Misting between double glazing panes is one of the most common maintenance issues UK homeowners face. The good news is that in most cases you do not need to replace the entire window frame — only the sealed glass unit itself needs to come out. In 2026, a standard unit replacement costs £80–£140 supplied and fitted by a glazier, making it one of the more affordable home repairs once you understand what drives the price.
This guide covers what a sealed unit replacement costs, why units fail, how size and specification affect the bill, and whether DIY is a realistic option.
Why Double Glazing Units Mist Up
A double glazed unit is a hermetically sealed assembly: two panes of glass with a spacer bar around the perimeter, held together with a primary and secondary sealant. The cavity between the panes is filled with dry air or an inert gas (typically argon) to reduce heat transfer.
Over time — usually between 10 and 25 years — the edge seals degrade. Moisture penetrates the cavity, condenses on the inner glass surfaces, and leaves the characteristic foggy or streaky appearance that cannot be wiped clean. Once the seal has failed, the unit’s thermal performance drops sharply and the misting will not resolve itself.
Common causes of premature failure:
- Poor-quality edge seals or sealants used during manufacture
- Direct UV exposure on south- or west-facing elevations
- Physical damage to the frame allowing water ingress around the unit
- Expansion and contraction stress on large units with inadequate movement allowance
- Poor installation (insufficient packers, gasket failure)
A failed seal is not a structural problem — the glass is not dangerous — but it does reduce thermal performance and can be visually intrusive.
2026 Cost by Unit Size
Glazier pricing is primarily driven by unit size. Labour is roughly constant per visit, so larger units carry a higher glass and spacer cost but proportionally lower labour overhead.
| Unit size | Typical use | Supply & fit cost (2026) |
|---|---|---|
| 400 × 600 mm | Small bathroom / fanlight | £55 – £90 |
| 600 × 900 mm | Standard casement window | £80 – £140 |
| 900 × 1,200 mm | Large casement / tilt-and-turn | £120 – £190 |
| 1,200 × 1,500 mm | Picture window / bay section | £175 – £260 |
| 1,800 × 1,200 mm | Wide fixed panel | £220 – £340 |
| 2,100 × 2,100 mm | Full-height patio door unit | £250 – £450 |
Prices include the glazier’s call-out, unit manufacture, and fitting. Most glaziers charge a minimum call-out of £50–£100 to cover travel and time — if you have multiple units to replace, grouping them into a single visit will substantially reduce the per-unit cost.
What Affects the Price
Glass specification. A standard 4-12-4 clear unit (4 mm glass, 12 mm argon cavity) is the baseline. Upgrading to Low-E coated glass (required to meet current Part L thermal standards for replacement units) adds £15–£30 per unit. Safety glass (toughened or laminated, required in critical locations under BS 6206 / BS EN 12600) adds £20–£40 per unit. Obscure or patterned glass is similar in cost to clear.
Triple glazing upgrade. If you want to upgrade to triple glazing while you have the glazier on site, expect to add £40–£80 per unit on top of the standard double glazed price. The existing frame must be capable of accommodating the additional weight and depth — not all frames can. Triple glazing delivers a U-value of 0.6–0.8 W/m²K versus 1.0–1.4 W/m²K for a modern double glazed unit.
Unit shape. Non-rectangular units (arched tops, triangular, raked heads) require bespoke cutting and typically add 30–60% to the glass supply cost.
Access and frame condition. Units on upper floors requiring ladder access, or frames in poor condition that complicate bead removal, will attract a labour premium of £20–£50 per unit.
Regional variation. London and south-east glaziers typically charge 15–25% above Midlands and northern England rates for equivalent work.
Labour Rates
A competent glazier charges £50–£100 per unit in labour, inclusive of removing the old unit, measuring, ordering, and fitting the replacement. Most glaziers will measure on the first visit and return when the unit is ready — typically 3–7 working days for standard sizes, longer for bespoke shapes.
Some companies offer a one-visit service where they carry common sizes in a van stock; if your unit matches a stocked size, the job is done on the day and the call-out cost covers everything.
Thermal Performance and Part L
When replacing a failed unit, current Building Regulations Part L (Conservation of Fuel and Power) requires the replacement to meet minimum thermal performance standards. For replacement glazing in existing dwellings, the notional requirement is:
- Whole window U-value: 1.4 W/m²K or better (using the BFRC Window Energy Rating system, this equates to a C-rated window or above)
- Centre-pane U-value for the unit alone: approximately 1.2 W/m²K
In practice, any modern Low-E argon-filled unit from a reputable manufacturer will comfortably meet these figures. Older replacement stock with clear glass and air fill (centre-pane U-value ~2.8 W/m²K) does not comply with Part L and should not be installed as a like-for-like replacement.
The BFRC (British Fenestration Rating Council) energy label system rates windows from A++ (most efficient) to E. Specifying a unit with a BFRC A or B rating gives you documented compliance evidence, which can be useful if you plan to sell the property.
[!tip] Checking your installer Installers registered with FENSA (Fenestration Self-Assessment Scheme) or Certass can self-certify that the replacement unit meets Building Regulations without a separate Building Control application. If your glazier is not registered, a Building Regulations application fee (typically £200–£400 through your local authority) may apply.
Worked Example: 3-Bed Semi-Detached
A typical three-bedroom semi-detached house built in the 1980s or 1990s might have 10–14 double glazed window units across the ground and first floors, plus a patio door. If the original units were installed 20–25 years ago and multiple seals have failed simultaneously:
| Item | Qty | Unit cost | Total |
|---|---|---|---|
| Standard casement units (600×900 mm) | 8 | £110 | £880 |
| Larger casement / bay units (900×1,200 mm) | 4 | £155 | £620 |
| Patio door unit (2,100×2,100 mm) | 1 | £350 | £350 |
| Total | £1,850 |
This falls comfortably within the £1,200–£2,000 range for a whole-house unit replacement on a typical 3-bed property. Grouping all units into one or two glazier visits avoids multiple call-out charges and typically produces a modest per-unit discount.
DIY vs Professional Replacement
Replacing a sealed unit is technically feasible as a DIY project — the glass bead is removed, the failed unit slides out, and the new unit is pressed into place with appropriate packers and re-glazing tape or silicone. However, there are important caveats:
Measurement accuracy is critical. Sealed units are manufactured to exact dimensions. A unit 2–3 mm undersized will rock in the frame; oversized and it simply will not fit. Glaziers measure with precision and take responsibility for fit — ordering a bespoke unit on a DIY measurement is a financial risk if the dimensions are wrong.
Safety glass rules apply regardless of who installs it. Critical locations (areas within 300–800 mm of floor level, beside doors, in bathrooms) require toughened or laminated glass under Part N of the Building Regulations. This applies to DIY installation just as it does to professional work.
Weight. A 2,100 × 2,100 mm unit weighs 50–80 kg depending on specification. Solo DIY installation is unsafe without appropriate handling equipment.
For standard casement windows in accessible locations, a competent DIYer can save the £50–£100 per-unit labour charge but must order the unit correctly and accept the dimensional risk. For large, heavy, or critical-location glazing, professional installation is strongly recommended.
Signs Your Unit Needs Replacing
- Persistent internal misting that does not clear with temperature changes
- Visible moisture droplets or water streaks trapped between the panes
- White or grey mineral deposits (dried condensate) on the inner glass surfaces
- Noticeably colder glass surface than a neighbouring intact unit
- Reduced sound insulation compared to other windows in the property
Misting that clears when the sun warms the panes can be normal surface condensation on the outside of energy-efficient glass in cold weather — this is not a seal failure. True inter-pane moisture remains regardless of external temperature.
Summary
Replacing misted double glazing units is straightforward and relatively affordable when addressed promptly. The typical 2026 cost of £80–£140 per standard casement unit makes it an accessible repair, especially when multiple units are replaced in a single visit. Specifying Low-E argon-filled units ensures compliance with Part L and restores the thermal performance the window was originally designed to deliver. For a whole house with 12 failed units, budget £1,200–£2,000 all-in.
