Can You Double Glaze Original Sash Windows

This is a question that we receive often at Ventrolla. You want the performance of a modern upvc double glazed sash window while retaining the charm and character of your original timber sash windows. And who can blame you.

Introduced to us from Germany in the 1980’s, the bulky white plastic window soon became a common site across Britain. Homeowners loved the thermal and sound deadening qualities that double glazed windows offered. Beautiful period wooden sash windows that had survived from the Edwardian, Victorian, and even Georgian eras were ripped out and dumped in skips to be replaced with less attractive white upvc imitations.

The problems with original sashes, and the reason people were so willing to skip them, was the thin glass and the gap necessary for the sashes to slide up and down. These two features of period windows meant they were thermally inefficient and often draughty which in turn leads to rain water leaking through and rot setting in. Add to that rattling in the wind and not being able to shut out the noise of the street. You can see why people flocked to the alternative and were willing to put up with the aesthetic flaws in favour of modern performance.

Can you double glaze period wooden sash windows?

Well yes and no.

The glass used in old sashes is very thin, 2mm – 4mm is common. That allows the glazing bars (the pieces of wood that separate each pane of glass) to also be thin and light. Modern double glazed units by comparison have two panes of glass (obviously) and an air gap in between. This is usually filled with an inert gas like Krypton, and it is this air gap that produces most of the thermal benefits. Each pane will typically be 4mm thick and the air gap anything from 6mm to 20mm. You might have seen notations for double glazed units written like this: 4 / 12 / 4. That means a 4mm pane of glass, a 12mm air gap and closing the sandwich, another 4mm pane making a total thickness of 20mm.

These are much wider and heavier than the original single pane glass and do not fit in the glazing bars. Indeed, if you routed out enough wood to slide the new unit in the bar would simply snap. You must also consider the total weight of the sashes, the lead counter weights hidden in the box frame are weighted precisely to aid a smooth opening a closing action. Adding more weight to the sashes means adding extra lead to the counter balance system too.

Right now, you’re probably thinking “Right, well if I can’t put double glazed units into my old sashes I’ll have to get upvc ones or just put on an extra jumper during the winter months.”

But wait! There is a third path!

New bespoke sash windows can be crafted that exactly match your existing sashes. Energy efficient slim double glazed units will be installed into them along with a hidden draught proofing system that has been tested to withstand winds up to 71 mph (a category 1 Hurricane starts at 74mph).

Once these handcrafted sashes are placed back into the original box frame you’re left with beautiful, unique timber sash windows that are full of charm and have all the modern benefits a upvc imitation would bring.

If you want a home that retains its period character but still benefits from modern energy performance, draughty proofing and sound deadening without having to compromise with unsightly white plastic windows get in touch today.