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The 50-year sash window

The ’50-Year Sash Window’ is our premium option. With over 30 years of research and development we have designed our sash window range to incorporate everything our customers had been asking for and more. With longevity and performance being the key factors, we combine traditional joinery methods with high performance double-glazing, draught sealing and the finest quality low maintenance micro-porous paint system, ensuring optimum durability.

Click on the highlighted areas to take a closer look at our premier box sash window features.

Click on the highlighted areas to take a closer look at our premier box sash window features.

Key benefits

  • EXTRA DURABLE – Timber guaranteed against rot for 50 years
  • EXTREME STABILITY – 75% reduction in shrinkage and swelling
  • LOW MAINTENANCE – 3-4 times longer lasting paint finish
  • ROBUST CONSTRUCTION – Traditional mortice and tenon joints
  • SYMPATHETIC – Traditional features replicated like for like
  • HIGH PERFORMANCE – A rated double-glazed units
  • NATURALLY INSULATING – Improved thermal insulation
Benefits Chart


Timber – Accoya® Wood

Because our customers want their windows to last we have always used the highest quality wood available. Now 95% of all our sash windows are made from the best wood available on the market – Accoya® wood. Accoya® wood is the result of more than 80 years of research and development that brought together a long established and extensively proven wood modification technique – acetylation – and leading-edge proprietary technology to create a high performance wood.

Accoya® wood is the leading high-technology wood. Created from sustainably sourced wood, Accoya® matches or exceeds the properties of the very best tropical hardwoods. See above for some of its performance benefits.

Performance benefits

Performance Benefits for Sash Windows & Casement Windows: Outstanding Durability


50 years above-ground durability and 25 years in-ground / freshwater durability.
Class 1 durability – matching (and even passing) the durability of hardwoods
such as Jarrah, Rosewood and Teak.
Performance Benefits for Sash Windows & Casement Windows: Dimensional Stability


Swelling and shrinkage reduced by 75% or more.
Doors and windows open effortlessly all year round. Reduced maintenance
costs because paints and varnishes last up to 4 times longer.
Performance Benefits for Sash Windows & Casement Windows: Retained Strength & Hardness


Acetylation process does not compromise the wood's strength.
Hardness is increased – Accoya® wood is 3 times harder than
Western Red Cedar and 1.5 times harder than Radiata Pine.
Performance Benefits for Sash Windows & Casement Windows: Consistent Quality Throughout


Each production batch sampled to ensure consistent treatment quality.
No need to apply chemical preservatives to cut or planed surfaces as
treatment goes right to the core.
Performance Benefits for Sash Windows & Casement Windows: Perfect For Coating


Easier to coat – less preparation and sanding between coatings required.
Improved dimensional stability and UV resistance improves the lifespan
of coatings. Perfect for transparent, translucent and opaque coatings.
Performance Benefits for Sash Windows & Casement Windows: Non Toxic & Recylable


Accoya® wood is non-toxic, no leeching of harmful chemicals.
Accoya® wood can be reused and recycled.

Glass & Glazing

Here at London Box Sash Windows we feel the glass that’s used in our sash windows has one of the most important roles of all, as most of our customers want to improve their quality of life and reduce energy costs by improving thermal performance. That’s why we glaze our premium sash windows to ‘A’ rated specifications, exceeding the latest building regulation requirements.

The windows are glazed with 24mm double-glazed units, comprising of low ‘E’ glass, argon filled ‘Swisspacer’ bar and Pilkington Optiwhite extra clear, low iron glass, which provides an indicative centre pane U-Value of 1.2 W/m2K.

To protect and prolong the life of our double-glazed units we have designed a unique drained and vented rebate glazing system to prevent premature failure of glazing unit seals by allowing any trapped moisture to drain through small, concealed drainage holes. This has allowed us to double our guarantee against premature failure.

Our sash window range can also incorporate a variety of other glass options to suit specific requirements:

  • Toughened glass or laminated glass.
  • Acoustic glass – Double-glazed units can incorporate acoustic insulating 6.8mm laminated glass (35db Rw). This product combines two sheets of clear glass bonded together with a totally transparent interlayer. The interlayer absorbs sound and reduces the level transmitted through the glass, helping to keep noise in or out.
  • Obscured or textured glass.
  • Stained glass
Sash Window Double Glazing Technical Data


London Box Sash Windows Teknos Low Maintenance Microporous Paint Systems

One of our customers’ biggest concerns is having to paint their windows every couple of years. To minimise this requirement we spray finish all our Accoya® sash windows with the finest Teknos low-maintenance microporous paint systems, coupled with end grain sealing and highly-skilled spray application, giving us an unrivalled, long-lasting paint finish. Because our premium sash window range is made using Accoya® exclusively, maintenance cycles are greatly extended and on redecoration only require one topcoat.

We can also colour match to Dulux, Farrow & Ball, Crown or RAL.

Draught Proofing

Our premium sash window range comes fully draught sealed. They have a perimeter brush pile polypropylene stripping, discreetly contained within concealed carriers installed to all staff and parting bead, maximising thermal and sound efficiency.