Double Glazed Sash Windows

Why Double Glazing in a Sash Window Is Different

Standard double glazed units are 28mm thick. That is fine for a casement window with deep rebates, but a traditional sash window was designed around a single pane of glass just 3–4mm thick. Fitting a 28mm unit into a Victorian sash means either butchering the timber profiles or accepting chunky, inauthentic proportions. Neither is acceptable for a period property.

Heritage slim double glazed units solve this. At 14–24mm total thickness, they fit inside original-depth sash rails without widening the frame or losing the slender sightlines that define a traditional sash window. The glass looks like single glazing from the street but delivers modern thermal and acoustic performance.

Glass Specification

Every sash window we build is double glazed as standard. The standard unit is a 24mm sealed unit: 4mm Low-E glass, 16mm argon-filled cavity, and 4mm float glass. For properties requiring slimmer profiles — particularly Conservation Area homes and Listed Buildings — we offer 14mm slim heritage units with a 6mm cavity.

How Double Glazing Affects Sash Weight

A double glazed sash weighs roughly twice as much as a single-glazed one. This matters because the counterweight system — whether traditional lead weights on cords or modern spiral balances — must be recalculated. We balance every window individually: the weights are cast or the spirals tensioned to match the exact glass specification. This is something mass-production factories often skip, resulting in sashes that slam shut or refuse to stay open.

U-Values and Building Regulations

Part L of the Building Regulations requires replacement windows to achieve a U-value of 1.4 W/m²K or better. Our standard 24mm unit achieves 1.2 W/m²K — comfortably exceeding the requirement. Slim heritage units at 1.8 W/m²K may qualify for exemption in Conservation Areas and Listed Buildings under the competent person scheme, though we always recommend checking with your local planning authority.

Pricing

Double glazed sash windows start from £815 for a standard 600×900mm window. Slim heritage units add approximately 15% to the glass cost. Acoustic laminated glass adds approximately 20%. Every quote includes the sealed unit, warm-edge spacers, argon fill, and Low-E coating as standard — there are no hidden upgrades.

Use our 3D configurator to build your exact specification and see the price instantly. You can compare standard vs slim units, add acoustic glass, choose spacer colours, and see the window in 3D before ordering.