What to think about when replacing sash windows

When looking into replacing timber sash windows, there are a few things to consider: not only the condition of the existing windows and their structural integrity but also the quality and finish of the replacement:

Frame thickness: When you change to double glazed windows you’ll have to increase the sash size to accommodate them and the box frame as well. This means pushing the window out closer to the outer brickwork and may have it protrude into the room. Find out how your provider will manage this so you’ll know how it will look and consider using the thinnest possible frame depth it is possible to minimise the visible difference.

Meeting rail thickness: The meeting rail is the horizontal section running effective across the top and bottom of the sash. This hides the rebate for the frame. With double glazing this will need to be big enough to ensure the glazing is properly finished. The sash with double glazing will be twice the weight – you won’t notice it as it’s counterbalanced – but it means the timber of the top meeting rail must be thick enough to take the weight of the window. 

Glazing bars: You have a couple of choices. Through or True bars work like the original bars separating the window into smaller panes. But because they must be bigger to accommodate the double glazing they don’t necessarily look authentic. Applied bars are bonded to the inner and outer panes of the glass. This means that there is one pane of glass per sash. The result is a slender bar that is less traditional than a Through bar but looks more like the original.  

Glass: The final look will be impacted by the reflectivity, tint and distortion of the glass. 

• Reflectivity – Single glazed windows appear less reflective than double glazed windows. 
• Tint – Modern glass tends to have a subtle tint thanks to solar control coatings. 
• Distortion – Original glass is imperfect giving a distorted view when you look through it. 

Consider Low Iron glass which is clearer and not considerably more expensive.

If you really want an authentic look, you might be tempted to consider Cylinder glass (it’s made as a glass cylinder then flattened). However it’s costly to make and it is brittle and prone to breaking. An alternative is Machine-drawn glass which still as an authentic feel and is available in larger sizes than Cylinder glass. It can also be toughened or laminated to make it stronger. Woodland use Goethe 3mm and 4.5mm machine drawn glass. 

Sash horns or joggles are a decorative feature of a sash window. Most companies who make their own range of sash windows will be able to replicate those on your windows. If they have to create a template for a unique design you can expect to pay more but check local companies as they will be used to matching designs found in local homes and may already have a record of your sash horn profile. The bigger companies tend to offer a standard range which may work out cheaper but won’t match your original sashes. As a guide, Woodland charges about £50 -£100 per sash for a bespoke horn profile.

Paint finish: Most original sash windows are finished in oil-based paint which builds up over repeated coats and which often conceals some of the original detailing. Most replacement sash windows will be finished in a water-based paint system which is less glossy, more environmentally friendly and allows the timber to breath. 

Fretwork and other bespoke detailing: Any original features can be reproduced – it’s really a matter of budget. As with sash horns, a larger company will offer you a limited range of options for lower cost. If you want to reproduce the unique detailing on your windows, try a local company with the technology in house and as they’re local, it’s worth going and seeing examples of their work.