FENSA-registered glazing experts. Made-to-measure sealed units, shipped UK-wide, backed by a 10-year warranty.
Slim double glazing (14mm krypton) — conservation-grade profile for listed sashes
Heritage-grade sealed units in a 14mm overall profile — narrow enough to retrofit into pre-Victorian timber sashes and crittal frames without altering muntin proportions or external sightlines. Standard 24mm and 28mm DG can't be specified in most listed buildings because the rebate is too shallow and the external profile would read as visually wrong. Slim DG (4mm-6mm-4mm krypton-filled, with a discreet low-E coating) drops into rebates as shallow as 11mm, retains the original timber muntin sightline within 1-2mm, and delivers a centre-pane U-value around 1.6-1.8 W/m²K — a meaningful upgrade on the 5.6 W/m²K of original single glazing while satisfying the great majority of conservation officers. Comparable to Histoglass and other heritage glazing systems on optical clarity and reflection signature.
Living in a listed or conservation property
Pre-2017, listed-building consent for any DG was rare. Since the Historic England guidance shift, slim DG in like-for-like sashes is increasingly permitted — typically subject to a spec sheet for the conservation officer's sign-off. We supply the spec sheet free, send the sample swatch on request, and offer free measurement review on the helpline. Most slim DG retrofits ship in 7-10 working days.
Configure slim DGConservation architect, listed-property glazier
UKCA-marked and BS EN 1279 compliant. Krypton fill is the standard not the upgrade — the 6mm cavity is too narrow for argon to perform efficiently. Trade tier pricing on multi-window listed retrofits, scheduled delivery, and a stamped spec sheet for conservation officer sign-off included with every order on request.
Trade pricing & bulk ordersWhere slim DG is the only viable answer
Six scenarios where standard DG can't be specified and slim DG is the conservation-compatible upgrade.
Georgian timber sash
True-sash vertical sliders. Slim 14mm fits the existing muntin profile without rebate modification.
Victorian / Edwardian sash
Single or double-light timber sashes with putty fillet. Retain the original profile and fillet line.
Crittal / steel frame
1920s-50s steel windows with very shallow rebates. Slim DG is the only DG option short of frame swap.
Leaded-light retrofit
Encapsulated leaded panels within a slim DG carrier. Retains the leaded appearance with thermal upgrade.
Listed arched primary
Listed-building arched fanlights. Shaped slim DG to template — see lead time uplift in FAQ.
Custom slim DG
Non-standard rectangles, shaped, or with applied Georgian bars. Configure live.
Measuring slim DG and the consent path
Slim DG measure is the same rebate-plus-6mm rule. The consent path is the more important pre-order step — confirm with your conservation officer in writing.
Pre-order consent and measure steps
- Email your conservation officer first. Describe the proposed retrofit (like-for-like sash, slim 14mm krypton-filled DG, with optional applied Astragal bars). Ask for written confirmation of consent before ordering. We supply a free spec sheet (build profile, coating signature, krypton fill, UKCA documentation) for officer sign-off — request via the helpline.
- Measure the rebate. Carefully ease back the putty fillet (timber sashes) or pop a glazing bead (UPVC or aluminium). Measure visible glass width at top, middle, bottom — smallest number wins. Same for height.
- Add 6mm to both dimensions (3mm of glass tucked into each side of the rebate). Verify the rebate depth — 11mm minimum for the 14mm slim build.
Pricing transparency
Slim DG is priced per unit on the configurator. Typically 30-45% more expensive per square metre than standard 28mm DG — the krypton fill (krypton is roughly 10x the cost of argon by weight) and the tighter manufacturing tolerances drive the premium. Listed-building consent and the spec-sheet service are included free. Trade-tier pricing applies on volume orders.
Slim double glazing — common questions answered
Q: What U-value do you get with slim DG and krypton fill?
Centre-pane U-value for our standard slim DG build (4mm-6mm-4mm krypton + soft-coat low-E) is around 1.6-1.8 W/m²K. That compares to roughly 5.6 W/m²K for original single glazing, 2.8 W/m²K for standard clear DG, and 1.2-1.4 W/m²K for standard low-E DG. So slim DG isn't quite as thermally efficient as standard low-E DG — the narrow 6mm cavity is the limit even with krypton — but it's a major upgrade on single glazing while keeping the heritage profile.
Q: Why krypton fill specifically? Why not argon?
Argon's thermal performance peaks in cavity widths of 14-16mm. In a narrow 6mm cavity, argon performs barely better than air (the molecular convection paths are too short for the gas density to matter). Krypton is denser and performs efficiently at narrow gaps — at 6mm cavity, krypton-fill delivers around 30% better thermal insulation than argon-fill. The trade-off is cost: krypton is roughly 10x the price of argon by weight. For wide-cavity standard DG (12mm+), use argon; for slim DG, krypton is the only sensible choice and is what we supply by default.
Q: Will my conservation officer approve slim DG?
Increasingly yes, but it depends on the building, the officer, and the elevation. Since Historic England's 2017 shift, Grade II conservation officers in most local authorities now permit slim DG in like-for-like sashes on primary elevations — particularly where the original glass is gone or the existing single glazing is failing. Grade I and Scheduled Ancient Monument consent is harder but not impossible. The route: email the officer first, supply our spec sheet (free), get written consent before ordering. We've never had a unit returned because consent was refused after a written pre-approval.
Q: How does slim DG compare to Histoglass?
Histoglass is a heritage-glazing specialist with strong conservation-officer recognition — they pioneered the slim-DG-for-listed-property category in the 2000s. Our slim DG is functionally equivalent on build spec (similar 4-6-4 krypton-low-E construction) and on certification (BS EN 1279, UKCA), and is typically 20-35% less expensive at retail because we manufacture in larger production batches. For conservation-officer approval purposes, both products are normally accepted on equivalent grounds — if your officer specifically requires "Histoglass branded", we can quote a comparable Pilkington Spacia unit. Ask via the helpline for the side-by-side spec comparison.
Q: Can slim DG include applied Georgian bars (Astragal)?
Yes — applied Astragal bars (lead, brass, or matt black; rectangular, fan-light, or diamond layouts) are added in production after the unit is sealed. The bars sit on the outer pane and don't intrude on the cavity, so the thermal performance is unaffected. Internal between-glass bars are not possible in slim DG — the 6mm cavity is too narrow to fit a structural bar. For internal bars, you'd need standard 24mm or 28mm DG which has a wider cavity. Discuss the bar option on the configurator or via the helpline.
Q: What about leaded lights — can slim DG encapsulate them?
Yes — we supply an "encapsulated leaded" slim DG variant where the existing leaded panel is captured between the inner and outer panes of the slim DG unit. The leaded panel itself does the visual job (matching the heritage appearance), the slim DG sandwich provides the thermal envelope. Lead time on encapsulated leaded slim DG is 12-16 working days — the leaded panel is removed, surveyed, re-leaded if needed, then sealed inside the new unit. Send a photograph and the panel dimensions for a quote.
Q: Is shaped slim DG (arched, raked) available for listed buildings?
Yes — shaped slim DG follows the same template-or-CAD spec route as our shaped TG. Send a paper template, photograph with reference measurements, or CAD file. Lead time on shaped slim DG: 14-18 working days (longer than shaped TG because of the tighter manufacturing tolerances). Useful for listed arched primaries, fan-light retrofits, and church-conversion gable lights where the original glazing follows a non-rectangular profile.
Q: Warranty, lead time, and documentation for slim DG?
10-year manufacturer warranty on edge-seal integrity (BS EN 1279-2 weathered durability — particularly important on slim DG because the narrow cavity is more vulnerable to seal failure under thermal cycling). 5-year warranty on low-E coatings. Standard rectangular slim DG: 7-10 working days. Shaped or encapsulated leaded variants: 14-18 days. Documentation: UKCA mark, BS EN 1279 compliance, conservation-officer spec sheet, Declaration of Performance, and krypton-fill certification — all on request, free of charge.
Related guides and specifications
Listed building? Get the spec sheet for your officer.
Call the helpline on 0117 330 3057 (08:00-18:00 Mon-Fri) or email trade@panerelief.co.uk for a free conservation-officer spec sheet.