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 2022.
Gas Fills
The gas fill between panes reduces convection and conduction across the cavity. Air (conductivity ≈ 0.024 W/mK) is the baseline.
| Gas Fill | Thermal Conductivity (W/mK) | Approximate U-value Improvement vs Air | Cost Premium |
|---|---|---|---|
| Air | 0.024 | Baseline | None |
| Argon | 0.016 | ~0.15–0.2 W/m²K improvement | Low (~5–10%) |
| Krypton | 0.009 | ~0.3–0.5 W/m²K improvement | Moderate (~20–40%) |
| Xenon | 0.005 | ~0.5+ W/m²K improvement | High (rarely specified) |
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. Gas retention in a properly sealed IGU is 90–95% over 25 years according to BS EN 1279-3.
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:
| Spacer Type | Conductivity (approx.) | Material |
|---|---|---|
| Aluminium (legacy) | ~160 W/mK | Extruded aluminium hollow section |
| Stainless steel | ~15 W/mK | Thin rolled section |
| TPS/foam tape | ~0.3–0.4 W/mK | Thermoplastic polyurethane foam |
| Swisspacer / polymer hybrids | ~0.2–0.3 W/mK | Glass-fibre reinforced polymer |
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 2022 for existing dwellings sets a maximum replacement window U-value of 1.4 W/m²K (whole-window). New builds must meet the fabric efficiency requirements in the notional building specification; typical targets are 1.2 W/m²K or better.
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 2022 (overheating) applies to new dwellings. Glazing with a high solar factor on south, east and west elevations can push a building into Category 1 or 2 overheating risk. 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 requires safety glazing (to BS EN 12150-1 or BS EN 14179) in critical locations:
- Within 1,500 mm of floor level on doors and side panels
- Within 300 mm of a door edge
- Any glazing in bathrooms or shower enclosures
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
| Specification | Whole-Window U-value* | Suitable Application |
|---|---|---|
| 4-12-4 air, no Low-E | 2.6–2.8 W/m²K | Legacy; does not meet Part L |
| 4-16-4 argon, Low-E soft coat | 1.0–1.2 W/m²K | Replacement windows (compliant) |
| 4-14-4 argon, Low-E, warm edge | 1.1–1.4 W/m²K | Standard new-build specification |
| 4-16-4-16-4 argon, 2× Low-E, warm edge | 0.6–0.8 W/m²K | Triple glazing, high fabric spec |
| 4-8-4 krypton, Low-E (slim unit) | 1.0–1.2 W/m²K | Slim-profile heritage or secondary |
*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.
