The A-rated upgrade. A 28mm low-E argon-filled sealed unit (4mm clear / 20mm cavity with planitherm soft-coat / 4mm clear) delivers a centre-pane U-value of 1.1 W/m²K and a Window Energy Rating of A under BFRC certification — the spec UK building inspectors expect for Approved Document L compliance on replacement glazing. "Low-E" means low-emissivity: a microscopically-thin metallic-oxide coating on the inside face of the inner pane that reflects long-wave heat radiation back into the room while letting visible light through. The Pilkington K-Glass (hard-coat) and Optitherm S1 / Planitherm Total+ (soft-coat) families are the UK-standard options — soft-coat performs better thermally but must be sealed inside the cavity (it oxidises if exposed to air, which is why it always faces the cavity, never the room). Technical specification Overall thickness 28mm Build-up 4mm clear / 20mm argon cavity / 4mm clear with Planitherm Total+ soft-coat (face 3) Gas fill 90% argon (BS EN 1279-3) Spacer bar Warm-edge composite, black (low thermal bridging) U-value (centre-pane) 1.1 W/m²K U-value (whole window with PVCu frame) 1.4 W/m²K g-value 0.62 Light transmittance ~78% BFRC Window Energy Rating A (when fitted in a compliant frame) Standards BS EN 1279, BS EN 673 (U-value), BS EN 410 (light/solar) Where this unit is required Building Regs Approved Document L — Part L1B requires replacement window units to meet a U-value of 1.4 W/m²K or better. A 28mm Low-E argon unit clears that with 0.3 W/m²K of headroom. New build and significant extensions — Local building control will check that replacement glazing in extensions ≥ 30% of floor area meets Part L. This unit is the default A-rated spec. Cold-room retrofits — North-facing bedrooms, single-glazed conservatories being upgraded, rooms above unheated garages. The 1.1 U-value cuts radiant heat loss roughly 22% vs a standard 24mm argon unit. EPC band improvement — Upgrading to A-rated glazing typically lifts an EPC by 5-10 points depending on the existing baseline. For landlords, this is a route to MEES C compliance ahead of the 2028 deadline. Energy-rated glass selection in depth is covered on the energy-rated glass topical page — U-value vs g-value tradeoffs, BFRC vs CE-label differences, and how the WER calculation actually works. Stock sizes and pricing Five UK-standard stock sizes are below. Each is dispatched 5-7 working days from a FENSA-registered manufacturing partner. Custom sizes via the configurator at the same per-m² rate. Trade accounts see tiered pricing on login. Certification and standards Every unit ships with a UKCA mark, a unique IGU edge-stamp identifying the manufacturer batch, and a Declaration of Performance referencing BS EN 1279 (parts 1, 2, 3, 5, 6). The Planitherm coating carries Saint-Gobain's batch certification. BFRC ratings are issued on the whole-window assembly (frame + glass); the glass alone contributes the U=1.1 centre-pane figure, with the frame contributing the rest. Frequently asked questions What's the difference between A-rated, B-rated, and C-rated glass? The BFRC Window Energy Rating combines U-value (heat loss), g-value (solar gain), and L-factor (air leakage). A-rated requires WER ≥ 0 — meaning solar gain offsets heat loss over a year. C-rated allows WER -20. A 28mm Low-E argon unit in a standard PVCu frame is the simplest path to an A rating. Soft-coat vs hard-coat low-E? Hard-coat (Pilkington K) is pyrolytic — fused into the glass at the float-line, durable, can be toughened, slightly worse U-value (~1.5 W/m²K in a 28mm unit). Soft-coat (Planitherm/Optitherm) is sputter-deposited, higher performance (1.1 W/m²K), must live inside the sealed cavity. This flagship uses Planitherm Total+ on face 3. Will fitting A-rated glass alone improve my EPC? Yes — replacement glazing scores under "windows" in the EPC RdSAP model. The improvement depends on the previous baseline: replacing single-glazed sash windows with A-rated DG can add 8-12 EPC points; replacing first-generation DG (1990s) typically adds 3-5 points. Can I use this in a listed building? Listed-building consent is local-authority-dependent. Conservation officers usually accept slim or low-iron variants where sightline matters. The 28mm Low-E is too thick for most original Georgian sash rebates — for those, consider the slim Krypton variant. Does the Planitherm coating tint the glass? Slightly. There's a faint neutral/bronze haze visible at sharp viewing angles, particularly with the sun behind you. From inside looking out, the tint is invisible. Light transmittance drops from ~80% (uncoated) to ~78% — imperceptible in normal use. Warranty? 10-year manufacturer warranty on edge-seal integrity (BS EN 1279-2 weathering) and 5-year warranty on coating integrity (Planitherm batch certification). Argon retention warranted to 80% of initial fill at 10 years.
