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When mock sash is the right choice

A Victorian semi-detached in Essex, undergoing an EnerPHit-oriented retrofit. Fourteen windows to replace, a large sliding door to a new rear extension, and a front elevation defined by arched sash openings that had shaped the character of the building for over a century. 

When sash windows reach end of life – rattling on cold nights, painted shut, leaking heat through single glazing – replacing them with a more thermally efficient window while preserving the aesthetic of the original can be achieved with mock sash windows.

Key Takeaways

  • Original single-glazed sash windows have a whole-window Uw of around 5.0 W/m²K - a significant source of heat loss in any period home
  • Even a modern double-glazed sliding sash usually sits between 1.2 and 1.5 W/m²K; the sliding mechanism is inherently less airtight than casement windows
  • A mock sash window uses a fixed top pane over a tilt-and-turn lower pane, replicating the external appearance of a traditional sash with better thermal performance
  • Norrsken's triple-glazed mock sash achieves whole-window Uw values of around 0.8 - 1.0 W/m²K, depending on the specification
  • Externally near-identical to a traditional sash; internally the hardware and operation differ

Why is a sash window less airtight than a casement window?

Because the sash must slide within its frame, the seal between sash and frame is a brush or pile strip rather than a compression gasket.
When a casement or tilt & turn window closes, the sash presses against a perimeter gasket, creating a continuous compressed seal. A sliding sash cannot achieve this: movement requires clearance, and clearance means the seal is never fully compressed. 

The thin sashes also prevent putting deeper glass units (which have greater insulation value) and if the glass is broken up with glazing bars, the thermal performance is worsened again – the warmest part of an insulated glass unit is at the centre of the pane. The more small panes & frame edges introduced, the worse the thermal performance. 

A modern double-glazed sliding sash reaches whole-window Uw values of around 1.5 W/m²K, clearing the Part L 2021 glazing backstop of 1.6 W/m²K at the better end.

Norrsken supplies an all-timber double-glazed sliding sash window suitable for heritage new builds and window replacements, and it’s a strong specification choice. But in an EnerPHit retrofit or heritage-style low-energy new build, better U-values are required. A mock sliding sash window is the solution.

Images: a Victorian house in Essex, before and after the replacement with mock sash windows.

Can you get sash window aesthetics with better thermal performance?

A mock sash uses a fixed top pane and an inward-opening tilt-and-turn lower pane to replicate the appearance of a traditional sliding sash – proportions, sash horns, and glazing bars where required.

Because the lower pane closes on a multi-point perimeter compression seal rather than a brush strip, it achieves airtight closure that the sliding format cannot match, even at equivalent glazing specification.

Norrsken's triple-glazed mock sash achieves typical whole-window Uw values of 0.8 – 1.0 W/m²K. They can meet EnerPHit retrofit targets and perform better than a true sliding sash at the same glazing spec.

The format also accommodates non-standard openings more readily, like the arched heads in the Essex project. Where the project includes both bespoke window shapes and performance targets, the mock sash format earns its place on both counts. For guidance on what's historically appropriate for your property, Norrsken's era guide covers a range of British residential architectural styles.

Do mock sash windows look like sash windows?

From the outside, a mock sash is near-identical to the original. The proportions, the horns, and the finish look like a traditional sash.
Inside, the difference is visible: tilt-and-turn hardware replaces sash lifts and fitch catches, and the lower pane opens inward rather than sliding upward. Inward-opening windows are generally easier to clean and maintain, but the interior operation will not replicate the feel of the original.

Installation depth also requires careful planning when replacing original box sash windows. Traditional box frames typically exceed 200 mm in depth; Norrsken mock sash units sit at approximately 190 mm all-in. The reveal, airtight sealing and making-good need to be worked out in advance.

Image: Internally, the tilt-and-turn window is visually distinct from a true sash window in the Victorian rtrofit.

Working on a period renovation?

If you're working through the window specification for a period renovation, our team is happy to talk through the options with you. Get in touch via any of the methods on our Contact page.

Looking for more inspiration? Browse completed projects in our inspiration gallery to see what's possible, or see what past customers have said on Trustpilot.

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