12mm Histoglass-Equivalent Slim Conservation Unit
Listed-building-approved sightline with double-glazed thermal performance. A 12mm slim double-glazed sealed unit (4mm clear / 4mm krypton cavity / 4mm clear with Planitherm Total+ soft-coat) — built to match the appearance and sightline of the original single-glazed unit it replaces, while delivering centre-pane U-value of ~1.6 W/m²K. The Histoglass-equivalent spec is the standard offering UK conservation officers approve for Grade II and Grade II* listed sash and casement windows. Histoglass is a registered trademark of Cambridge-based Slim-Lite Glass, the original UK slim-cavity-Krypton specialist. The "Histoglass equivalent" specification is the industry shorthand for a 12mm sealed unit with a krypton-filled narrow cavity (4mm) and a low-E inner pane, manufactured to match the sightline of a 4mm or 6mm single-glazed pane within ±1mm tolerance. Conservation officers regularly approve this spec because the unit reads as single-glass-equivalent from the street, while the homeowner gets meaningful thermal performance. Technical specification Overall thickness 12mm ± 0.5mm Build-up 4mm clear / 4mm krypton-filled cavity / 4mm clear with Planitherm Total+ soft-coat (face 3) Gas fill 90% krypton (BS EN 1279-3) Spacer bar Narrow steel spacer, black powder-coated — reads as a dark hairline at sightline edge U-value (centre-pane) ~1.6 W/m²K g-value 0.60 Light transmittance ~77% Sightline match ≤1mm from 4mm single-glazed original — conservation-officer-approved appearance Standards BS EN 1279, BS EN 673, BS EN 410, Historic England Technical Guidance Note (HETGN) on slim-cavity glazing Where this unit is the right call Grade II and Grade II* listed buildings with conservation-officer approval for slim DG retrofit — the standard spec your local authority's conservation officer will likely permit. Confirm in writing before ordering; consent requirements vary by authority. Conservation areas with Article 4 directions restricting window changes — slim DG with sightline-match spec typically clears Article 4 requirements where full-fat double does not. Georgian and Victorian sash windows being upgraded thermally without losing the original lambs-tongue glazing-bar profile — the 12mm slim unit drops into rebated bars where 24mm units force bar profile changes. Period casement windows in heritage settings — Edwardian cottage casements, Arts and Crafts ranges, Lutyens and Voysey heritage projects. Properties where full single-pane replacement is uneconomic but planning constraints rule out 24mm DG — the slim 12mm sits in the gap: better than single, conservation-compliant, half the cost of true Histoglass-branded equivalent. For even thinner builds where 12mm overall is still too visible at the rebate, the 11.5mm Ultra-Slim Conservation Double Glazed Unit is the next step down — at the edge of commercial manufacturing yield. For 14mm slim with more cavity volume (slightly better thermals, slightly more visible at sightline), see the 14mm Slim Krypton Conservation Double Glazed Unit. The slim double glazing topical page walks the conservation-glazing decision tree in depth. Stock sizes and pricing Four heritage-window-appropriate stock sizes below — the smaller bespoke market for listed-building glazing means we cap at 1200×1000mm; for larger heritage apertures, custom-sized units are priced via the configurator. Dispatch is 14-21 working days from order (slim-cavity krypton-fill manufacturing is sequential and yield-sensitive). Trade and conservation-architect accounts: tiered pricing on log-in. Certification and standards UKCA mark, BS EN 1279 Declaration of Performance, krypton-fill certificate (BS EN 1279-3 tested at ≥85% initial fill), narrow steel spacer identification, Planitherm low-E coating face identification (face 3), and a Listed-Building Compatibility Statement on request — referencing the Historic England Technical Guidance Note (HETGN) on slim-cavity glazing for sash and casement windows. For conservation-officer submission, we supply a CAD-rendered sightline comparison (original single vs replacement 12mm slim) on request. Frequently asked questions Will my conservation officer actually approve this? Approval is case-by-case and authority-by-authority. The 12mm Histoglass-equivalent spec is on the more-permissive end of conservation glazing — most authorities permit it where 24mm DG would be refused, on the basis that the sightline match preserves the original window appearance. We supply a Listed-Building Compatibility Statement with each order for conservation-officer submission. Get the consent in writing before ordering; conservation-officer rules vary widely between Westminster, Cambridge, Bath, and rural authorities. How is this different from genuine branded Histoglass? Genuine Histoglass (Slim-Lite Glass, Cambridge) is the trademark-protected original — manufactured to a specific specification with documented production records used in many high-profile listed-building consents. Our 12mm Histoglass-equivalent uses the same build-up (4-4-4 krypton + low-E) and meets the same sightline tolerance, manufactured to BS EN 1279, but cannot use the Histoglass brand name. Performance is equivalent; the brand is the difference. For projects where the consent specifies "Histoglass or equivalent", our spec satisfies the "equivalent" clause. What's the lifespan of slim-cavity krypton? 10-15 years before edge-seal degradation, vs 15-25 years for standard double glazing. The narrower cavity puts more thermal cycling stress on the edge seal, and krypton's higher molecular weight (vs argon) means slower fill loss but tighter seal-integrity tolerances. We warrant 10 years on edge-seal integrity; realistic field life is 12-15 years before misting becomes likely on south-facing aspects. U-value vs full-fat double glazing — what am I giving up? Centre-pane U-value: 12mm slim krypton = ~1.6 W/m²K vs 28mm Low-E argon double = 1.1 W/m²K. The slim unit is ~45% worse on paper. In practice, the slim unit is still ~3× better than the original single-glazed pane (single = 5.7 W/m²K). For heritage applications, the conservation-officer-compliance constraint trumps the thermal-optimisation argument — you can't fit 28mm in the original rebate without losing the listed-building consent. Will I see the spacer at the edge? The narrow black powder-coated steel spacer reads as a dark hairline at the rebate edge — typically 4-6mm wide visible from inside, less from outside (the original putty or beading covers most of it). Most homeowners find this acceptable; some conservation officers require black aluminium foil tape over the spacer to further reduce visual signature. Can I get this in shaped (arched, raked) for Gothic-revival windows? Yes — at the edge of manufacturing yield. Arched and raked slim-cavity krypton units cost roughly 1.8-2.0× the equivalent rectangular unit, with lead times of 21-28 working days. Send a physical cardboard template or accurate CAD for shape-cut. Warranty in field service? 10 years on edge-seal integrity (BS EN 1279-2 accelerated weathering). 10 years on krypton retention to ≥80% of initial fill. 10 years on Planitherm low-E coating performance. Transit damage covered 14 days from delivery — slim units are more fragile than full-fat double; brief the installer on handling protocol before delivery.