Window Condensation: Inside, Outside, or Between?

Answer First: Window condensation shows up in three places, and each one means something completely different. Inside condensation means your home is too humid for the glass temperature. Fix the humidity, not the window. Outside condensation means your glass is so well-insulated that the outer surface stays cold enough to collect dew. That is a compliment to your windows. Between-the-panes condensation means the factory seal has failed, the argon gas has escaped, and the insulated glass unit is dead. You cannot wipe it, you cannot defog it, you need to replace the sealed unit.

Three Locations. Three Diagnoses.

Most people treat all condensation the same. They wipe it down, maybe crack a window, and move on. That is like treating a headache and a brain tumour with the same aspirin.

Where the water shows up tells you everything. Here is the cheat sheet for Toronto homeowners in 2026.

Location	What It Means	Whose Fault	The Fix
Inside surface	Too much humidity in the home	Yours (lifestyle/ventilation)	Lower RH, run exhaust fans
Outside surface	Glass is colder than outdoor dew point	Nobody's. It's physics.	Nothing. Enjoy the show.
Between panes	Seal failure. IGU is dead.	The window (age/quality)	Replace the sealed unit

Quotable Nugget #1: If you can wipe it off, it's a humidity problem. If you can't wipe it off, it's a window problem.

Inside Condensation: The Humidity Mirror

This is the one that panics people every November. You wake up, and every window in the house is dripping. The sills are wet. There might be frost in the corners.

Take a breath. Your windows are probably fine.

The Dew Point, Explained Simply

Air holds water vapour. Warm air holds more. Cold air holds less. When warm, humid indoor air touches a cold glass surface, the air right next to the glass drops below its dew point temperature and dumps its water onto the glass.

That is it. That is the entire science. The glass is just a cold surface doing what cold surfaces do.

Here is a practical example. Your living room is 21C with 45% relative humidity. The dew point of that air is about 9C. If the interior surface of your window drops below 9C, you get condensation.

A standard double-pane window with argon gas might keep its interior surface at 12-14C when it is -10C outside. No condensation. But a single-pane or a failed double-pane might sit at 5C on the interior. Dripping wet.

The Toronto Humidity Problem

Toronto has a specific challenge. Lake Ontario acts as a massive humidifier. In autumn and early winter, the lake is still warm relative to the air. That warm water pumps moisture into the atmosphere. Couple that with modern building practices that seal houses tight for energy efficiency and you have a recipe for interior condensation.

Every shower adds about 0.5 litres of moisture to the air. Cooking dinner adds another 0.5 litres. A family of four just breathing generates roughly 8 litres of water vapour per day. In a well-sealed Toronto home, all that moisture has nowhere to go.

Quotable Nugget #2: A family of four generates 8 litres of water vapour per day just by existing. In a sealed home, that moisture has to land somewhere.

CMHC Humidity Guidelines for Canadian Winters

Canada Mortgage and Housing Corporation does not mess around on this topic. Their guidelines are temperature-dependent.

Outdoor Temperature	Maximum Indoor RH
-10C	35%
-20C	25-30%
-30C	20%

Those numbers feel low to most people. That is because most people are used to living at 40-50% RH, which feels comfortable but is too wet for cold glass surfaces.

The practical fix:

Run your bathroom exhaust fan for 20 minutes after every shower. Not 5 minutes. Twenty.
Use your kitchen range hood when cooking. The recirculating ones that blow back into the room do not count.
Check your HRV. If your home has a Heat Recovery Ventilator, make sure it is actually running. We see Toronto homes where the HRV has been off for years because the homeowner thought it was "wasting heat."
Buy a $15 hygrometer. Stick it on your kitchen counter. If it reads above 40% when it is -10C outside, you are going to see condensation.

When Inside Condensation Actually Means Bad Windows

There is one exception. If only some of your windows have inside condensation while others in the same room do not, the wet ones might have a problem. Possible causes:

Failed argon gas fill dropping the interior glass temperature
Poor weatherstripping allowing cold air infiltration right at the glass
Single-pane or original glass in a home where other windows have been upgraded

In that case, you are looking at a residential window replacement for those specific openings, not a whole-home humidity fix.

Outside Condensation: The Compliment You Don't Recognize

This one confuses people. You just spent $12,000 on new triple-pane windows and now the outside is covered in dew every morning. Something must be wrong, right?

Wrong. This is your windows working exactly as designed.

How It Happens

Your fancy new Low-E coated, argon-filled windows are so good at keeping heat inside your home that the outer pane stays cold. On a calm, clear night, that outer pane can actually drop below the outdoor dew point through radiative cooling.

Result: dew forms on the outside of your window. Same reason dew forms on your car windshield overnight.

Quotable Nugget #3: Outside condensation on new windows is not a defect. It is proof that your money was well spent. The glass is so insulating that the outer pane stays cold.

When Does It Happen?

Exterior condensation is most common during:

Spring and fall mornings when nighttime temperatures drop but humidity is moderate
Clear nights with no cloud cover (clouds act as a blanket, keeping surfaces warmer)
Low-wind conditions where there is no air movement to evaporate the dew

It almost always burns off by mid-morning. If it bothers you aesthetically, there is genuinely nothing to do about it. Some European manufacturers apply hydrophilic coatings to the outer pane that cause water to sheet off rather than bead. These are not common in Canada yet.

Old Windows Never Did This

Your previous single-pane or basic double-pane windows leaked so much heat that the outer pane stayed warm. Warm glass, no dew. It was a sign of energy waste, not quality.

Think of it this way: your old windows were like wearing a T-shirt in January. No frost on the T-shirt because all your body heat is escaping. Your new windows are like wearing a proper winter coat. The outside of the coat is cold. That is the point.

For a deeper look at how argon and Low-E coatings create this insulating effect, see our breakdown in Does Argon Gas Actually Work? The Physics of Low-E Coatings.

Between-Pane Condensation: The Death Certificate

This is the only type of condensation that means your window has failed. And it is the most common service call we get in the GTA.

Here is how to tell: you cannot wipe it off from either side. The moisture is trapped inside the sealed unit, between the two (or three) panes of glass. It often shows up as a milky white haze, water droplets, or sometimes a rainbow oil-slick pattern.

The Anatomy of Seal Failure

Every insulated glass unit (IGU) is a sandwich:

Pane 1 (interior glass, usually with Low-E coating)
Spacer bar (separates the panes, holds desiccant beads)
Argon gas fill (insulating gas between panes)
Pane 2 (exterior glass)
Primary seal (polyisobutylene, PIB)
Secondary seal (polysulphide or silicone)

The seals around the edges keep the argon in and the humid air out. The desiccant beads inside the spacer bar act as a backup, absorbing any trace moisture that sneaks past the seal.

The Three-Stage Failure

Stage 1 (Years 1-8): The Silent Leak Daily temperature swings cause the gas between the panes to expand and contract. This is called thermal pumping. Every sunny day, the gas heats up and pushes outward. Every cold night, it contracts and pulls inward. Over thousands of cycles, micro-fractures develop in the seal.

Argon atoms start escaping. You cannot see this happening. The window looks fine.

Stage 2 (Years 8-12): Desiccant Saturation As the seal degrades further, humid outdoor air seeps in. The desiccant beads absorb this moisture, doing their job. But they have a finite capacity. Think of them like tiny sponges. Eventually they are full.

Stage 3 (The Fog): Game Over Once the desiccant is saturated, incoming moisture has nowhere to go except onto the glass surfaces inside the unit. You see fog, droplets, or haze. On cold mornings it is worse. On warm afternoons it might seem to "clear up," tricking you into thinking it fixed itself. It did not. The moisture is still there; it just evaporated back into the air space temporarily.

Quotable Nugget #4: A failed window seal loses approximately 1% of its argon fill per year. By the time you see fog, the unit may have lost 30-50% of its insulating gas.

The Energy Cost of Ignoring It

A properly sealed double-pane argon-filled window has an R-value of about R-3.4. Once the seal fails and the argon escapes, you are left with an air-filled dead space. The R-value drops to roughly R-2 initially and continues declining as moisture degrades the Low-E coating.

In a Toronto winter, where outdoor temperatures regularly hit -15C to -20C, that degraded window is bleeding heat. Across a home with 15-20 windows, even 3-4 failed units can add $200-$400 per year to your heating bill.

The Fix: IGU Replacement, Not Defogging

We have written about this in detail before, but it bears repeating. "Defogging" services that drill holes in your glass are a cosmetic band-aid. They vent the moisture but do not restore the argon gas, do not fix the seal, and do not bring back your R-value.

The proper fix is replacing the Insulated Glass Unit. This means:

The old glass sandwich comes out
A new factory-sealed unit with fresh argon gas, new desiccant, and a modern warm-edge spacer goes in
The vinyl or aluminum frame stays in place

Cost? Roughly $350-$650 per unit depending on size, glass type, and Low-E spec. Compare that to $1,200-$1,800 for a full window replacement with frame.

If your frames are still in good condition, a glass replacement is the move. If the frames are cracked, warped, or painted shut, then yes, you are looking at a full residential window replacement.

The Diagnostic Flowchart

Before you call anyone, run through this.

Step 1: Can you wipe it off?

Yes, from the inside -> Interior condensation. Humidity issue.
Yes, from the outside -> Exterior condensation. Your windows are great.
No -> Between-pane condensation. Seal failure. Go to Step 2.

Step 2: How many windows are affected?

One or two -> Likely individual seal failures. IGU replacement.
Most or all -> Could be age (all installed at the same time) or could be a humidity problem masquerading as seal failure. Get a professional assessment.

Step 3: How old are the windows?

Under 5 years -> May be under warranty. Check with your installer.
5-15 years -> Prime "failure cliff" age, especially for builder-grade units.
Over 15 years -> Expected end-of-life. Consider whether the frames justify IGU-only replacement or if a full upgrade makes more sense.

The Toronto-Specific Factors

Living near the Great Lakes comes with unique condensation challenges that people in, say, Calgary never deal with.

Lake-Effect Humidity

Lake Ontario does not freeze in winter. That open water pumps moisture into the GTA air mass all season. Toronto's average winter relative humidity hovers around 70-75% outdoors, compared to 50-60% in inland cities. More moisture in the outdoor air means more moisture infiltrating your home through air changes.

The Condo Factor

Toronto has roughly 400,000 condo units. Many built during the 2005-2015 boom use builder-grade windows with metal spacer bars that are now hitting the 10-15 year failure cliff. If you live in a high-rise and see between-pane fogging, you are not alone. Your neighbours probably have the same problem.

Quotable Nugget #5: Toronto has ~400,000 condo units. A significant percentage of those built between 2005-2015 are entering the "spacer failure cliff" right now.

The New-Build Paradox

Homes built to 2026 Ontario Building Code standards are sealed tighter than ever. Better insulation, better air barriers, better windows. But that airtightness traps moisture. If your HRV is not sized correctly or is not running, you will see interior condensation on even the best triple-pane windows.

This is not a window defect. It is a ventilation deficiency. The window is just the messenger.

Prevention: The Cheap Stuff That Actually Works

You do not need to spend thousands to manage condensation. Start here.

Hygrometer ($12-$20 at Canadian Tire). Measure before you panic. If your indoor RH is under 35% in winter and you still have interior condensation, then you might have a window problem.
Exhaust fans on timers. Bathroom fans should run 15-20 minutes after the last shower. Hardwire them to a timer switch so nobody has to remember.
Open your blinds. Closed blinds and heavy curtains trap a layer of cold air against the glass, dropping the surface temperature below the dew point. The prettiest drapes in Rosedale are sometimes the cause of the worst condensation.
Move furniture away from heating vents. If a couch is blocking the floor register under a window, warm air never reaches the glass surface. That window stays cold. Cold glass, wet glass.
Run your HRV. Seriously. It is not optional in a modern home. If yours is off, broken, or set to minimum, your house is a sealed container full of human-generated moisture.

When to Call Us

Not every condensation issue needs a professional. But these scenarios do:

Between-pane fogging on any window. The seal is gone. It will not heal itself.
Interior condensation that persists even at 30% RH. The glass may be underperforming.
Ice forming on the inside of the glass. This suggests the window R-value is critically low.
Water pooling on the sill and damaging the frame. Sustained moisture rots wood frames and warps vinyl.
Condensation on only one or two windows in an otherwise dry room. Those units are likely failed.

We do free assessments across Toronto, Mississauga, Brampton, Markham, and Vaughan. No pressure. We will tell you if it is a humidity problem (not our department) or a window problem (very much our department).

FAQ

Is condensation on the inside of my windows dangerous? Not immediately, but sustained interior condensation above 50% RH promotes mould growth on frames, sills, and surrounding drywall. CMHC recommends keeping indoor humidity between 30-50% in winter to prevent this.

Why do my windows fog up on the outside in summer? Exterior condensation happens when the glass surface is colder than the outdoor dew point. This is actually a sign your windows insulate well. It disappears once the sun hits the glass.

Can foggy windows between the panes be repaired without replacing them? Defogging services drill holes and vent the moisture, but they do not restore the argon gas fill or the R-value. The seal is still broken. We recommend replacing the sealed unit (IGU), not the whole window.

How long do window seals last in Toronto's climate? Factory-sealed IGUs with modern warm-edge spacers typically last 15-20 years in the GTA. Builder-grade units from the 2005-2015 condo boom often fail in 8-12 years due to cheaper metal spacers.

What indoor humidity level prevents window condensation in winter? When it drops below -10C outside, keep indoor RH at 30-35%. At -20C, aim for 25-30%. A cheap hygrometer from Canadian Tire will save you thousands in mould remediation.

Does condensation between panes affect my energy bill? Yes. A failed seal means the argon gas has escaped and your double-pane window now insulates like a single pane, roughly R-1 instead of R-3.4. That can increase heat loss through that window by up to 32%.

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