How to tell it's really between the panes
Wipe the inside of the glass with a dry cloth. If the haze comes off, it's surface condensation, not a failed seal. Why condensation forms on the inside of a window covers that problem. Check the outside surface too, but only if you can reach it from the ground or a safe position, without leaning out of an upstairs window. Haze that stays put after both checks is inside the unit.
Make sure it's actually a sealed double-pane unit. If you have storm windows over or inside older single-pane sashes, fog between the storm and the original window is a different problem. Pacific Northwest National Laboratory (PNNL) notes that moisture can collect between an interior storm panel and the original window when the storm isn't airtight. That's fixed by sealing the storm, not by buying new glass.
Why the seal failed
A double-pane unit is two sheets of glass sealed around the edges, sometimes with an insulating gas such as argon in between, according to the Department of Energy's window guide. PNNL's sash retrofit guide says fogging between the two layers of glass shows the seal has failed, and that the failure is commonly caused by the seals sitting in contact with water for long periods. The sash is supposed to let water drain to the outside.
If water pools at the bottom of the glass because the sash doesn't drain, a new unit can fail the same way. PNNL also lists water staining on the interior window frame, peeling paint, and stained trim or flooring below a window as signs of water getting in or of condensation. If you see those along with the fog, ask for the leak to be found, not just the glass replaced.
Three ways to fix it
- Replace the glass unit only. A glass shop orders a new sealed unit to fit the existing sash. This fits when the sash and frame are sound and drain properly.
- Replace the sash. The sash is the part of the window that holds the glass. PNNL's sash replacement guide describes swapping in new sashes with new insulated glass while keeping the existing frame, and warns that the fit has to be tight or the window will leak air.
- Replace the whole window. Consider this when the frame or sill is rotted or water is getting in around the window. PNNL's window replacement checklist points out that many existing windows were never air-sealed or flashed around the rough opening, and that a replacement set inside the old frame leaves that gap as it was. A full-frame replacement exposes the opening so it can be sealed and flashed.
If the windows are original wood sashes, Should drafty original wood windows be repaired or replaced? goes through that decision. For cracked or broken glass rather than fog, see What to ask before replacing a broken window pane.
What to ask about "resealing" or "defogging"
Some companies offer to clear fogged glass without replacing it. Ask whether that restores the sealed, insulating unit or only improves the look, and get any insulation claim in writing. A clearer view alone isn't proof the seal works again. Don't drill the glass, take the unit apart or heat it to try to dry it out yourself.
Before paying for a repair, check the manufacturer's warranty. Find the label or etching on the glass or frame and the purchase paperwork, and ask whether labor is covered or only the glass.
Philadelphia permit and lead rules
In Philadelphia, the City's window and door permit standard says no permit is needed to replace windows on a one- or two-family home when the opening keeps the same location, shape and size and there's no structural change. Properties listed by the Philadelphia Historical Commission are excluded from that easy route, so check before changing windows on a historic property. The same City standard calls for replacement windows with a U-factor of 0.32 or lower and safety glass in hazardous locations. They're good specs to ask for even without a permit. In Bucks or Montgomery County, ask your township, since permit rules vary by town.
If the house was built before 1978, window work that disturbs painted surfaces can release lead dust. EPA's Renovate Right guidance says renovators working on painted surfaces in homes built before 1978 must be certified and follow lead-safe work practices. Ask the company for its EPA Lead-Safe certification before it removes any sashes or trim.
What to ask the window or glass company
- Is the fog inside the sealed unit, and is the sash or frame damaged or holding water?
- Can you replace just the glass unit or sash, and if not, why not?
- Why do you think the seal failed, and will the new glass drain properly?
- What are the U-factor and glass type of the replacement, and does it need safety glass?
- If the house is pre-1978, are you EPA Lead-Safe certified?