Double Glazed Units Replacement When, Why, and How

Double glazed units replacement is one of the most common home-improvement jobs carried out across the United Kingdom, yet it is also one of the most misunderstood. Many homeowners assume that when their double glazing fails they need to rip out the entire window – frame and all – and start from scratch. In the vast majority of cases that simply is not true. The sealed glass unit itself can be replaced while the existing frame stays exactly where it is, saving a considerable amount of money and disruption. Whether you have noticed condensation forming between the panes, felt cold draughts near a window that used to be perfectly sealed, or simply want to upgrade the glass specification for better energy efficiency, understanding when and how to replace your sealed units puts you in control of the process and helps you avoid overpaying for work that may not be necessary.

This guide covers everything you need to know – from the expected lifespan of a sealed unit and the causes of failure, through the step-by-step replacement process, right through to costs, building regulations, and whether this is the ideal moment to upgrade your glazing specification. It is written for UK homeowners, landlords, and property managers, with all pricing, regulations, and standards relevant to England, Wales, Scotland, and Northern Ireland.

How Long Do Sealed Units Last?

The typical lifespan of a double glazed sealed unit in the UK is between 15 and 25 years, although this range varies considerably depending on several key factors. Understanding what determines longevity helps you set realistic expectations and spot early warning signs before a minor seal issue becomes a major problem.

Seal quality. The edge seal is the single most critical component in determining how long a sealed unit will last. A properly constructed unit has a dual-seal system: a primary seal – usually polyisobutylene (PIB) – that acts as a moisture barrier, and a secondary seal – typically polysulphide, silicone, or hot-melt butyl – that provides structural bonding and additional moisture resistance. Units manufactured with high-quality sealants applied under controlled factory conditions, in compliance with BS EN 1279-2 (moisture penetration) and BS EN 1279-3 (gas leakage rate), will consistently outlast units produced with cheaper materials or less rigorous quality controls. A well-sealed unit can retain over 90% of its original gas fill for more than 20 years.

Spacer bar type. The spacer bar sits around the internal perimeter of the unit, separating the two panes and maintaining the correct cavity width. Traditional aluminium spacer bars are inexpensive but highly conductive – they create a thermal bridge at the edge of the glass that leads to temperature differentials and increased stress on the seal. Warm-edge spacer bars, made from composite polymers, stainless steel, or hybrid materials, conduct far less heat and reduce the thermal stress on the seal by keeping temperatures more consistent across the unit. Units built with warm-edge spacers typically last several years longer than those with aluminium equivalents, and they also reduce edge condensation on the interior surface of the glass.

Gas retention. Argon, krypton, and xenon gas fills degrade in performance as gas gradually escapes through the seal over time. BS EN 1279-3 sets the acceptable rate of gas leakage for certified units – a maximum of 1% per year measured by volume. At this rate, a unit filled with 90% argon at manufacture would still contain approximately 70% argon after 20 years, which is sufficient to deliver meaningful thermal and acoustic benefits. Units that leak faster than this threshold – often due to poor-quality seals or spacer bar defects – lose their gas fill more rapidly and fail sooner.

Exposure and orientation. South-facing and west-facing windows are exposed to significantly more solar radiation than north-facing ones, and the resulting thermal cycling – repeated expansion and contraction of the glass and seal materials – accelerates seal degradation. Properties in exposed coastal locations face additional challenges from salt spray and high winds, both of which place extra stress on the seals. Units in sheltered, north-facing positions can reasonably be expected to last towards the longer end of the 15–25-year range, while those on exposed southern elevations may need replacing sooner.

Installation quality. Even a perfectly manufactured unit will fail prematurely if it is not installed correctly. The unit must be seated on the correct packers to distribute its weight evenly, the glazing beads must hold it securely without excessive pressure, and the frame rebate must be clean and properly drained. Poor installation accounts for a significant proportion of premature seal failures, particularly in uPVC windows where incorrect packer placement can create stress points on the glass edge.

Causes of Sealed Unit Failure

Understanding why sealed units fail helps you identify which windows in your property are most at risk and whether replacement can be deferred or needs urgent attention.

Seal degradation over time. All sealant materials degrade gradually when exposed to UV radiation, temperature cycling, and moisture. This is a natural ageing process and is the most common cause of sealed unit failure in units that are 15 years old or more. The primary PIB seal slowly loses its plasticity and develops micro-cracks through which moisture vapour penetrates into the cavity. Once moisture is inside the unit, the desiccant in the spacer bar absorbs it – but only up to a point.

Desiccant saturation. Every sealed unit contains a desiccant – typically molecular sieve or silica gel – packed inside the spacer bar. Its purpose is to absorb any residual moisture trapped inside the unit during manufacture and any small amounts that permeate through the seal over time. Once the desiccant becomes fully saturated, it can no longer absorb additional moisture, and water vapour begins to condense on the inner surfaces of the glass. This is the point at which you see visible misting between the panes – the classic sign of a failed unit.

Spacer bar failure. In units with aluminium spacer bars, the metal expands and contracts at a different rate to the glass and sealant, creating shear stress at the interface between the spacer bar and the seal. Over many years of thermal cycling, this stress can cause the seal to separate from the spacer bar, opening a path for moisture ingress. Warm-edge spacer bars experience significantly less differential movement because their thermal properties are closer to those of the glass and sealant, which is one of the main reasons they are associated with longer unit lifespans.

Thermal stress cracking. Thermal stress cracks occur when one part of a pane of glass is significantly hotter than another – for example, when direct sunlight heats the centre of the pane while the edges remain cool within the frame rebate. The resulting differential expansion can crack the glass, especially if the edge of the pane has been damaged or chipped during installation. South-facing and west-facing windows are at highest risk, particularly large panes with dark-coloured frames that absorb additional heat. Toughened glass is approximately four to five times more resistant to thermal stress than standard annealed glass.

Poor original installation. Units that were not properly supported with packers, or where the glazing beads were fitted too tightly, are under constant mechanical stress that accelerates seal failure. If the frame itself is slightly twisted or racked – common in older timber frames that have settled over time – the sealed unit is forced into a shape it was not designed for, placing uneven loads on the seal and spacer bar.

Physical damage and impact. Accidental impact – a football, a bird strike, a stone thrown up by a lawnmower – can crack or shatter a pane of glass. Even if the glass does not visibly crack, a hard impact can damage the edge seal, leading to delayed failure weeks or months later. If you notice misting appearing in a unit shortly after an impact event, the two are almost certainly connected.

Signs Your Units Need Replacing

Some signs of sealed unit failure are obvious; others are subtle and easy to dismiss. Here are the indicators to watch for:

• Misting or condensation between the panes. This is the single most common and recognisable sign of a failed sealed unit. If you can see moisture, fog, or water droplets between the two panes of glass – not on the inside room surface or the outside – the unit's seal has failed and the desiccant is saturated. The misting may come and go with temperature changes initially, but it will become progressively worse and eventually permanent. For a detailed explanation of what causes this and your options, read our guide to misted double glazing.
• Draughts near the window. If you can feel cold air near a window that previously felt well sealed, the unit may have deteriorated. While draughts can also be caused by worn window hardware or perished rubber gaskets, a failed sealed unit is a common culprit – particularly if the draught coincides with visible misting.
• Cold spots on the glass. Hold the back of your hand close to the inner surface of the glass on a cold day. If certain areas feel noticeably colder than others – particularly near the edges – the insulating performance of the unit has degraded. This is often caused by gas loss from the cavity, reducing the unit's thermal resistance.
• Increased energy bills. A single failed sealed unit will not dramatically affect your heating costs, but if several units across the property have failed, the cumulative impact on your home's thermal envelope can be significant. If your energy bills have crept up and you cannot attribute the increase to tariff changes or lifestyle factors, deteriorated glazing is worth investigating.
• Visible seal degradation. Examine the edge seal of your units from the inside by looking through the gap between the glass and the glazing bead. If you can see cracks, gaps, or sections where the sealant has pulled away from the spacer bar, the unit is likely to fail soon if it hasn't already.
• Increased external noise. Double glazing provides a meaningful reduction in external noise. If traffic, aircraft, or neighbourhood sounds that were previously well attenuated now seem louder, the gas fill in the cavity may have escaped – reducing both the thermal and acoustic performance of the unit.

The Replacement Process – Step by Step

Replacing a sealed unit is far simpler and less disruptive than most homeowners expect. The entire process – from initial enquiry to a clean, finished window – follows six clearly defined steps.

Step 1: Enquiry and quotation. Contact a reputable sealed unit supplier or installer – you can request a quote from us here – and describe the windows that need attention. Provide the number of units, approximate sizes, and any specific requirements (for example, toughened glass, obscure glass, or a particular gas fill). Most suppliers will give you an indicative price at this stage and arrange a survey.

Step 2: Survey and measurement. An experienced surveyor visits the property to take precise measurements of each sealed unit that needs replacing. This includes the width and height of the glass, the overall thickness of the unit (both panes plus cavity), and the glass specification. The surveyor will also assess the condition of the frame to confirm that it is suitable for reglaze – checking for rot in timber, warping in uPVC, or corrosion in aluminium. If you are measuring units yourself for a supply-only order, follow our measuring guide for step-by-step instructions on the three-point measuring technique.

Step 3: Manufacture. Once measurements and specifications are confirmed, the replacement units are manufactured to order. Standard lead times are typically 5 to 10 working days for common configurations. Bespoke shapes, triple glazed units, or specialist glass types such as acoustic laminate may take 10 to 14 working days. All units should be manufactured to BS EN 1279 and carry the appropriate quality markings on the spacer bar.

Step 4: Old unit removal. On the installation day, the fitter removes the internal glazing beads from the window frame using specialist tools – usually a glazing shovel or stiff blade. The beads clip into uPVC frames and are pinned or screwed into timber frames. With the beads removed, the old sealed unit is carefully lifted out of the frame rebate, taking care not to damage the frame itself. The rebate is then cleaned of any old sealant, debris, or packer residue.

Step 5: New unit installation. The new sealed unit is positioned in the frame on correctly sized packers – small plastic blocks that support the weight of the unit and ensure it sits centrally in the rebate with an even gap all round. Correct packer placement is essential: the unit must be supported at the correct load-bearing points to prevent stress on the glass edge and seal. The fitter checks that the unit is level and properly aligned before proceeding.

Step 6: Beading replacement and clean-up. The glazing beads are refitted around the new unit. In uPVC frames, the beads click back into their retaining channels with a rubber mallet. In timber frames, they are pinned or screwed back into position and may require a small amount of touch-up paint. The fitter cleans both faces of the new glass, removes any protective film, and tidies up the work area. The entire process per window typically takes 20 to 40 minutes.

Do You Need New Frames or Just New Glass?

This is one of the most important questions in double glazed units replacement, and the answer saves many homeowners hundreds – sometimes thousands – of pounds. In the vast majority of cases, only the sealed glass unit needs replacing, not the frame.

When glass-only replacement is sufficient. If your window frames are structurally sound – no rot, no warping, no cracking – then replacing the sealed unit alone is all that is needed. The frames, hinges, handles, and locking mechanisms can all remain in place. This applies equally to uPVC, timber, and aluminium frames. Glass-only replacement is faster, cheaper, and far less disruptive than a full window replacement. There is no need to disturb the internal plasterwork, external rendering, or decorative trim around the window.

When frames need replacing. In some cases, the frame itself has deteriorated to the point where fitting a new sealed unit into it would be a waste of money. Common scenarios include:

• Rot in timber frames. If the bottom rail or cill of a timber frame has rotted, it may not provide adequate support for a new sealed unit. Localised rot can sometimes be repaired with a timber splice or epoxy filler, but extensive rot typically means the frame needs replacing.
• Warped uPVC frames. uPVC frames can warp over time – particularly on south-facing elevations where prolonged heat exposure softens the plastic. A warped frame will not hold a sealed unit squarely, leading to premature seal failure and draughts.
• Damaged aluminium frames. While aluminium is resistant to rot and warping, it can corrode – especially in coastal environments – and the thermal break in older aluminium frames may have deteriorated, negating the benefits of a new unit.

Cost comparison. Replacing the sealed unit only costs a fraction of a full window replacement. As a rough guide, a glass-only replacement might cost £60 to £180 per unit depending on size and specification, whereas a full window replacement – new frame, new glass, new hardware, and making good to the surrounding finishes – typically runs from £350 to £800 or more per window. If your frames are in good condition, glass-only replacement is almost always the sensible choice. Visit our services page for details of both supply-only and supply-and-install options.

Upgrading During Replacement

If you are already replacing one or more sealed units, it is well worth considering whether to upgrade the glass specification at the same time. The labour cost of fitting a higher-performance unit is identical to fitting a basic one – the only additional expense is the difference in the cost of the glass itself. This makes replacement an ideal opportunity to improve your home's energy efficiency, acoustic insulation, or security without the upheaval and cost of full window replacement.

Upgrade to Low-E glass. If your existing units contain standard clear float glass with no Low-E coating, switching to Low-E glass during replacement is one of the most cost-effective energy upgrades you can make. A standard 4-16-4 unit with clear float glass has a centre-pane U-value of approximately 2.8 W/m²K. The same unit with a Low-E coating and argon gas fill achieves around 1.1 W/m²K – a reduction of more than 60% in heat loss through the glass. Browse our full range on the products page.

Add or upgrade gas fill. If your existing units are air-filled, or if the argon fill has leaked out over the years, specifying argon (or krypton for narrow cavities) in the replacement units improves both thermal and acoustic performance. Argon is included as standard by reputable suppliers and adds little or nothing to the unit cost. For a detailed comparison of argon, krypton, and xenon, see our gas fills guide.

Switch to toughened or laminated glass. Building Regulations require toughened or laminated safety glass in critical locations – below 800 mm from floor level, within 300 mm of a door, and in doors themselves. If your existing units in these locations contain standard annealed glass (common in older properties installed before current regulations), replacement is the opportunity to bring them up to current safety standards. Laminated glass also provides improved security against break-in attempts, as the PVB interlayer holds the glass together even when shattered.

Upgrade to acoustic glass. If external noise is a problem, replacing standard units with acoustic laminated glass – typically a 6.4 mm laminated pane paired with a 4 mm float pane – can reduce noise ingress by an additional 5 to 10 dB compared to a standard sealed unit. That may sound modest in numerical terms, but a 10 dB reduction is perceived as roughly halving the loudness of the sound. Properties near busy roads, railways, or flight paths benefit significantly from this upgrade.

Replacement Costs

The cost of double glazed units replacement depends on several factors, including the size of the unit, the glass specification, whether you need supply only or supply and installation, and where in the UK you are located. The table below gives indicative price ranges for 2026.

Unit Type	Supply Only	Supply & Install
Standard clear float (4-16-4)	£35 – £75	£80 – £145
Low-E with argon (4-16-4)	£45 – £95	£90 – £165
Toughened Low-E with argon	£55 – £120	£110 – £190
Acoustic laminated	£70 – £140	£130 – £220
Triple glazed Low-E with argon	£90 – £180	£160 – £280

Factors affecting price. Larger units cost more than smaller ones – pricing is calculated per square metre. Non-standard shapes (arches, circles, trapezoids) carry a premium due to the additional manufacturing complexity. Toughened and laminated glass costs more than standard annealed. Location plays a role too: unit replacement cost tends to be higher in London and the South East due to higher labour rates. Delivery charges vary by supplier and order size – many offer free delivery for orders above a certain threshold.

For a more detailed breakdown of pricing by glass type, size, and region, visit our comprehensive replacement costs guide.

Building Regulations

One of the advantages of replacing double glazing glass on a like-for-like basis is that it generally does not require building regulations approval. However, there are important nuances to be aware of.

Like-for-like replacement. If you are simply replacing a failed sealed unit with a new unit of the same (or better) specification in the same frame, this is classified as a repair rather than a new installation. Repairs do not normally require building regulations approval. You are free to carry out this work yourself or hire any installer.

When approval may be needed. If you are replacing more than 25% of the total glazed area of the property – for example, as part of a large-scale refurbishment – the work may be classified as a new installation rather than a repair, and Part L of the Building Regulations (Conservation of Fuel and Power) may apply. Part L sets minimum thermal performance standards for replacement glazing: currently a maximum whole-window U-value of 1.4 W/m²K for windows in England and Wales. In Scotland, the requirements are set out in Section 6 of the Building Standards, with slightly different thresholds.

Safety glass requirements. Regardless of whether the work is classified as a repair or a new installation, the glass used must comply with the safety requirements of Approved Document N (England and Wales) or the equivalent Scottish and Northern Irish standards. This means that glazing in critical locations – below 800 mm from floor level, within 300 mm of a door, and in doors themselves – must be toughened or laminated safety glass conforming to BS EN 12150 (toughened) or BS EN 14449 (laminated).

Building Regulations self-certification. Competent person schemes are government-authorised programmes that allow registered installers to self-certify that replacement window installations comply with Building Regulations, without you needing to involve your local authority's building control department. If your replacement work goes beyond a simple like-for-like repair – for example, you are changing window sizes, replacing frames, or upgrading a significant proportion of the glazing – using a qualified installer simplifies the compliance process and provides you with a certificate that you can present when selling the property.

We supply and install replacement double glazed units across London, Manchester, Birmingham, Nottingham, Sheffield, and areas throughout the UK. Whether you need a single unit or a full property reglaze, our team is ready to help.

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