Window Condensation in Winter: Why It Happens and How to Stop It

Quick answer: first figure out which side is wet

If your windows are wet in winter, the location of the moisture matters more than the size of the puddle.

• Room-side glass: usually means moist indoor air is meeting a cold window surface. Start with indoor moisture, ventilation and airflow around the glass.
• Outside glass: can be normal on efficient windows during cool, calm, humid conditions. Indoor dehumidification will not fix outdoor dew.
• Between panes: on a sealed double- or triple-glazed unit, persistent inaccessible fog or moisture can indicate a failed insulating-glass seal.
• Trim, wall or head of the window during rain: treat that as possible water intrusion until you know otherwise.

Natural Resources Canada (NRCan) explains that condensation risk rises as outdoor conditions get colder and/or indoor humidity increases. It also notes that airflow across the glass helps keep the interior surface warmer. Deep sills, closed drapes and blinds, nearby furniture and blocked heat sources can all make condensation worse. See NRCan’s condensation guidance.

Window condensation checker

Why is my window wet?

Start with what you can actually observe. The tool distinguishes likely room-side condensation, exterior condensation, sealed-glass problems and possible water intrusion. It does not inspect the window or assign a fake confidence score.

Where is the moisture?This is the most important question. Dry the accessible glass first if you are not sure.Choose oneOn the room-side surface of the glassOn the outdoor-facing surfaceBetween panes where I cannot wipe itMostly on the frame, sill, trim or surrounding wallI cannot tellWhen does it show up?Choose oneMostly during cold weatherMostly overnight or in the morningMainly during rain, wind-driven rain or snowmeltIt can happen year-roundNo clear patternHow widespread is it?Choose oneOne windowSeveral windows in one roomMany windows around the homeWhat form does the moisture take?Choose oneLight fog or hazeWater droplets or beadsRunning water or pooling on the sillFrost or iceAre you adding humidity?Choose oneA furnace / central humidifier is runningA portable room humidifier is runningNo humidifierWhat ventilation does the home have?Choose oneHRV or ERVKitchen / bathroom exhaust fans onlyNo mechanical ventilation I know ofNot sureWhat is happening around the glass?Choose oneHeavy blinds / curtains are usually closed or heat is blockedCoverings are open/light and heat can reach the glassNo meaningful covering or blockageAny recent building change?Choose oneThe home is newly builtWindows were replaced recentlyNo recent new-home / window changeAny visible damage?Choose oneNo obvious damageVisible mould around the windowPeeling finish, staining, soft wood or damaged drywall

Optional: what is the indoor dew point?

Enter indoor temperature and relative humidity if you know them. Dew point explains the physics, but it does not tell us the actual temperature of your window glass.

Indoor temperature (°C)Relative humidity (%)

Reset answers

Start with the moisture location

Answer the observation questions to get a conservative next-step path.

The checker is intentionally more interested in where and when the water appears than in selling you a gadget.

This is homeowner decision support, not a window inspection, building-envelope assessment, HVAC commissioning report, mould assessment, or warranty determination.

Why condensation forms on the inside of windows

Condensation is not water leaking through intact glass. It is water vapour already in the indoor air changing into liquid water when that air meets a cold enough surface.

NRCan’s Keeping The Heat In: How your house works describes the dew point as the condition where air reaches 100% relative humidity and can no longer hold all of its water vapour. When indoor air contacts a cold window, the air immediately beside the glass cools. If it cools to its dew point, some vapour condenses on the window. If the surface is cold enough, the moisture can appear as frost.

That is why two homes at the same indoor temperature can behave very differently:

• one has lower indoor moisture and stays dry;
• one has higher indoor moisture and fogs every window;
• one has warmer high-performance glazing and tolerates more moisture;
• one has an older or locally cold window that condenses first.

The useful question is not simply “What humidity should my house be?” It is “Can my actual windows stay warm enough at today’s outdoor temperature for the moisture level I have indoors?”

Dew point without fake precision

The checker above can calculate indoor dew point from indoor temperature and relative humidity using the Alduchov-Eskridge Magnus approximation. That is an educational calculation, not a window-performance model.

If the checker says your indoor dew point is 7°C, it means a room-side surface around 7°C or colder can develop condensation under those indoor conditions. It does not mean ReadyHome knows the temperature of your glass, spacer, frame or wall.

We deliberately do not estimate a home’s window-surface temperature from generic glazing labels. Real surface temperature varies across the glass and frame and depends on the actual window, installation, air movement and outdoor conditions.

How much indoor humidity is too much in a Canadian winter?

There is no single number that works for every Canadian house all winter.

Health Canada recommends maintaining indoor relative humidity generally between 30% and 50% as part of controlling indoor dampness and mould. Its moisture and mould guide specifically includes condensation on cold surfaces such as window panes as a sign to address moisture and says a dehumidifier may be useful when condensation is occurring or RH is above 50%.

NRCan adds an important cold-weather wrinkle: the amount of humidity a window can tolerate before condensation depends strongly on outdoor temperature and window performance.

Its current Keeping The Heat In operating guide gives the following example limits at an indoor temperature of 21°C:

Do not treat that table as a prescription to drive your house to 10% RH. It is a condensation-performance illustration for particular window categories at 21°C. Your actual windows, frames, edge spacers, installation and airflow can behave differently.

NRCan’s practical advice is more useful than chasing a magic setpoint: occasional condensation is not automatically a problem, but excessive condensation or frosting is a sign to reduce moisture production or increase appropriate ventilation.

A better winter humidity rule

Use these signals together:

1. Measure indoor RH with a hygrometer if you have one.
2. Watch whether condensation is occasional or persistent.
3. Notice whether it affects one window, one room or most of the house.
4. Account for major outdoor temperature changes.
5. Adjust moisture production and ventilation when the house is telling you the current balance is too wet.

A Vancouver home at +5°C and a Winnipeg home at -30°C do not have the same condensation conditions. Neither do a 1970s double-glazed window and a modern triple-glazed unit.

What should I do first when the inside of my windows is wet?

Start with the reversible things that address indoor moisture and local surface temperature.

1. Use bathroom and kitchen exhaust where the moisture is created

Health Canada identifies showering, bathing, cooking and washing as common indoor moisture sources, especially when kitchen and bathroom fans do not work, do not vent outdoors or are not used.

Use the exhaust fan during the moisture-producing activity and make sure the system actually exhausts outdoors. A recirculating range hood can filter some grease or odour, but it does not remove the cooking moisture from the house.

2. Check the clothes dryer and indoor laundry drying

A clothes dryer should vent outdoors. Hanging wet laundry indoors can also raise indoor RH. If widespread window condensation is already telling you the house is carrying too much moisture, indoor clothes drying is an obvious source to reduce.

3. Open heavy blinds and curtains

This one is wonderfully counterintuitive.

Closing a thick curtain may make the room feel warmer, but it can isolate the glass from warm room air. NRCan says deep sills, closed drapes and blinds, and nearby furniture can block airflow and let cool air pool near the window.

Health Canada’s mould-prevention guidance is even more direct: keep window coverings open so warm air can reach the windows, keep baseboards or heating vents clear of furniture, and leave interior doors open where that helps airflow.

4. Dry standing water off the sill

Do not leave repeated runoff sitting on wood, drywall, caulking or finishes while you solve the larger humidity problem. Health Canada recommends drying window frames and sills so water does not keep feeding mould growth.

5. Check obvious moisture sources

Persistent whole-house humidity can be fed by more than showers. NRCan and Health Canada point to damp basements, plumbing leaks, exposed earth in crawlspaces, standing water, humidifiers, houseplants and other household moisture sources.

If the moisture problem is coming from a basement leak or seepage, a window treatment is not going to fix it. Use the water-damage hub to follow the actual source.

Will a dehumidifier stop window condensation?

Sometimes. But only if excess indoor humidity is actually the problem.

This matters because Canadian search results are full of people jumping straight from “wet window” to “buy dehumidifier.” That can be useful, irrelevant or completely wrong depending on the moisture location.

A dehumidifier can help when

• condensation is on the room-side surface;
• indoor RH is high for the current conditions;
• moisture is widespread rather than limited to one unusually cold window;
• source control and available ventilation are not enough;
• the dehumidifier is appropriate for the temperature of the room where it will operate.

NRCan’s dehumidifier guidance says the ideal RH for a building is generally 30% to 50%, with roughly 30% to 40% suggested during winter heating in colder climates to help prevent window condensation.

A dehumidifier will not fix

• condensation on the outside of the glass;
• moisture trapped inside a sealed glazing unit;
• wind-driven rain or a failed flashing detail;
• a single locally cold window when the rest of the home is already reasonably dry;
• a broken HRV, blocked exhaust fan or obvious moisture source that should be corrected instead.

If the actual problem is a damp basement or you need to choose equipment, continue to the basement dehumidifier guide. That page owns sizing, low-temperature operation, drainage and appliance selection.

Can a humidifier cause condensation on windows?

Absolutely.

NRCan’s current humidity guidance tells homeowners with a furnace humidifier to set it lower when indoor moisture is too high.

The important part is that the outdoor conditions change. A humidifier setting that causes no visible problem during mild November weather may be too aggressive during a January cold snap because the interior surface of the windows gets colder.

If room-side condensation suddenly appears across many windows while a central or portable humidifier is running:

1. reduce or pause added humidification;
2. use a hygrometer to see what the indoor RH is doing;
3. use kitchen and bathroom exhaust correctly;
4. check the windows again as outdoor temperature changes.

Do not blindly copy somebody else’s humidistat dial number. Their climate, house tightness and windows are not yours.

What about an HRV or ERV?

Mechanical ventilation can be central to the answer in a tight Canadian home, but “HRV good, ERV bad” is much too simplistic.

Health Canada’s Ventilation and the indoor environment says inadequate air exchange can contribute to high humidity and cold-weather window condensation. It also explains that increasing an HRV’s ventilation rate can reduce winter RH in a humid home because dry outdoor air dilutes indoor moisture.

But Health Canada also warns that an HRV can over-dry a house in winter. An ERV transfers both heat and some moisture and therefore tends not to dry the home as aggressively.

So the homeowner path is:

• If you have an HRV/ERV: make sure it is operating and maintained, then use its controls according to the manufacturer instructions and the conditions in the house.
• If condensation is widespread and persistent: ventilation performance is worth investigating.
• If the house is already very dry: more ventilation is not automatically better.
• If you are unsure how the system is balanced or controlled: have an HVAC/ventilation professional assess it instead of guessing at fan speeds or dampers.

Why do blinds and curtains make window condensation worse?

Because warm air has to reach the glass to help keep its room-side surface warmer.

NRCan notes that heat sources are often located beneath windows specifically because airflow across the glass helps reduce condensation. Close a heavy covering tightly and you can create a colder pocket between the covering and the window.

If one bedroom window is wet every morning but similar windows elsewhere are dry, try the boring experiment before buying anything:

• open or raise the covering;
• clear the baseboard or heating vent;
• move furniture away from the window;
• compare the result under similar outdoor conditions.

That does not prove the window is perfect. It tells you whether local airflow is an important part of the pattern.

Why did condensation start after I installed new windows?

New windows can make an old humidity problem more visible.

NRCan’s window-upgrade guidance says some homeowners install new windows and see worse condensation because the old leaky windows had been reducing indoor humidity through uncontrolled air leakage. The replacements seal the house more tightly, so indoor humidity rises unless moisture production and ventilation are managed.

That does not mean energy-efficient windows create water.

It means:

• the old windows may have been acting as accidental ventilation;
• the new windows reduced that uncontrolled air exchange;
• the house now needs its moisture and ventilation managed intentionally.

The same basic issue can occur in a new or substantially air-sealed home. Health Canada notes that tighter buildings depend more on effective mechanical ventilation for indoor air quality and moisture control.

Before blaming the glass, confirm that the moisture is truly on the room-side surface and check the whole-house pattern.

Condensation between window panes

This is where the exact window construction matters.

Sealed double- or triple-glazed unit

NRCan says that if condensation occurs inside a sealed double-glazed unit because the seal is broken, the best correction is generally to replace the glazing unit. It also recommends checking whether the window remains under warranty.

That means:

• room dehumidification does not repair the seal;
• drilling the glass is not ReadyHome’s homeowner fix;
• the sash or entire window does not automatically need replacement;
• a glazing professional can determine whether the insulated glass unit can be replaced within the existing assembly.

Older storm windows or non-sealed double-window systems

Different mechanism.

NRCan notes that condensation between panes of non-sealed glazing units or storm windows can result from moist house air leaking past the inner pane and condensing on the colder outer pane. Weatherstripping the inner sash and keeping designed storm-window drainage open can be part of that fix.

That distinction is why the checker says “likely glazing issue” instead of declaring every between-pane fogged window a failed sealed unit.

Why is there condensation on the outside of my windows?

Exterior condensation can look alarming precisely because it often shows up after somebody paid for better windows.

NRCan says low-e coatings reduce heat loss through high-performance windows. During a cool, calm, humid night, the outer glass can remain cold enough for condensation to form on its outside surface around sunrise.

If:

• the room-side surface is dry;
• the moisture is truly on the outdoor face;
• it appears under cool, humid conditions;
• it clears as the glass warms;

then there is usually nothing indoors to dehumidify.

I would not call exterior dew “proof” that every aspect of the window is perfect. It is simply a normal physical possibility with well-insulated low-e glazing.

Frost or ice on the inside of windows

Frost is condensation with a colder surface.

NRCan explains that if a window surface is extremely cold, window condensation can appear as frost. Its operating guidance treats excessive condensation or frosting as a sign to reduce moisture production or increase ventilation.

The pattern again matters:

Many windows frosting: investigate indoor moisture, added humidification and ventilation first.

One window frosting while similar windows remain dry: investigate local airflow, weatherstripping, glazing performance or installation.

Frost repeatedly melts onto wood or drywall: dry the affected materials and fix the recurring moisture balance before mould or material damage becomes the second problem.

Do not use frost as a licence to drive indoor humidity as low as possible. The target is a house that controls condensation without creating unnecessarily dry indoor conditions.

Window condensation vs a water leak

Condensation and water intrusion can both leave a wet sill, so the useful clues are timing and where the water first appears.

NRCan warns that a poorly installed window can cause condensation, drafts or allow water to leak into a home. If the moisture starts in trim, drywall or wall material instead of forming on the glass, do not assume humidity explains it.

If precipitation is the trigger, move to the water-damage hub rather than buying another moisture-control appliance.

What if mould is growing around the window?

Persistent condensation can keep window sills and nearby materials damp enough for mould to grow.

Health Canada says moisture problems can come from condensation on cold surfaces such as windows and emphasizes that unless the cause of the moisture problem is identified and solved, mould will reappear.

For a window-condensation problem, the order is:

1. correct the moisture source, humidity balance or airflow issue;
2. dry the sill and surrounding material;
3. clean visible mould using Health Canada’s appropriate procedure for the size and material;
4. escalate when growth is extensive, keeps returning, may be hidden, or the corrective action is unclear.

Use the mould-in-home guide for the actual cleanup and area decision. This page deliberately does not duplicate that guidance.

When does the window itself need repair or replacement?

Condensation does not automatically equal failed window.

Start thinking about the window or installation itself when the evidence is localized or physically tied to the assembly:

• persistent inaccessible moisture inside a sealed glazing unit;
• one window behaves much worse than comparable windows under the same conditions;
• damaged or missing weatherstripping is creating a real air leak;
• the frame, sash or seals are visibly deteriorated;
• water enters through the installation during rain;
• condensation remains severe even after reasonable indoor moisture and airflow issues are addressed.

Higher-performance windows can reduce condensation because their interior surfaces stay warmer. NRCan lists features such as triple glazing, low-e coatings, gas fills, thermally improved spacer bars and insulated frames as contributors to better energy performance and reduced condensation.

But replacing windows is an expensive answer to a diagnosis that may actually be too much indoor moisture. Figure out which problem you have before turning this into a renovation quote.

Frequently asked questions

Why do my windows have condensation every morning?

Overnight and early-morning conditions can combine higher room moisture with colder glass. Bedrooms can also have a local moisture load from occupants while doors and window coverings are closed. Start by confirming the moisture is on the room-side surface, then compare that room with the rest of the home.

Is condensation on the inside of windows bad?

A small amount occasionally is not automatically a building emergency. NRCan specifically says occasional condensation does not pose a problem. Persistent heavy condensation, pooling or frosting is more useful as a signal that moisture production, ventilation, airflow or window performance needs attention.

Should I wipe condensation off my windows?

Yes. Drying the glass, frame and sill protects finishes and reduces the time nearby materials stay wet. But wiping is symptom control. If water returns day after day, fix the moisture or cold-surface cause as well.

Will opening a window stop condensation?

Opening a window can lower indoor humidity in cold weather because it increases outdoor air exchange, but Health Canada notes that relying on window opening for ventilation can waste significant heating energy and makes humidity control harder. Use designed exhaust or mechanical ventilation when available rather than treating a cracked window as a permanent winter control system.

Can new windows cause condensation?

They do not create moisture, but NRCan says tighter replacement windows can reduce the uncontrolled air leakage that was previously lowering indoor humidity. If ventilation does not keep up, the indoor RH can rise and condensation can appear even though the new glass is more efficient.

Does condensation between panes mean I need new windows?

Not necessarily a whole new window. On a sealed insulated unit, a broken seal may be corrected by replacing the glazing unit, depending on the assembly and condition. Check the warranty and have a glazing professional assess the repair path.

Why is only one window wet?

One window behaving differently from similar windows points more strongly toward a local condition: blocked airflow, a colder room, damaged weatherstripping, glazing differences, installation details or a localized water path. Compare it with nearby windows before treating the whole house.

Should I buy a hygrometer?

It is useful and inexpensive context. Health Canada recommends a hygrometer for measuring indoor humidity. Just remember that RH alone does not diagnose exterior condensation, a failed sealed unit or rain intrusion.

Methodology

How ReadyHome built this guide: Topic scope was checked against Canadian search demand on September 10, 2026. Technical guidance was then checked against current Natural Resources Canada and Health Canada material. The diagnostic checker uses observable patterns rather than a numeric risk score. Its optional dew-point calculation uses the Alduchov-Eskridge Magnus approximation, but ReadyHome does not estimate glass-surface temperature or claim the tool can inspect a window.

Sources

• Natural Resources Canada: Condensation — interior/exterior condensation, window features, airflow and broken sealed-glass guidance.
• Natural Resources Canada: Keeping The Heat In — How your house works — dew point, condensation physics and indoor moisture sources.
• Natural Resources Canada: Keeping The Heat In — Upgrading windows and exterior doors — new-window tightness, sealed vs non-sealed between-pane condensation and glazing-unit replacement.
• Natural Resources Canada: Keeping The Heat In — Operating your house — winter humidity/condensation table, moisture reduction and ventilation guidance.
• Natural Resources Canada: Humidity levels — humidifier, exhaust, ventilation and moisture-control guidance.
• Natural Resources Canada: Choosing the right dehumidifier — dehumidifier role and general/winter RH context.
• Health Canada: Guide to addressing moisture and mould indoors — 30–50% RH guidance, window-condensation prevention, dehumidifier use and mould/moisture boundaries.
• Health Canada: Ventilation and the indoor environment — ventilation, winter condensation, HRV/ERV behaviour and over-drying caution.
• Alduchov & Eskridge (1996): Improved Magnus Form Approximation of Saturation Vapor Pressure — equation basis for the optional indoor dew-point calculation.