You can, in part. But I must expain:

If someone is really determined beyond reason, they could remove the existing glass from the existing sashes and insert the (much) thicker sealed glass double glazing units into these old sashes.

However, to do so, you have to create such a large rebate in the existing (and no doubt slim) timber sashes that the four corner joints are weakened to the extent that their life will be considerably shortened. You can look forward to the probability of their failure at some future point.

Also, both the sash top and bottom rails, and side stiles, are completely mutilated when forming this new larger rebate, which, by the way is where the putty is supposed to go. And nobody tells you. Putty rots the edge sealer of glass units, so you have another material failure coming your way.

It is amazing to me that some companies offer this service. If you are tempted, ask for a comprehensive ten year guarantee, and start saving up to have the work done again properly!

But you CAN make use of a most important part of your existing sash window - the weight box frame. Even if this has deteriorated, it can usually be renovated.

Then the proper, sensible way to install sealed glass double glazing units is to bite the bullet and have the timber sliding sashes remade anew, with special design mortice and tenon corner joints, strong enough to support the new thicker, heavier glass. You install these new sashes into your existing (renovated) weight box frame, thereby not destroying the surrounding plaster and decorations.

Also the glass units can be fitted from the inside of the house, a burglar's nightmare, which provides additional comfort to know.

I must add here that the above is a specialised job and cannot be done properly by DIY or by a non-specialised builder, but it's worth it.

I'm sure all our visitors, both contributors and readers, send you their best wishes for your project. Can you send us a diary of how you get on? Then I can personally offer any help via this page, should you need it.

Kindest regards and thank you again for your most interesting question.