Window condensation in winter is almost always a humidity problem rather than a window defect — and the surface the water sits on tells you which. Moisture on the room side of the glass means the indoor air is carrying more vapour than the cold glass can hold. Fog between the panes that will not wipe off is the one symptom that points to a failed sealed unit. Moisture on the outside face is not a fault at all.
Across Simcoe County the calls start the first week overnight lows push past -10 °C. Homes on the Lake Simcoe shoreline in Innisfil and Oro-Medonte usually see it before inland subdivisions off Highway 400. Late-1970s and 1980s builds see it worst: original double glazing with aluminum spacers plus a modern furnace humidifier is the classic combination. Here is how to read the glass before you spend a dollar.
Three places condensation appears, three different answers
Wipe the glass with a dry cloth. If the water comes away, it is sitting on a surface you can reach. If the haze stays put, it is sealed inside the glass unit and nothing you change in the room will fix it.
| Where the moisture sits | What it means | What to do |
|---|---|---|
| Room side of the glass | Indoor relative humidity is higher than the glass surface temperature will tolerate | Lower the humidity and get room air moving across the glass |
| Between the panes | The insulated glass unit has lost its seal — the argon has escaped and the desiccant is saturated | Replace the glass unit; check the manufacturer warranty first |
| Outside face of the glass | The window is insulating well; the outer pane sits near the outdoor dew point | Nothing. It normally clears by mid-morning |
| Frost on the frame, sash corners or trim | Air leakage through the frame or a failed perimeter seal at the installation | Have the perimeter seal, insulation and capping inspected |
Why the room side of the glass sweats
Air holds water vapour in proportion to its temperature. Cool that air and it reaches a point — the dew point — where it can no longer hold what it carries, and the surplus lands on the nearest cold surface. Glass is the coldest interior surface in most houses, so it fogs long before the walls do.
That makes your windows a moisture gauge for the whole building, which is genuinely useful. The glass is warning you before that same vapour starts condensing inside a wall cavity where nobody can see it.
How much indoor humidity a Canadian winter will tolerate
Health Canada puts the winter target at roughly 30-35% relative humidity. Treat that as an average, not a ceiling — the colder it gets outside, the lower the number has to go.
| Outdoor temperature | Practical indoor RH limit | Typical Simcoe County situation |
|---|---|---|
| 0 °C to -7 °C (32 to 19 °F) | 35-40% | Comfortable, most homes have no fogging |
| -7 °C to -12 °C (19 to 10 °F) | 30-35% | First fogging on older double glazing |
| -12 °C to -18 °C (10 to 0 °F) | 25-30% | Edge frost appears on aluminum-spacer units |
| -18 °C to -24 °C (0 to -11 °F) | 20-25% | Humidifier must come down or sills stay wet |
| -24 °C and colder | 15-20% | Deep-freeze week; even good windows show corner frost |
Measure with a hygrometer in a central room at 21-22 °C, not on the windowsill — the sill reads several degrees colder and will mislead you. Older double glazing with an aluminum spacer will fog several points below these figures. A triple-pane unit with a warm-edge spacer will comfortably run above them.
When winter condensation is normal
- The first hard cold snap. The house has not dried down yet and the humidistat is still on its shoulder-season setting.
- New builds and recent renovations. Concrete, drywall compound and framing lumber release moisture for a year or more after occupancy.
- Right after a shower, a roast dinner or a laundry load dried indoors. A short burst that clears within an hour is normal.
- Behind closed blinds and heavy drapes. Trapped air next to the glass drops several degrees below room temperature.
- A thin band along the bottom edge of the glass. The spacer is the coldest point of any sealed unit, so it fogs first.
- Dew on the exterior face on a still, clear morning. This is a performance indicator, not a defect — the outer pane is being kept cold because so little heat is escaping through the unit.
When condensation is telling you something is wrong
- Frost that never melts through a full day of sun.
- Water pooling on the sill, staining the trim or lifting paint and MDF casing.
- Black spotting on the caulk line or in the drywall corners around the opening.
- Ice forming inside the frame or at the sash lock — that is air leakage, not room humidity.
- One window streaming while identical windows on the same wall stay clear.
- Wet corners at a patio door, especially at the bottom of the fixed panel where the sill pan should be draining.
Fog between the panes: the sealed unit has failed
An insulated glass unit is two or three panes bonded to a spacer bar, with a primary and secondary seal, a desiccant inside the spacer to absorb residual moisture, and argon between the panes. When the secondary seal fails, the gas leaves and humid outdoor air enters. Once the desiccant is saturated, it stops absorbing and the moisture shows on the inner glass surfaces where you cannot reach it.
Some argon loss is expected even in a sound unit — industry testing commonly cites around 1% a year, so a 20-year-old window has measurably less insulating gas than it left the factory with. A seal failure turns that slow drift into a collapse: the unit loses most of its thermal advantage and the glass gets cold enough to fog on days it never used to.
How to confirm it before you call anyone
- Clean both the interior and exterior faces properly. If the haze survives both, it is inside the unit.
- Look for streaky mineral deposits or a permanent milky patch — these appear after repeated wetting and drying cycles between the panes.
- Watch the pane in direct sun. Failed units often clear at midday and fog again at dusk.
- Find the etched code in a corner of the glass. It usually carries the fabricator and manufacturing date, which is exactly what a warranty claim needs.
A failed unit cannot be repaired. Drilling-and-drying services clear the visible haze but do not restore the argon fill, the desiccant or the R-value. The correct fix is a new sealed unit — and in most cases the frame, hardware and installation stay untouched, so it is far cheaper and faster than a full window replacement. Most sealed-unit warranties run somewhere between 10 and 20 years depending on the manufacturer, so check your paperwork before you assume you are paying.
Fixes, in the order that actually works
- Turn the humidifier down first. On most furnace humidistats a single step is 5-10% RH. Adjust it when the forecast changes, not once a season.
- Run the exhaust fans and let them finish. Bathroom fan for 20 minutes after a shower, range hood every time you boil or fry. Confirm both terminate outdoors and not into the attic — this is a common find in older Barrie and Bradford housing stock.
- Give the glass room air. Open blinds during the day, leave a hand’s width behind drapes, and pull furniture off the sill.
- Service the HRV or ERV. Wash the core, replace the filters and set it to run continuously on low through the coldest months.
- Cut the hidden moisture sources. Store firewood outside, vent the dryer properly, cover the sump pit, and check for a damp crawlspace.
- If the hygrometer reads 30% and the glass still streams, the problem is the window, not the air.
If the windows themselves are the limit
When you get to replacement, the specifications that control condensation are not the same ones that sell on a brochure. Since 2020 ENERGY STAR Canada uses a single national standard instead of climate zones: a certified window needs a U-factor of 1.22 W/m²·K (0.21 imperial) or better, or an Energy Rating of 34 or higher. The Most Efficient tier sits at 1.05 W/m²·K (0.18) with an ER of 40 or more.
For condensation specifically, the spacer matters more than most buyers realise. Independent testing has shown that swapping an aluminum spacer for a foam warm-edge system lifts the edge condensation-resistance rating from the high teens into the mid-40s — and the glass edge is precisely where frost starts. Ask for the Condensation Resistance number, which runs on a 30 to 80 scale where higher is better, rather than accepting adjectives. If you are weighing double against triple glazing, our guide to energy-efficient windows for Barrie winters works through where the extra pane pays for itself.
What to ask before you sign anything
- What is the Condensation Resistance rating and the spacer type on the unit you are quoting?
- Is the warranty on the sealed unit separate from the warranty on frames and hardware, and how many years is each?
- Does the installation warranty come from the company that does the work, or from a subcontractor?
- Will you replace the glass unit only, or is a full window the only option offered?
- How is the perimeter air-sealed and insulated — low-expansion foam, backer rod and sealant, or both?
Your next step
If the haze wipes off, start with the humidistat and the exhaust fans, and give it two weeks. If it does not wipe off, photograph the window, find the etched date code and get it assessed — a failed unit in one window is often the first of several from the same production batch.
TopView Windows & Doors has been installing across Simcoe County and York Region for more than ten years with our own crews and no subcontracting, so the same team that measures your opening is accountable for the seal around it. We cover both the product and the installation in writing, and we currently pay the HST on window and door replacements. Book a free assessment at (647) 448-8439 or 1-800-917-0317, browse our window replacement options, and keep the glass working with our fall window and door maintenance checklist.
