Glazing Explained: U-values, Low-E, Argon & Spacers

The thermal performance of a window depends on every layer in the assembly, the glass panes, the gas trapped between them, the coating applied to the glass surface and the edge spacer holding the panes apart. Each element contributes to the overall U-value (the rate of heat loss per square metre per degree of temperature difference, measured in W/m²K). Understanding how these components interact allows you to specify a window that meets Building Regulations, controls solar gain under Part O and delivers long-term energy savings.

Glass Build-Ups: Single, Double and Triple

Single glazing (one pane, 4–6 mm glass) has a centre-pane U-value of approximately 5.8 W/m²K. It is now only found in older properties and is not compliant with Part L for any replacement scenario.

Double glazing consists of two panes with an air or gas-filled cavity between them, typically 12–20 mm wide, sealed within an Insulated Glass Unit (IGU). A basic 4-12-4 air-filled unit (4 mm glass, 12 mm air gap, 4 mm glass) achieves around 2.6–2.8 W/m²K centre-pane. Cavity width matters: the optimum for air-filled units is 14–16 mm, wider cavities allow convection and reduce performance.

Triple glazing adds a third pane and two gas-filled cavities. A typical 4-16-4-16-4 argon-filled Low-E unit achieves 0.5–0.8 W/m²K centre-pane. The additional weight (typically 30–40% heavier than equivalent double glazing) requires heavier frames and stronger opening mechanisms. Triple glazing adds cost of roughly £60–£100 per m² over equivalent double-glazed units.

Low-E Coatings

Low-emissivity (Low-E) coatings are microscopically thin metallic oxide layers, typically silver or tin oxide, applied to one surface of the glass inside the sealed unit. They work by reflecting long-wave infrared radiation (heat) back into the room while allowing short-wave solar radiation (visible light and near-infrared) through from outside.

There are two principal manufacturing routes:

• Pyrolytic (hard coat) Low-E: baked into the glass surface during manufacture. More durable and can be cut after coating. Lower performance than soft coat, typical emissivity (ε) of 0.15–0.20.
• Sputtered (soft coat) Low-E: applied in a vacuum deposition process after manufacture. Achieves emissivities as low as 0.02–0.04 but must be sealed within the IGU as it is easily damaged if exposed.

The coating is identified by its position within the unit. For double glazing, the coating is placed on surface 3 (the inner face of the outer pane, counting from outside to inside). For triple glazing, coatings are placed on surfaces 2 and 5.

Solar factor (g-value): Low-E glass also has a solar factor, the proportion of solar energy that passes through. A high g-value (e.g. 0.60–0.72) admits more solar heat gain (desirable on south-facing elevations in winter) while a low g-value (0.30–0.45) limits solar gain (used in Part O calculations to prevent overheating). Specifying the wrong g-value on south or west elevations can lead to overheating failures under Part O (the 2021 edition, in force from 15 June 2022).¹

Gas Fills

The gas fill between panes reduces convection and conduction across the cavity. Air (conductivity ≈ 0.024 W/mK) is the baseline.

Argon is by far the most common fill in UK residential glazing. It is inert, non-toxic, cheap relative to krypton, and the optimum cavity width for argon is 14–16 mm, narrower than air-optimised units, which suits thinner frame designs. BS EN 1279-3 is the standard behind the gas retention claims, and it is worth knowing what it actually says: it sets a maximum leakage rate, not a lifetime retention figure, and the limit is 1% of the fill per year. A unit at exactly that limit is down to about 78% of its original argon after 25 years, so the 90 to 95% over 25 years that gets quoted is a manufacturer’s performance against the standard rather than something the standard promises. Ask for the tested leakage rate rather than the retention headline.²

Krypton is used where thin cavities are required (e.g. secondary glazing retrofits or slim-profile triple-glazed units) because its optimum cavity is 8–10 mm. It is roughly five to eight times the cost of argon.

Warm-Edge Spacers

The edge spacer separates the panes around the perimeter of the IGU and is bonded into the hermetic seal. Aluminium was historically the standard material, still found in older units, but aluminium is highly conductive (conductivity ≈ 160 W/mK) and creates a thermal bridge around the window edge, lowering the edge U-value and causing cold-edge condensation.

Warm-edge spacers replace aluminium with lower-conductivity alternatives:

The linear thermal transmittance of the edge (the psi-value, ψ, in W/mK) determines how much the edge spacer affects the whole-window U-value. Warm-edge spacers reduce ψ from approximately 0.08 W/mK (aluminium) to 0.04–0.05 W/mK (stainless) or 0.03–0.04 W/mK (polymer foam). On a small window this may improve the whole-window U-value by 0.1–0.2 W/m²K, meaningful for compliance calculations.

BFRC (British Fenestration Rating Council) whole-window U-values include the edge effect; manufacturer centre-pane U-values do not. Always request the whole-window U-value when specifying for Part L compliance.

Part L (Thermal) and Part O (Overheating)

Approved Document L Volume 1 (2021 edition, in force from 15 June 2022, so often called the 2022 rules) sets a limiting U-value of 1.4 W/m²K for a replacement window in an existing dwelling, or Window Energy Rating Band B as an alternative route. In a new dwelling the limiting value is looser at 1.6 W/m²K, because new build is controlled by the target rates rather than by the limit: the notional dwelling that sets those targets assumes 1.2 W/m²K, which is why 1.2 is the figure a new build usually has to reach in practice.³ Both are whole-window values.

Standard compliance routes:

• Single window replacement: must achieve ≤1.4 W/m²K whole-window
• Extension or new build: follows SAP/SBEM calculation, but a 1.2 W/m²K window specification is typical for a compliant elemental approach

Approved Document O (2021 edition, in force from 15 June 2022) applies to new dwellings. Its risk category is not something glazing can push you into: under the simplified method the category is fixed by location (a high risk location is urban and some suburban parts of London, with guidance for parts of central Manchester; everywhere else in England is moderate risk) and by whether the dwelling has cross-ventilation, meaning openings on opposite façades.¹ What glazing decides is whether you then pass, because the category selects a maximum glazing area for the building and for the most glazed room, taken on the most glazed façade’s orientation. The mitigation options include reducing window-to-floor-area ratio, specifying lower g-value glass, adding external shading, or relying on thermal mass. Large south-facing rooflights and bifold/sliding door walls are common triggers. A dynamic thermal model (CIBSE TM59 methodology) may be required if the simplified method fails.

Safety Glass: Part K

Approved Document K protects against impact with glazing in what it calls critical locations, and the list is narrower than the one that gets repeated on merchant websites:

• In a door or a door side panel, from floor level up to 1,500 mm
• In a side panel within 300 mm of the edge of a door, up to 1,500 mm
• Elsewhere in a wall or a window, from floor level up to 800 mm

Being in a bathroom is not itself a critical location. What matters is the height above the floor and the proximity to a door, and a bathroom window below 800 mm qualifies for that reason rather than because of the room it is in. Safety glazing is also only one of four ways Part K allows you to satisfy it: glass that breaks safely, glazing that is inherently robust, glazing in small panes, or glazing that is permanently protected. Where the safe-breakage route is used, the classification to look for is Class 3 of BS EN 12600 or Class C of BS 6206, or Class 2 / Class B in a door or side panel wider than 900 mm.⁴ BS EN 12150-1 and BS EN 14179 are the product standards for toughened and heat-soaked toughened glass, which is a different question from whether the pane passes the impact test.

Safety glass must be either toughened (tempered) or laminated. Laminated glass (PVB interlayer) is generally preferred for overhead applications and rooflights where broken fragments must remain in place; toughened glass shatters into small blunt pieces but falls freely.

Typical Glazing Specifications and U-values

*Indicative whole-window values based on typical PVC-U casement frames. Timber and aluminium frames vary.

For most UK residential replacement projects, a 4-16-4 argon-filled unit with a soft-coat Low-E and a polymer warm-edge spacer in a uPVC or timber frame reliably achieves 1.0–1.2 W/m²K whole-window and represents the best value-for-money specification at roughly £80–£130 per m² supply-only for the IGU, plus frame and installation costs.

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.