Is the existing box frame suitable?
The first step is establishing whether the existing frame provides a sound basis for the new sashes. During the survey, we assess the sill, pulley stiles, linings and joints, as well as the overall condition and alignment of the frame. Localised defects can often be repaired as part of the work, but more extensive decay or distortion may make complete replacement the better approach.
We also consider the surrounding finishes. Retaining the original box can reduce disruption to plasterwork, architraves, shutters and decoration—one of the practical advantages of retrofit where the existing frame is worth preserving.
Double glazing changes more than the glass
Introducing double glazing affects the construction of the sash itself. The glazing is thicker and heavier than a traditional single pane, so the sash sections, glazing rebates and balance weights all need to be designed accordingly. Fine glazing bars and other original details also need careful consideration to ensure the new sashes remain sympathetic to the proportions of the window. The glass specification can then be selected according to the needs of the room, including options for safety, privacy or improved acoustic performance where appropriate.
Rather than designing the joinery around a standard glazing unit, the glass, timber profiles and proportions should be considered together.
Retrofit or complete replacement?
Retrofit replaces the moving sashes while retaining and servicing a sound existing box frame. Complete renewal replaces the entire window. Where suitable, retrofit can be less disruptive and preserve more original joinery while still introducing double glazing and improved draught sealing. Complete replacement may be more appropriate where the box frame has extensive decay, significant distortion or is itself a poor-quality previous replacement.
Importantly, every window in a property does not have to follow the same approach. A sound original bay might be suitable for retrofit while another window in poorer condition requires complete renewal. At STILE, we assess each opening individually so that original box frames are retained where they remain sound—and replaced where there is a clear reason to do so.