Sealed unit replacement
Fog in a foyer, arched or grilled window usually means new glass, not a new window
A milky haze inside the tall window over the stairs, or across the half-round above the front door, looks worse than it is. Fog between the panes means the sealed unit, the double-glazed glass in a home window, has lost its seal. It rarely points to hidden damage. We connect you with an independent local glass professional who checks the frame, matches the shape and grilles, and quotes a new unit. One honest note: glass-only work doesn't earn a window rebate.
- Often the glass alone, with frame and trim untouched
- Grille patterns and arched shapes matched
- Plain answers on rebates and heritage rules
Diagnosis
Fog between the panes versus condensation on the inside: how to tell
Run a dry cloth over the room side of the glass. If the moisture wipes away, it came from the room. If it stays, it's inside the sealed unit.
Water on the room side, mostly in winter
Beads on the inside of a bedroom or ensuite window on a cold morning are indoor humidity meeting cold glass. The seal is fine. Running the fan and keeping air moving across the glass usually helps.
Haze between the panes that comes and goes
Fog that appears inside the unit when the sun warms it, then fades, is an early broken seal. Moist air is moving past the edge, and it won't dry out on its own.
White film or streaks that never clear
A chalky residue on the inner faces means the seal failed a while ago, and the haze may have etched the glass.
Why now
Why so many Markham windows are fogging now
Nearly half of Markham's homes went up between the early 1980s and the mid-2000s. Many still have their original sealed units, and that generation of glass has reached the age when edge seals give way, which is why one fogged window is often followed by others.
The weather adds to it. Mornings here are humid year-round and the heating season is long, so the air inside each unit keeps expanding and contracting. Over time the seal lets moisture in.
You may hear the unit called a thermopane or a thermal sealed unit; Unionville's heritage plan still uses the older word. It's the same thing: two panes, a spacer and a seal around the edge.
The swap
What a sealed-unit swap involves
The frame and sash stay in the wall. Only the glass unit changes, in three stages.
Measure
The pro measures the glass, not the opening, and notes thickness, coating, grille pattern and spacer colour. Shaped glass gets a template traced from the unit.
Order
Sealed units are made to order, so there's a wait between visits. Safety glass goes in where the Building Code calls for it, such as a sidelight by the front door.
Install
The old unit comes out from inside or outside, depending on how it's held. The new one is set on blocks, sealed and stopped in, and the trim usually stays put. On a casement, the sash may come off so the glass can be changed on a bench.
Matching
Grilles-between-glass, arches and transoms: matching what's there
A new unit should look as if it had always been there.
Grilles between the glass
The bars are sealed inside the unit, so they're replaced with it. Colonial and prairie patterns come in several bar widths and colours; a close-up lets the new grid line up with its neighbours. In Cornell, Angus Glen and Cathedraltown, where houses were designed to look historic, a mismatched grille shows from the street.
Half-rounds, eyebrows and arched tops
Shaped units are built from a template of the old glass, and the spokes of a sunburst grille have to meet the bars of the window below. Arched and transom glass goes further into shapes.
Two-storey foyer units
The tall window over the stairs is the one owners notice most. It can take a long ladder, staging on the stairs or an extra pair of hands, and the quote reflects that access.
Before and after
A fogged picture window, then clear again
Same frame, same trim, same view; only the sealed unit changed.
More on picture windowsFrame first
When the frame is the problem and a new window is smarter
New glass in a failing frame is money spent twice. The pro should check what surrounds the unit: soft wood at the sill, split vinyl corners, water stains below. A sash that won't close tight may only need hardware work; a frame that has moved is another matter.
If several windows on one wall are fogging and the frames show their age, replacement may beat a series of glass swaps. Our guide to repairing the glass or replacing the window sets the two side by side, and the windows overview covers every other kind of window fix.
Your quote
What to send for a useful quote
Every sealed unit is made to measure, so the price follows the window you have. These details let a pro quote from photos before visiting:
- Size and shape: a photo of the whole window, rough width and height, and whether it's rectangular, arched or half-round
- Grilles: a close-up showing the pattern, bar width and colour
- The spacer stamp: any printing on the metal strip between the panes
- Glass type: any tint, and whether it's in or beside a door
- Height and access: which floor it's on, and whether it's over a stairwell
A photo quote is a starting point. Once the pro measures, the scope can change, and they should explain why before any work goes ahead.
Rebates
Why glass-only work doesn't qualify for window rebates
Ontario's Home Renovation Savings program pays a rebate on complete ENERGY STAR certified windows and doors, counted per rough opening and usually tied to a home energy assessment. A new sealed unit in your existing frame isn't a new window, so it doesn't qualify, and we don't know of any current program that pays for glass-only work.
That doesn't make the glass swap the wrong choice: it's the smaller job, and it keeps the frame, trim and grilles you have. If most of your windows are near the end, compare both paths on the replacement windows page, and check the program site for current rules.
Heritage villages
A fogged window in one of the four heritage districts
In Unionville, Markham Village, Thornhill and Buttonville, changes to the outside of a property, windows included, go through a City heritage permit. The district plans prefer repairing original wood windows over swapping in sealed units, so a fogged window on an older house there starts with a call to the City's heritage staff, not a glass order. Our heritage district guide explains the permit.
Questions
Questions about fogged sealed units
Explore your options
Related window fixes
Local pros replace sealed units across Markham, including Milliken Mills, Berczy Village, Angus Glen, Cornell and Box Grove.
Photograph the fogged pane and send it over
Include the whole window, the grilles and the stamp on the spacer bar if you can read it. We'll pass it to a local pro who replaces sealed units, and they'll quote the glass, or tell you if the frame needs more.