{"product_id":"11-5mm-ultra-slim-conservation-double-glazed-unit","title":"11.5mm Ultra-Slim Conservation Double Glazed Unit","description":"\n\u003cp class=\"pr-flagship__lede\"\u003e\u003cstrong\u003eThe thinnest commercially viable sealed unit for the most demanding listed-building consents\u003c\/strong\u003e. An 11.5mm slim double-glazed sealed unit (3mm clear \/ 5.5mm krypton cavity \/ 3mm clear with Planitherm Total+ soft-coat) — the smallest cavity-and-pane combination that still passes BS EN 1279 edge-seal integrity testing. Below this thickness, manufacturing yield collapses and edge-seal warranties become uneconomic. Specified for Grade I and Grade II* listed buildings where 12mm overall is still too visible at the rebate.\u003c\/p\u003e\n\n\u003cp\u003eThe 11.5mm build is the bottom of the slim-cavity-krypton market. Most slim-DG suppliers stop at 12mm because the yield rate drops sharply below that — 3mm panes are at the edge of manufacturing tolerance for laminated edge-seal application, and 5.5mm cavities require tighter spacer-bar precision than standard production. We list this flagship because some Grade I consent processes specifically require sub-12mm overall thickness as a condition of approval. For most listed-building work, the \u003ca href=\"\/products\/12mm-histoglass-equivalent-slim-conservation-unit\"\u003e12mm Histoglass-Equivalent\u003c\/a\u003e is the right call; the 11.5mm exists for the cases that require it.\u003c\/p\u003e\n\n\u003ch2\u003eTechnical specification\u003c\/h2\u003e\n\u003ctable class=\"pr-spec-table\"\u003e\n  \u003ctr\u003e\n\u003cth\u003eOverall thickness\u003c\/th\u003e\n\u003ctd\u003e11.5mm ± 0.5mm\u003c\/td\u003e\n\u003c\/tr\u003e\n  \u003ctr\u003e\n\u003cth\u003eBuild-up\u003c\/th\u003e\n\u003ctd\u003e3mm clear \/ 5.5mm krypton-filled cavity \/ 3mm clear with Planitherm Total+ soft-coat (face 3)\u003c\/td\u003e\n\u003c\/tr\u003e\n  \u003ctr\u003e\n\u003cth\u003eGas fill\u003c\/th\u003e\n\u003ctd\u003e90% krypton (BS EN 1279-3, tighter fill-precision tolerance)\u003c\/td\u003e\n\u003c\/tr\u003e\n  \u003ctr\u003e\n\u003cth\u003eSpacer bar\u003c\/th\u003e\n\u003ctd\u003eUltra-narrow steel spacer, black powder-coated — typically 3-4mm visible at sightline\u003c\/td\u003e\n\u003c\/tr\u003e\n  \u003ctr\u003e\n\u003cth\u003eU-value (centre-pane)\u003c\/th\u003e\n\u003ctd\u003e~1.7 W\/m²K\u003c\/td\u003e\n\u003c\/tr\u003e\n  \u003ctr\u003e\n\u003cth\u003eg-value\u003c\/th\u003e\n\u003ctd\u003e0.60\u003c\/td\u003e\n\u003c\/tr\u003e\n  \u003ctr\u003e\n\u003cth\u003eLight transmittance\u003c\/th\u003e\n\u003ctd\u003e~78%\u003c\/td\u003e\n\u003c\/tr\u003e\n  \u003ctr\u003e\n\u003cth\u003eSightline match\u003c\/th\u003e\n\u003ctd\u003eSub-12mm — appears single-glazed from street at typical viewing distances\u003c\/td\u003e\n\u003c\/tr\u003e\n  \u003ctr\u003e\n\u003cth\u003eStandards\u003c\/th\u003e\n\u003ctd\u003eBS EN 1279, BS EN 673, BS EN 410, Historic England Technical Guidance Note (HETGN) on ultra-slim heritage glazing\u003c\/td\u003e\n\u003c\/tr\u003e\n  \u003ctr\u003e\n\u003cth\u003ePane thickness note\u003c\/th\u003e\n\u003ctd\u003e3mm clear is the edge of UK float-glass spec for sealed-unit production; sourced from Pilkington heritage-glass batch runs\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003c\/table\u003e\n\n\u003ch2\u003eWhere this unit is the right call\u003c\/h2\u003e\n\u003cul\u003e\n  \u003cli\u003e\n\u003cstrong\u003eGrade I listed buildings where the conservation-officer consent specifies sub-12mm overall thickness\u003c\/strong\u003e — typically late-Georgian and Regency town houses, country houses, ecclesiastical properties.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eGrade II* (Star-listed) properties with strict sightline-preservation conditions\u003c\/strong\u003e — heritage glazing where the 12mm spec was reviewed and rejected as still too visible at the rebate edge.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eOriginal crown-glass and cylinder-glass replacement\u003c\/strong\u003e in pre-Victorian sash windows where the original glazing was 2-3mm thick and the rebate is sized to match.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eLambs-tongue glazing-bar profile retention\u003c\/strong\u003e in fine Georgian and Federalist sash bars where 12mm overall is too thick for the rebate-shoulder profile.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eScheduled Monument and Grade A (Scotland) listed building consents\u003c\/strong\u003e where the heritage consent process specifies sub-12mm as a hard requirement.\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003cp\u003eFor most listed-building work, the \u003ca href=\"\/products\/12mm-histoglass-equivalent-slim-conservation-unit\"\u003e\u003cstrong\u003e12mm Histoglass-Equivalent Slim Conservation Unit\u003c\/strong\u003e\u003c\/a\u003e is the right call (cheaper, faster lead time, longer warranty). The 11.5mm is for the most demanding consents. For 14mm slim with slightly better thermals and full standard manufacturing yield, see the \u003ca href=\"\/products\/14mm-slim-krypton-conservation-double-glazed-unit\"\u003e\u003cstrong\u003e14mm Slim Krypton Conservation Double Glazed Unit\u003c\/strong\u003e\u003c\/a\u003e. The \u003ca href=\"\/pages\/slim-double-glazing\"\u003e\u003cstrong\u003eslim double glazing topical page\u003c\/strong\u003e\u003c\/a\u003e walks the spec-tier comparison.\u003c\/p\u003e\n\n\u003ch2\u003eStock sizes and pricing\u003c\/h2\u003e\n\u003cp\u003eFour small-format stock sizes below — the 11.5mm market is dominated by small-pane Georgian sash bars and narrow Edwardian casements, so we cap at 1200×1000mm. For larger heritage apertures, custom-sized units are priced via the \u003ca href=\"\/pages\/configure\"\u003econfigurator\u003c\/a\u003e with manufacturing-yield premiums where the unit area exceeds 1.2m² (yield drops sharply above this size). Dispatch is 21-28 working days (slow due to specialist 3mm float-glass batch scheduling).\u003c\/p\u003e\n\n\u003ch2\u003eCertification and standards\u003c\/h2\u003e\n\u003cp\u003eUKCA mark, BS EN 1279 Declaration of Performance, krypton-fill certificate (≥85% initial fill, tighter tolerance than standard slim), ultra-narrow steel spacer identification, Planitherm low-E coating face identification, and a Listed-Building Compatibility Statement on request — specifically referencing Historic England HETGN guidance on ultra-slim heritage glazing for Grade I and II* consent submissions. For Scheduled Monument or Grade A consents, additional bespoke documentation can be prepared on request.\u003c\/p\u003e\n\n\u003ch2\u003eFrequently asked questions\u003c\/h2\u003e\n\u003ch3\u003eWhy not just specify the 12mm equivalent?\u003c\/h3\u003e\n\u003cp\u003eFor most listed-building consents, you should — it's cheaper, has better manufacturing yield, longer effective warranty, and faster lead time. The 11.5mm exists for consents where a conservation officer has specifically reviewed and rejected 12mm as too visible. If your consent reads \"sub-12mm overall thickness\", you need this. If your consent reads \"slim-cavity glazing acceptable\" without a hard millimetre limit, the 12mm equivalent will likely satisfy.\u003c\/p\u003e\n\n\u003ch3\u003eWhat's the realistic field lifespan?\u003c\/h3\u003e\n\u003cp\u003e8-12 years before edge-seal degradation likely causes misting on south-facing aspects, vs 12-15 years for the 12mm spec. The 3mm panes flex marginally more under thermal cycling than 4mm panes, and the 5.5mm cavity puts higher stress on the edge seal per cycle. We warrant 10 years on edge-seal integrity but realistic expectation is 10-12 years before signs of seal age appear.\u003c\/p\u003e\n\n\u003ch3\u003eWhy does the spec specify 3mm panes when 4mm is standard?\u003c\/h3\u003e\n\u003cp\u003eTo keep overall thickness sub-12mm while preserving cavity volume for thermal performance. A 4-4-4 build is 12mm overall; a 3-5.5-3 build is 11.5mm with more cavity (5.5mm vs 4mm). The cavity volume is the thermal performance driver, so the trade is: marginally thinner panes for a marginally wider gas-fill gap. The panes are sourced from Pilkington heritage-glass batch runs (manufactured for stained-glass and conservation lead-light work) where 3mm float is a standard product.\u003c\/p\u003e\n\n\u003ch3\u003eU-value vs 12mm Histoglass equivalent?\u003c\/h3\u003e\n\u003cp\u003eCentre-pane U-value: 11.5mm = ~1.7 W\/m²K vs 12mm = ~1.6 W\/m²K. The difference is marginal — within measurement noise on the test rig. Both are roughly 3× better than the single glass they replace (5.7 W\/m²K). For consent compliance the difference is irrelevant; for actual thermal performance you'll feel no perceptible difference between 11.5mm and 12mm in service.\u003c\/p\u003e\n\n\u003ch3\u003eLead time honesty?\u003c\/h3\u003e\n\u003cp\u003e21-28 working days, sometimes longer. The 3mm float-glass batch runs at Pilkington are scheduled monthly; if your order arrives in week 1 of a batch cycle, lead time is faster (closer to 21 working days), if in week 3 then closer to 28. We confirm a tighter lead-time estimate at order acknowledgement.\u003c\/p\u003e\n\n\u003ch3\u003eWhat's the cost premium over 12mm?\u003c\/h3\u003e\n\u003cp\u003eRoughly 10-15% over the equivalent 12mm Histoglass-equivalent unit. The premium reflects: lower manufacturing yield (more units rejected at QC), specialist 3mm pane sourcing, and tighter spacer-bar production tolerance. For high-profile listed-building consent work the premium is small relative to project scope; for routine slim-DG retrofits the 12mm is the better economic spec.\u003c\/p\u003e\n\n\u003ch3\u003eAvailable shaped?\u003c\/h3\u003e\n\u003cp\u003eYes, but at significant cost premium and lead-time extension. Shaped 11.5mm units (arched, circular) cost ~2.0-2.5× the rectangular equivalent and lead time stretches to 28-35 working days. For shaped heritage glazing, the 14mm slim spec offers better manufacturing yield with only marginally more visible sightline — generally the right call for shaped over rectangular.\u003c\/p\u003e\n","brand":"Pane Relief","offers":[{"title":"600 × 400 mm","offer_id":61855201362250,"sku":"DG115-ULSL-600x400","price":45.0,"currency_code":"GBP","in_stock":true},{"title":"800 × 600 mm","offer_id":61855201395018,"sku":"DG115-ULSL-800x600","price":72.0,"currency_code":"GBP","in_stock":true},{"title":"1000 × 800 mm","offer_id":61855201427786,"sku":"DG115-ULSL-1000x800","price":119.0,"currency_code":"GBP","in_stock":true},{"title":"1200 × 1000 mm","offer_id":61855201460554,"sku":"DG115-ULSL-1200x1000","price":179.0,"currency_code":"GBP","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0895\/3515\/2458\/files\/double-glazed-sealed-unit-igu-product-cutaway_4674ab57-f157-46d5-8bc7-d3e8e3985ea7.webp?v=1779039920","url":"https:\/\/panerelief.co.uk\/products\/11-5mm-ultra-slim-conservation-double-glazed-unit","provider":"Pane Relief","version":"1.0","type":"link"}