Energy-Rated Glass Units — U-Values, WER Classes, and the UK Specification Stack for Thermal Performance
Energy-rated glass units are sealed insulating glass assemblies whose thermal performance is calculated to BS EN 673 and, in a whole window, rated against the British Fenestration Rating Council (BFRC) Window Energy Rating (WER) scheme — a U-value in W/m²K plus a class from G up to A++. This page covers the specification predicates that drive thermal performance — low-E coatings, gas fills, warm-edge spacers, cavity widths, frame Uf-values — how they combine to meet Approved Document L 2021 for replacement glazing, and where the cost-benefit lines fall between standard double, A-rated double, and triple glazing for UK projects.
U-values, WER classes, and Approved Document L
The U-value is heat loss per square metre per degree of temperature difference. Lower is better. For glazing, U-values are calculated to BS EN 673:2011 for the centre-pane (Ug), BS EN ISO 10077-1/2 for the whole-window (Uw) including frame and edge effects, and BS EN ISO 12567 when hot-box tested. Centre-pane Ug on a datasheet flatters the whole-window Uw a building inspector checks — spacer and frame add 0.2–0.4 W/m²K that Ug ignores.
In England and Wales, Approved Document L 2021 (effective 15 June 2022) sets the statutory thermal floor. For replacement windows in an existing dwelling, Uw must be no worse than 1.4 W/m²K, or the window must achieve a BFRC Band B rating or better. For new dwellings the limiting Uw is tighter at 1.2 W/m²K and forms part of the SAP calculation. Scotland (Section 6) and Wales currently sit at 1.4 W/m²K for replacement glazing with their own SAP frameworks.
The BFRC Window Energy Rating scheme is the consumer-facing label. It combines solar heat gain (g-value), thermal transmittance (Uw), and air leakage (L-factor) into a single Energy Index in kWh/(m²·year), mapped onto an A++ → G band. Most volume uPVC windows hit WER A with a standard build-up (4 mm soft-coat low-E / 16 mm argon / 4 mm clear, warm-edge spacer, 70 mm five-chamber uPVC frame). WER A+ needs an upgraded frame, premium low-E (e.g. Planitherm One), or krypton; WER A++ typically requires triple glazing and a timber-aluminium-clad or premium uPVC frame.
Three physical predicates do most of the thermal work. Low-E coating is a vacuum-deposited metallic film: silver-based "soft-coat" sputtered low-E (Pilkington Optitherm, Saint-Gobain Planitherm) gives Ug ≈ 1.1 W/m²K in a 4/16Ar/4 build but must be sealed inside the unit because it tarnishes; pyrolytic "hard-coat" (Pilkington K Glass) is more robust but performs less well at Ug ≈ 1.6. Cavity gas fill is argon at 90% (the BFRC-declared standard) or krypton (denser, narrower cavity). Argon at 16 mm is optimum for double; krypton at 10–12 mm matches it in a slimmer unit; triple glazing uses two 18–20 mm argon cavities. Warm-edge spacer replaces aluminium with a low-conductivity composite — Swisspacer Ultimate, Edgetech Super Spacer, or Thermix TGI — improving edge psi-value by ~0.05 W/m·K and lifting whole-window Uw by ~0.1.
Specification predicates buyers compare on
The attributes below are the buyer-intent predicates we see most often in trade enquiries, building-control submissions, and architect specifications. Use them to compare like-for-like — a Ug figure quoted in isolation is not directly comparable to a Uw figure.
| Predicate | Typical range | Where it matters |
|---|---|---|
| Centre-pane U-value (Ug, W/m²K) | 1.0 – 1.2 double low-E argon; 0.5 – 0.7 triple low-E argon | Glass-only comparison; BS EN 673 calculated |
| Whole-window U-value (Uw, W/m²K) | 1.2 – 1.4 A-rated double; 0.8 – 1.0 triple in good frame | Approved Document L compliance; SAP calculation input |
| Window Energy Rating (WER, BFRC) | A++ / A+ / A / B (compliance band B+ for Part L) | Consumer-facing label; trade promotion; FENSA certificate |
| Low-E coating type | Soft-coat (Pilkington Optitherm, SG Planitherm); hard-coat (Pilkington K) | Soft-coat for sealed units; hard-coat for secondary / toughened |
| Cavity gas fill | Argon 90% (standard); krypton (premium narrow cavity); xenon (rare) | Drives Ug; argon = 16 mm optimum; krypton = 10–12 mm optimum |
| Cavity width | 12 mm krypton; 16 mm argon double; 18–20 mm argon triple | Convection optimum; narrower or wider degrades performance |
| Warm-edge spacer | Swisspacer Ultimate, Edgetech Super Spacer, Thermix TGI | Edge psi-value; condensation resistance at perimeter |
| Solar factor (g-value) | 0.50 – 0.65 standard low-E; 0.30 – 0.45 solar-control | Passive solar gain trade-off; overheating risk in south elevations |
| Light transmittance (LT%) | 70 – 80% standard low-E; 50 – 65% solar-control | Daylight quality; affects BREEAM Hea 01 and SAP daylight credit |
| Frame Uf-value (W/m²K) | 1.0 – 1.4 uPVC; 1.6 – 2.2 thermally-broken aluminium; 1.0 – 1.3 timber-aluminium clad | Whole-window Uw — frame is 25–30% of the area |
| Air permeability class (BS EN 12207) | Class 3 minimum; Class 4 for passivhaus | Air leakage L-factor in BFRC index; affects Approved Document L1B |
| Approved Document L 2021 compliance | Uw ≤ 1.4 (replacement) or WER ≥ B; Uw ≤ 1.2 (new build) | Building Control sign-off; FENSA self-certification |
Where the spec lands — retrofit, new-build, and heritage
Retrofit into existing frames
The most common Pane Relief order is a sealed unit replacement into an existing uPVC, aluminium, or timber frame where the original IGU has misted or its perimeter seal has failed. Sash-rebate depth is fixed by the frame — a 44 mm triple unit will not fit a sash designed for 24 mm double. Best-in-class for a 24 mm replacement is 4 mm soft-coat low-E / 16 mm argon (90%) / 4 mm clear, warm-edge spacer, Ug ≈ 1.1 W/m²K, giving whole-window Uw around 1.3–1.4 in a 70 mm uPVC frame — comfortably inside the Part L floor. If the frame accepts 28 mm, 4/20/4 drops Ug to ≈1.0. Triple is rarely feasible as a retrofit; "best double" beats "shoehorned triple". Configure a replacement A-rated unit for retrofit pricing.
New build and full-window replacement
When frame and glass are both replaced, the spec can be optimised holistically. The new-build target is WER A or A+ at Uw ≤ 1.2 W/m²K, hit with a 70 mm uPVC frame (Uf ≈ 1.0) plus 4/16Ar/4 soft-coat low-E. WER A++ needs a timber-aluminium-clad frame (Uf ≈ 1.0–1.2) plus a 4/18Ar/4/18Ar/4 triple-glazed dual low-E unit (Ug ≈ 0.6), giving Uw ≈ 0.8. Thermally-broken aluminium tops out around Uw 1.4 even with triple. For passivhaus retrofit, triple with two soft-coat surfaces at 18 mm argon, in a passivhaus-certified frame, is baseline.
Listed buildings and conservation areas
Heritage trade-offs are the hardest in this category. Approved Document L permits departures from the 1.4 W/m²K limit where compliance "would unreasonably alter the character" of a listed building or building in a conservation area. Three paths: slim-cavity double (4 / 8–12 krypton / 4, Ug ≈ 1.5–1.7) for historic sashes; secondary glazing (hard-coat low-E single pane inside the original sash, combined Uw ≈ 1.4–1.8, fully reversible — often the conservation officer's preference); or vacuum-insulated glass (Pilkington Spacia, NSG FineoVIG; Ug ≈ 0.7–1.1 in 6–8 mm overall). The LPA is the binding decision-maker. See Conservation area glazing.
Standard double vs A-rated double vs triple — like-for-like
The three build-ups below are the most-quoted UK replacement-window thermal tiers. All assume a 70 mm five-chamber uPVC frame (Uf ≈ 1.0 W/m²K); whole-window Uw will shift ±0.1 depending on frame and opening.
| Build-up | Ug (centre-pane) | Uw (whole-window) | Typical WER class |
|---|---|---|---|
| Standard double — 4/12Air/4 clear, aluminium spacer (pre-2002 legacy) | 2.8 W/m²K | 2.8 – 3.0 W/m²K | Below G — does not meet Part L |
| Building Regs minimum — 4/16Ar/4 hard-coat low-E, warm-edge spacer | 1.6 W/m²K | 1.6 – 1.8 W/m²K | C / D — fails 2021 Part L by margin |
| A-rated double — 4/16Ar(90%)/4 soft-coat low-E, warm-edge spacer | 1.1 W/m²K | 1.2 – 1.4 W/m²K | A (standard) / A+ (premium frame) |
| Triple-glazed — 4/18Ar/4/18Ar/4 dual soft-coat low-E, warm-edge spacer | 0.6 W/m²K | 0.8 – 1.0 W/m²K | A++ (with quality frame) |
The honest trade-off: moving from Building Regs minimum to A-rated double is the highest-leverage thermal upgrade — 8–15% extra cost for a 30–35% heat-loss reduction, payback 4–8 years at current 2026 gas tariffs. Moving from A-rated double to triple adds 25–40% cost for 15–20% further reduction; payback 12–25 years. For most southern-England replacement projects A-rated double is optimum; for new-build, passivhaus, Scottish, and exposed sites, triple is the better answer. Run a thermal estimate.
Standards bodies and regulatory references
This page references the following UK standards bodies, regulators, manufacturers, and trade associations. Their published documents are the authoritative source for specification decisions.
- British Fenestration Rating Council (BFRC) — operator of the UK Window Energy Rating scheme. Publishes the Energy Index calculation method, A++ to G band labels, and the register of certified products.
- Approved Document L — Conservation of fuel and power — statutory thermal performance baseline for buildings in England. Sets the Uw ≤ 1.4 W/m²K floor for replacement glazing and limiting fabric values for new dwellings.
- BSI Group — publisher of BS EN 673 (thermal transmittance calc), BS EN ISO 10077-1/2 (whole-window calc), BS EN ISO 12567 (hot-box test), BS EN 1279 (IGU durability), BS EN 12207 (air permeability).
- Glass and Glazing Federation (GGF) — UK trade body. Publishes the GGF Glazing Manual and bulletins on energy-rated glazing and Part L compliance routes.
- FENSA — Fenestration Self-Assessment Scheme. FENSA-registered installation self-certifies compliance with Approved Documents K and L for replacement windows in dwellings.
- Pilkington (NSG Group) — UK manufacturer of K Glass (hard-coat low-E), Optitherm (soft-coat low-E), Spacia (vacuum-insulated glass), and Suncool (solar-control low-E).
- Saint-Gobain Building Glass — manufacturer of Planitherm (soft-coat low-E), Planitherm One (premium dual-silver), and Cool-Lite (solar-control). Publishes whole-window Uw modelling tools.
- Swisspacer — warm-edge spacer manufacturer. Publishes psi-value tables for BS EN ISO 10077-2 whole-window calculations.