You wake up on a cold Ontario morning and notice water collecting along the bottom of your windows.

Sometimes the glass is foggy.

Sometimes there are actual droplets running toward the frame.

And on extremely cold days, you may even see frost.

The first question most homeowners ask is:

Why are my windows sweating?

Window condensation is common during Canadian winters, but where the condensation appears and how often it happens can tell you very different things about your home.

Sometimes the problem is simply excess indoor humidity.

Sometimes the window surface is getting extremely cold.

Sometimes air leakage or poor installation is involved.

And if moisture is trapped between two sealed panes of glass, the insulated glass unit may have failed.

Natural Resources Canada explains that winter condensation commonly occurs when warm, moisture-containing indoor air contacts a colder window surface. As the air cools, it can no longer hold the same amount of water vapour, causing moisture to condense on the glass.

Here’s how to tell what your windows may be trying to tell you.

Why Does Condensation Form on Windows in Winter?

Warm indoor air contains water vapour.

During an Ontario winter, the inside surface of a window can be significantly colder than the surrounding room.

When humid indoor air reaches that cold surface, its temperature drops.

Once the air cools enough, some of its water vapour changes into liquid water.

That’s condensation.

It’s the same basic reason moisture appears on the outside of a cold drink during summer.

The difference is that repeated condensation around windows can affect materials surrounding the glass.

Natural Resources Canada notes that if the window becomes cold enough, condensation can even freeze and appear as frost.

So the presence of condensation does not automatically mean your window is defective.

You need to determine where it is forming and what other symptoms are present.

First: Where Is the Condensation?

This is the easiest way to begin diagnosing the issue.

There are three very different situations:

1. Condensation on the Inside Surface

Moisture is on the glass you can touch from inside the home.

This is usually related to the combination of:

• Indoor humidity
• Cold glass temperature
• Outdoor temperature
• Air movement around the window

2. Condensation Between the Panes

The fog or moisture appears inside the sealed glass unit, where you cannot wipe it away.

This can indicate a failed seal.

Natural Resources Canada states that when condensation develops inside a sealed double-glazed unit because the seal has broken, the problem is generally corrected by replacing the glazing unit.

3. Condensation Around the Frame

Water may appear near:

• Window edges
• Frames
• Trim
• Sills

This deserves additional investigation because it may involve surface temperature, air leakage, installation details, or indoor humidity.

Poorly installed windows can contribute to condensation, drafts, and even water leakage.

These three situations should not be treated as the same problem.

1. Your Indoor Humidity May Be Too High

One of the most common causes of condensation on the inside of windows is simply too much moisture inside the home for the current outdoor temperature.

Everyday activities add moisture to indoor air.

Examples include:

• Showering
• Cooking
• Boiling water
• Drying laundry indoors
• Humidifiers
• Houseplants
• Normal breathing

Modern homes can also hold moisture more effectively because they tend to be more airtight than older homes.

Natural Resources Canada notes that moisture can build up inside tighter homes, particularly during winter when windows remain closed.

When outside temperatures fall, window surfaces become colder.

That means a humidity level that caused no problem in October might suddenly produce condensation during a cold January morning.

What Indoor Humidity Should You Aim For?

There isn’t one perfect relative-humidity level for every home and every outdoor temperature.

As temperatures outside become colder, the amount of indoor humidity that can be maintained without window condensation generally decreases.

Natural Resources Canada provides winter humidity guidance and notes that relative humidity around 30% to 40% is often appropriate during heating season, although conditions may require adjustment to prevent condensation.

The practical rule is:

If your windows are repeatedly wet, indoor humidity deserves attention.

A simple digital hygrometer can help you monitor relative humidity inside the home.

2. Your Windows May Be Getting Too Cold

Even when indoor humidity is reasonable, older or less efficient windows can have colder interior surfaces.

The colder the glass becomes, the easier it is for moisture to condense.

Window performance can be influenced by:

• Number of glass panes
• Insulating glass design
• Low-emissivity coatings
• Gas fill
• Spacer system
• Frame construction
• Weatherstripping
• Installation quality

Natural Resources Canada notes that higher-performance windows maintain warmer interior surfaces and are generally less prone to condensation than lower-performance windows.

Envirotech’s current replacement-window systems use insulated glass packages, Low-E glass, argon gas options, and a non-metal spacer system designed to reduce heat transfer at the edge of the glass.

This doesn’t mean:

“Condensation = buy new windows.”

But window performance should become part of the discussion when condensation is combined with other signs of deterioration.

3. Your Window May Be Drafty

Stand near the window during cold weather.

Do you feel noticeably colder air?

Drafts can occur around:

• Window sashes
• Frames
• Weatherstripping
• Trim
• The rough opening around the unit

Natural Resources Canada identifies cold drafts as one potential sign of poor window performance or installation.

Air leakage can affect comfort even when the glass itself is still intact.

You may notice:

• One room always feels colder
• Curtains move slightly near the frame
• Cold air near the sill
• Frost forming around specific edges
• Heating equipment working harder to maintain comfort

Sometimes weatherstripping or air sealing may be enough.

Other situations may point toward more significant window deterioration or installation issues.

The important thing is to identify where the air is actually entering.

4. Condensation Between Window Panes Is Different

This is one of the clearest signs homeowners should understand.

If you see fog or moisture between the panes of a double- or triple-glazed window, and you cannot wipe it away from either surface, the sealed glass unit may have lost its seal.

An insulated glass unit is designed to maintain a sealed space between panes.

Once the seal fails, moisture can enter that space.

Natural Resources Canada specifically states that condensation inside a sealed double-glazed unit generally indicates a failed seal and recommends replacing the glazing unit.

Common appearance:

• Persistent fogging
• Haze between panes
• Water droplets trapped inside
• Condensation that never wipes away

This is not the same as indoor humidity causing moisture on the room-side surface.

Turning down your humidifier will not repair a broken glass seal.

Do You Need to Replace the Entire Window?

Not necessarily.

If only the insulated glass unit has failed but the frame and surrounding window remain in excellent condition, replacement of the glazing unit may sometimes be possible.

However, full-window replacement may make more sense when other problems are present, such as:

• Deteriorated frames
• Repeated seal failures
• Air leakage
• Warping
• Difficult operation
• Poor energy performance
• Water intrusion
• Multiple failing windows

The correct decision should consider the whole window, not one symptom.

5. Poor Installation Can Cause Window Problems

Even a high-quality window can perform poorly if it is not installed correctly.

Natural Resources Canada warns that poor installation may contribute to:

• Condensation
• Cold drafts
• Water leakage
• Damage surrounding the window

That’s why replacement windows should not be judged only by the glass package.

Installation details also matter.

A window must integrate properly with:

• The wall opening
• Air barriers
• Flashing
• Exterior cladding
• Interior finishes
• Insulation around the frame

Buying a better window but installing it poorly defeats much of the purpose.

6. Air Circulation Around the Window Matters

Look at what’s happening around the glass.

Is the window covered by:

• Heavy curtains?
• Closed blinds?
• Furniture?
• Deep window coverings?

Restricting warm room air from reaching the interior glass can make the surface colder.

That can increase the likelihood of condensation under certain conditions.

During periods of heavy winter condensation, try:

• Opening blinds
• Leaving curtains slightly open
• Allowing warm air to circulate near the window
• Avoiding furniture that completely blocks heat registers near windows

This may help reduce surface moisture, although it will not fix failed glass seals or structural window problems.

7. Kitchens and Bathrooms Often Show Condensation First

Not every room produces the same amount of moisture.

Bathrooms and kitchens frequently have higher humidity because of:

• Showers
• Baths
• Cooking
• Dishwashers
• Boiling water

Natural Resources Canada notes that window condensation is particularly likely in humid areas such as kitchens and bathrooms.

Make sure exhaust fans:

• Work properly
• Vent outdoors
• Are used during moisture-producing activities

Managing moisture at the source can make a significant difference.

Is a Little Window Condensation Normal?

Occasional light condensation during extremely cold weather does not necessarily mean your windows need replacing.

For example:

If the temperature drops dramatically overnight and you see a small amount of condensation at the bottom of the glass the next morning, indoor humidity and outdoor temperature may simply have reached the condensation point.

More concerning patterns include:

• Condensation every day
• Water pooling on the sill
• Persistent frost
• Wet drywall or trim
• Peeling paint
• Mould around the frame
• Fog trapped between panes
• Strong drafts
• Water entering during rain

These suggest that the issue deserves more attention.

Can Window Condensation Cause Damage?

Repeated moisture should not simply be ignored.

Condensation can wet:

• Wood frames
• Window trim
• Drywall
• Paint
• Sills
• Surrounding finishes

CMHC has noted that severe window condensation can contribute to deterioration of adjacent building materials and fungal growth.

If you regularly find water pooling along the sill, wipe it up and address the underlying moisture issue.

The goal is not simply to keep cleaning the window every morning.

It is to understand why so much moisture is forming.

Does Condensation Mean I Have Mould?

Not automatically.

Condensation is moisture.

Mould requires suitable moisture conditions and other factors to grow.

However, repeated wetting around windows can create conditions that deserve attention.

Health Canada recommends addressing persistent moisture and mould problems rather than allowing materials to remain damp.

If you see visible mould growth around a window, identify and correct the moisture source rather than simply painting over it.

How to Reduce Condensation on Windows in Winter

Start with the simplest steps.

1. Monitor Indoor Humidity

Use a hygrometer.

If humidity is consistently high and windows are wet, reduce indoor moisture.

2. Use Bathroom Exhaust Fans

Run the fan during showers and allow it to continue afterward.

3. Use Your Kitchen Exhaust

Cooking can release substantial moisture into indoor air.

Vent it outdoors where your system allows.

4. Adjust Your Humidifier

If you use a whole-home or portable humidifier, reduce the setting during colder weather when condensation appears.

5. Improve Air Circulation

Open blinds and curtains periodically so room air reaches the glass.

6. Check Weatherstripping

Damaged or missing seals may contribute to cold drafts.

7. Look for Air Leaks

Check around the frame rather than focusing only on the glass.

8. Inspect the Glass

Determine whether moisture is:

on the surface

or

between the panes.

That distinction changes the solution.

When Is Window Replacement Worth Considering?

Window replacement becomes more reasonable when condensation is only one part of a broader performance problem.

Consider an assessment if you have several of these symptoms:

Condensation Between the Panes

A likely sealed-unit issue.

Persistent Cold Drafts

Especially when weatherstripping or basic air sealing does not solve them.

Windows Are Difficult to Operate

They stick, bind, or no longer close properly.

Frames Are Warped or Damaged

Physical deterioration can compromise performance.

Water Is Entering the Home

Rainwater infiltration should be investigated promptly.

Multiple Windows Are Failing

Repeated isolated repairs may become less practical.

The Windows Are Older and Perform Poorly

Newer ENERGY STAR-certified windows are designed for higher energy performance. Natural Resources Canada currently states that ENERGY STAR-certified windows are about 20% more energy efficient than an average window.

What Makes an Energy-Efficient Window?

When comparing replacement windows, don’t focus only on whether they are double or triple glazed.

Look at the complete assembly.

Important components include:

Low-E Glass

Low-emissivity coatings help reduce heat transfer through the glazing.

Insulating Gas

Argon or other gases can be used between panes to improve thermal performance.

Spacer System

The material separating the panes affects heat transfer around the glass edge.

Frame Construction

Frames and sashes also influence thermal performance.

Weatherstripping

Proper seals help limit unwanted air leakage.

Installation

A high-performance unit still needs proper installation.

Natural Resources Canada recommends selecting high-performance ENERGY STAR-certified windows, doors, and skylights as one way to improve energy efficiency and reduce condensation potential.

Double-Pane vs. Triple-Pane Windows

More panes can improve thermal resistance, but the number of panes is only one part of window performance.

A quality window may combine:

• Double or triple glazing
• Low-E coatings
• Insulating gas
• High-performance spacers
• Efficient frames

Envirotech currently offers both double- and triple-glazed options within its insulated glass packages.

The correct choice depends on:

• Home location
• Window orientation
• Existing wall construction
• Comfort goals
• Budget
• Desired energy performance

Instead of automatically choosing the option with the most glass, compare the overall performance specifications.

What About Window Condensation After Installing New Windows?

This can surprise homeowners.

They replace old windows and suddenly notice condensation.

Does that mean the new windows are bad?

Not necessarily.

Natural Resources Canada explains that new windows can make a home more airtight. If indoor moisture levels remain high, the reduced uncontrolled air leakage can allow indoor humidity to rise.

In that situation, the solution may be improved humidity control or ventilation rather than replacing the new windows.

This is another reason condensation needs diagnosis rather than assumptions.

Is Frost on the Inside of a Window Normal?

Frost means the interior surface has become cold enough for condensed moisture to freeze.

Natural Resources Canada notes that extremely cold window surfaces can result in frost rather than liquid condensation.

A tiny amount during unusually cold weather may occur.

Frequent or heavy frost deserves investigation, particularly when combined with:

• Strong drafts
• Older single- or double-pane windows
• Damaged seals
• High indoor humidity
• Deteriorated weatherstripping

Why Is Only One Window Condensing?

This can provide a useful clue.

If every window in the house has condensation, indoor humidity may be a major factor.

If only one or two windows have persistent problems, investigate those individual units.

Possible differences include:

• Orientation
• Shade
• Air circulation
• Window age
• Installation
• Seal condition
• Glass package
• Nearby humidity sources

For example, a bathroom window may behave differently from a living-room window simply because the room regularly contains more moisture.

Why Are My Windows Wet at the Bottom?

The lower edge of the glass often becomes one of the colder portions of a window.

Moisture can also run downward after forming elsewhere on the pane.

That’s why homeowners frequently first notice:

• Droplets along the bottom
• Dampness on the sill
• Moisture near the glass edge

Window spacer technology is one factor that affects temperature around the perimeter of an insulated glass unit.

Envirotech’s current Regency window offering uses a non-metal Super Spacer system designed to reduce conductive heat transfer around the glass edge.

Repair or Replace: A Quick Guide

What You See	What to Investigate
Light condensation on interior glass	Indoor humidity + surface temperature
Heavy condensation across many windows	Humidity + ventilation
Draft around one window	Weatherstripping / installation / window condition
Fog trapped between panes	Sealed glass unit
Water entering during rain	Installation / flashing / exterior seal
Warped or damaged frame	Repair vs. replacement
Multiple old failing windows	Replacement assessment

The answer should be based on the cause, not the appearance alone.

7 Questions to Ask Before Buying Replacement Windows

If replacement does become the right option, ask:

1. Are the windows ENERGY STAR certified?

Compare actual performance rather than relying only on marketing language.

2. What glass package is included?

Ask about:

• Double vs. triple glazing
• Low-E coatings
• Gas fill

3. What spacer system is used?

This influences performance around the glass edge.

4. What frame construction is used?

The glass isn’t the entire window.

5. How will the opening be insulated and sealed?

Installation matters.

6. What warranty applies?

Ask about glass, frame, hardware, and installation coverage separately.

7. What exactly is included in the estimate?

Confirm removal, installation, finishing, and disposal.

Replacement Windows From Envirotech Exteriors

Envirotech Exteriors currently installs replacement windows in Burlington, Hamilton, Niagara, Kitchener, Waterloo, and Guelph. Available styles include casement, single- and double-hung, slider, awning, bay/bow, and basement/hopper windows.

Current window features include options for:

• Double or triple glazing
• Low-E glass
• Argon-filled insulated glass
• Non-metal spacer technology
• Fusion-welded vinyl frames
• Multiple window styles and colours

If your windows are repeatedly:

Fogging.
Drafting.
Leaking.
Freezing.
Or trapping moisture between panes,

don’t automatically assume the only solution is replacement.

First determine:

Is the issue indoor humidity?

Is the window surface simply very cold?

Is the weatherstripping failing?

Has the sealed glass unit failed?

Is the frame damaged?

Or has the complete window reached the point where replacement makes more sense?

Diagnose the Condensation Before Replacing the Window.

If you’re dealing with persistent window condensation, drafts, failed seals, or aging windows, request a free window estimate from Envirotech Exteriors.

Frequently Asked Questions

Why do my windows have condensation in winter?

Winter condensation commonly forms when warm, humid indoor air comes into contact with a colder window surface. Lower outdoor temperatures, indoor humidity, and window performance all influence how much condensation develops.

Does condensation mean my windows need replacing?

Not necessarily. Condensation on the interior surface may primarily indicate high indoor humidity or a cold glass surface. Condensation trapped between panes is more suggestive of a failed sealed glass unit.

Why is there fog between my double-pane windows?

Fogging or condensation between panes can occur when the seal of the insulated glass unit fails and moisture enters the sealed space. Natural Resources Canada recommends replacing the glazing unit when this occurs.

How do I stop my windows from sweating?

Start by monitoring and controlling indoor humidity, using kitchen and bathroom exhaust fans, improving airflow around the window, and checking for drafts or damaged weatherstripping. If the problem persists, have the window assessed.

Can new windows reduce condensation?

Higher-performance windows generally have warmer interior glass surfaces and are less prone to condensation under comparable indoor conditions. Natural Resources Canada recommends ENERGY STAR-certified windows as one way to improve energy efficiency and condensation resistance.

Why do my new windows have condensation?

New windows can make the building envelope more airtight. Natural Resources Canada notes that this can allow indoor humidity to rise if moisture and ventilation are not properly managed.

Should I replace a window if it is drafty?

Not automatically. Weatherstripping, caulking, or installation-related issues may sometimes be repairable. Replacement becomes more reasonable when drafts occur alongside physical deterioration, poor performance, failed glass units, or recurring problems.

Where does Envirotech install replacement windows?

Envirotech currently provides window and door installation in Burlington, Hamilton, Niagara, Kitchener, Waterloo, and Guelph.