FENSA-registered glazing experts. Made-to-measure sealed units, shipped UK-wide, backed by a 10-year warranty.
Passive House double glazing — the rare DG that hits the Passivhaus thermal envelope
Most double glazing tops out around U=1.2 W/m²K and that's nowhere near Passivhaus territory. But there's a narrow band of high-spec double-glazed sealed units — slim cavity (12-16mm), krypton fill, dual low-E coating on faces 2 and 3, and premium warm-edge spacer — that get a 28mm DG unit to U=0.9-1.0 W/m²K. Not quite the standard PHI triple, but close enough to qualify in PHPP for component zones where triple isn't viable: heritage and conservation-area frames with shallow rebates, vertical-sliding sash windows where triple weight isn't supported, and EnerPHit retrofit jobs where you can't open up the frame profile. Pane Relief manufactures these specialist DG units for self-builders, conservation architects, and retrofit coordinators who need Passivhaus-grade performance in the slimmest possible build.
Listed building, conservation area, EnerPHit retrofit
You have original timber sash, casement, or steel-frame windows that conservation officers won't let you replace. Replacement single-pane glass would bring no thermal gain; standard DG won't fit the rebate. Slim Passivhaus-grade DG drops in at 14-20mm overall thickness, gets U-values that PHPP will accept on a PHPP retrofit zone, and looks indistinguishable from single glazing externally. Phone helpline for spec validation, 10-year edge-seal warranty.
Configure my slim DG unitEnerPHit coordinator, heritage installer, sash specialist
Net 30 trade accounts. Passivhaus desk for PHPP component validation, useful for EnerPHit-Plus retrofit zones and conservation-officer documentation. Slim 14mm-20mm builds with WER A or A+ certification possible on the whole window. Bespoke heritage profiles, leaded-look overlays, and Crittall steel-frame conversions. Coating data sheet and gas-fill certificate on request.
Trade pricing & bulk ordersWhich slim Passivhaus DG build do you need?
Five build options spanning conservation-grade slim DG to PHPP-component double — pick by frame depth and target U-value.
Heritage 14mm slim DG
4mm low-E + 6mm krypton + 4mm. U=1.4 W/m²K. For sash and conservation steel-frame retrofits where rebate is <16mm.
Sash 16mm slim DG
4mm low-E + 8mm krypton + 4mm. U=1.2 W/m²K. Designed for vertical-sliding sash retrofit. Conservation-friendly profile.
Passivhaus 24mm dual low-E
4mm low-E + 16mm krypton + 4mm low-E. Dual low-E. U=1.0 W/m²K. PHPP component-grade DG.
Passivhaus 28mm dual low-E
4mm low-E + 20mm krypton + 4mm low-E. U=0.9 W/m²K. The deepest DG that still saves you from a triple build.
Conservation leaded DG
Slim DG with internal leaded-bar overlay, Georgian bars, or applied bevels. Heritage-friendly aesthetic + modern thermal.
Move up to triple
If you have the rebate depth and frame weight rating, triple glazing always wins on U-value. See Passivhaus triple collection.
When to spec slim DG instead of jumping to triple
There's a small set of scenarios where slim Passivhaus DG is the right answer — and a larger set where triple wins. Here's how to tell which is which.
The 5 scenarios where slim DG wins
- Heritage / conservation-area frames: conservation officers often refuse triple glazing because external sightline changes the building character. Slim DG at 14-16mm overall passes visually as single glazing.
- Vertical-sliding sash: sash counterweight cords and the sash mass calculation often can't accommodate triple-glazing weight. Slim DG at ≤20 kg/m² works without redesigning the sash mechanism.
- Crittall steel frame conversion: original 1920s-1940s Crittall frames have rebates around 6-12mm. Slim 14mm DG drops in with bespoke gaskets; triple needs frame replacement.
- EnerPHit retrofit with frame retention: when the PHPP retrofit strategy keeps existing frames in service for cost or carbon reasons, slim DG is often the only viable upgrade.
- Listed-building consent: some Grade II and most Grade II* / Grade I listings require glass that's visually authentic. Slim DG with restoration-grade restoration float (Pilkington Optiwhite or Schott Restover) is the accepted solution.
Pricing & documentation
Prices on this collection are "From £X" at the smallest stock size on the 16mm slim-DG krypton build. Custom heritage profiles, leaded overlays, and bespoke restoration-glass outer panes priced live in the configurator. PHPP-ready data sheet, krypton-fill certificate, and conservation-officer documentation pack shipped on request.
Passive House double glazing — common questions answered
Q: Can you really get Passivhaus performance from a double-glazed unit?
You can get close — typically U=0.9-1.0 W/m²K on a 28mm slim DG with dual low-E and krypton fill. That's about 20-30% short of a full PHI triple (U=0.7-0.8) but a long way ahead of standard A-rated DG (U=1.2). For PHPP retrofit zones (EnerPHit) where you can't fit triple — heritage, sash, conservation, Crittall — slim Passivhaus DG will get the PHPP component number you need; the heat balance then has to be made up elsewhere (more wall insulation, better airtightness).
Q: Will PHPP accept slim DG as a Passivhaus-grade component?
Yes — Passivhaus is a whole-building performance standard, not a component-by-component requirement. PHPP will accept any glazing U-value you input, provided the rest of the heat balance still hits Passivhaus targets. In an EnerPHit retrofit with retained heritage frames, slim DG is the standard tool for the glazing zone — the building gets there on insulation, airtightness, and MVHR. Send your PHPP file and we'll cross-check the glazing entry before manufacture.
Q: How thin can you actually go?
Pane Relief stock the 14mm slim DG build (4-6-4 krypton) as the thinnest standard. Bespoke 12mm builds (4-4-4 krypton) are available to order for the tightest Crittall steel-frame conversions — U-value lifts to about 1.6 W/m²K at that thickness so the thermal gain is more modest. Below 12mm, vacuum glazing (Pilkington Spacia at 6.5mm, U=1.1) is the right product — see the slim cavity guide for comparison.
Q: How heavy are these units vs standard DG?
Slim 14mm DG: about 20 kg/m². Slim 16mm: about 20-22 kg/m². 24mm dual-low-E: about 22 kg/m². 28mm dual-low-E: about 22-24 kg/m². All within sash and casement weight tolerances for standard timber and steel frames. The thicker 28mm DG is about 30% lighter than a 36mm Passivhaus triple — useful where frame weight rating is borderline.
Q: Krypton or argon — does it matter for slim DG?
For slim cavities (6-8mm), krypton is significantly better than argon — the narrow cavity is below argon's convection optimum, so argon's thermal conductivity dominates. Switching from argon to krypton in a 6mm cavity drops the U-value by about 0.3-0.4 W/m²K. For deeper cavities (16-20mm) the krypton premium narrows but krypton still wins on PHPP. Pane Relief specifies krypton on all slim-DG and Passivhaus DG builds as standard.
Q: Will it look the same as single glazing from outside?
From normal viewing distance (2m+), yes — slim 14-16mm DG is visually indistinguishable from single glazing. Up close, there's a slim warm-edge spacer line around the perimeter (Swisspacer or Edgetech) which can be specified in matching colour (black, grey, brown, white). For conservation officer or Grade II* listing approval, ask for the conservation pack — we'll provide photo documentation and a written technical statement.
Q: How does this compare to vacuum glazing (Pilkington Spacia, LandVac)?
Different niches. Vacuum glazing achieves U=0.7-1.1 W/m²K in just 6-8mm total thickness — ideal for the slimmest possible heritage retrofits. Higher capital cost per m² (typically 2-3x slim DG), more brittle edge seal, narrow standard size range. Slim Passivhaus DG is cheaper per m², more durable, wider size range, but thicker (14-28mm). For Grade II listed and conservation-area heritage where rebate is <10mm, vacuum wins. For 10-28mm rebates, slim DG wins on cost and durability.
Q: Lead time for slim Passivhaus DG?
Stock-size 14-16mm slim DG: 10-14 working days. 24-28mm dual-low-E builds: 14-21 working days. Custom heritage profiles or leaded overlays: 21-28 working days. Crittall conversion bespoke slim DG: 28-35 working days. Conservation pack (documentation for listed-building consent): no extra time, prepared during manufacture.
Related guides and specifications
Need slim DG for an EnerPHit or heritage project?
Call the Passivhaus desk on 0117 330 3057 (08:00-18:00 Mon-Fri), or request a quote. Heritage and conservation consultancy accounts available.