To dry out a blown window from inside, first confirm that moisture is trapped between the panes rather than on the room-facing glass. A failed insulated glass unit cannot be permanently dried with a household dehumidifier; the reliable permanent remedy is replacing the sealed glass unit, while gentle warming may provide temporary cosmetic clearing.
Key facts
A blown window is an insulated glass unit (IGU) with a failed perimeter seal.
Moisture between panes is sealed away from indoor air, so a room dehumidifier cannot remove it.
A hair dryer can sometimes clear visible fog temporarily, but it cannot restore the seal or insulating gas.
Drilling glass is a specialist repair with a real risk of cracking, injury, and warranty loss.
Replacing the IGU usually preserves the existing uPVC, timber, or aluminium frame.
Persistent condensation on the room side may indicate excessive indoor humidity rather than a blown unit.
What Is a Blown Window?
A blown window is a double- or triple-glazed insulated glass unit whose edge seal has failed. The unit normally contains two or more panes, a spacer bar, desiccant, and sealed air or argon-filled cavities; once the seal fails, humid air enters and condensation forms between the panes.
The phrase does not usually mean that the entire window frame has failed. In many installations, the frame remains serviceable and only the removable glass sandwich needs replacement. That distinction can reduce the scope and cost of the repair substantially.
How does the seal failure create moisture?
A sealed IGU is manufactured with a spacer around its edge. The spacer contains desiccant that absorbs a limited amount of residual moisture during manufacture. When the primary or secondary seal deteriorates, outside air enters the cavity, the desiccant eventually becomes saturated, and water vapour condenses on the colder glass surface.
Argon loss also reduces thermal performance, but argon is not the cause of the visible water. The failed seal is the root problem. Replacing the seal, where technically possible, is uncommon for ordinary domestic units because the glass assembly is factory-made and replacement is more predictable.
Is the Moisture Really Between the Panes?
Condensation between panes remains visible after wiping both accessible glass surfaces. Look at the window from a shallow angle and inspect the edge of the glass, because internal fogging often appears as a grey film, droplets, streaks, or a milky band that changes with outdoor temperature.
Use this diagnosis before buying equipment:
- Wipe the room-facing glass with a dry microfiber cloth.
- Wipe the accessible exterior surface if safe to reach.
- Wait 10 minutes and inspect the same mark.
- Check whether droplets move when the sash opens or closes.
- Look for moisture around the spacer bar rather than only in the centre.
| Where the moisture appears | Likely cause | Correct first action |
|---|---|---|
| Room-facing glass | Indoor humidity and a cold surface | Ventilate, heat evenly, and measure humidity |
| Between sealed panes | Failed IGU edge seal | Arrange IGU replacement or specialist defogging |
| Exterior glass in the morning | Efficient glazing and humid outdoor air | Usually leave it alone |
| Frame, sill, or wall | Drainage, plumbing, or persistent leakage | Inspect seals, drainage holes, and surrounding masonry |
| Corners of room glass | Surface condensation or thermal bridge | Reduce humidity and check ventilation |
A hygrometer helps separate the problems. Indoor relative humidity around 40-60% is a practical residential target in many climates, but condensation also depends on room temperature, glass temperature, ventilation, and thermal bridging. A 50% reading does not prove that an IGU has failed.
Before Attempting Any Repair
The safest approach is to identify the failed unit, document its condition, and obtain a glass-only replacement price before drilling or applying heat. Most cosmetic drying attempts trade a visible problem for a less visible one, such as cracked glass, damaged coatings, blocked drainage, or a voided warranty.
Preparation checklist
| Item | Typical requirement | Why it matters |
|---|---|---|
| Hygrometer | 1 unit, approximately £10-£30 or $15-$40 | Separates room condensation from cavity failure |
| Microfiber cloth | 2-3 clean cloths | Prevents surface marks from confusing diagnosis |
| Photos and measurements | 3-5 photos plus visible dimensions | Helps obtain quotes and document warranty condition |
| Hair dryer, temporary use | Low or medium setting | Provides gentle, moving heat only |
| Replacement quote | 1-3 written quotes | Establishes whether defogging is financially sensible |
| Safety equipment for specialist work | Eye protection and cut-resistant gloves | Required when handling or drilling glass |
Do not use a heat gun, open flame, boiling water, abrasive cleaner, or concentrated heat on one point of the pane. Do not remove glazing beads from an upper-floor or large window unless you understand how the glass is supported and can control its weight.
Can a Hair Dryer Clear a Blown Window?
A hair dryer can sometimes evaporate visible cavity moisture temporarily, but it cannot repair a failed IGU seal. Use the method only for short-term presentation, such as clearing a window before a viewing, and stop immediately if the glass develops a crack, popping sound, or sharply uneven temperature.
Safe temporary procedure
- Close the window and remove curtains, blinds, and combustible objects.
- Set a household hair dryer to low or medium heat, never a high-temperature heat gun.
- Hold it at least 15 centimetres, or 6 inches, from the glass.
- Keep the airflow moving continuously across the pane and perimeter.
- Warm the glass gradually for 5-10 minutes, then stop and inspect it.
- Let the pane cool naturally rather than adding cold water or opening it abruptly.
- Wipe only accessible surfaces after the fog has cleared.
You will know the attempt has worked when the visible haze reduces without a new crack, seal distortion, or frame movement. The moisture commonly returns within hours or days because the cavity remains connected to humid air through the failed edge seal.
A hair dryer is not a desiccant replacement. It cannot restore argon, reactivate saturated spacer desiccant, or prevent the next condensation cycle. It is a cosmetic measure only.
Should You Drill the Glass From Inside?
Most homeowners should not drill a blown window themselves. Drilling an IGU requires controlled glasswork, appropriate bits, cooling, protection from shards, and a method for managing the cavity afterward; an incorrect hole can crack tempered or coated glass and turn a repair into a replacement.
Professional defogging services may drill small access holes, flush or dry the cavity, and install vents or plugs. Results vary because the original hermetic seal is not restored. A vented unit may remain clearer under some conditions, but it no longer has the same factory-sealed construction or gas retention.
Alcohol injection and heat are especially poor DIY recommendations. Alcohol is flammable, can damage coatings or sealants, and does not guarantee that dissolved contaminants leave the cavity. A household hair dryer does not provide controlled temperature measurement across a pane.
When can specialist defogging make sense?
Defogging may be considered when the glass is unusually expensive, historically significant, difficult to source, or temporarily needed before a larger renovation. It is less attractive when a standard replacement unit is readily available and the frame is sound.
| Option | Typical cost | Typical duration | Result | Main risk |
|---|---|---|---|---|
| Household hair dryer | £0-$30 | 10-30 minutes | Temporary visual clearing | Thermal cracking |
| Specialist defogging | £80-£250 per unit | 1-3 hours | Variable cosmetic improvement | Recurring fog and altered unit |
| Standard IGU replacement | £100-£400 per unit | 30-90 minutes onsite | Long-term sealed glazing | Measurement or fitting error |
| Whole-frame replacement | £500-$2,000+ per opening | 1-2 days | Frame and glazing renewal | Unnecessary cost if frame is sound |
Prices are typical broad ranges, not quotations. Local labour, access, glass size, Low-E coatings, laminated or acoustic glass, Georgian bars, and regional currency differences can change the total substantially.
Why Is IGU Replacement Usually the Permanent Fix?
Replacing the failed insulated glass unit removes the saturated, contaminated cavity and installs a new factory-sealed assembly. The existing frame can usually remain if it is straight, structurally sound, compatible with the new unit, and free from major drainage or seal failures.
A glazier normally removes the internal or external glazing beads, supports the old glass, checks the rebate, installs setting blocks, places the measured replacement unit, and refits the beads. The replacement must match more than width and height: thickness, pane arrangement, safety classification, Low-E coating, spacer specification, and gas-fill design may all matter.
What should be measured?
A reliable order normally records:
- Overall glass width and height, measured in millimetres.
- Approximate IGU thickness, often 20-28 millimetres in older domestic double glazing.
- Number of panes and cavity count.
- Toughened or laminated safety markings.
- Low-E coating and solar-control requirements.
- Decorative bars, obscured patterns, or acoustic construction.
- Frame material, bead position, and drainage condition.
The safest measurement method is often a glazier’s site visit. Measuring only the visible glass can produce an undersized order because the sealed unit extends behind the glazing bead.
Which Option Should You Choose?
Choose IGU replacement for a durable repair, specialist defogging for a limited or unusual situation, and gentle hair-dryer warming only for temporary appearance. Do not confuse a low initial cost with a permanent result.
| Situation | Recommended action | Avoid | Reason |
|---|---|---|---|
| Standard uPVC window, clear internal fog | Replace the IGU | Drilling at home | Replacement is usually straightforward |
| Rented property | Photograph and notify landlord | Altering glass or seals | Damage may become the tenant’s liability |
| Listed or historic window | Obtain conservation-grade advice | Standard modern unit without review | Glass appearance and frame fabric may matter |
| Cracked or heavily stained unit | Replace the IGU | Repeated drying attempts | Etching and damage do not disappear |
| Sale or inspection within days | Gentle temporary warming | Heat gun or concentrated heat | Cosmetic clearing may buy short-term visibility |
| Sound frame with many failed units | Compare batch replacement quotes | Replacing whole frames automatically | Glass-only work may reduce disruption |
What should renters do?
Renters should notify the landlord or property manager, provide dated photographs, and report any water on the sill or wall. A renter should not drill the unit, remove glazing beads, inject chemicals, or use concentrated heat because these actions can create damage beyond ordinary maintenance.
The landlord should also investigate whether the issue is only failed glazing or whether blocked drainage, failed frame seals, or building movement contributed to the problem.
How Much Does a Blown Window Repair Cost?
A standard glass-only replacement commonly costs approximately £100-£400 per unit in the UK or $150-$600 in the US, depending on size, access, glass specification, and labour. A whole-window replacement usually costs considerably more and is unnecessary when the frame remains serviceable.
| Cost factor | Lower typical range | Higher typical range | Price effect |
|---|---|---|---|
| Small clear double-glazed unit | £100 / $150 | £200 / $300 | Lowest common replacement category |
| Large or difficult-access unit | £200 / $300 | £600 / $900 | Access and handling increase labour |
| Laminated or acoustic glass | £250 / $375 | £800 / $1,200 | Special glass and thicker construction |
| Defogging service | £80 / $120 | £250 / $375 | Variable outcome and warranty terms |
| Whole-frame replacement | £500 / $750 | £2,000+ / $3,000+ | Includes frame, installation, and finishing |
Request an itemised quote that names the glass type, thickness, safety classification, spacer colour, gas fill if specified, disposal, access equipment, and guarantee period. A low quote may omit survey, scaffolding, or nonstandard glass.
How Long Does a Replacement Take?
A straightforward IGU replacement usually takes 30-90 minutes onsite per unit after the glass has been manufactured. Ordering often takes approximately 5-15 working days for standard glazing and longer for laminated, acoustic, shaped, or decorative units.
A replacement unit should remain clear after installation under ordinary conditions. New condensation on the room-facing surface can still occur if indoor humidity is high, but moisture returning between the panes indicates a defect in the new unit, installation, or surrounding frame.
What if the new glass fogs again?
Check the guarantee first and photograph the moisture before anyone removes the unit. Confirm whether the fog is between panes, inspect the bead and sealant, and ask the installer to verify that the replacement dimensions and orientation match the order.
A second failure may arise from poor storage, damaged edges, incorrect packers, frame movement, or a defective manufactured unit. Repeated replacement without checking the frame wastes money.
Common Mistakes and How to Fix Them
Mistake 1: Treating cavity moisture like room humidity
A dehumidifier lowers water vapour in the room but cannot pull moisture through intact glass. Use one to protect the room and frame, not to dry the sealed cavity.
Mistake 2: Using an industrial heat gun
High-temperature airflow can create a large temperature difference between the exposed centre and shaded edge of the pane. That thermal stress can crack glass or deform a uPVC frame. Use no heat gun.
Mistake 3: Drilling without identifying glass type
Tempered glass can shatter into many small pieces when damaged, while laminated glass behaves differently and may require specialist handling. Coatings and edge seals can also be damaged before the cavity is meaningfully dried.
Mistake 4: Assuming all white marks can be cleaned
White powder or cloudy deposits may be degraded desiccant, mineral residue, or permanent glass etching. Once the inner glass surface is chemically or physically marked, drying alone will not restore optical clarity.
Mistake 5: Replacing the whole window automatically
A sound frame does not need removal merely because the IGU has failed. Check frame straightness, drainage, hinges, locks, gasket condition, and rebate damage before approving a complete window replacement.
How Do You Prevent Another Failure?
Prevention begins with keeping water away from the edge seals and maintaining normal indoor humidity. Clean frame drainage slots, avoid blocking ventilation, repair leaking roofs or masonry, and do not use abrasive tools that cut gaskets or damage sealant.
Use extractor fans during cooking and bathing, keep bathroom doors closed while steam clears, and maintain stable heating near cold windows. Surface condensation is a separate problem from a blown IGU, but prolonged wetting can damage timber beads, plaster, paint, and nearby seals.
An important practitioner rule is to inspect the bottom corners first. Drainage blockages and standing water place repeated stress on seals, while condensation at the top of the cavity more often reflects general seal failure rather than a single blocked drain.
FAQ
Can a blown window dry naturally?
A failed IGU may appear clearer on a warm, dry day, but natural warming does not restore the perimeter seal. Moisture commonly returns when the cavity cools or outdoor humidity rises. Treat temporary clearing as cosmetic and arrange replacement if clarity and insulation matter.
Will rice or silica gel dry moisture between panes?
Rice and loose household silica gel cannot reach the sealed cavity without opening the IGU. Placing packets on the sill may absorb surface water, but it will not remove condensation between panes and may create dust or a false impression of repair.
Can I replace only one pane of a double-glazed unit?
Usually, no. The two panes, spacer, desiccant, and seals form one factory-made IGU. A glazier normally replaces the complete sealed unit rather than separating the panes onsite, because separating them destroys the original edge seal and controlled cavity.
Does a blown window cause mould?
A blown IGU does not automatically cause mould because the moisture is inside the glass cavity. Mould becomes more likely when condensation reaches the room-facing surface, frame, sill, wall, or curtains, especially where indoor humidity remains high and ventilation is poor.
Does a blown window increase heating bills?
A failed IGU generally insulates less effectively than a sound argon-filled unit, but the exact energy penalty depends on pane size, orientation, frame area, wind exposure, and the rest of the building envelope. Replace it for clear glazing and restored performance, not from an assumed universal percentage.
The Bottom Line
To dry out a blown window from inside, first verify that the fog is between the panes. A hair dryer may clear the appearance for a short time, but drilling, alcohol injection, and DIY vent plugs carry avoidable risks and do not recreate the original sealed IGU. For a lasting repair, obtain a measured glass-only replacement quote, keep the existing frame when it is sound, and address indoor humidity or drainage problems that could damage the replacement.
