Swap only the glass unit when the frame is sound and one or two units fog. Replace the whole window when the frame is rotted or warped, the hardware is worn or several units have failed.78 A glass-unit swap runs about 0 to 0 per unit in Montreal-area price ranges, and a glazier does it.45 Rénoclimat pays 0 only for a new ENERGY STAR Most Efficient window, never for glass alone.2 These are market ranges, not Factory Direct prices. Factory Direct sells and installs whole windows and does not advertise glass-only work, so this guide tells you plainly when a glazier is the better call.
Foggy window: what each fix costs
Why is my window foggy between the panes?
Fog between the panes means the seal around the sealed glass unit has failed. Natural Resources Canada says condensation can form between the panes if the seal is broken, and the fix is to replace the insulating glass unit.1 In Quebec that unit is called a thermos.
Moisture got in through the seal, so you cannot wipe it off. Wipe the glass on the inside and the outside. If the haze stays, it sits between the panes. Early failure can look like a faint haze on cold mornings that clears by afternoon.8
Sealed units also lose gas slowly as they age. The long-standing industry benchmark is 1% of the gas fill per year, which leaves about 80% after 20 years.9 Fog is moisture, not missing gas. Fog means the seal let air and water in.
Is it a failed unit or just condensation on the glass?
Fog on the room side of the glass is a humidity problem, not a seal problem.8 Natural Resources Canada says it forms when moist indoor air touches a cooler surface, and the risk rises as it gets colder outside or indoor humidity goes up.1 A low-e coating on the room-side glass raises that risk too.1 Condensation on the outside of the glass around sunrise, after a cool, calm, humid night, also happens.1
What helps: keep air moving across the glass, open the blinds, clear deep sills and run your heat recovery ventilator (HRV) if you have one.1 A new glass unit will not fix a humidity problem. For all causes and fixes, read our guide on window condensation and humidity.
Can I defog a window instead of replacing the unit?
Defogging means drilling small holes in the unit and injecting a cleaning liquid. Monsieur Thermo, a South Shore glazier, says the holes stay visible and the fog can come back.6 It also says a glazier may refuse to defog because of the unit size, the low-e film or the amount of moisture inside.6 It quotes about $120 to defog against about $180 to replace the unit.6
Vitrerie Langevin calls defogging a temporary fix that does not restore the original insulation.5 Both firms sell replacement, so weigh their view with that in mind. Natural Resources Canada says replace the unit.1
When is replacing only the glass unit the right call?
Choose the swap when most of these are true:
- The frame and sash are sound. No rot, no warping, no repeated leaks. The window opens, closes and locks.58
- The window is still fairly young. Windows Canada says a swap is often sensible on a young window with a sound frame, and favours a new window when the frame is tired.8 Vitrerie Langevin says a unit older than 15 to 20 years can show fatigue.5
- One or two units failed, not a whole wall.
- The glass is still under warranty. Check your paperwork first. Vaillancourt notes that sealed-unit warranties are sometimes 10 or 15 years or longer. That page dates from 2019, so confirm your own terms.10
- You want the lowest cost and accept that the old frame stays.
A glazier (vitrier) does the swap. The glazier measures your unit, orders a new one made to size and fits it in your existing frame. Arched and round shapes need custom units and cost more.4
When should you replace the whole window?
- The frame is rotted, warped or cracked. New glass in a bad frame still leaks.78
- Drafts or water keep coming back after the caulking and weatherstripping are ruled out.8
- The hardware is failing: crank, balances or locks.7
- The sash no longer opens or locks, because the frame has moved.8
- Many units have failed. Installix lists this as a reason to replace the window and gives no count.7
- You want the window itself to perform better. Vaillancourt says a full change is the way to the best energy efficiency, sound control and airtightness. It adds that a unit swap in a poorly installed window brings little gain.10
- You want the Rénoclimat money. See below.
We found no rule of thumb in any source for how many fogged units tip the decision, so this guide gives none. Count your failed units, get a glazier’s price for all of them and compare it with a whole-window quote.
How much does a glass unit cost compared with a new window?
Quebec glaziers publish about $150 to $450 per unit. An Ontario installer prices the same job at $800 and up. We found only these sources for 2025 and 2026, so we show them side by side.
| Source | Where and when | Glass unit, installed | New window, installed |
|---|---|---|---|
| Changement de thermos inc.4 | Greater Montréal, Jul 2025 | $150 – $450 (small $150 – $200, standard $200 – $300, large or patio door $300 – $450) | $800 – $2,000 |
| Vitrerie Langevin5 | Québec City, 2026 (page undated) | $150 – $450 | Can easily pass $1,000 |
| Monsieur Thermo6 | South Shore of Montréal, 2026 (page undated) | About $180 (indicative) | Not given |
| Installix7 | Toronto area, Jul 2026 | $800 – $1,600 by size and glass. $500 – $775 each when several are done on one visit | Not given |
| SoumissionRénovation11 | Quebec, Apr 2026 | Not given | $800 – $1,500 (24 by 48 inch window) |
| HomeStars12 | Canada, Aug 2026 | Not given | $300 – $1,000 (standard replacement) |
Size and glass type move the glass-unit price. Low-e and argon add about 15 to 20% over clear glass.7 Triple, laminated or tempered units cost more.7 Doing several units on one visit lowers the price each. Changement de thermos quotes about $300 for one unit and about $250 each for five.4
Worked example (a made-up house, arithmetic only). Five units fail. At about $250 each, the swap is about $1,250.4 Five whole windows at $800 to $1,500 each are $4,000 to $7,500.11 Rénoclimat returns $1,500 on those five, which leaves $2,500 to $6,000.2 The swap is cheaper today. The new windows also bring new frames, new hardware and the subsidy. The swap keeps the old frame.
Does Rénoclimat pay for a new glass unit?
No. The 2026 Rénoclimat participant guide lists replacing the glass (the thermos) alone as not eligible. Replacing only the frame, and installing windows that are only ENERGY STAR, are not eligible either.2
The aid is $300 per rough opening for a window listed as ENERGY STAR Most Efficient. The cap is $4,500 for a house and $6,000 for a duplex, a triplex or a multi-unit building.23 The window must go into an existing rough opening. Work done before October 1, 2026 does not qualify.2 Check each model on the Natural Resources Canada list and keep the ENERGY STAR sticker and your invoices.2 Our Rénoclimat checklist walks through the steps.
Take a window that costs $800 to $1,500 installed in the market ranges above. After $300 back, it is $500 to $1,200.112 Your written quote shows your real number. For per-window and whole-house prices, see our window replacement cost guide.
What should I ask a glazier before a glass-unit swap?
- The exact size and glass spec: clear, low-e or triple, and argon or air. Ask for the original spec or better. Read our low-e glass guide and our double vs triple pane comparison.
- Where the low-e coating sits. Natural Resources Canada says a coating on the room-side glass raises the risk of condensation.1
- The warranty in writing, for the unit and for the labour. One glazier states 10 years on manufacturing defects and 1 year on labour.5
- The price for every unit on the house, not only the fogged one.
- Who removes the old unit and what happens if the frame turns out to be damaged.
How do I decide?
- Wipe the glass. Fog that stays is between the panes.
- Look at the frame and the hardware. Any rot, warping or leaks? Does it open and lock?
- Count the fogged units on the house.
- Find the window age and the warranty paperwork.
- Get two prices: a glazier for every fogged unit, and a whole-window quote. Subtract $300 per eligible window for Rénoclimat.
If the frame is sound and only one or two units fog, the glazier’s price usually wins. If the frame is tired or the units are failing across the house, the whole-window quote deserves a close look.
FAQ
Why is there fog between my window panes?
The seal of the sealed glass unit has failed and moisture got in. Natural Resources Canada says to replace the insulating glass unit.1
How long does a sealed glass unit last?
We found no study with a number. Vitrerie Langevin says 15 to 25 years for a well-installed unit.5 Windows Canada says windows often enter the replacement conversation around 20 to 25 years and last about 30 years in Canadian conditions, and that condition matters more than age.8
Does my warranty cover a fogged unit?
It may. Read your window paperwork and ask the manufacturer or installer. Sealed-unit warranties are sometimes 10 or 15 years or longer.10 Warranty terms differ by company.
Will a new glass unit stop the fog on the inside of the room?
No. Fog on the room side comes from indoor humidity meeting a cool surface.18 Run your HRV, keep air moving across the glass and keep humidity down.
How many fogged windows mean I should replace them all?
No source gives a number. Installix names “many units on a wall” as a reason to replace.7 Price both routes for your house and compare.
Does Rénoclimat pay for glass only?
No. It pays $300 per new ENERGY STAR Most Efficient window, up to $4,500 for a house.2
Not sure which one you have? Book a free in-home estimate with Factory Direct Montreal. Factory Direct sells and installs whole windows, so we are the right call when frames show rot or warping or several units have failed. For a single fogged unit in a sound frame, call a glazier. Factory Direct has completed 9,200+ projects across Greater Montréal over 35+ years (RBQ 5735-2130-01).
Sources
- Natural Resources Canada, Condensation on windows, doors and skylights, modified January 16, 2025: natural-resources.canada.ca.
- Gouvernement du Québec, Rénoclimat Guide du participant, édition 2026 (French), in force October 1, 2026: cdn-contenu.quebec.ca.
- Gouvernement du Québec, Rénoclimat, Nouveautés en vigueur depuis le 1er octobre 2026 (French), updated October 13, 2026: www.quebec.ca.
- Changement de thermos inc., Combien coûte le remplacement d’un thermos de fenêtre à Montréal (French), posted July 28, 2025: changementdethermos.com.
- Vitrerie Langevin, Remplacement de thermos et fenêtres embuées (French), undated page, © 2026. A glazier that sells this service: vitrerielangevin.com.
- Monsieur Thermo, Désembuage ou remplacement de vos thermos (French), undated page, © 2026. A glazier that sells this service: www.monsieurthermo.com.
- Installix, Sealed unit replacement cost, GTA price guide, July 21, 2026. Ontario prices: installixwnd.ca.
- Windows Canada (Canadian Choice Windows & Doors), When to replace windows, August 26, 2026: www.windowscanada.com.
- Vitro Glass, Technical document TD-126, Argonomics of insulating glass units, revision 4, December 14, 2023: www.vitroglazings.com.
- Vaillancourt Portes et Fenêtres, Changement de fenêtre, thermos ou désembuage (French), published August 6, 2019. Older than 2020, so confirm warranty terms today: www.vaillancourt.ca.
- SoumissionRénovation, Prix remplacement fenêtres Québec 2026 : guide complet (French), modified April 13, 2026: soumissionrenovation.ca.
- HomeStars, Cost of window installation, updated August 21, 2026: r.homestars.com.
Related guides: Window replacement cost in Montreal · Double vs triple pane windows in Montreal · Basement and egress windows in Montreal
