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What Is Double Glazing? How It Works & Why It Matters
Quick Answer
Double glazing is a window construction method that uses two panes of glass separated by a sealed gap (typically 12-20mm) filled with air or an insulating gas such as argon. The sealed unit, also called an insulated glass unit (IGU), significantly reduces heat transfer through the window compared to single glazing. Double glazing is the minimum standard for all new and replacement windows in UK homes under Building Regulations and is the most effective single upgrade for improving a home's energy efficiency, comfort, and noise insulation.
At a Glance
Double glazing has become so ubiquitous in British homes that it is easy to take for granted, but the technology behind it is genuinely remarkable. A modern double-glazed window can reduce heat loss through the glass by up to 50-70% compared to single glazing, significantly cutting heating bills and improving comfort. For the millions of UK homes that still have single-glazed windows, or older double-glazed units from the 1980s and 1990s that are past their best, upgrading to modern double glazing is one of the most cost-effective home improvements available. This guide explains exactly how double glazing works, the science behind its insulating properties, how to understand energy ratings and U-values, current costs, and when triple glazing might be a better option.
How Double Glazing Works
Heat moves through windows by three mechanisms: conduction (heat travelling through solid materials like glass and frame), convection (warm air circulating within or around the window), and radiation (infrared heat radiating from warm surfaces to cold surfaces). Single glazing is a poor insulator because glass conducts heat readily and there is nothing to interrupt the heat flow from the warm interior to the cold exterior. Double glazing addresses all three heat transfer mechanisms. The sealed air or gas cavity between the two panes acts as an insulating barrier, dramatically reducing conduction. The narrow cavity width (optimally 16mm) is designed to minimise convective air currents within the gap. Low-emissivity (low-e) coatings on the inner surface of the outer pane reflect infrared radiation back into the room rather than allowing it to escape. The combination of these features is what makes double glazing so effective. A single-glazed window has a U-value of approximately 5.0-5.8 W/m²K. A modern double-glazed unit achieves 1.0-1.6 W/m²K, representing a 70-80% reduction in heat loss.
Components of a Double-Glazed Unit
A sealed double-glazed unit consists of several precision-engineered components. The outer pane is typically 4mm clear float glass, though thicker glass (6mm, 6.4mm laminated) can be specified for security or acoustic purposes. The inner pane is coated with a microscopic low-emissivity (low-e) metallic oxide layer, usually on surface 3 (the inner face of the inner pane in a standard unit), which reflects radiant heat back into the room. The spacer bar maintains the precise distance between the panes and contains desiccant (a moisture-absorbing material) that keeps the cavity dry and prevents internal condensation. Modern warm-edge spacers use thermally broken materials (stainless steel, composite, or foam) rather than aluminium, reducing heat loss at the glass edge by up to 65%. The cavity is typically filled with argon gas (replacing air), which has 34% lower thermal conductivity than air, improving the U-value by approximately 0.3 W/m²K. Krypton gas offers even better performance but at significantly higher cost. The entire assembly is sealed with a primary butyl rubber seal and a secondary polysulphide or silicone seal to prevent moisture ingress and gas leakage.
Understanding U-Values and Energy Ratings
The U-value measures how much heat passes through a window per square metre per degree of temperature difference between inside and outside. A lower U-value means better insulation. For replacement windows in England and Wales, Building Regulations (Approved Document L) require a maximum whole-window U-value of 1.4 W/m²K, or a minimum Window Energy Rating (WER) of Band C. The Window Energy Rating scheme, operated by the British Fenestration Rating Council (BFRC), provides a simpler A++ to E scale that accounts for heat loss (U-value), solar heat gain (g-value), and air leakage (L-value). A-rated windows are net energy gainers over a typical UK heating season, meaning they let in more energy from the sun than they lose. Most quality modern double-glazed windows achieve A or A+ ratings. When comparing windows, the whole-window U-value (which includes the frame) is more meaningful than the centre-pane U-value (which only measures the glass centre and ignores edge and frame losses). Always ask for the whole-window U-value when getting quotes.
Benefits of Double Glazing
Energy savings are the primary benefit. Replacing single glazing with modern double glazing can reduce heat loss through windows by 50-70%, potentially saving £100-£200 per year on heating bills for a typical three-bedroom semi-detached house. Comfort improves because double glazing eliminates cold spots near windows, reduces draughts, and raises the temperature of the inner glass surface, making rooms feel warmer even before the thermostat is adjusted. Condensation on the inner glass surface is significantly reduced because the inner pane stays warmer with double glazing. Noise reduction of 25-35 dB makes a noticeable difference for homes near roads, railways, or flight paths. Asymmetric glazing (different thickness panes) and laminated glass can improve this to 35-45 dB. Security is enhanced because double-glazed sealed units are significantly harder to break than single panes, and the internal glazing method used in modern windows means the glass cannot be removed from outside.
Types of Double Glazing Glass
Low-emissivity (low-e) glass has a microscopically thin metallic oxide coating that reflects infrared heat radiation. It is standard on all modern double-glazed units and is the single most important factor in achieving low U-values. Hard-coat low-e glass is applied during manufacture (pyrolytic process) and is slightly less effective but more durable. Soft-coat low-e glass is applied in a vacuum (sputtered process) and offers superior performance but must be protected within a sealed unit. Toughened (tempered) safety glass is heat-treated to be four to five times stronger than standard glass and breaks into small, relatively harmless granules rather than sharp shards. It is required by Building Regulations in certain locations including doors, side panels, low-level glazing, and windows near baths or showers. Laminated safety glass consists of two panes bonded with a polyvinyl butyral (PVB) interlayer that holds the glass together if broken. It offers superior security and acoustic performance. Self-cleaning glass has a special coating that breaks down organic dirt using UV light and allows rainwater to sheet off rather than forming droplets, reducing cleaning frequency.
Signs Your Double Glazing Needs Replacing
Misting or condensation between the panes is the most obvious sign of seal failure. When the perimeter seal breaks down, moist air enters the cavity and condenses on the inner glass surfaces, creating a cloudy or misty appearance that cannot be cleaned. This means the insulating gas has escaped and the unit's thermal performance is significantly degraded. Draughts around the window frame indicate that the weatherseals have deteriorated or the frame has moved, allowing air to pass between the sash and frame. Difficulty opening or closing the window may indicate that the frame has warped, hinges have worn, or the mechanism has failed. Visible damage to the frame such as cracking, warping, or discolouration (particularly with older uPVC) suggests the material is degrading. Excessive external noise that should be blocked by double glazing may indicate seal failure or an older unit with poor acoustic performance. If your double glazing was installed before 2002, it likely uses an older specification that falls well below current Building Regulations standards, and replacement will deliver significant energy savings.
Double Glazing and Building Regulations
Under Building Regulations in England and Wales (Approved Document L), replacement windows must achieve either a whole-window U-value of 1.4 W/m²K or better, or a Window Energy Rating of Band C or above. Installation must be carried out by a FENSA-registered installer or a local authority building control approved installer, who will provide a FENSA certificate as proof of compliance. This certificate is important for property sales, as solicitors routinely request evidence that replacement windows meet Building Regulations. Scotland has slightly different requirements under Section 6 of the Building Standards, with a maximum U-value of 1.6 W/m²K for replacement windows. Exemptions exist for listed buildings and buildings in conservation areas where compliance would unacceptably alter the building's character, but you must demonstrate to the local authority that you have explored all reasonable options to improve thermal performance.
How Long Does Double Glazing Last?
The sealed glass units in modern double-glazed windows typically last 20-35 years before seal failure occurs. Premium sealed units from manufacturers with comprehensive quality control may last even longer. The window frames themselves often outlast the sealed units: uPVC frames last 25-35 years, timber frames 40-60+ years with maintenance, and aluminium frames 40-50 years. When a sealed unit fails (evidenced by misting between panes), it can often be replaced within the existing frame without replacing the entire window, at a cost of approximately £50-£100 per unit plus fitting. This makes double glazing maintenance relatively affordable over its lifespan. The hardware (hinges, locks, handles) is typically the first component to require attention, usually after 10-15 years, and can be replaced at modest cost to extend the window's useful life.
Pros and Cons
Advantages
- Reduces heat loss through windows by 50-70% compared to single glazing
- Meets current Building Regulations for all UK replacement windows
- Significantly reduces external noise (25-35 dB reduction)
- Eliminates or greatly reduces condensation on interior glass surfaces
- Enhances security with toughened or laminated glass options
Disadvantages
- Significant upfront cost (£3,000-£8,000 for a full house)
- Sealed units have a finite lifespan of 20-35 years before potential seal failure
- Not as thermally efficient as triple glazing in the coldest UK locations
- Cannot be repaired if the seal fails - the entire sealed unit must be replaced
- Heritage restrictions may limit double glazing options in conservation areas
Cost Guide
Double glazing costs in the UK depend on window type, frame material, and the number of windows being replaced. Per window (fully installed): uPVC casement £300-£500, uPVC sash £600-£1,200, timber casement £500-£1,500, timber sash £800-£2,800, aluminium casement £600-£1,400. For a full house: a two-bedroom flat (4-6 windows) costs £1,500-£3,500, a three-bedroom semi (8-10 windows) costs £3,000-£6,000, and a four-bedroom detached house (12-15 windows) costs £5,000-£12,000, all in uPVC. Timber adds 50-100% and aluminium adds 40-80% to these prices. These figures include VAT, FENSA-registered installation, and disposal of old windows. Energy bill savings of £100-£200 per year mean a typical payback period of 15-30 years, though the comfort and noise benefits are immediate.
Is It Right for Your Home?
Double glazing is the right choice for virtually every UK home that currently has single glazing or outdated double glazing installed before 2002. It is the most cost-effective window upgrade available and is required by Building Regulations for all replacement windows. If you live in an extremely exposed or cold location (northern Scotland, high altitude) or are building a Passivhaus, triple glazing may offer worthwhile additional benefits. For most of England and Wales, high-quality double glazing with low-e glass, argon gas, and warm-edge spacers provides an excellent balance of performance and value. If your property is listed or in a conservation area, specialist slim-profile double-glazed units or secondary glazing may be the most appropriate option.
Frequently Asked Questions
Double glazing is a window construction using two panes of glass separated by a sealed gap (typically 16mm) filled with argon gas. This construction reduces heat loss by 50-70% compared to single glazing, cuts external noise by 25-35 dB, and reduces condensation. It is the minimum standard for UK replacement windows.
Double glazing works by trapping a layer of insulating gas between two panes of glass. The gas has low thermal conductivity, reducing heat conduction. The sealed cavity minimises convective air currents. A low-emissivity coating on the glass reflects infrared heat back into the room, reducing radiant heat loss.
Per window: uPVC casement £300-£500, uPVC sash £600-£1,200, timber £500-£2,800. Full house: three-bedroom semi £3,000-£6,000 (uPVC). Prices include installation and VAT. London adds 10-20%. Triple glazing adds £100-£200 per window.
Sealed glass units last 20-35 years. uPVC frames last 25-35 years, timber frames 40-60+ years, aluminium frames 40-50 years. Hardware (hinges, locks) may need replacement after 10-15 years. Failed sealed units can often be replaced within existing frames at £50-£100 per unit.
Condensation between the panes (misting) means the perimeter seal has failed, allowing moist air into the cavity. The insulating gas has escaped and thermal performance is reduced. The failed sealed unit needs replacing - it cannot be repaired. Replacement units can often be fitted into existing frames.
Double glazing is not mandatory for existing windows, but Building Regulations require that any replacement windows achieve a minimum energy performance (U-value of 1.4 W/m²K or WER Band C). In practice, this means double glazing as a minimum. Single-glazed windows can remain until they are replaced.
Triple glazing adds a third pane and second gas cavity, achieving U-values of 0.8-1.0 W/m²K versus 1.2-1.4 for double. It costs 20-40% more per window. For most of England and Wales, high-quality double glazing is sufficient. Triple glazing offers meaningful benefits in very cold or exposed locations.
Yes, standard double glazing reduces external noise by 25-35 dB. For better sound insulation, specify asymmetric glazing (different thickness panes, e.g., 6mm outer and 4mm inner) or laminated glass, which can achieve 35-45 dB reduction. The wider the cavity and the greater the pane thickness difference, the better the acoustic performance.
A FENSA certificate is proof that replacement windows have been installed by a FENSA-registered company and comply with Building Regulations. It is required by solicitors during property sales. If your installer is not FENSA registered, you need approval from local authority building control instead, which is more expensive and time-consuming.
Yes, but with restrictions. Conservation areas may require specific frame materials (usually timber), glazing bar details, and slim-profile double-glazed units. Slim units (14-16mm total) are available that fit within original timber frame rebates. Always consult your local planning authority before ordering replacement windows in a conservation area.
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