Double Glazed Sash & Case Windows Edinburgh

Slim-Line · U-Value 1.2 · Invisible from the Street · From £445 per sash

Yes — original Edinburgh sash and case windows can be double-glazed. Slim-line argon-filled sealed units 12mm or 14mm deep retrofit into the original 19mm glazing rebate, bringing the centre-pane U-value down to 1.2 W/m²K (equivalent to a modern A-rated window) without changing the look of the window from the street. It's conservation-area-compliant and increasingly approved on listed buildings.

How slim-line double glazing works

A traditional sash and case window has a 19mm-deep glazing rebate cut into the timber, designed for a single 4mm pane bedded in linseed putty. Modern double-glazed units are typically 24–28mm — too deep for a heritage rebate. Slim-line units are purpose-made for heritage retrofit: 12mm or 14mm total depth, two 4mm panes separated by a 4mm or 6mm argon-filled cavity with a warm-edge spacer bar. They sit inside the original rebate with no profile change, no router-out of the sash, and no extra weight to throw the sash counterbalance.

Centre-pane U-values and thermal performance

Original 4mm single glazing has a centre-pane U-value of around 5.8 W/m²K. A 12mm argon-filled slim-line unit drops that to 1.6 W/m²K. A 14mm krypton-filled unit drops it to 1.2 W/m²K — the same as a modern A-rated double-glazed casement window. On a typical Edinburgh tenement flat with seven sashes, that takes the whole-window heat loss from about 1,800 kWh/year to 380 kWh/year — a saving of roughly £150 a year on gas at 2026 prices, and a major comfort improvement in the room because the cold-glass downdraught stops.

Brand options: Slimlite, Histoglass, FineO

Three slim-line unit brands dominate the Edinburgh heritage market. Slimlite (Sashforce) is the standard 12mm argon-filled unit, lowest cost, 10-year sealed-unit warranty. Histoglass adds a hand-blown 'restoration grade' outer pane that mimics the optical ripple of Cylinder glass — essential for listed buildings where the council won't accept perfectly flat modern glass. FineO is a vacuum unit only 6.5mm deep with a U-value of 0.7 W/m²K — the highest performance, lowest visible spacer bar, but 3× the price. We size and quote per sash so each window gets the right unit.

What the council allows

In Edinburgh conservation areas, slim-line double glazing in original timber sashes is routinely approved because the streetscape doesn't change. On listed buildings, consent is now granted in most cases provided the original glass is salvaged and re-used elsewhere in the building, the spacer bar is colour-matched to the putty line, and a hand-blown outer pane is used on principal elevations. We complete the Listed Building Consent paperwork as part of every quote.

Combining slim-line glazing with draught-proofing

Double-glazed slim-line units alone solve the cold-glass radiation problem but not the air leakage problem. A typical Edinburgh sash leaks 25–40% of the room air per hour through worn parting beads and a sloppy meeting rail — no amount of glazing fixes that. We always quote slim-line glazing and brush-pile draught-proofing as a single package: the combination cuts whole-window heat loss by 75–85% and is what actually makes the room comfortable.

Cost, lead time and warranty

From £445 per sash for 12mm Slimlite argon units fitted and putty-pointed. £585 per sash for 14mm krypton units. £945 per sash for FineO vacuum units. Lead time is 4–6 weeks because each unit is made to template. 10-year sealed-unit warranty on Slimlite, 15 years on FineO, plus a 10-year Heatscape workmanship guarantee.

Frequently asked questions

Can original Victorian or Georgian sash windows really be double-glazed?
Yes — slim-line units 12–14mm deep fit into the original 19mm glazing rebate without altering the sash profile, the astragal pattern or the streetscape appearance. The window looks identical from outside; the only change is a slight extra reflection in the glass.
How much does slim-line double glazing cost per window?
From £445 per sash for 12mm argon-filled Slimlite units, £585 for 14mm krypton units, £945 for FineO vacuum units. A typical pair of sashes (top and bottom) is £890–£1,890 depending on the unit chosen.
Will slim-line glazing be approved in an Edinburgh conservation area?
Yes — it's routinely approved because the streetscape doesn't change. On listed buildings, consent is granted in most cases with a hand-blown outer pane and colour-matched spacer bar. We handle the consent paperwork.
How much will I save on heating bills?
On a typical Edinburgh tenement with seven sashes, around £150/year on gas at 2026 prices, plus a major comfort improvement because the cold-glass downdraught stops. Payback is 12–18 years on glazing alone, faster if combined with draught-proofing.
Do slim-line units have a guarantee?
10 years on Slimlite argon units, 15 years on FineO vacuum units. Heatscape adds a 10-year workmanship guarantee on the joinery and re-bedding.
Is vacuum glazing (FineO) worth the extra cost?
On a listed building or a noise-affected street, yes — the U-value is half a slim-line argon unit and the acoustic performance is significantly better. On a non-listed conservation area property, the cost-benefit usually favours 14mm krypton slim-line.

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