Summary

A misted double-glazed unit is one of the most common domestic call-outs a glazier or window fitter gets, and one of the easiest jobs to under-price if you don't separate what it actually costs you to source and fit from what you charge the customer. The fault itself is simple: the hermetic seal around the edge of the sealed unit (the insulating glass unit, or IGU) has failed, moisture has entered the argon-filled cavity, and the fogging or streaking between the panes cannot be cleaned or repaired — only the unit itself can be replaced.

The pricing trap is treating this like a full window replacement. It isn't. Replacing the glass unit within an existing, sound frame is a materials-led job with a short, predictable labour time — measure, order, return and fit. Replacing the entire window (frame and glass together) is a different job entirely, with FENSA/CERTASS notification, trickle vents, make-good and a much higher price tag. Confusing the two scopes on a quote either scares off a customer who only needed a unit swap, or leaves you underpriced when the frame turns out to need replacing too.

The second common misconception is that a misted unit can be "topped up" or dried out. Some services advertise drilling a small hole into the cavity and injecting desiccant — this is a temporary cosmetic fix, not a repair, and the fogging returns within months. The only genuine fix is a new sealed unit, made to the measured size, with the Low-E coating and spacer bar spec matching (or bettering) what's already in the frame. This guide covers how to measure, source, price and margin the job correctly, and where it sits relative to a full window or frame replacement — covered in detail in double glazed unit replacement guide.

Key Facts

Quick Reference Table

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Scope Unit Spec Labour Quoted Price (Regional) Quoted Price (London)
Single small unit (up to 700×700mm), ground floor 4-16-4mm Ar, warm-edge, Low-E 1–1.5 hrs across two visits £220–£320 £280–£400
Single mid-size unit (approx. 900×1200mm) as above 1.5–2 hrs £280–£420 £360–£520
Single large unit / bay pane (1500×1200mm+) as above 2–2.5 hrs £380–£580 £480–£720
Two units, same window, same visit as above 2–2.5 hrs combined £360–£520 £460–£650
Whole bay window (3 units, same visit) as above 4–5 hrs £750–£1,100 £950–£1,400
Upper floor, tower or scaffold access add to above scope +1–2 hrs plus access hire +£100–£220 +£150–£300
Acoustic laminated upgrade (per pane) 6.4/8.4mm acoustic laminate no significant extra labour +£70–£150 +£90–£190
Security laminated upgrade, P5A (per pane) laminated inner pane, BS EN 356 no significant extra labour +£80–£170 +£100–£220
Triple-glazed unit replacement 4-16-4-16-4mm Ar Low-E as size above, +0.25hr £420–£720 £550–£920
French/patio door glazed panel large IGU, toughened both faces 2–3 hrs £320–£560 £420–£720

Detailed Guidance

Confirming it's genuinely a seal failure before you quote

Condensation that forms between the panes and doesn't wipe off (and, on close inspection, sometimes shows white mineral deposits or a permanent haze) is a failed seal — that's the job this guide prices. Condensation on the room-facing surface of the glass is a ventilation/humidity issue, not a glazing fault, and no amount of unit replacement will fix it. Confirming which one you're looking at before you quote avoids selling a customer a unit replacement that won't solve their actual problem, or missing a genuine seal failure because the room was steamy on the day you visited. Full diagnostic steps — including how to tell the two apart on site — are covered in window condensation.

Unit-only replacement vs whole-frame replacement — get the scope right

This is the single most important line on the quote. If the frame is sound (no rot, no warping, hardware working, no draughts beyond the failed unit itself), you're pricing a unit-only job: source a new IGU to the measured size, fit it into the existing frame. If the frame itself has failed — rotten timber, corroded steel, cracked or bowed uPVC, or the customer wants a different style, colour or opening configuration — you're pricing a full window replacement, a different job with FENSA/CERTASS notification, trickle vents, make-good and a materially higher price (see window replacement pricing guide for that scope). Don't blur the two on a single quote line; itemise them separately if both are in scope, and be explicit with the customer about which regulatory route applies to which part of the job.

Measuring and specifying the replacement unit

Remove the interior glazing bead (a flat putty knife or plastic trim tool at a corner, working along each side) to expose the unit sitting in the rebate on its setting blocks. Measure the unit itself — width, height and edge thickness — not the frame opening, and note the spacer bar colour and any visible Low-E coating (check by holding a light to each glass face; a coated surface reflects differently). Specify to your glass supplier: overall size, thickness build-up (e.g. 4-16-4mm), spacer type (warm-edge unless matching an older aluminium-spacer original), Low-E coating and surface position, and whether toughened or laminated glass is needed for a critical location under BS 6262-4:2018. Most suppliers will make the unit to order within 24–72 hours — this lead time is why most jobs need two visits.

Labour time, lead times and scheduling

Budget a short first visit to diagnose, measure and quote (15–30 minutes on site), then a second visit to fit once the unit arrives. The fit itself is quick on a straightforward ground-floor casement: lift the old unit out, clean the rebate, set new plastic setting blocks at the quarter points, offer the new unit in, refit the beads, and check for square and rocking. Allow 30–60 minutes for a standard-size unit on easy access, more for larger panes (two-person lift) or awkward frames. Where several units are failing in the same window or across a bay, price them together in one visit — the marginal labour cost per extra unit is much lower than a separate call-out for each.

Access and scaffolding for upper floors

A ground-floor unit swap needs nothing beyond a step or short ladder. First-floor and above changes the job: a tower hire (roughly £80–£120 a week) covers most single-window jobs, while multiple upper-floor openings on one elevation may justify full scaffold (£400–£900 per elevation per week, plus erect/strike). Always price access as a separate line — folding it into the unit cost hides the real driver of the price difference between a £280 ground-floor job and a £450 first-floor equivalent, and makes it harder to explain to the customer why two seemingly similar jobs cost different amounts.

Pricing worked example: single kitchen window, two misted units

A tradesperson quoting a straightforward job — two failed units in the same ground-floor kitchen casement window, standard 600×900mm each, matching the existing 4-16-4mm Ar Low-E spec — might build the job like this:

Cost to the tradesperson:

Total cost to the tradesperson: £238.

Quoted price to the customer:

Quoted subtotal: £359.40, plus VAT if registered — in practice most businesses round this to a quoted price of roughly £360–£380. The Quick Reference Table above reflects quoted prices to the customer throughout, not the tradesperson's raw cost. Keep these two figures separate on your own worksheet and never let the customer-facing quote show a cost number that hasn't had margin applied — for a two-unit job like this, that's the difference between a £238 cost and a genuinely profitable £360+ quote.

Frequently Asked Questions

Can I just replace one misted unit, or does the whole window need replacing?

In the great majority of cases, just the unit. A misted double-glazed unit is a sealed glass failure, not a frame failure — if the frame is sound, replacing only the glass unit within it is the correct and far cheaper fix. The whole window only needs replacing if the frame itself has failed (rot, corrosion, warping) or the customer wants a different style or configuration, which is a separate, larger job.

Does replacing a misted unit require FENSA or CERTASS notification?

Generally no. FENSA and CERTASS notification applies to replacement of the controlled fitting — the window itself (frame and glass together) — under Building Regulations. Swapping the glass unit within an existing, unaltered frame is treated as maintenance rather than installation of a new window, so it typically falls outside that notification route. This isn't universal for every unusual case, so confirm with local Building Control if you're at all unsure, particularly on anything beyond a straightforward like-for-like swap.

How long does a replacement unit take to arrive, and can it be fitted the same day?

Most local glass suppliers make units to order once you've measured and specified, typically within 24–72 hours. Same-day fitting is only realistic if you're carrying stock sizes for a very common opening, which is uncommon for made-to-measure domestic units — budget for and quote two visits (measure, then fit) as the default, and set the customer's expectations accordingly at the first visit.

Will the new unit match the rest of the house's windows?

It should, if you specify it correctly. Match the overall thickness, spacer colour and Low-E coating position to the existing units elsewhere in the property so the new unit doesn't stand out visually or perform differently thermally. If the house's other units are an older, lower-spec build-up, you can usually upgrade the replacement to a better-performing spec (warm-edge spacer, current Low-E coating) without it looking mismatched — worth raising with the customer as a modest, worthwhile upsell.

Is drilling and injecting the unit to remove the fog a genuine alternative to replacement?

No. This is a temporary cosmetic treatment, not a repair — it dries the cavity out for a period, typically months rather than years, but the seal that failed is still failed, and moisture re-enters through the same route. It doesn't restore the gas fill or the unit's original thermal performance, and it doesn't come with a warranty comparable to a new unit. Presenting it as a genuine like-for-like alternative to a customer risks a callback and a damaged relationship when the fog returns.

Regulations & Standards