What will retrofitting original sash windows with double glazed units do for heat insulation?

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What will retrofitting original sash windows with double glazed units do for heat insulation?

You’ve arrived on my article because you’re considering upgrading your single glazed wooden sash windows but you don’t necessarily want the expense of entire replacement sash. ‘You can expect significant heat and sound improvements from retrofitting sash windows with double glazing. Not only that, you’ll improve the security of your property too.’ 

Following on from our page that highlights double glazing existing sash windows, I wanted to talk about the benefits of using your original sash windows and upgrading them rather than replacing.

It could also be that like me, you loathe the idea of replacing your original sash windows that are otherwise excellent condition. But you do want the benefits that come from double glazed units. That’s where converting your original wooden sash to double glazing comes in.

Why would you retrofit your original wooden sash windows?

Your original sash have stood the test of time. We are talking a hundred and twenty years plus now in most cases throughout London. They may well have the odd sash window repair or sill replacement. They may well have had the odd sash rail splice repair too but it makes the salesman popping round and proudly telling you how amazing his windows are, and they will last twenty years no hassle pretty cringeworthy doesn’t it?

Let’s set the history of your sash windows aside for a minute. If you’re not convinced by their quality, then we should talk about cost. If you look at sash window prices in London, you’ll quickly realise using the original sash and upgrading this way is half the price of entire replacement sash windows or less. Performance wise, it’s normal to achieve over 85% of the energy efficiency of brand new sash windows from your original sash. When you consider you’re paying half the cost of new windows however, it becomes instantly clear this is where the value for money really is.

How will a sash window look after it’s retrofitted and converted with double glazed units?


Hands down, the biggest benefit of double glazing your original sashes is the look. A retrofitted sash is as close as you’ll get to keeping the original look. Here’s an example with a standard double glazed unit.

We have converted the original sash with a 4-6-4, Argon filled, low-e, soft coat, toughened, double glazed units. We can in fact make the double glazed unit look less prevalent if you’re really into your conservation. This is with a thin sight line unit (you may know this as slimline). However, you’re probably looking at these pictures and thinking I am splitting hairs and the result is excellent.

Here’s a look from inside and at another angle of what you can expect. You’ll notice these windows are still waiting for a top coat. They already look pretty tidy:

You would be hard pressed to tell the difference between the two. Some professionals may not be able to tell the difference at first glance either!

As you can see, from straight on you wouldn’t know they are double glazed with the exception of the shiny panes of glass causing a reflection. This would be the only tell tale give away. However, this same look will occur on single glazed sash windows that have secondary glazing installed and that looks plain ugly, even if very effective against noise and heat pollution. Some property owners have no choice though. The best they can do is have a draught proofing sash window service in combination with secondary glazing because of listed building regs, however, I do find these days more listed buildings approved for retro fit than ever before!

What are the drawbacks of retrofitting and converting single glazed sash windows to double glazing?


There’s only one real drawback to converting and retrofitting original sash windows to double glazing and that is the depth of your sash. You’ll need at least 35mm but 38mm and up is ideal. This is because of the mortice and tenon. When we double glaze your existing sash window we do not want to structurally weaken the sash. So the deeper the sash, the better the overall result and stronger the sash will be once we’ve finished the process.

In some cases you may have lower sash horns, these are a decorative moulds that afford a full width mortice and tenon. This may mean we can double glaze sashes with less depth but the only way to be sure is to give us a call and we can take a look for you.

Are there any other benefits to converting sash windows to double glazing?


Yes, for a start, where we can use toughened glazing, this will improve your sash window security considerably. Having two panes of glazing that are toughened represent a far more difficult challenge for any potential intruder. Breaking toughened glazing takes considerable effort. You would need at a minimum, a standard claw hammer with a huge amount of force. It would be more than enough noise and fuss to alert neighbours and passers by.

Overall, the balance of retrofitting sash windows with double glazing is the best value for money. You’ll improve the thermal performance of your windows near inline with new sash, and acoustically there will be a good improvement as well. You can expect to dull out the noise of cars, people shouting, and to an extent reduce the noise of diesel vans and trucks. You’ll still know they are there if you listen for them, they just won’t feel like they are inside your home anymore which is the most important improvement by far!