Heritage-aperture double glazing where rebate depth is the constraint. A 14mm slim krypton-filled sealed unit (4mm clear / 6mm krypton cavity with soft-coat low-E / 4mm clear) is the conservation-officer-friendly double-glazing spec — engineered for Georgian and Victorian timber sashes, listed-building consents, and conservation-area replacement where a 24mm or 28mm unit will not fit the original rebate. Slim double-glazed units exist for one reason: the original glazing rebate in a pre-1920s timber sash was cut for 4mm single glazing — typically 8-12mm deep. A standard 24mm modern unit simply will not seat in that rebate without re-cutting the sash (which most listed-building consents prohibit) or fitting bulky surface-mounted secondary glazing (which conservation officers usually reject on sightline grounds). Slim units squeeze a true argon-or-krypton-filled IGU into a 14mm overall depth — sometimes less — and let original joinery stay intact. Why krypton, not argon, in this build Cavity gas performance is a function of cavity depth. In a 16mm cavity (the standard 24mm unit) argon is optimal — krypton offers no further U-value benefit and costs five times more. Below 10mm, argon's convective performance degrades and krypton becomes the better insulator. At a 6mm cavity (this unit), krypton drops centre-pane U-value from ~2.0 (argon) to ~1.6 (krypton) — a meaningful 20% improvement at a cavity depth where every fraction of a watt matters. Technical specification Overall thickness 14mm (nominal — tolerance ±0.5mm) Build-up 4mm clear / 6mm krypton cavity / 4mm clear with Planitherm Total+ soft-coat (face 3) Gas fill ≥90% krypton (BS EN 1279-3 verified) Spacer bar Warm-edge composite, black slim-profile (typ. 5-6mm) U-value (centre-pane) 1.6 W/m²K U-value (whole window, sash + glass) ~1.9-2.1 W/m²K depending on frame g-value 0.63 Light transmittance ~78% Sightline / cavity-edge appearance ~12-15mm visible from outside (matches single-glazed sightline within heritage tolerance) Standards BS EN 1279 (parts 1-6), BS EN 673 (U-value), Historic England slim-DG guidance compatible Where this unit is the right call Listed buildings (Grade II, II*, I) where the conservation officer has accepted slim double glazing as a "like-for-like" replacement of failed single glazing. Conservation area consents in cities like Bath, Edinburgh, Brighton, and Oxford where original sash sightlines are protected. Georgian, Regency, and Victorian timber sashes with original glazing rebates 8-14mm deep — fits without re-rebating. Heritage shopfronts and ecclesiastical glazing where leaded-light, crown-glass, or stained-glass aesthetics must be preserved at the sightline. Properties in Article 4 direction areas where windows are subject to planning control and full-thickness DG would breach consent conditions. Slim double glazing's tradeoffs, conservation-officer evidence-pack templates, and the case for slim Krypton over secondary glazing are covered in depth on the slim double glazing topical page — including a checklist for evidencing slim DG to a conservation officer. Stock sizes and pricing Five UK-standard stock sizes are listed below. Each is dispatched 10-14 working days from order (slim-cavity krypton manufacturing is a specialist process — longer lead time than standard 24mm DG). Custom sizes — including arched-top, raked, leaded-effect overlay, and conservation-bar-compatible variants — are priced via the online glass configurator. Note on listed-building consent: these units are manufactured to a specification accepted by most UK conservation officers under "minimal intervention" principles, but consent is always property-specific. We can supply a technical pack (cavity-depth diagram, gas-retention test data, BS EN 1279 DoP) for inclusion in your application — request via our contact form at order. Certification and standards UKCA-marked, BS EN 1279 compliant across all six parts. Krypton gas fill is verified per BS EN 1279-3 at initial fill ≥90% with annual loss
