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Double vs triple glazing — compared on real Ug values

Every figure below comes from the same validated calculation engine (EN 673 vertical boundary conditions, calibrated against 1,908 certified manufacturer rows) — not marketing claims. Configure either class in the configurator below and add it to your RFQ cart.

Product guide

Sealed units, specified from outside to inside

Build a replacement or new sealed-unit request with separate pane, cavity, spacer and decoration choices. Match the finished unit depth to the frame.

A modest glazed window with a garden view.
AI-generated illustration. Appearance varies by specification; this is not an exact product sample.

Double-glazed sealed units, triple glazing and replacement insulating glass units

A sealed glass unit combines separate panes, sealed cavities, spacer bars and edge seals. Double glazing has one cavity and triple glazing has two. When replacing a misted unit, the overall depth, finished dimensions, edge detail and frame compatibility matter as well as the number of panes.

Specify the unit from outside to inside. A weather-facing pane may carry a solar-control or self-cleaning function, while another pane may provide low-emissivity coating, privacy, safety or acoustic performance. Coating position and processing need to match the complete build.

Warm-edge spacers, gas fills, obscure patterns, leaded designs, Georgian bars and pet-flap openings are separate specification choices. Slimline heritage units and shaped or stepped units need their own construction review. A generic U-value does not describe every possible combination.

Sealed units, specified from outside to inside for your project

Common enquiry briefs include replace a misted unit, new window or door, conservatory or rooflight, slimline / heritage. Describe the intended use and the supporting construction so the supplier can assess the complete specification. Different applications can need different glass processing, fixings and installation details.

Glass size, cavity and total depth

They are three separate measurements. For example, 4 / 16 / 4 has a 16 mm gap and a 24 mm nominal overall depth.

Design each face

Place appearance and weather-facing functions on the appropriate pane, and confirm coating positions with the fabricator.

Plan decoration early

Bars, leading, patterns, holes and stepped edges need a drawing and may change the manufacturing route.

What to include in a made-to-measure quote

Compare quotes using the same dimensions, quantities, finish, glass construction and installation scope. If you have a drawing or an existing specification, include its reference and identify which details still need advice.

  • Number of panes: Double glazing has one cavity; triple glazing has two.
  • Outer pane: Outside-facing glass. Options vary by processor and the complete build.
  • Inner pane: Room-facing glass; confirm safety processing and the coating surface with the supplier.
  • Middle pane if triple: Choose not applicable for a double unit.
  • Cavity / spacer width: This is the gap, not the total unit thickness. For triple units, record unequal gaps in the notes.
  • Spacer type and colour: Some suppliers restrict warm-edge bars in very narrow cavities.
  • Cavity gas: Gas choice and performance depend on the complete sealed build.
  • Special features: Add pattern names, bar layouts, steps and hole details in your notes. Supplier drawings are required for fabrication.

Cut-to-size dimensions, edges and fabrication

Measure the finished glass only when the fitting allowances and support details are known. Otherwise identify the opening or existing panel you measured and request a survey. Give each different size its own basket line, with its quantity and drawing reference.

Holes, cut-outs, shaped edges and interfaces with hardware should be settled before manufacture. The finished glass construction and any coatings must suit the intended environment. A screen preview or concept photograph cannot replace a sample, survey or product-specific installation drawing.

Supplier checks before manufacture

  • Measure the finished glass unit, not just the visible opening. Confirm fitting clearances with the installer.
  • Total depth is all panes plus all cavities; glazing beads, edge seals and sightline also need to fit.
  • Slimline, roof, shaped and pet-flap builds require supplier confirmation; no generic U-value is promised.

Requesting comparable glass quotes

Save the choices you know and ask for help with the rest. The quote basket records a specification request rather than placing a manufacturing order. A firm quote needs an agreed final build, dimensions, delivery arrangements and any installation or design work.

When comparing proposals, check that glass-only supply and supply with fitting cover the same scope. Ask who provides measurement, drawings, supports, compatible hardware, lifting access and finishing work. Keep the final approved specification with the order.

Sources and further reading

BuildMakeupDepthUg (W/m²K)Rw (dB)Weight (kg/m²)
Air-filled uncoated double4-16-424mm2.7330.620
Low-E argon double (UK default)4-16Ar-4 low-E24mm1.230.620
Krypton slim double4-12Kr-4 low-E20mm1.1430.120
Argon triple, matched cavities4-12Ar-4-12Ar-4 low-E36mm1.1834.730
Deep-cavity argon triple4-16Ar-4-16Ar-4 low-E44mm1.0835.830
Krypton triple (Passivhaus band)4-10Ar-4-10Kr-4 low-E32mm1.2234.230

Choose a double when…

  • Replacing windows in an existing home — 1.6 W/m²K whole-window Part L is comfortably met.
  • Budget and weight matter; frames and installers are standard.
  • You want slimmer sightlines (24mm units fit most timber frames).

Choose a triple when…

  • New build or deep retrofit targeting ≤1.2 W/m²K whole-window.
  • Very exposed/coastal or high-noise sites (with the right cavity design).
  • Passivhaus or low-energy certification is the goal.

01 · Unit type

02 · Glass panes

Outer Clear 4mm · Inner Laminated 6.8mm

Outer pane

Clear float

Toughened

Laminated safety

Acoustic

Low-iron

Inner pane

Clear float

Toughened

Laminated safety

Acoustic

Low-iron

Low-E coating position

One low-E coating is modelled (the validated engine tier). Two-coat builds are quoted verbatim by suppliers — add a note at checkout.

03 · Cavity, gas & spacer

Cavity width

1 × 16mm

Gas fill

the default — cheap and effective

Spacer bar

Spacer colour

Standard colours shown. Bespoke RAL-coded bars can be requested on specialist projects — add it as a note at checkout.

Warm-edge spacers lower the whole-window U-value (linear ψ 0.06 W/mK). Colour choice doesn't change performance.

04 · Unit size

Examples and guidanceFinished panel dimensions

Record the finished width and height of each distinct panel. For a circle use its diameter; shaped work needs a dimensioned drawing or agreed template.

Panel dimension referenceFinished rectangular panel width and height. Opening dimensions are a different reference.Width (mm)Illustrative schematic, not to scale or a fabrication / fixing specification.

What your choice means

An opening, frame and finished glass are different references. Request a survey when the finished manufacturing dimensions remain unknown.

Examples you may seeOne size per basket line; quantity records identical pieces.Check units and the dimension reference before confirming manufacture.Guardian: glass types (opens in a new tab) ↗
Examples and guidanceFinished panel dimensions

Record the finished width and height of each distinct panel. For a circle use its diameter; shaped work needs a dimensioned drawing or agreed template.

Panel dimension referenceFinished rectangular panel width and height. Opening dimensions are a different reference.Height (mm)Illustrative schematic, not to scale or a fabrication / fixing specification.

What your choice means

An opening, frame and finished glass are different references. Request a survey when the finished manufacturing dimensions remain unknown.

Examples you may seeOne size per basket line; quantity records identical pieces.Check units and the dimension reference before confirming manufacture.Guardian: glass types (opens in a new tab) ↗
Overall sealed-unit depth 26.8mm — check your frame rebate (the slot the unit sits in) accepts it.Weight 25.8 kg/m²1000 × 1000 mm = 1 m² actual glass area

Common questions

How much better is triple glazing than double?

Centre-pane: a 4-16Ar-4 low-E double sits near 1.1-1.2 W/m2K; a 4-12Ar-4-12Ar-4 triple near 0.7-0.8. Roughly a 30-40% improvement in glass heat loss — but the whole window (frame + edges) improves by less, because frames dominate.

Is triple glazing worth the extra cost?

For replacements in average UK homes, usually not: the marginal Ug gain is small against the price premium, extra weight and shallower light transmission. It pays off for new builds targeting low U-values, very noisy or exposed sites, and Passivhaus projects.

Do triples weigh much more?

A 4-12Ar-4-12Ar-4 triple weighs 30 kg/m2 versus 20 kg/m2 for a 4-16Ar-4 double — 50% heavier. Check sash hinges and trickle-vent fit; most residential frames accept 36-44mm triple depths with the right gaskets.

Which is better for noise?

Triple glazing is not automatically quieter — cavity widths matter more than pane count. Acoustic laminated doubles (e.g. 4-16Ar-10.8 acoustic) often beat standard triples on Rw.