Failed sealed unit: replace the glass, or the whole window?

What a failed sealed unit actually is

A modern window does not have one pane, it has two or three, separated by a space filled with insulating gas and bonded together around their edge. That assembly is called a sealed unit — in Québec, everyone calls it a thermos.

When we say a sealed unit has failed, the glass is not broken. It is the perimeter seal that gave way. The insulating gas escapes, humid air takes its place, and that moisture condenses between the two panes — exactly where nobody can wipe it.

How to recognize the real problem

Three signs leave no doubt:

• The fog is BETWEEN the panes. Run a cloth over both faces of the window: if the haze stays, it is inside the unit. Fog on the indoor face points to too much humidity in the house, not a failed sealed unit.
• It comes and goes with the temperature. It appears in the morning, clears in the afternoon, returns the next day. A healthy sealed unit never does that.
• A whitish deposit eventually settles in. That is the mineral residue of water that has evaporated and condensed dozens of times. At that stage the mark is permanent: no cleaning will remove it.

What it actually costs you

A window with a broken seal loses a good part of its insulating power. The insulating gas is gone, and the space between the panes is now filled with ordinary air, which insulates far less well.

It is not only a matter of bills. Trapped moisture eventually attacks the spacers, then the frame. And a fogged window is visible from the street: it is the kind of detail a visitor notices before even stepping inside.

The real question: the unit alone, or the window?

Replacing only the sealed unit is technically possible, and in some cases it is the right move. Let us be upfront: it is not a service Fenestria offers. We replace the window, not the pane. You still deserve a straight answer, so here is how to decide — and if your case points to the sealed unit alone, a glazier in your area is the right person to call.

Replacing the unit alone makes sense when…

• The window is under fifteen years old and its frame is sound — no warping, no discoloration, no rot.
• The opening mechanism still works properly and the window closes tight.
• Only one or two windows in the house are affected.
• The model is still manufactured, so a replacement unit exists in the right dimensions.

Replacing the whole window makes sense when…

• Several windows fogged up at the same time. If three units fail in one year, the others will follow: same age, same sealant.
• The window is over twenty years old. Frame, weatherstripping and hardware have reached the end of their useful life, sealed unit or not.
• You feel a draft near the closed window, or it sticks when you open it. The fogging is then one symptom among several.
• You are aiming for a grant — and this is the point almost nobody knows.

The detail that changes the math: Rénoclimat

Since October 1, 2026, Rénoclimat pays $300 per opening for a window certified ENERGY STAR^® Most Efficient. But the program is explicit: the assistance targets replacing the window. Changing only the glazing (the sealed unit) or the sash does not qualify.

In other words, the apparent saving of the unit-only route shrinks as soon as you count the lost grant — and it disappears entirely if the other windows in the house fail within two years.

Two clarifications that prevent expensive mistakes: the work must be done by a contractor holding the appropriate RBQ licences, and you must keep your invoices and the ENERGY STAR^® sticker from each window to file the claim. And Hydro-Québec's LogisVert does not apply to doors and windows, whatever many sites suggest.

Why we give $300 per opening

The real cost of a failed sealed unit is not the repair: it is the time spent waiting to do it. Every winter passed with a window that has lost much of its insulating power shows up on the heating bill, while the trapped moisture keeps working on the spacer, then the frame.

And that is where waiting for the grant weighs: Rénoclimat is claimed after the work, once the window is paid for, in a portal whose opening is only announced for the fall of 2026, and the file is then reviewed before payment. The window does not wait.

Fenestria is currently applying $300 off per opening — door or window — directly to the quote. That discount exists for one reason: to remove the reason to postpone. No file to build, no portal, no waiting. The price drops the day you decide.

• Sealed unit alone: no discount. The program excludes glazing on its own, and a sealed unit is not an opening.
• Complete replacement: $300 per opening, taken off the quote — and the government gives you another $300 per eligible window.

On an eight-window house, that is $2,400 off the price, plus $2,400 from the government — and a house that stops heating the outdoors this winter instead of next.

If you do replace, what to demand

A sealed unit is not just a sealed unit. What separates one that lasts twenty years from one that fails in eight:

• The spacer. It is the piece that separates the two panes all the way around, and it decides how long the seal lives. A low-conductivity spacer such as Swisspacer limits the temperature swings that fatigue the sealant — a standard aluminum spacer does the opposite.
• The thickness. At least 1 1/4 in. double-pane, 2 1/2 in. triple.
• The Low-E treatment. Built into the glass — like Climaguard 8071 — rather than applied to its surface, where it wears.
• The frame that holds it. Welded frames of at least 5 5/8 in. in pure, non-recycled uPVC: a frame that moves fatigues the seal of the unit it carries.
• The warranty. 20 years, non-degressive. A warranty that shrinks with time is a warranty that abandons you exactly when the seal fails.

In short

One or two recent windows with a sound frame: the unit alone makes sense. Several windows, or windows over twenty years old, or a tired frame, or the intention to claim the grant: complete replacement costs less in the end.

When in doubt, have the whole assembly looked at rather than the fogged pane. The sealed unit is the most visible symptom, rarely the only one.