The hidden sins of retrofit double glazing

We’re always being asked about double glazing or thicker glass for old timber, wooden and sash windows. Find out how they can be double glazed successfully and what to look for when upgrading your heritage windows.

There are plenty of companies out there that will sell you retro fit double glazing for old timber and wooden windows.

But many of them don’t know how your heritage windows work and they end up breaking them in the process. All to often, we get called in to make the windows work again!

The problem is that when people fit thick glass and double glazing units into old wooden window sashes, it makes the sash much heavier. This stops the sash window for operating because it becomes to heavy for the original mechanisms.

To keep the sash operating smoothly, technically, you have to add additional weight to the other end of the pulley and rope system.

With all that extra weight, the ropes and the pulleys come under a lot more strain. And the space for the counterweight become overcrowded. Put lightly, double glazing sash windows simply can’t work like this.

Two weights on one cord jam in the weight pockets, stopping windows from operating.
Weights are wired together onto a conventional cotton cord. This won’t be long before the cord breaks, leaving the window inoperable. 

We often see shoddy workmanship in these situations. Ropes that are not strong enough to last; original lightweight pulleys that are left in place; and windows that won’t work properly because the counterweights have stuck.

One common trick in the double-glazing retrofit industry is to permanently screw the top sash shut, and to steal the counterweights from the top sash. Trouble is, the top sash will never work again, and the bottom sash is also likely to jam from carrying a bulky and unbalanced counterweight.

How we retrofit double glazing in timber, wooden and sash windows

At Sealasash we understand how sash windows are meant to work and when we fit heavier glass we re-weight each sash properly with lead counterweights. With the additional weight of lead, all the sashes work effortlessly.

We’ve also been able to obtain access to the latest vacuum insulated double glazing technology from Panasonic, that is a thinner and light glass overall. But it performs better than triple glazing standards.

When you are considering re-glazing your windows with new glass, it’s important that you ask the service provider if they guarantee they will make your sashes balanced and all work properly. They should be effortless to lift, perfectly smooth and there shouldn’t be any rattles.

Fixing shut sashes that are meant to move, particularly bottom sashes, will cause long-term rot issues and lead to more expensive repairs. So we don’t recommend this!

If you would like to talk to a timber window expert who understands all your options, and who will listen to your priorities, then please don’t hesitate to contact us.

Looking for double glazing for your sash windows or wooden windows?

Sealasash can double glaze your old timber, wooden and sash windows with new high-performance glass.  We also upgrade windows to include draught and acoustic seals that further improve the comfort and energy efficiency of your windows. 

Learn more about Sealasash:


Vacuum Insulated Double Glazing

Our ultra-high performance double glazing fits perfectly within your existing timber work, eliminating any bulky or expensive modifications.


Fix Those Annoying Rattles

With our draught and acoustic seals you’ll achieve amazing new comfort levels by reducing infiltration of outside air, noise and dust without the annoying rattles.

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Using traditional craftsmanship and modern materials, we’ll restore your windows and make them better than new.  This includes high-performance double glazing and draught seals that will dramatically improve the comfort and energy efficiency of old timber, wooden and sash windows.