Double Glazed Unit Replacement: A Guide

Summary

"Misted windows" is one of the most common call-outs a glazier or window installer gets, and one of the easiest to get wrong without diagnosing it properly first. Moisture visible between the panes of a double-glazed unit — sealed unit failure — is a completely different problem from condensation on the inside face of the glass, which is a ventilation and humidity issue with nothing wrong with the unit itself. Quoting a unit replacement for what is actually a ventilation problem wastes the customer's money; missing a genuine seal failure leaves them with a fogged window and a complaint.

This guide covers how sealed units are built, how to measure accurately for a replacement, what spec to order, the Building Regulations position on notification, and the practical steps of extracting an old unit and fitting a new one without cracking the glass or leaving it rocking in the frame.

Sealed double-glazed units (correctly called Insulating Glass Units, or IGUs) have a working life of roughly 10–25 years depending on installation quality and exposure. As the housing stock installed in the uPVC boom of the 1990s and 2000s reaches the end of that life, unit-only replacement — as opposed to whole-window replacement — is a growing, profitable niche: quicker, cheaper for the customer, and generally not requiring FENSA paperwork.

Key Facts

• IGU (Insulating Glass Unit) — the correct term for a sealed double- or triple-glazed unit: two or more panes separated by a spacer bar, hermetically sealed at the edge, cavity filled with air or an inert gas
• Misting between the panes — always means the edge seal has failed; the unit cannot be repaired (only temporarily masked) and must be replaced
• Condensation on the room-facing inner surface — a ventilation/humidity issue, not a unit fault; do not quote a replacement unit for this
• Edge seal construction — a primary seal (usually polyisobutylene/butyl, on the spacer bar as the gas/vapour barrier) and a secondary structural seal (polysulphide, polyurethane, or silicone) holding the assembly together
• Desiccant — a molecular sieve packed inside the spacer bar that absorbs residual moisture; once saturated (seal failure), it can no longer keep the cavity dry and misting appears
• Spacer bar types — aluminium (cheap, thermally conductive, higher cold-edge condensation risk and seal stress) vs warm-edge/thermally-broken spacers (Swisspacer, Thermix, TGI, Super Spacer) which reduce edge heat loss and seal stress
• Gas fill — argon is the standard fill for modern double glazing; krypton is denser and used mainly in narrow-cavity slimline/triple units; gas fill degrades gradually but is not itself a "fault" unless the seal has actually failed
• Low-E coating — a near-invisible metal oxide coating (usually on surface 3, the inner pane's cavity-facing side) that reflects heat back into the room; specify soft-coat (standard for modern units) unless matching an older hard-coat unit
• Double vs triple glazing — double glazing with argon and Low-E typically achieves ~1.2–1.8 W/m²K centre-pane; triple glazing typically achieves ~0.8–1.2 W/m²K, but is heavier and thicker, not always compatible with older frame rebates without frame replacement too
• Toughened glass — required in Building Regulations "critical locations" (see glazing regs): glazed doors and side panels, low-level glazing below 800mm, glazing near stairs — must meet BS EN 12600 Class 1 (toughened) minimum
• Laminated glass — required for overhead glazing (rooflights, glass roofs) and where enhanced security or acoustic performance is needed; two panes bonded with a PVB interlayer, stays largely intact if cracked
• Glass-visible size vs bead-pocket size — the "glass size" (daylight size) is what's visible once glazed; the bead-pocket/rebate size is the actual unit dimension including the portion hidden under the bead — always order to the bead-pocket (overall unit) size
• Standard tolerance — glaziers typically cut/order units to within ±1–2mm of measured size; leave adequate clearance in the rebate (typically 3–5mm perimeter gap once glazed)
• Setting/glazing blocks — rigid PVC packers placed under and around the unit at defined points to hold it square, transfer its weight correctly, and maintain drainage paths — never use bare rebate or timber packers
• Weep holes — drainage slots in the bottom rail of PVCu (and some timber/aluminium) frames that let water past the outer seal drain away; must be clear before and after reglazing or trapped water accelerates the next seal failure
• U-value target for replacement — ≤1.4 W/m²K (or WER Band B or better) under Approved Document L:2021 for existing dwellings
• Unit weight — a standard 4-16-4mm double-glazed unit weighs roughly 20–25 kg/m²; plan for two people to lift and offer units above roughly 1m²
• FENSA/CERTASS — competent person schemes for self-certifying whole-window (frame + glass) installations; unit-only replacement within an existing, sound frame generally sits outside their notification scope

Quick Reference Table

Detailed Guidance

Diagnosing Seal Failure vs Ventilation Condensation

Before quoting anything, establish exactly where the moisture is. Three positions, three completely different causes:

1. Between the panes, inside the sealed cavity — cannot be touched or wiped away, often with a hazy or streaky appearance that doesn't clear regardless of weather, sometimes with white mineral deposits (evaporated moisture residue) visible at the edges. This is edge-seal failure. The unit must be replaced; there is no repair.
2. On the room-facing inner surface — wipes off with a cloth, appears in cold weather or high-humidity conditions (cooking, showering, drying laundry, poor extraction), and typically worse on north-facing or poorly ventilated rooms. This is a ventilation/humidity issue — see window condensation — and is not a unit fault. Fitting a new unit here will not fix the underlying humidity problem.
3. On the outer face of the glass — usually early morning, clears as the day warms up. This is normal external condensation and is actually a sign the unit is insulating well (the outer pane stays cold because heat isn't escaping through it). No action needed.

Photograph the fault with the light behind you before quoting — it protects both you and the customer, and it's useful if there's a warranty claim to pursue with the original installer first.

Measuring for a Replacement Unit

Getting the measurement wrong means a second visit and a wasted unit, so take care here.

1. Remove the glazing bead from the room side with a stiff putty knife or plastic trim tool, starting at a corner and working along — beads are push-fit/clip-fit in most PVCu and many timber and aluminium systems.
2. With the bead off, measure the glass-visible size (what shows once glazed) separately from the bead-pocket (rebate) size — the true unit size under the bead on all four sides. Always order to the overall unit size, not the visible glass size; confusing the two is the single most common ordering error.
3. Measure width and height at more than one point — older frames are rarely perfectly square, and you need the largest dimension that will still fit with clearance, not an average.
4. Measure the unit thickness at the edge (e.g. 4mm glass – 16mm cavity – 4mm glass = 24mm overall).
5. Note the spacer bar colour and material (silver aluminium vs a coloured/black warm-edge spacer) so the replacement matches or upgrades appropriately.
6. Check for a Low-E coating by holding a lighter or bright point-source light near the glass at an angle — a coated surface reflects the flame with a slightly different tint than the other surfaces.
7. Confirm whether the existing unit is toughened (look for an etched kite mark or manufacturer's stamp, usually in a corner) — replacement in a critical location must match or exceed this.

Order with a standard tolerance allowance from the glass supplier — most cut to within 1–2mm of the stated size — and always specify the bead-pocket size, thickness build-up, spacer colour, gas fill, and Low-E position/coat type.

Sealed Unit Construction and Spec Choices

A modern IGU is built from a spacer bar (holding the panes apart and setting cavity width), a primary butyl seal against the spacer for the gas/vapour barrier, and a secondary structural seal (polysulphide, polyurethane, or silicone) bonding the assembly together at the edge. The cavity is filled with dry air, argon, or occasionally krypton in narrow-cavity applications, and a desiccant inside the spacer absorbs residual moisture at time of manufacture.

Two spec decisions matter most for a like-for-like or upgrade replacement:

• Spacer bar — an aluminium spacer is cheaper but thermally bridges the edge of the unit, increasing cold-edge condensation risk and thermal-cycling stress on the seal (which shortens its life). A warm-edge/thermally-broken spacer costs a little more but reduces both — worth recommending when replacing an aluminium-spacer unit that failed prematurely.
• Gas fill and coating — argon with a soft-coat Low-E on the inner pane's cavity-facing surface is the standard modern spec and comfortably meets the ≤1.4 W/m²K target. Triple glazing or krypton fill should only be specified where the existing rebate depth and frame strength can take the extra weight and thickness — retrofitting triple glazing into a frame built for a 24mm double unit often isn't possible without replacing the frame too.

Where the unit sits in a Building Regulations critical location — glazed doors, side panels within 300mm of a door, low-level glazing below 800mm, or glazing near stairs — it must use toughened glass to BS EN 12600 Class 1 as a minimum; overhead glazing must use laminated glass. See glazing regs for the full critical-location reference table. Don't downgrade safety glazing when replacing a unit, even if the original installer cut corners.

Building Regulations: Is Unit-Only Replacement Notifiable?

This is the point installers most often get wrong, so it's worth being precise. Whole-window replacement (frame and glazing together) is notifiable work under Building Regulations Part L and must be carried out either under a competent person scheme (FENSA or CERTASS self-certification) or submitted to local authority Building Control.

Replacing only the sealed glazed unit within an existing, sound frame — leaving the frame itself untouched — is treated as maintenance/repair rather than installation of a new window, and in the great majority of cases is not separately notifiable, provided the replacement unit meets current thermal performance requirements (≤1.4 W/m²K) and any applicable safety glazing standard for its location. This is consistent with the position for misted-unit replacement covered elsewhere in this knowledge base (see window condensation and upvc window repair).

That said, this is a genuinely nuanced area and the exact boundary can depend on local authority interpretation, the scale of works, and whether the frame itself needs attention alongside the glass. If in doubt, a call to the local authority Building Control department costs nothing and avoids an awkward conversation later.

Installation: Removal, Reglazing, and Getting the Fit Right

1. Remove the bead — lever gently with a stiff putty knife or plastic trim tool at the midpoint of each side; beads typically clip out from a corner. Keep beads in order — they're often handed (specific lengths/profiles per side) and mixing them up causes fitting problems later.
2. Extract the old unit — with the bead off, the unit sits loose in the rebate on its setting blocks. Lift out with a second person for anything over roughly 1m² — a standard unit weighs 20–25 kg/m², and glass edges are sharp enough to cause serious injury if dropped or mishandled. Wear cut-resistant gloves.
3. Clean and inspect the rebate — remove old, perished setting blocks and sealant residue. This is also the point to check for frame problems (see below) before reglazing.
4. Set new glazing blocks — position rigid PVC setting blocks at the quarter points along the bottom of the rebate to support the unit's weight, keep it centred, and maintain drainage paths. Never substitute timber packers — they compress, rot, or block drainage.
5. Offer the unit in — tilt the bottom edge in first, then bring the top in square. Check it's level and centred with consistent bead-pocket coverage (typically 3–5mm) on all sides.
6. Refit the beads — clip back in the correct order and orientation; a snug, gap-free fit confirms the unit is correctly seated.
7. Check for rocking — gently press each corner; movement indicates an uneven or missing setting block and must be corrected, or the unit will eventually crack or the seal will be stressed prematurely.
8. Seal appropriately for the frame type — PVCu frames rely on the bead's integral rubber gasket (check it's correctly seated; don't add silicone in the rebate, it traps moisture against the unit edge); timber frames typically need a compatible low-modulus silicone bead between glass and bead.
9. Confirm weep holes are clear — blocked drainage slots in the bottom rail or sill let water sit against the new unit's edge seal, one of the most common causes of premature failure.

Checking for Frame Problems Before You Quote

Not every misted window is a straightforward unit swap. Before pricing the job, check:

• Frame distortion — bowed or twisted PVCu or timber frames put the sealed unit under uneven load, which both caused the original seal failure and will shorten the life of the replacement if not corrected. A unit fitted into a distorted rebate is a false economy.
• Timber frame rot — particularly at the bottom rail and sill, where water sitting against a failed seal often migrates into the timber. Probe with a bradawl; soft timber means the frame — not just the unit — needs attention.
• Whether the whole frame needs replacing — if it's failing structurally, badly distorted, or the rebate can't accommodate a like-for-like or upgraded unit, quote for whole-window replacement instead. This changes the job scope, the cost, and the Building Regulations position (now notifiable via FENSA/CERTASS or Building Control) — flag this to the customer rather than fitting a new unit into a frame that won't support it.

Indicative costs for straightforward unit-only replacement typically run from roughly £50 for a small standard pane up to £200+ for larger or upgraded units — see the companion pricing guide misted double glazing unit replacement pricing guide for a full breakdown.

Frequently Asked Questions

Can a misted double-glazed unit be repaired instead of replaced?

Not properly. Some services offer to drill a small hole and inject desiccant or draw out the moisture. This can improve clarity for a period — typically a few months up to a couple of years — but it doesn't restore the gas fill or repair the failed seal, so misting returns. It's a temporary cosmetic fix, not a repair, and generally isn't worth recommending over a proper unit replacement.

Do I need to be FENSA or CERTASS registered to replace a sealed unit?

No. FENSA and CERTASS registration relates to self-certifying whole-window (frame plus glazing) installation under Building Regulations Part L. Replacing a sealed unit within an existing, sound frame is not whole-window installation and does not require scheme registration or notification in the great majority of cases. If you're also replacing the frame, that work does fall under the scheme requirements.

The unit was only fitted a few years ago — why has it already failed?

A quality IGU with correct installation should last 10–20+ years. Early failure usually points to water sitting in the rebate against the seal (blocked weep holes or poor drainage), an incompatible sealant reacting with the edge seal, an aluminium spacer under heavy thermal cycling (south-facing, exposed elevation), or occasionally a manufacturing defect. Check whether the original installer offered a warranty on the unit — many do, typically 10 years or more.

Can I upgrade to triple glazing when replacing a failed double-glazed unit?

Sometimes, but check the rebate depth and frame rating first. Triple glazing is thicker and heavier, and older frames — particularly PVCu systems built around a 24–28mm unit — may not have the rebate depth or sash strength for it. If the frame can't accommodate it, either stick with an upgraded double-glazed spec (warm-edge spacer, argon, Low-E) or quote for a full window replacement.

Does the replacement unit need to match the exact spec of the original?

It needs to meet current Building Regulations minimums (≤1.4 W/m²K, plus any applicable safety glazing requirement) — it does not need to match an older, lower-performing original exactly. Most replacements are an improvement on the original spec by default, since older units (especially pre-2000s air-filled, non-Low-E) fall well short of current standards.

Regulations & Standards

• Building Regulations Approved Document L:2021 — energy conservation; sets the ≤1.4 W/m²K (or WER Band B) U-value requirement for replacement glazing in existing dwellings

• Building Regulations Approved Document K (2013) — protection from falling, collision and impact; defines critical locations requiring safety glass (incorporates the former Approved Document N)

• BS EN 12600 — impact test classification for safety glass; Class 1 (toughened) and Class 2 (laminated) ratings referenced for critical-location glazing

• BS 6262 (Parts 1–7) — code of practice for glazing for buildings, including guidance on unit design, glazing methods, and safety

• BS EN 1279 — insulating glass units: quality, durability, and test method standards for sealed unit manufacture

• FENSA / CERTASS — competent person schemes for self-certifying whole-window installation under Part L; unit-only replacement in an existing frame generally sits outside scheme scope

• Planning Portal: Building Regulations — Approved Document L and K guidance

• FENSA — competent person scheme scope and notification requirements

• Glass and Glazing Federation (GGF) — technical guidance on IGU specification, installation, and glazing methods

• BSI Group — BS EN 12600, BS 6262, and BS EN 1279 standard details

• CERTASS — alternative competent person scheme for window and door installers

• window condensation — full diagnostic guide distinguishing seal failure from room-side and external condensation

• upvc window repair — uPVC hardware repairs including misted unit replacement basics

• glazing regs — critical locations and safety glass requirements in full

• misted double glazing unit replacement pricing guide — indicative pricing for unit replacement vs whole-window replacement