Window Replacement London Ontario: Avoiding Condensation Issues

03 April 2026

Condensation on windows looks simple enough, a little fog on the glass when it is cold, maybe a drip on the sill after a long shower. In practice, chronic condensation is a red flag. It tells you the building, the indoor air, and the window system are out of balance. If you are planning window replacement London Ontario homeowners wrestle with two goals at once: stop drafts and improve comfort, without creating a condensation problem that stains trim, feeds mould, or rots frames.

Over the past two decades working on window installation London Ontario wide, from mid-century bungalows in Glen Cairn to new infill in Wortley Village, I have seen everything from picture windows that rain on the inside at minus 15 to triple panes that never sweat, even behind heavy drapery. The difference is rarely one silver bullet. It is a combination of product choice, installation details, and indoor humidity control that suits our climate.
Why condensation forms, in plain language
Warm air holds more moisture than cold air. When warm, moist indoor air meets a cold surface, the air at the surface cools. As it cools, it cannot hold the same amount of moisture, so water vapour turns into liquid on the coldest part first. On a winter morning in London, when outdoor temperatures sit anywhere from minus 5 to minus 15 Celsius, your window glazing becomes the coldest exposed surface in the room. If the interior glass temperature dips below the dew point of the indoor air, you see fog, then droplets.

Two factors drive this: how much moisture is in your indoor air, and how warm the interior glass stays relative to the room. The second factor depends on the window’s thermal performance and on air movement around it. The first depends on ventilation, activities, and the building envelope. You have control over both, but only if you think about them before the window and door replacement London projects go ahead.
The London, Ontario context
Our city sits in a humid continental zone. Winters are cold enough to matter, often with long stretches below freezing, and summers are warm and sticky. That swing is hard on windows. In January, the indoor dew point might sit around 5 to 10 Celsius if your relative humidity is 30 to 35 percent, which is comfortable for most homes. If your interior glass drops to 0 to 3 Celsius, it will fog. In July, condensation appears on the outside of high performance windows after a cool night followed by a humid morning. That one is harmless, a sign the glass is doing its job holding cool inside air.

Because London homes range widely in age and construction, the right target for wintertime indoor humidity varies. A tight new build in Riverbend with spray foam and an HRV can run 35 percent RH at minus 5 Celsius without trouble. A 1920s Old North brick house, even after window replacement, may need to keep 25 to 30 percent RH during cold snaps to avoid dripping sills. That difference is not failure, it is physics meeting building reality.
What better windows actually do
A modern insulated glass unit creates a warmer interior glass surface, which raises the threshold at which condensation starts. Several design choices matter here.

Double pane vs. Triple pane. Triple pane glazing, with two low emissivity coatings and argon or krypton gas fill, can keep the interior glass 3 to 7 degrees warmer than a comparable double pane on a cold night. That margin often moves you from visible fog to a clear view, especially on large north-facing windows.

Low E coatings. The right low E stack bounces long-wave heat back into the room, keeping the interior glass warmer. Not all low E coatings are equal. In southern Ontario, low solar gain coatings on most orientations keep cooling bills down in summer. On south-facing glass with roof overhangs, a moderate solar gain coating can be justified to capture winter sun without overheating in July.

Warm edge spacers. The metal that separates the panes at the perimeter is a condensation magnet if it is aluminum. Modern stainless steel, composite, or foam spacers improve edge-of-glass temperatures. The difference shows up as dry corners on a January morning.

Frames and sashes. Fiberglass and high quality vinyl frames insulate far better than aluminum. Wood interior with aluminum-clad exterior performs well if the glazing and weatherstripping are up to current standards. Pay attention to thermal breaks in patio doors and garden door thresholds, which see different stresses.

When you compare products for window replacement London, check Canadian ratings you can trust. Energy Star in Canada and the CSA A440/A440.2 performance standards provide a baseline. Look at U-factor in W/m²·K and the Energy Rating value. In practice, for our climate, whole-window U-factors in the range of 1.1 to 1.6 W/m²·K perform well. Lower is better, but do not ignore air leakage ratings and hardware quality in the chase for a number.
Installation details that make or break condensation control
Even excellent glass can sweat if the installation creates cold spots or pathways for moist air. I have pulled interior casing off a new bow window that dripped every morning, only to find a gap at the head with no air seal, just a wad of fiberglass. Warm, moist air moved into that cavity and hit the back of cold sheathing. The sill was black within a season.

Several field-proven details reduce risk:

A continuous interior air seal. Closed-cell foam or sealant at the interior perimeter stops room air from getting into the rough opening. This is your primary condensation control at the junction.

Proper exterior water management. Back dams, sill pans, and correctly installed flashing tape keep water out of the opening. Water in a jamb cavity cools the frame and accelerates interior condensation.

The right foam. Low-expansion polyurethane foam insulates without bowing the frame. Over-expanding foam can pinch sashes, but the bigger risk is insufficient foam. Cold gaps create cold edges, and that is where droplets start.

Weep paths. Factory drainage should not get blocked by caulking or siding. If the weeps cannot drain, water sits at the coldest edge and telegraphs as interior sweating.

Integration with existing cladding. This is where siding companies London contractors add value. Reflashing and rewrapping the opening correctly around vinyl or fiber cement ensures wind washing does not chill the frame.

I have seen homeowners invest in premium triple panes only to skip the sill pan, then wonder why the bottom rail ices up. Conversely, I have replaced foggy builder-grade units in a 1990s Masonville home, installed double panes with warm-edge spacers and a meticulous air seal, and watched morning condensation disappear at the same indoor humidity.
Ventilation and humidity control after the upgrade
Better windows and tighter installations cut infiltration. That is good for comfort and bills, but it reduces the uncontrolled drying effect that older leaky houses had. If you do nothing else, your indoor humidity will rise in winter. That is why many window installation London Ontario projects are paired with ventilation adjustments.

Bathroom fans that actually move air to the outside matter. So does using them. https://zanekahr073.lowescouponn.com/steel-door-installation-london-ontario-insulation-and-threshold-basics https://zanekahr073.lowescouponn.com/steel-door-installation-london-ontario-insulation-and-threshold-basics A family of four can add several litres of moisture to indoor air on a winter evening from cooking, showers, and laundry. Run the fans during and 15 minutes after each shower. Use the range hood when boiling water, ideally vented outdoors. If your home has a heat recovery ventilator, set it to run on a schedule during cold snaps. Portable dehumidifiers help in basements, but whole-house control is better if you can manage it.

Window coverings also play a role. Tight-fitting blinds and heavy draperies trap air against the glass. In January that small pocket becomes very cold. Crack the blinds or raise them an inch to encourage a gentle wash of room air on the glass. In one Byron two-storey we serviced, simply lifting cellular shades overnight cut the morning drip line in half.

If you need a simple winter target for indoor relative humidity, use outdoor temperature as your guide:

Around 0 to minus 5 Celsius outside, 35 to 40 percent RH indoors usually stays safe for most double or triple pane windows.

Around minus 10 to minus 15, stay near 30 percent, dropping to 25 percent if you notice persistent edge condensation.

Colder than minus 20, which we see occasionally, trim to the low 20s temporarily, then bring it back up when the cold snap passes.

These are guidelines, not absolute rules. Watch your own glass and sills. If the bottom rail glistens before breakfast, act. You are protecting your trim, your paint, and your indoor air quality.
Recognizing the kind of condensation you are dealing with
Not all fog is equal. Surface condensation on the room side of the inner pane is the most common. That is the kind you can wipe with a finger. It responds to humidity control, warmer glass, or better air circulation.

Condensation between panes indicates a failed insulated glass unit seal. No amount of dehumidifying will fix it. The desiccant in the spacer has saturated, and moisture is in the cavity. On older london ontario windows that have served 15 to 25 years, this is a frequent reason for replacement. Sometimes you can replace just the sealed unit, but if the frame is warped or the sash is out of square, a full window replacement London is the better long-term play.

Exterior condensation forms in spring and fall mornings on high performance glass when the exterior pane is colder than the dew point of the outdoor air. It vanishes as the sun hits the glass. Consider it a compliment to your window’s insulating value. Do not confuse it with interior moisture problems.
Special cases: large windows, bay and bow, and patio doors
Big glass pushes the limits. A 6 by 6 picture window on the north elevation presents far more cold area than a casement half that size. Triple pane makes a noticeable difference here. On bay and bow windows that project outside the insulated wall line, use insulated head and seat boards, and include a thermal break under the seat. We add a smart vapor retarder at the interior and treat the projection as a mini roof with proper flashing. Otherwise, the interior pane of the flankers becomes the coldest surface in the room.

Sliding patio doors are notorious for edge-of-glass condensation along their bottom rail, especially in older vinyl units with metal-reinforced sills. Modern patio door installation with thermally broken thresholds, warm-edge spacers, and careful air sealing changes that picture. In a White Oaks semi we updated last winter, moving from a rattly aluminum slider to a well-made triple pane patio door eliminated the habitual puddle that formed after every deep freeze.

For homeowners considering steel doors London Ontario options, remember that the panel’s foam core and thermal breaks determine interior surface temperature as much as the skin. A steel door installation London Ontario done right includes an insulated jamb, continuous sill pan, and weatherstripping that seals without over-compressing. Metal is unforgiving. Any thermal bridge at the frame shows up as a cold stripe that sweats.
Sequencing work when multiple trades are involved
Window and doors London Ontario projects often happen alongside exterior updates. If you are recladding, coordinate with siding companies London contractors so the window flashing ties into the new weather barrier correctly. The best sequence I have found:

Set and seal the windows and doors first with temporary head flashings and sill pans.

Bring in the siding crew to integrate housewrap and tapes with the window flanges and trim, then install exterior casings.

Finish interior air sealing, insulation, and casing once the exterior is watertight.

This order keeps the opening protected and gives each trade a clean handoff. Backward sequencing leads to cut housewrap at corners, missing head flashings, and wind washing around frames, all of which cool the interior glass edge.
Matching products to house types around London
Old North and Woodfield homes often have deep walls, brick or plaster interiors, and heritage proportions. When we handle window and door replacement London on these, we pay attention to jamb depth, interior stool details, and sightlines. A slimline fiberglass or wood-clad unit preserves character while giving you double or triple pane performance. Because these homes breathe differently, we keep an eye on winter RH and recommend an HRV if original chimneys and vents were sealed during other renovations.

Suburban builds from the 1990s and 2000s across Masonville, Stoney Creek, and Westmount typically came with builder-grade vinyl windows. The frames can still be sound while the sealed units fail. If the budget is tight, glass-only replacements improve performance modestly. If air leakage, hardware, or warping is an issue, full-frame replacement with warm-edge double pane or triple pane pays back in comfort. These homes often benefit from modest HVAC balancing after new windows reduce draft load.

Newer infill and custom builds already have decent envelopes. Here the margins are in choosing the right glazing on different orientations, verifying professional installation, and setting up mechanical ventilation to match the now tighter house. You will also be less tolerant of exterior condensation in spring, so setting expectations matters.
Practical habits that reduce interior condensation
No product or installer can compensate for a few daily habits that flood a house with moisture.

Keep the dryer vent clear and truly terminating outside. A loose hose in a basement is a recipe for wet windows.

Cover pots when boiling, and run a vented range hood. Recirculating hoods help with odours but not with moisture.

Do not shut your blinds completely on the coldest nights. Leave a small gap to let room air bathe the glass.

Pull furniture and plants a few inches off exterior walls and window sills. Still pockets of humid air condense first.

If your home has a humidifier, set it to an outdoor-reset mode or turn it down during cold snaps.

These changes cost little and prevent most day-to-day fogging that worries homeowners after a successful window replacement.
When condensation is a symptom of something bigger
Sometimes window condensation points to a deeper building issue. I once visited a south London split-level with new london windows and doors. Every winter, frost bloomed along the base of the living room windows. The culprit was not the glass. A disconnected bath fan dumped moist air into the attic above, saturating the insulation at the exterior wall. The cold sank down the stud bays behind the drywall. The window frames were simply the visible edge of a cold, moist wall. We fixed the fan, replaced the wet insulation, improved attic ventilation, and the window issue vanished.

Wet basements produce similar patterns. If your lower-level sliders fog worse than upstairs, or if the basement smells musty, solve that first. Address exterior grading, downspout extensions, and sump performance. A dehumidifier can hold the line, but you want to stop the source. Windows respond to the environment they are in. Make that environment stable.
Costs, choices, and honest trade-offs
Triple pane units add cost and weight. Hardware needs to be up to the task, especially on large casements and patio doors. Frames must be plumb and level to avoid long-term hinge stress. If you run 30 percent indoor RH and your home sees long stretches below minus 10, triple pane often pays back in avoided condensation, quieter rooms, and lower heating costs. If your home spends most of winter around 0 to minus 5 and you can manage RH with ventilation, a well-specified double pane with warm-edge spacers may be enough.

Argon gas fills are standard and help. Krypton is more expensive and mainly reserved for tight air spaces or very high performance goals. Laminated interior glass adds a layer that marginally warms the interior surface, while also boosting sound control and security. It is a nice upgrade on street-facing rooms downtown.

On doors, a well-insulated steel slab with a high-quality frame and sill system can perform as well as a fiberglass unit, at a friendlier price. For steel door installation London Ontario homeowners should look past the slab gauge and ask about the foam core, thermal breaks, sill pan, and the installer’s air sealing practices.
Choosing the right partner
There are many london window and door companies, and a wide range of product lines. Reputation matters, but so does the way a contractor talks about humidity, ventilation, and installation details. If a sales pitch dodges the condensation question or blames it entirely on your lifestyle, keep asking. A pro will discuss glass packages, spacers, frame materials, and the exact air sealing approach for your walls. They will also give you a straight answer about interior RH targets and how to control them after the work is done.

If your project ties into exterior upgrades, coordinate with siding companies London trades that understand flashing, not just finishes. Ask to see a sill pan on site, not just a line item on a quote. The difference shows up years later in dry, clean window corners instead of stained drywall.
A short pre-installation checklist
Before you sign for window replacement London Ontario or patio door installation, confirm these points:

Product specifications list whole-window U-factor, glass coatings, gas fill, and spacer type, not just brand names.

Installation includes a formed or fabricated sill pan, interior air seal, low-expansion foam, and integrated flashing.

Ventilation strategy post-install is clear, with bathroom fans confirmed to exhaust outdoors and HRV settings reviewed if present.

Window coverings and furniture placement around large units are discussed to prevent trapped cold air pockets.

Warranty covers sealed unit failure and installation workmanship, with realistic service timelines.
What success looks like
After a proper london ontario windows and doors upgrade, your winter mornings should be quiet and clear. The glass remains free of sweat except during showers or dinner rush, and even then it dries within minutes. Trim stays bright, paint holds, and the bottom rail of that big north-facing picture window does not sparkle with ice when the forecast dips. You are not micromanaging a hygrometer hourly, just following a few good habits and letting the building systems you paid for do their work.

That outcome is not luck. It is the sum of smart product choices, careful installation, and a plan for indoor humidity that fits the way you live. Done right, window replacement London Ontario projects make homes warmer, healthier, and easier to enjoy through every season we get here, from the lake-effect flurries to the dog days of August.

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Address: 3392 Wonderland Rd S, London, ON N6L 1A8, Canada

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https://mccallumaluminum.on.ca/

McCallum Aluminum Ltd is a quality-driven window and door installation company serving the London Ontario region. 
For door replacement in London, Ontario, contact McCallum Aluminum Ltd at (519) 433-4223 or visit https://mccallumaluminum.on.ca/. 
McCallum Aluminum Ltd provides quality-driven service for windows, helping homeowners improve comfort across nearby communities. 
To find McCallum Aluminum Ltd on Google Maps, use: https://www.google.com/maps?cid=10246687099425416717. 
Looking for a community-oriented installer near you? Call (519) 433-4223 and learn more at https://mccallumaluminum.on.ca/.

<h2>Popular Questions About McCallum Aluminum Ltd</h2>

What does McCallum Aluminum Ltd specialize in? 
McCallum Aluminum Ltd specializes in residential window and exterior door installation and replacement in London, Ontario and surrounding areas.

Where is McCallum Aluminum Ltd located? 
3392 Wonderland Rd S, London, ON N6L 1A8, Canada. Google Maps: https://www.google.com/maps?cid=10246687099425416717

What areas do you serve? 
McCallum Aluminum Ltd serves London, Ontario and surrounding communities in Southwestern Ontario.

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How do I request a quote or estimate? 
Call +1 (519) 433-4223 tel:+15194334223 or visit https://mccallumaluminum.on.ca/ and use the contact form.

Do you install patio doors and entry doors? 
Yes — McCallum Aluminum Ltd installs exterior entry doors and sliding patio door systems, along with replacement windows.

How can I contact McCallum Aluminum Ltd? 
Phone: +1 (519) 433-4223 tel:+15194334223 
Email: inquiries@mccallumaluminum.on.ca mailto:inquiries@mccallumaluminum.on.ca 
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<h2>Landmarks Near London, Ontario</h2>

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