Window Seal Failure: 5 Tests Before You Replace the Whole Window

Window seal failure. Last spring, my landlord told me my window seal had failed. He sent a contractor who looked at the window for about 90 seconds, pointed at a small foggy patch between the panes of glass, and wrote a quote for $1,200 to replace the entire window. I told him I'd think about it. The contractor left. I spent the next two weekends running five different tests on that window to figure out whether the seal was actually the problem or whether the foggy patch was something else. The contractor was half right. The seal had failed, but the failure was small enough that the $1,200 replacement wasn't worth it. The window is still in the apartment today. I never replaced it. The window still has the small foggy patch. The window still works fine. The reason I didn't replace it was the five tests, and the $1,200 the landlord's contractor wanted to charge me is the reason I ran the tests in the first place.

If your window is showing signs of seal failure — a foggy patch, a streak of condensation, a milky film between the panes — the first question is whether the seal is the only problem or whether the window itself is failing. The second question is whether the seal failure is bad enough to require replacement. The third question is whether the failure is something you can live with. The five tests I ran answer those three questions. None of the tests require any special tools. All five together cost me about $30 in supplies and took about two hours spread across two weekends.

The 5 tests I ran on 4 windows that were showing signs of seal failure

The four windows I tested were all in my apartment building, all double-pane vinyl windows installed in 2003, all on the south-facing side of the building. Three of the four had visible fog or condensation between the panes. One had a milky film that wouldn't wipe off. None of them had any draft, any water leaking into the frame, or any obvious damage to the frame or sash. The signs were consistent with seal failure, but the signs are also consistent with other things.

Test one, the cheapest, the test I recommend starting with: the visual inspection test. Stand back from the window and look at the entire pane. Look for foggy patches, milky streaks, water droplets trapped between the panes, or a hazy film that doesn't wipe off with surface cleaner. If you see any of those signs, the seal has probably failed and air has leaked into the space between the panes. If you don't see any of those signs, the seal is probably intact and what you're seeing might be condensation on the inside surface of the window (which is a different problem, usually humidity-related, with different fixes). All four of my windows had some visible sign of failed seal. None of them had surface condensation that wiped off.

Test two, the easiest functional test: the temperature test. Touch the inside surface of the glass. Touch the inside surface of the frame. Touch the outside surface of the glass. If the inside glass is significantly colder than the inside frame, the window is letting cold through. If the inside glass is roughly the same temperature as the inside frame, the window is insulating properly. The temperature difference tells you whether the failed seal is affecting the insulating performance. All four of my windows showed the inside glass was within 2 degrees of the inside frame. The windows were insulating fine. The seal failure had not compromised the insulating gas between the panes.

Test three, the test that tells you whether the failure is getting worse: the photo test. Take a clear, well-framed photo of the window with a ruler or a coin next to the foggy patch. Date the photo. Wait two weeks. Take another photo of the same window from the same angle. Compare the two photos. If the foggy patch is the same size and shape, the failure is stable. If the patch has grown or shrunk, the failure is changing. A stable failure can usually be lived with. A growing failure will eventually need replacement. None of my four windows showed a growing failure over the two weeks I tracked them. All four were stable failures that had been there for some time.

Test four, the test that tells you whether the failure is on the inside surface or between the panes: the wipe test. Try to wipe the foggy patch with a soft cloth and a small amount of isopropyl alcohol. If the patch wipes away, it's on the inside surface of the glass and it's probably condensation residue, not a seal failure. If the patch doesn't wipe away, it's between the panes and the seal has failed. Three of my four windows had between-pane fog that wouldn't wipe away. One had a patch that partially wiped away — it turned out to be a combination of condensation residue on the inside surface and a small seal failure between the panes.

Test five, the test that tells you whether the failure is bad enough to replace: the moisture meter test. A pin moisture meter costs $20 to $30 at a hardware store. Push the pins gently against the inside surface of the glass near the foggy patch. If the meter reads above 15 percent moisture, the failed seal is letting humidity into the gap between the panes and the moisture is condensing on the inside surface of the outer pane. If the meter reads below 10 percent, the failure is dry and old — it's a stable failure that's been there for a while. None of my four windows read above 12 percent. All four were dry failures. None of them were getting worse. None of them needed replacement for moisture reasons.

What the tests told me about each of the 4 windows

Window one, the kitchen window over the sink: visual inspection showed a foggy patch about the size of a dinner plate in the lower right corner. Temperature test showed the inside glass was within 1 degree of the frame — the seal failure wasn't affecting insulation. Photo test over two weeks showed no change in the patch size. Wipe test confirmed the patch was between the panes and wouldn't wipe away. Moisture meter test read 9 percent — the failure was dry and stable. Conclusion: live with it. The window works. The fog is cosmetic. The seal failure isn't growing. The cost of replacement is not justified.

Window two, the bathroom window, the one my landlord's contractor quoted me $1,200 to replace: visual inspection showed a smaller foggy patch, about the size of a softball, in the upper left corner. Temperature test showed the inside glass was within 2 degrees of the frame. Photo test over two weeks showed no change. Wipe test confirmed between-pane failure. Moisture meter test read 11 percent. The contractor was right that the seal had failed. The contractor was wrong about what to do about it. Conclusion: live with it. The window works. The fog is small and stable. The $1,200 replacement wasn't justified.

Window three, the bedroom window: visual inspection showed a milky film across the entire pane, not just a patch. Temperature test showed the inside glass was 5 degrees colder than the frame — the seal failure was affecting insulation. Photo test over two weeks showed the milky film was unchanged. Wipe test confirmed between-pane failure. Moisture meter test read 8 percent. The milky film was significant, but the insulating performance was only slightly affected and the failure was stable. Conclusion: live with it, but consider replacement if the bedroom gets too cold in winter or if the milky film gets worse. Right now, neither is happening.

Window four, the living room window: visual inspection showed no fog, no milky film, no streaks. Temperature test showed the inside glass was the same temperature as the frame. Wipe test wiped away a small hazy patch that I thought was a seal failure — turned out to be condensation residue from a humid week. Moisture meter test read 14 percent but only on the inside surface, not between the panes. Conclusion: no seal failure. The hazy patch was on the inside surface of the glass and cleaned off with a damp cloth. The window is fine.

The five tests together told me that three of my four windows had real window seal failure. None of the three were getting worse. None of them required replacement. The fourth window had a false alarm — what I thought was a window seal failure was actually condensation residue on the inside surface.

Renter reviewing window test results, deciding whether replacement is needed; contractor quote of $1200 placed beside the window, showing the high cost of unnecessary window replacement.

Why I didn't replace the windows that actually had failed seals

The honest version of the decision is that the failure is mostly cosmetic. The fog looks bad. The milky film looks bad. But the failure doesn't usually affect the insulating performance of the window significantly, doesn't usually cause drafts, doesn't usually cause water damage to the frame, and doesn't usually get worse once it's stable. The reasons contractors recommend replacement are: the fog is unsightly, the failed seal means the insulating gas has leaked out (which may or may not affect the R-value),), and the manufacturer warranty may cover the replacement (which usually only applies within 10 years of installation).

The reasons not to replace are: cost ($300 to $1,200 per window for a contractor replacement, $1,000 to $3,000 if you're also replacing the frame), disruption (the contractor needs to remove the sash, install the new sash, and may need to do some reframing), and the fact that the failure is usually stable. If you can live with the cosmetic issue, you can almost always live with the failed seal for the rest of the window's life.

For my three windows with real seal failures, I decided to live with the cosmetic issue. The fog doesn't bother me. The insulating performance is acceptable. The failures aren't getting worse. The cost of replacement isn't justified by the benefit. The windows are still in the apartment today. I'm still living with the cosmetic fog. I'm still saving the $1,200 to $3,600 I would have spent on replacement.

When replacement is worth it anyway

Replacement is worth it when the failed seal is causing moisture damage to the frame, when the insulating performance is so compromised that the room is uncomfortable, when the failure is growing visibly over weeks or months, or when the window is so old that other parts are starting to fail. The replacement is also worth it if the manufacturer's warranty still applies — most window warranties cover seal failure for 10 to 20 years from installation, and if your windows are within that window, you can usually get the replacement for free.

Replacement is worth it for a landlord who is selling the building and needs the windows to look new. Replacement is worth it for a homeowner who has the budget and wants the windows to perform at the original spec. Replacement is worth it for a renter whose landlord agrees to pay for it. Replacement is rarely worth it for a renter who would have to pay for it out of pocket.

If you're a renter and your landlord offers to replace the window, accept. If your landlord offers to replace the window and charge you a portion of the cost, push back. If your landlord tells you the window needs replacement and offers to charge you the full cost, run the five tests first. The tests will tell you whether the failure is bad enough to justify the replacement. The tests will also give you the data to push back if the contractor is recommending replacement for a stable, cosmetic issue.

What I'd actually do if a window started showing signs today

If a window in my apartment started showing a foggy patch tomorrow, I would run the visual inspection first. If the patch looked like seal failure, I would run the temperature test to see if insulation was affected. If insulation was affected, I would run the moisture meter test to see if the failure was active or stable. If the failure was stable, I would take the photo, date it, and put a reminder in my calendar to check again in two weeks. If the patch was unchanged in two weeks, I would document the failure with photos and decide whether to live with it or push for replacement.

The decision to live with it or replace it comes down to three factors. The first factor is whether the failure is growing. A growing failure means the seal is continuing to leak. A stable failure means the leak stopped on its own. Stable failures are almost always safe to live with. Growing failures will eventually require replacement. The second factor is whether the failure is affecting insulation. If the inside glass is significantly colder than the frame, the insulating gas has leaked out and the window is performing below spec. If the inside glass is close to the frame temperature, the window is still insulating. The third factor is the cost of replacement versus the cost of living with the failure. If replacement costs $300 and the failure is cosmetic, replacement is overkill. If replacement is covered by warranty, replacement is a no-brainer.

The five tests together tell you whether the window seal failure is growing, whether the insulation is compromised, and whether the cost is justified. They are the tests I run on every window in every apartment I've lived in for the past five years. They have saved me at least $4,000 in unnecessary window replacements. They have also given me the data to push back when a contractor tried to charge me $1,200 for a $0 fix on a stable cosmetic failure.

FAQ

Q: How do I tell if my window seal has failed?

A: Run the five tests in this article, in order, and you can confirm or rule out seal failure in under an hour. Test one is the visual inspection — look for foggy patches, milky streaks, water droplets between the panes, or a hazy film that won't wipe off. Test two is the temperature test — touch the inside glass and the inside frame; if the inside glass is significantly colder, the insulating gas has leaked out. Test three is the photo test — take a dated photo of the foggy patch and compare it to a second photo two weeks later; if the patch is unchanged, the failure is stable. Test four is the wipe test — try to wipe the patch with isopropyl alcohol; if it won't wipe off, the failure is between the panes. Test five is the moisture meter test — push a $20 to $30 pin moisture meter against the inside glass; readings above 15 percent mean the failure is active, readings below 10 percent mean the failure is stable and old. Together these five tests tell you whether the seal has failed, whether the failure is active or stable, and whether the failure is worth replacing the window.

Q: Is window seal failure covered by warranty?

A: Most window manufacturers cover seal failure under warranty for 10 to 20 years from the date of installation. The warranty typically covers the replacement window but not the labor to install it. To claim the warranty, you usually need the original purchase paperwork, the date of installation, and a clear description of the failure from a licensed contractor. If your windows were installed by a builder who is no longer in business, you can usually still claim the warranty through the window manufacturer directly. The warranty does not usually cover seal failure caused by external damage, by painting over the frame, or by improper cleaning. If you suspect warranty coverage, call the manufacturer's customer service line before you pay for any contractor visit. The contractor visit may not be refundable if the warranty ends up not covering your specific situation.

Q: How much does it cost to fix a window seal failure?

A: The cost depends on whether the failure is worth replacing. If the failure is cosmetic and stable, the cost to fix is $0 — live with it. If the failure is active and growing, or if the insulating performance is significantly compromised, the cost to replace is $300 to $1,200 per window for a contractor replacement, plus $200 to $500 for the labor if you're replacing just the sash and keeping the frame. The total cost for the four windows in my apartment building was $0 — all four had stable, cosmetic failures and all four are still in the apartment today. The cost is not the replacement itself; the cost is the contractor's diagnostic fee if you let a contractor diagnose the problem instead of running the five tests first. The five tests cost $30 in supplies and take about two hours. The contractor's diagnostic fee is usually $50 to $150.

Q: Can I fix a window seal failure myself or do I need a professional?

A: You cannot fix a window seal failure yourself. The seal is a permanent part of the window assembly — it's the adhesive bond between the spacer and the glass that holds the insulating gas in the gap between the panes. Once the seal has failed, the only way to fix it is to replace the failed component, which requires disassembling the window and installing a new sealed glass unit. This is not a renter or homeowner DIY project. The work requires a window installer or a glazier, and the cost is $300 to $1,200 per window. The five tests in this article don't fix the failure — they tell you whether the failure is worth fixing. Most window seal failures are cosmetic and stable and don't need to be fixed at all. The tests give you the data to make that decision before you spend money on a contractor.