window frames, sloping the grade so water drains away from the house, clearing debris from eavestroughs and downspouts, always using bathroom and kitchen exhaust fans and confirming they vent outdoors, always venting the clothes dryer outside, and running a dehumidifier or air conditioner in damp areas such as the basement.
Trusted Sources
Humidity targets and moisture guidance above come from Health Canada and CMHC. These are the primary documents.
Health Canada — Relative Humidity Indoors
The factsheet behind the numbers: summer vs. winter targets, moisture sources, and how to prevent high RH.
Read the factsheetHealth Canada — Mould
Health risks of damp homes, the 30–50% humidity range, and when a mould patch needs a professional.
Read the pageHealth Canada — Moisture & Mould Guide
The full homeowner guide to finding moisture problems and addressing them properly.
Read the guideCMHC — Mould In Housing
Canada’s housing agency on condensation, dew point, and why windows are where it shows up first.
Read the pageLinks open official federal government and housing-agency pages in a new tab. Home Pros Group isn’t affiliated with these organizations.
Your Windows Are Talking About Your Air
Home Pros Group has kept furnaces, ducts, dryer vents and A/C coils healthy across Spruce Grove, Stony Plain and the Greater Edmonton Area since 2002. If your home feels damp, smells musty, or your dryer vent hasn’t been checked in years, that’s our end of the problem — and we’ll tell you honestly if it isn’t.
This article is general information about indoor humidity and home maintenance, not medical or professional advice. Humidity targets, moisture sources and mould guidance are summarized from Health Canada’s Relative Humidity Indoors factsheet, Health Canada’s guidance on mould and CMHC. Every home is different, and window seal failure should be assessed by a qualified glass or window professional. If you have health concerns related to dampness or mould, speak with your doctor or call Health Link at 811.