Foggy thermos · Montreal
Fog inside your window thermos? Often only the glass needs changing
In a Montreal plex, the sealed double-glass unit in each window is what everyone calls the thermos. When fog sits between its two panes and won't wipe off, the edge seal has let go. The frame is often fine, and a new thermos made to measure can go into the window you already have. We connect you with an independent local glass professional who tells you plainly whether new glass will do.
- Glass-only replacement when the frame is sound
- Défogging, a new thermos and a new window compared evenly
- Plain answers on RénoPlex and Rénoclimat
Which side of the glass?
Fog between the panes or on the glass: two different problems
Wipe the pane from inside with a dry cloth: what comes off belongs to the flat, what stays belongs to the thermos. If it turns out to be something else, the windows overview sorts problems by symptom.
On the room side: humidity in the flat
A flat with cooking, showers and laundry drying indoors holds a lot of moisture, and on a cold Montreal morning it settles on the coldest glass, usually low on a bedroom window, where it feeds mould on the sill. Écohabitation recommends keeping winter humidity moderate; ventilation helps more than new glass.
Between the panes: a broken edge seal
Fog or a white haze inside the unit means a point in the seal has opened. At first it comes and goes with the weather; later it leaves streaks that never clear, and it won't dry out by itself.
Why it happens
Why a thermos eventually fogs
A thermos is two panes held apart by a spacer, the intercalaire, with a sealed, dry space between them. A drying agent in the spacer soaks up stray moisture until it can hold no more.
Montreal winters test that seal. The heating season is long, the temperature crosses freezing on many days, and freezing rain comes every winter. Each swing makes the glass and trapped air expand and contract until a point in the seal gives way. Water left standing in the bottom of the frame adds to the strain.
When one thermos in a flat fogs, look at the others from the same round of work: units installed together tend to age together.
Three options
Défogging, a new thermos or a new window
Montreal's glass trade tends to split into camps, each selling its own answer. Here are the three side by side; the pro we match you with confirms which fits.
Défogging the existing thermos
Small holes are drilled in the glass, the space is cleaned and dried, and the holes are sealed. It can improve the view on a unit that isn't badly etched, but it doesn't restore the factory seal or any insulating gas. Ask what is covered, and for how long.
A new thermos in the existing window
Frame, sash, hardware and trim stay; a new unit is made to size and set in. The usual fix when the frame is sound and only a few units have failed.
A new window
When the frame is soft or air leaks around the edges, new glass is money in the wrong place. See replacing windows in Montreal, borough permit first.
Glass only
Replacing only the thermos: when it's the right call
A glass-only swap makes sense more often than people expect.
The frame is solid
Wood that stays firm when pressed, PVC or aluminum without cracks, a sash that closes tight. A sound frame deserves new glass, not a new window.
Only some units have fogged
Two fogged units in a flat is a glass job. Every window going at once is a different conversation.
You want to keep the look
Where your borough expects the original appearance, changing only the glass is often the simplest path. Ask the borough whether your work needs a permit first.
Frame first
When the frame or sash means replacing the window
Glass needs something sound to sit in. If the sill is soft, the sash has twisted, or cold air comes in around the frame rather than through the glass, a new thermos only postpones the decision. The same goes when several windows and their hardware are failing at once. Our guide to repairing the glass or replacing the window takes the questions in order.
What affects the quote
What changes the price of a thermos
Each thermos is made to measure, so the price depends on:
- Size of the glass, and any arched, angled or oversized shape
- Glass type: low-E coating, tint, or tempered or laminated glass where the code calls for it
- How the unit is held: glazing stops, a sash that has to come out, or a door panel
- The floor, and whether the pro can work from a balcony or gallery or only from inside
- How many units are replaced on the same visit
Photos and rough dimensions let a pro give a first idea before measuring.
Where thermos units sit
Thermos units in casement, guillotine, patio-door and transom glass
In a casement (battant), the thermos sits in the opening sash; the casement windows page covers the crank side. Each guillotine sash carries its own unit, though older wood guillotines may still have single glass.
The large units in a porte-patio can be replaced alone when the door still slides and locks well; if not, see patio door repair. The imposte above a plex door fogs too, and shaped units are measured on site.
Subsidies
Subsidies: why a thermos alone doesn't qualify
Both programs that help with windows here are built around replacing the whole window or door. Québec's Rénoclimat doors and windows grant states that replacing only the glass or the sash does not qualify. The City's RénoPlex program pays a set amount per window, door or patio door replaced on eligible small plexes, with approval required before work starts. Neither lists a thermos-only swap.
A new thermos is still often the sensible repair; our subsidy guide explains who qualifies for what.
Before the soumission
Preparing your quote: measurements, photos, floor
A good photo saves a visit: the whole window, a close-up of a corner, and one with a common object against the pane for scale. Add rough dimensions, the floor, and whether a balcony or gallery gives access.
Settle the details at the soumission, not after: intercalaire colour, low-E coating or tint, and matching the neighbouring units. Ask for the warranty terms in writing.
Questions
Foggy thermos questions
Explore your options
Other window problems we can help with
Local glass pros replace window thermos units across the island and nearby, including Rosemont–La Petite-Patrie, Villeray, Laval, the West Island and Longueuil.
Send a photo of the foggy thermos
Hold something against the glass for scale and tell us the floor. We pass it to an independent glazier who replaces thermos units and comes back to you with a soumission.