To clarify, replacing sash windows refers to the process of replacing all parts of a sash window – both the window sashes and the box frame. Replacing sash windows with more energy efficient glazing can be extremely beneficial in the long term.

In most cases, we can make completely new sashes fitted with standard double glazing. However, if you are looking for a premium glazing option that offers the best in thermal efficiency, 0.7 W/m²K U-Value slim double glazing is the ideal choice. This option will make your sash windows as efficient as triple glazed new windows whilst retaining the slimness of a single pane. The new wooden sashes will also undergo full draught proofing.

Before we take a closer look at our sash window replacement process, take a moment to recap the components of a sash window. This will help you to fully understand our explanation of the process.

The general sash window replacement process at Parsons Joinery and our sister company Wandsworth Sash Windows is as follows:  

Replacing the window sashes including the box frame is a very popular service at Parsons Joinery. The brand-new fully painted wooden sashes and box frame will include double glazed/vacuum insulated glass and the service also includes full draught proofing.

Here’s what is included in our sash window replacement service, step-by-step:


The existing sashes and entire box frames are carefully removed.


The first part of the installation involves installing the new fully painted timber frame.


Wedges are used to secure the frame in place.


Expanding foam is used to insulate around the window.


New weights are fitted into the pockets of the box frame. They are counterbalanced to the weight of the sashes to ensure they slide up and down perfectly.


The new sashes are hung on white nylon sash cords.


Beading is fitted to ensure the windows are draught proofed.


Any areas affected by the installation of our new windows are ‘made good’ by our team.


The new frame is sealed to the exterior brick work with silicone or sand and cement.


Your choice of ironmongery is fitted.


New architrave is fitted on the inside of the windows on the frame and this is painted in the colour that matches the new joinery.


The fitter will check that the new sashes run up and down nice and smoothly. We recommend that you do this daily while they settle in.


The result? Beautiful, brand-new timber box sash windows expertly fitted by Parsons Joinery.

View the full video of our complete sash replacement process here:

For more information on our sash window repair and replacements please feel free to contact us. We will be happy to chat through your requirements or provide an obligation-free quote.