Windows aren’t just holes in the wall with glass. They are sealed systems that keep heat in, draughts and noise out, and moisture where it belongs. When that sealed system fails, you see it: misting between the panes, hazy crescents around the edges, or a full “fishbowl” effect after a cold night. Homeowners often call this blown double glazing. The next question usually comes quickly: is it covered under warranty, and does that mean someone will fix it without charge?
I’ve been on both sides of the phone. I’ve fielded calls as a contractor and I’ve made them as a homeowner when my own kitchen window fogged up after just four winters. What I’ve learned is that warranties on double glazed units are more generous than many people expect, but they come with boundaries that matter. If you want a smooth, successful claim, it helps to understand how the warranties are structured, how to gather evidence, and what repairs are realistic. Let’s unpack that in plain language.
What “blown” actually means, and what it doesn’t
A modern double glazed unit is a sealed insulated glass unit, or IGU. Two panes of glass are separated by a spacer around the edge, the perimeter is sealed with a primary sealant, and the cavity between panes is filled with dry air or an inert gas like argon. The spacer usually contains desiccant to mop up any stray moisture. As long as the perimeter seal keeps water vapour out, the unit stays crystal clear.
When the perimeter seal fails, moisture sneaks into the cavity. On a cool morning, that moisture condenses on the inner faces of the glass, which you can’t wipe because the condensation is trapped between panes. That’s the signature look of a blown unit. Sometimes you’ll notice it first as faint fogging at the corners, or a dirty smear that cleaning won’t shift because it’s inside the cavity. In severe cases, droplets run down like rain.
It’s important to separate a genuine sealed unit failure from surface condensation. If you can touch the damp patch with your finger and it wipes, your glazing isn’t blown. You’ve got room-side condensation, usually a sign of high humidity indoors or poor ventilation. I’ve carted away perfectly good IGUs only to find the new ones “mist” again after a week because the kitchen had no extractor and the tumble dryer vented indoors. Get the diagnosis right Misted Window Repairs first, because a manufacturer won’t replace a unit that hasn’t actually failed.
What most warranties cover, and for how long
Manufacturers and installers tend to split coverage into several buckets. The glass unit has one warranty, the frames another, the hardware a third, and labour sometimes sits apart. The terms vary by company, but there are patterns.
- Sealed unit warranty. Many manufacturers warrant the IGU against failure of the hermetic seal for 5 to 10 years. On higher-spec units and reputable brands, 10 years is common. Some offer 15 to 20 years in the UK and Ireland, but read the small print; extended terms sometimes require registration or apply only to the glass, not labour. Frame and hardware. uPVC frames often carry 10-year warranties against discolouration or warping. Aluminium usually similar. Timber can be shorter or tied to maintenance obligations, like repainting every set number of years. Handles, locking mechanisms, and hinges are typically covered 1 to 5 years. Workmanship or installation. Installers sometimes offer their own guarantees, separate from the manufacturer. In the UK, FENSA or CERTASS registrations can include insurance-backed guarantees for a period, usually 10 years, that protect you if the installer vanishes. This matters when the failure relates to poor fitting rather than the glass unit itself.
Sealed unit failure is precisely the event that “blown double glazing” describes, so if you’re within the stated term and you can show it is misting between the panes, you usually have a valid case. The coverage generally applies to the replacement of the IGU, not the whole window. That means the sash or frame stays, the glass unit gets swapped.
The line between manufacturer and installer responsibilities
Here’s where claims go sideways. Many homeowners call the manufacturer directly, only to be sent back to the installer. The division is simple if you think about cause. If the perimeter seal failed because of a manufacturing fault, the manufacturer’s sealed unit warranty is in play. If the problem stems from installation errors, such as the sash being racked out of square, glazing packers missing, or the unit sitting in water because drainage holes are blocked or never drilled, the installer is usually on the hook.
CST Double Glazing Repairs4 Mill Ln
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A practical example: I once inspected a patio door that misted within three years. The bottom rail was full of standing water due to clogged weep holes, and the glazing unit had no setting blocks to keep it off the wet sill. That is installation. The manufacturer initially refused the claim, and rightly so. The installer replaced the unit and corrected the packers and drainage. The replacement unit lasted, which proved the point.
If your installer has gone out of business, manufacturer support can still help, but they may ask for independent evidence that the seal failure is not due to poor fitting. Photographs of packers, glazing channels, and weep holes help. So do inspection notes from a qualified glazier. Be patient here. Sorting out responsibility takes a little back-and-forth.
How to tell if your unit is blown, without guesswork
You don’t need lab gear to confirm a failed IGU, though a few checks increase your odds of a successful warranty claim.
- Check the location of the mist. If it sits between the panes and you can’t wipe it off from either side, that’s the core sign of sealed unit failure. Look for debris or marks within the cavity. Flecks of desiccant beads, streaks you can’t clean, or dusty smears trapped inside the glass confirm a breakdown. Compare rooms on the same side of the house. If one unit fogs and adjacent ones do not after a cold night, that points to a single failed unit rather than a general humidity issue. Inspect the edge seal. From inside, shine a torch around the spacer. Cracked or crumbling sealant, or a gap in the butyl line, strengthens your case. Note timings. Units that come clear midday and fog again at dawn behave like typical failed seals. Constant haze even on warm afternoons sometimes indicates a different fault, like contamination in the cavity from the factory.
Take photos when the misting is most visible, usually early morning. Keep a date log for a week. Warranties rarely require this, but clarity helps the person processing your claim.
What the warranty usually covers in practice
When a claim succeeds, most manufacturers or installers replace the sealed unit like-for-like. This means:
- Glass type. Clear float glass swaps for clear float. If you had a low-E coated pane, they’ll match it. If the unit was argon filled, the replacement should be too. Dimensions and sightlines. The new IGU will match the old in thickness and spacer size so your beading and gaskets fit properly. Safety rating. Toughened or laminated glass gets replaced with the same safety grade. Expect the installer to check for safety kitemarks if the unit sits within critical zones like doors, low-level windows, and bathrooms.
What they typically do not include:
- Upgrades. If you want to bump from 4-20-4 to 4-16-4 with warm-edge spacer and triple glazing, that’s a quoted extra, not a warranty item. Labour or callout in borderline cases. Some manufacturers cover the glass supply only, leaving the installer or homeowner to handle fitting costs. Others cover both parts and labour for the covered term. It varies. Damage unrelated to the seal. Scratches, small chips, and cracks from impact rarely qualify. Thermal stress cracks are a grey area; some brands cover them for a short initial period, others do not.
Read your paperwork. If you don’t have it, check archived emails or call the installer for copies. The job number often sits on the original invoice or on a warranty card with a QR code or serial label.
What about Misted Double Glazing Repairs without a warranty?
Outside warranty, your options are straightforward and the economics matter. Replacement of the IGU is the cleanest, most durable fix. In most UK markets, standard-size replacement units fall in the range of £80 to £150 for supply only, and £150 to £300 fitted, depending on glass spec, access, and sash type. Larger patio doors, shaped units, or laminated safety glass cost more. Leaded designs or Georgian bars add time and money.
You’ll see companies advertising hole-drilling and venting for Double Glazing Repairs, where they drill the outer pane, flush the cavity to dry it, and insert a vent or valve. I’ve tested these on rental stock where budgets were tight. They clear the mist for a while, but the unit is no longer sealed and the thermal performance drops. In damp climates, the haze tends to creep back over months. For a home you plan to keep, proper replacement wins. The only time I recommend the drill-and-vent method is on awkward heritage frames where removal risks damage, or as a short-term patch before a planned refurbishment.
Timing and lead times you can expect
Once a claim is approved, the process is simple. A surveyor measures the visible glass size, checks spacer thickness, and notes any safety glass requirements. Factories cut, toughen if needed, assemble the unit with spacer and seal, and gas-fill. Lead times range from a few days to two weeks for standard units. Add a week for toughened safety glass because it needs heat soaking and separate processing. Busy seasons and holidays can stretch those timelines.
Fitting a single sash unit usually takes 30 to 60 minutes. Hinged casements are easiest. Fixed lights take longer because the beads can be stubborn, and silicone may need cutting. Sliding sashes and tilt-and-turn frames need care but are still a one-visit job. Good installers bring glazing packers in different thicknesses to keep the weight of the unit off the bottom edge and align sightlines. Poor packing is a known cause of repeat failures, especially on tall units where the glass can slump and stress the corners.
What can void a sealed unit warranty
Manufacturers protect themselves with exclusions. They aren’t trying to trap you, but certain conditions genuinely ruin IGUs. Here are the most common pitfalls I’ve seen:
- Aftermarket tint films. Adding an internal film can raise the surface temperature enough to cause thermal stress, especially on large south-facing panes. Check with the manufacturer before adding films. Some recorders of the batch will note any later film application. Unapproved glazing compounds. Silicone smeared onto the edge seal or solvents used to clean frames can attack the perimeter sealant. If you had a painter who used aggressive thinner and it wicked into the bead, that becomes your problem. Persistent standing water. Blocked weep holes in uPVC, no drainage in aluminium, or timber sills that trap water will age seals quickly. Manufacturers often exclude failures caused by poor maintenance. Structural movement. If a lintel fails or the frame racks so far that glass binds in the channel, expect finger pointing. The glass maker will cite movement, the installer will point to the building. You’ll need a builder to correct the cause before any replacement lasts. DIY alterations. Drilling through frames, fitting vents that weren’t designed for the sash, or replacing beads with off-brand parts can muddy the water.
If you’re unsure, ask in writing before making changes. A short email beats a denied claim.
How to prepare a strong warranty claim
I’ve helped customers get claims approved in days because they sent the right information the first time. When you contact your installer or the manufacturer, have the following to hand:
- Order details. Invoice number, installation date, and the address of the property. Photos. Clear images of the misting from different angles, ideally morning shots when it’s worst. One or two close-ups of the spacer and any labels. Unit location. Room name and specific opening (e.g., first-floor rear bedroom, left-hand casement). If there are multiple failures, list them all. Access notes. Whether the unit is at height, requires special ladders, or is within reach of a balcony. This helps scheduling. Any maintenance history. If you’ve cleaned drainage slots or had frames serviced, mention it. It shows you’ve done your part.
Be polite, precise, and patient. Warranty departments are bureaucratic by nature, but people on the other end want to close cases cleanly. If they ask for a site visit to confirm failure, that’s normal. Keep all communication in writing.
If you bought the house with existing windows
A common snag: the windows are only six years old, they’re misting, and the paperwork is nowhere to be found because you’re not the original purchaser. Many warranties are transferable, but only if the transfer happened within a set period after the sale, often 6 to 12 months. Some require a small admin fee or a registration form.
It’s still worth asking. Provide proof of the installation date if you can, like a certificate number, which might be stamped on a label within the sash, or a FENSA certificate linked to your address. If the manufacturer confirms the date and the warranty allows transfer, you may get coverage. If not, ask whether they offer goodwill pricing on replacement units. I’ve seen brands supply glass at cost in these scenarios, with the homeowner paying for fitting.
Energy performance and what a replacement can improve
When an old unit fails, homeowners sometimes use the opportunity to increase performance. A good installer can replace like-for-like under warranty and quote separately for an upgrade to a better spec at your cost. The price delta between a basic 4-16-4 air-filled unit with aluminium spacer and a 4-20-4 argon-filled low-E unit with warm-edge spacer is not huge on a single window. Typical uplift runs £20 to £60 per unit for supply, more for triple glazing or laminated options.
If condensation on the room-side glass has always been a winter nuisance, ask about low-E coatings and warm-edge spacers. Both reduce the surface temperature drop that causes internal condensation at the edges. That won’t fix ventilation problems, but it can help.
Can an isolated blown unit hint at a wider problem?
One misted sash in a house of twenty windows is usually just bad luck, a manufacturing defect that escaped quality control. If you see a pattern, like the three largest south-facing panes all misting within a year, look for stresses they share: intense solar gain, underperforming drainage, or frames installed too tight in the opening.
I once surveyed a modern build where six tall casements on a coastal front failed in the same winter. Salt-laden air and driving rain found their way into the glazing rebates, and the drainage channels were undersized. We upsized the weep holes and added better packers to lift the IGUs off the sill. The replacements have held for five winters. Patterns matter. They help you fix causes, not just symptoms.
Prevention: small habits that protect sealed units
Double glazed units are sealed at the edges, but the environment around them influences life expectancy. Two or three habits make a measurable difference over a decade.
- Keep drainage slots clear. Twice a year, run a plastic card or a thin brush through the weep holes and the track at the bottom of uPVC or aluminium frames. A blocked weep hole can leave the bottom edge of the glass bathing in water after heavy rain. Avoid aggressive cleaners on beads and seals. Mild soapy water works. Solvent-based products can soften gaskets and degrade sealants. Don’t wedge sashes. Leaning heavy objects against opening lights or forcing them to close over warped blinds twists the sash and transfers stress to the glass edges. Manage humidity indoors. Use extract fans when cooking and bathing, crack trickle vents, and dry clothes outside or on a vented dryer. While this mainly prevents surface condensation, it also reduces the driving force for moisture infiltration through microscopic weaknesses. Service moving parts. Lubricate hinges and locking mechanisms annually. A smooth-closing sash distributes pressure evenly around the unit rather than slamming one corner.
None of these will rescue a bad seal, but they will delay the day it fails.
What to do when a claim is denied
Even good cases get rejected. The letter usually cites an exclusion. Your options:
- Ask for the technical basis. Request the inspection notes or ask for a site recheck by another technician. If the initial assessor missed drainage problems or mislabeled surface condensation, a second look can reverse a decision. Provide additional evidence. Photos from winter mornings, videos showing trapped droplets, or an independent glazier’s report can sway the outcome. Negotiate a shared-cost solution. Many companies will offer a goodwill split, especially if you’re just outside the term or you have multiple units from the same batch. Consider small claims only as a last resort. Legal paths are slow and sour relationships. Use them if the sums are significant and the evidence is strong.
From my experience, a measured, evidence-led appeal succeeds more often than not, particularly when the unit is within a year either side of the stated term.
A brief word on safety and DIY
Swapping a sealed unit isn’t rocket science, but it does demand care. Beads can chip, glass edges are sharp, and toughened units can explode if mishandled. On upper floors, manual handling becomes risky. If you decide to DIY outside of warranty, use proper glazing suckers, packers of varied thicknesses, and eye protection. Always mark the low-E face orientation if you remove a unit to measure and refit, so you don’t reverse it. Better yet, pay a pro for an hour. I’ve corrected too many well-meant attempts that doubled the cost.
Where Double Glazing Repairs intersect with home value
Buyers clock fogged panes quickly. On surveys, “misted double glazing” reads like a small red flag. The actual cost to put it right is often modest, but it gives buyers leverage. If you’re selling in six months and three sashes are misted, sort them now. The house will feel better cared for, and you won’t be negotiating credits under time pressure. When you have a valid warranty, this is the easiest win of all. File the claim, keep the paperwork, and hand the new owner a neat folder that shows the replacements and remaining coverage.
The short answer to the title question
Can you fix blown double glazing under manufacturer warranty? Often, yes. If the sealed unit is within the warranty period and the misting is inside the cavity, most manufacturers or installers will replace the IGU at no cost or a very modest fee. The key is to route the claim to the right party, document the failure clearly, and avoid exclusions that shift blame away from the product.
If your coverage has lapsed, don’t panic. Proper replacement remains a sensible, durable fix, and it doesn’t have to break the bank. Be wary of quick fixes that promise miracles for pennies. They rarely last, and they usually sacrifice the performance you paid for in the first place.
When handled with a bit of method, Misted Double Glazing Repairs are more routine than dramatic. The glass can be swapped, the view returns, and the house goes back to holding heat as it should. The warranty you already own is there to make that happen. Use it.