Replicating the proportions and fine details of historic sash windows while achieving modern levels of thermal and acoustic performance is one of the biggest challenges in the design and manufacturing of timber sliding sash windows.
Conventional modern double glazing is much thicker and heavier than historic single glazing, requiring wider and thicker frames & sashes to accommodate the glass, this alters the proportions of the window along with its overall look and feel. This can cause replacement windows to project beyond the internal wall or ‘stick out’, altering sightlines and proportions which changes the feel of the room. It can also make it difficult when installing behind traditional shutters, shutter boxes or timber panelling.
We have engineered and designed a new sash box that solves this problem without sacrificing either form or function; our ultra slim sliding sash window combines both an ultra slim box width with high thermal and acoustic performance.
Key features and benefits:
- 134mm overall sash box frame depth (the depth taken from the back of frame to front of the frame). This 134mm frame depth closely matches the typical dimensions of an historic single glazed sash box.
- Incorporating 8.3mm thick vacuum glazing, currently one of the most thermally efficient glazing options available.
- U-Value of 0.76 W/m²K (whole-window Uw).
- Improved acoustic performance with an 39 dB acoustic rating.
- Authentic historic appearance and proportions.
- More compatible with traditional interior features such as shutters, shutter boxes and timber panelling
For comparison: our 24mm double-glazed sash box has a frame depth of 150mm, and our 36mm triple-glazed sash box is 190mm.
Why this matters: in period properties with single-skin 9-inch walls, original single-glazed sash window frames are typically around 120-130mm deep. Replacing them with a 150mm double-glazed sash will cause the frame to project into the room. Our in-house installers usually hide this by adding architrave and packing behind it, but that changes the original appearance of the window. With our ultra-slim 134mm sash box there’s no compromise – the modern, vacuum‑glazed replacement window sits in the same plane as your historic frames and preserves the period look.
How we achieved it: delivering this slim, high-performance solution required multiple design iterations, tight manufacturing tolerances, and investment in new CNC tooling.
If you’d like to learn more about this or some of our other unique designs, or request a tailored quote, our sales team is ready to help.