A “blown” window is one where the hermetic seal between the panes of glass has failed. Moisture-laden air enters the sealed unit and condenses on the inner surfaces of the glass, producing the characteristic milky or foggy appearance that cannot be wiped away. It is purely a structural failure of the insulated glass unit (IGU) itself, not a problem with the frame, and in most cases can be fixed without replacing the entire window.

In 2026, the typical cost to repair a blown window in the UK is £130–£370 per unit, covering a new IGU and the labour to fit it. The wide range reflects differences in glass size, specification, and whether the existing frame is in good enough condition to accept a new unit without additional work.

What Does “Blown” Actually Mean?

Double and triple glazed windows consist of two or three panes of glass separated by a spacer bar and sealed at the edges to create an airtight cavity. That cavity is either filled with dry air or an inert gas (argon is most common in standard units; krypton in high-performance ones) which reduces heat transfer.

Over time, typically 15–25 years, the sealant around the perimeter degrades. Once the seal fails, external moist air is drawn into the cavity whenever temperatures change. The moisture condenses on the cooler internal glass surfaces and, because it cannot escape, the unit becomes permanently cloudy. At this stage no amount of drilling holes, injecting desiccant, or using a hairdryer will provide a lasting fix. The IGU must be replaced.

Blown Window Repair Cost Breakdown

The figures below cover unit supply and fitting for a typical UK home. Costs assume a FENSA-registered glazier working in England or Wales.

Repair ScenarioUnit CostFitting CostTotal Range
Small casement (up to 0.5 m²), double glazed£80–£130£50–£80£130–£210
Medium casement (0.5–1.0 m²), double glazed£120–£200£60–£100£180–£300
Large casement or picture window (1.0–1.5 m²), double glazed£180–£250£80–£120£260–£370
Sash window IGU (bespoke, typically slim-profile)£150–£300£80–£150£230–£450
Triple glazed unit (medium casement)£200–£350£70–£120£270–£470
Triple glazed unit (large)£300–£400£100–£150£400–£550
Bay window (per panel, as above)As above × panelsPer panelMultiply by panel count

Fitting costs are per unit and assume the frame is in sound condition. If the bead (the plastic or timber trim that holds the glass in the frame) is damaged or brittle, glaziers typically charge an additional £20–£50 to replace it.

Glazier Day Rates in 2026

Most blown-unit jobs are charged as a fixed price per unit rather than on a day-rate basis. Where a glazier does quote by time, typical 2026 rates are:

RegionHourly RateDay Rate
London & South East£55–£85/hr£380–£600
Midlands & East of England£40–£65/hr£280–£460
North of England£38–£60/hr£260–£4201
Scotland£40–£60/hr£280–£420
Wales£38–£58/hr£260–£400

A single blown unit typically takes 1–2 hours to replace (remove bead, extract old unit, fit new unit, rebed and re-bead). Booking several units in one visit significantly reduces the effective per-unit cost.

Worked Example

A semi-detached house in the East Midlands has four blown casement windows, two medium units and two large units, all in uPVC frames that are otherwise sound.

ItemCost
2× medium double-glazed IGUs (£160 each)£320
2× large double-glazed IGUs (£220 each)£440
Fitting: 4 units × £90 average£360
Total£1,120

Had the homeowner replaced all four full windows (frame and glass), the cost would typically have been £300–£800 per window, or £1,200–£3,200 for four, potentially three times as much for no practical gain if the frames are structurally sound.

What Drives the Price?

Glass size and specification, larger units require more glass and more gas fill, and are heavier to handle. Toughened or laminated glass (required in low-level glazing under Building Regulations Part K, which treats glass below 800 mm in a wall, and below 1500 mm in a door or its side panel, as a critical location) costs 30–50% more per unit than standard annealed glass.2

Glazing configuration, double glazing is the baseline. Triple glazing uses an extra pane and a wider spacer bar, which increases both material and handling costs. Argon-filled units are now standard in replacement IGUs; krypton-filled units for slim-profile heritage sashes cost more.

Window type, casement windows are the most straightforward to re-glaze because the sash can usually be unhinged and laid flat. Sash windows often require bespoke slim-profile units (typically 4 mm or 6.4 mm glass with a 4 mm cavity) which must be made to measure. Bay windows involve multiple panels at angles, which adds labour. Roof windows (Velux-style) are usually repaired using manufacturer-supplied replacement IGUs that clip directly into the existing frame.

Frame condition, if the outer frame has warped, the drainage channels are blocked, or the rubber glazing gaskets have perished, the glazier may charge extra to remedy these. A completely failed frame is a reason to consider full window replacement instead.

Location and access, ground-floor windows are cheapest to work on. First-floor windows may require steps or a small scaffold platform; very high or awkward glazing can add £50–£150 in access costs per visit.

Temporary Fixes: Are There Any?

No temporary fix restores the thermal performance or appearance of a blown unit. Some glaziers offer a “defogging” or “restoration” service that involves drilling small holes through the spacer bar, injecting desiccant, and polishing the inner pane surfaces. At £60–£100 per unit this is cheaper short-term, but it does not restore the gas fill or the sealed cavity, so the unit’s U-value (thermal transmittance) will not meet modern standards and the fogging is likely to return within a few years. This approach is only worth considering in a window that is about to be replaced anyway and you simply want to improve visibility in the interim.

IGU-Only Replacement vs Full Frame Replacement

Replacing only the IGU is almost always the right choice when:

  • The uPVC, aluminium, or timber frame is less than 15–20 years old and in good structural condition
  • The frame seals, hinges, and handles are functioning correctly
  • The rest of the window is draught-free

Full frame replacement becomes the better option when:

  • The frame is warped, rotted (timber), or corroded (steel) beyond economic repair
  • Multiple units in the same frame have failed or are close to failure
  • The existing window does not meet current Building Regulations thermal standards (U-value of 1.4 W/m²K or better for the whole window under Part L 2021) and you want the energy saving, not just a cosmetic fix
  • You are changing the window style or size as part of a wider renovation

Building Regulations Part L and FENSA

Under Building Regulations Part L (Conservation of Fuel and Power), any replacement window, including a full frame replacement, in an existing dwelling must achieve a minimum whole-window U-value of 1.4 W/m²K (or a Window Energy Rating of Band B or better).3 This requirement does not apply to an IGU-only swap where the existing frame is retained: Part L counts only the entire unit, frame included, as a controlled fitting.3

However, a full frame replacement is notifiable work under the Building Regulations. The simplest compliance route is to use an installer registered with a competent persons scheme: six are authorised for glazing in England and Wales, of which FENSA (Fenestration Self-Assessment) and CERTASS are the largest for replacement windows and doors.4 A registered installer can self-certify their work and issue a certificate, which you will need when you sell the property.

Always Ask for the Certificate

If your glazier is FENSA or CERTASS registered, the installation certificate should be provided automatically. If you have lost a certificate for previous work, replacements can be obtained from FENSA (£30 including VAT) or CERTASS using the address and approximate installation date.5

DIY replacement of a full frame window is possible but the work is still notifiable, meaning you must apply to your local authority building control for approval and arrange a separate inspection, typically adding £200–£400 in fees and administrative delay.

When to Repair and When to Replace

SituationRecommended Action
One or two units blown, frame soundIGU replacement
Multiple units blown in same frameIGU replacement (cost-effective to do all at once)
Frame warped, rotted, or corrodedFull window replacement
Window over 25 years old, many seals at end of lifeFull replacement (consider thermal upgrade)
Wanting to upgrade to triple glazingFull replacement (IGU swap into existing frame rarely viable for triple)
Heritage or listed buildingConsult conservation officer; slim-profile double-glazed sash units often permitted

Getting Quotes

Get at least three written quotes specifying: the number of units, glass size, specification (double or triple, argon-filled, toughened if applicable), and whether the price includes disposal of the old units. Ask whether the company is FENSA or CERTASS registered, this matters only for full frame replacements, but it is a useful indicator of a professional operation. Avoid any glazier who cannot provide a written quotation or who asks for a large cash deposit upfront.

References (5)

Figures in this note were checked against the sources below on 10 September 2026. Superscript numbers in the text point to them. Every source the site cites, by topic.

  1. 1costvery strongOffice for National Statistics.RN-MMT1DOA price, true at 2026-09. The ASHE Table 14 dataset gives annual estimates of paid hours worked and earnings for UK employees by sex and by full-time or part-time status, broken down to four-digit Standard Occupational Classification, which is the level that isolates individual construction trades such as bricklayers, carpenters, plasterers and electricians. Released 23 October 2025 as the 2025 provisional edition (zip, about 10.7 MB), with revised and provisional editions back to 2017. A correction dated 19 December 2025 suppressed annual pay estimates for SOC 3312 police officers in some areas; construction occupations are unaffected.
  2. 2technicalvery strongMinistry of Housing, Communities and Local Government.RN-XH6E39The standard that decides whether a loft ladder is legal and when a raised deck needs a balustrade. Published 3 January 2013, it covers protection from falling, collision and impact including stairs, ladders, ramps, guarding and vehicle barriers. Paragraph 1.31 states that retractable ladders must not be used as a means of escape. Paragraph 1.32 permits a fixed ladder, with fixed handrails on both sides, only for access in a loft conversion containing one habitable room, and only where there is not enough space without altering the existing space for a compliant stair, which is the rule that catches most loft ladder installations. Paragraph 1.29 restricts alternating tread stairs to loft conversions. For a private stair the maximum pitch is 42 degrees (Table 1.1 note 1), the normal relationship between rise and going is twice the rise plus the going between 550mm and 700mm, and for dwellings external tapered steps forming part of the building need a going of at least 280mm. Paragraph 3.1 requires guarding wherever it is reasonably necessary for safety at the edge of a floor, gallery, balcony, roof, light well, basement or similar sunken area. Paragraph 3.2 requires guarding to be at least the height in Diagram 3.1, allows any wall, parapet or balustrade to serve as guarding, and requires it to resist the loads in BS EN 1991-1-1 with its UK National Annex and PD 6688-1-1, with BS 6180 for barrier and infill panel design. Handrails should be positioned 900mm to 1000mm from the pitch line or floor.
  3. 3technicalvery strongMinistry of Housing, Communities and Local Government.RN-L0FCJXThe in-force U-value tables a DIY insulation or door note must quote, with the exact numbers. Table 4.2, limiting U-values for new fabric elements in existing dwellings, sets roof 0.15, wall 0.18, floor 0.18, swimming pool basin 0.25, window 1.4 or Window Energy Rating Band B minimum, rooflight 2.2, doors with more than 60 per cent of the internal face glazed 1.4 or Doorset Energy Rating Band C minimum, and other doors 1.4 or Doorset Energy Rating Band B minimum. Table 4.3, for existing elements, gives a roof threshold of 0.35 and an improved value of 0.16, which is the figure a loft insulation note needs: paragraph 4.12 says a retained element whose U-value is higher than the column (a) threshold, for example through a loft or garage conversion, should be upgraded to the column (b) value. Paragraph 4.13 allows a lesser upgrade only where the improved value is not technically or functionally feasible or would not achieve a simple payback of 15 years or less. Paragraph 4.8 requires a replacement element to be no worse than the one it replaces as well as meeting Table 4.2. Paragraph 4.10 gives the heritage exception: where character must be maintained, a centre pane U-value of no more than 1.2 or low-emissivity secondary glazing. Note 9 records that the timber window concession of 1.6 or Band C ended on 14 June 2023. The 2021 edition came into force on 15 June 2022; a 2026 edition exists at https://assets.publishing.service.gov.uk/media/69c122a6cfa346b9d4704a55/ADL1_2026.pdf.
  4. 4technicalvery strongMinistry of Housing, Communities and Local Government.RN-SAMO3MThe official list of competent person schemes whose members can self-certify certain building work instead of getting building regulations approval, published 4 April 2013 and last updated 12 March 2018. Named schemes with contact numbers include ATTMA, APHC, Assure Certification, Blue Flame Certification, the British Board of Agrement, BESCA, Capita Gas Registration and Ancillary Services trading as Gas Safe Register, CIGA, Certass, Certsure LLP trading as NICEIC, the Elmhurst Airtightness Scheme, Fensa Limited, HETAS Limited, NAPIT Registration Limited, the National Federation of Roofing Contractors trading as CompetentRoofer, and OFTEC. Two footnotes matter: the Gas Safe Register is not technically a competent person scheme although in practice its installers carry the same building regulations responsibilities, and Benchmark Certification Limited ceased to operate as a competent person scheme on 10 February 2017, becoming part of NAPIT Registration Limited that day. Installers joining an electrical scheme are added to the single electrical competent person register.
  5. 5technicalstrongFENSA (Fenestration Self-Assessment Scheme).RN-FEJOZAFENSA is the competent person scheme for replacement windows and doors. The certificate confirms that the windows and doors an installer replaced comply with the Building Regulations, use energy efficient products, are registered with the local council, and that the warranty is insured. Certificates can be searched by property address or by FENSA certificate details. A replacement certificate costs £30.00 including VAT whether paid online or by cheque, the cheque price including an administration charge. The page also lists why a search returns nothing: the address entered is wrong, the job was more than a straight window or door replacement, the work was done by an installer who is not FENSA approved, or the company never notified FENSA that the job was finished. That last category is the one that surfaces during a conveyancing search.

  1. Earnings and hours worked, occupation by four-digit SOC: ASHE Table 14, Office for National Statistics, accessed 21 September 2026. Priced at 2026-09, so no calibration is needed. ons.gov.uk

  2. Approved Document K: Protection from falling, collision and impact, 2013 edition, HM Government (GOV.UK), accessed 10 September 2026. assets.publishing.service.gov.uk

  3. Approved Document L: Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, HM Government (GOV.UK), accessed 10 September 2026. assets.publishing.service.gov.uk

  4. Competent person scheme: current schemes and how schemes are authorised, Ministry of Housing, Communities and Local Government (GOV.UK), accessed 10 September 2026. gov.uk

  5. FENSA Certificate search and replacement, FENSA, accessed 10 September 2026. forms.fensa.org.uk