It’s been a while since I shared my thoughts and I recently did a pretty interesting job. The customer was focusing on good value for money so chose a mixture of retrofitting double glazing into the original sash and installing new double glazed sash windows into the existing frame. The results were pretty interesting actually. In my opinion I preferred the slightly less than perfect look of the original sash. They fit in with the property nicely looking pretty tidy. I feel the new double glazed sash into the existing frame looked a bit too ‘new’. And don’t get me wrong, they looked great, it’s just in terms of value for money the retrofitting the sash with double glazing probably was better but let’s break it down:
Thickness of double glazed unit when comparing retrofitting double glazing and new double glazed sash windows
In this particular example the new double glazed sash windows and the retrofitting both benefitted from a 4-8-4 Argon filled, low-E, toughened unit, and a white spacer bar. The only difference I suppose glazing wise was the sandblast glazing in the original sash. So from a heat perspective as well as an acoustic they match identically. Given they are both installed with a sash window draught proofing system there’s not really a great deal in it other than aesthetics. So let’s take a look at those:
If you look at the picture below you can see there’s visually not much difference:
So I put a ring in red around the original sash with new double glazed units retrofitted and in blue is the new sash into the original frame. I think they both look great and if I am perfectly honest definitely default back to the better value
So you have to question, unless there’s a load of sash window repair that would affect the price point, why throw your original sash away? Well there are a few reasons why retrofitting might not be better than new double glazed sash into the original frame.
Some sashes are too thin for retrofitting double glazing into your original sash
When your sashes are too thin you can ‘normally’ get new sashes made and keep the old frame. This is because we can offset the mortise and tenon and play around with the size of the internal profile on the sash, reducing it enough to install a double glazed unit still. This can’t be done on an original sash as we are dictated to by where the size of the profile and where toe mortise and tenon sits.
How thin is too thin to retrofit double glazing and new double glazed sash?
If you want to install double glazing into your original sash then you ideally want 35mm and up. Personally I prefer to install into 37mm but 35mm works fine for a 4-6-4 Argon filled double glazed unit. You’ll get some good noise efficiency as well as 90% of the heat protection in practical terms when comparing to entirely new sash windows. When you compare to just replacement sashes though, you’ll quickly realise there’s virtually no difference in thermal efficiency at all!
Here’s a look at some 35mm sashes that we recently double glazed. It leaves the profile in tact and looks pleasant externally. If I had one gripe it would be the glazing bead is a little flat, but I am most definitely being very critical of my work here 🙂
Apart from the fact these look like new sash now from a few metres away, it’s also pretty difficult to tell these are even double glazed. Which to me is the biggest advantage of all over new double glazed sash. If you’re trying to improve the thermal performance of your property but want to maintain a traditional look then retrofitting is definitely the way forward!
Which is better on price: retrofitting double glazing into original sash or new double glazed sash windows?
It won’t take much looking to realise double glazing the existing sash and retrofitting can save as much as 50% on new double glazed sash and at a minimum 25%. That adds up to a fair old saving if you have a whole house to handle.
Will retrofitting the original sash or new double glazed sash last longer?
Interesting question. Since I started writing about pricing and new sash my mood has definitely switched more toward hardwood sash. The main reason is so few people are maintaining their windows properly after we install because they are left so nice, they rarely bother with a top coat on the frames. So when I make the this comparison I am judging hardwood double glazed sash vs retrofitting the original sash.
I’d say they are absolutely equal. Since we use a high quality double glazed unit that’ll last realistically twenty years or so and have a failure rate of less than 1 in 10,000 within ten years it just comes down to ensuring you keep painting your sash windows.
We always repair and remove any rot from the original sash as we install double glazing. Here’s a picture before the rail was painted from the above comparison. We just spliced a new one in but you would not have known unless shown or told, and may still not have believed it without seeing 🙂 Here’s a look at the repair on the retrofitted double glazed sash:
Here’s a look at that same sash painted and built after 🙂
So overall it doesn’t really matter which option you choose, if you’re looking for sensible value for money then it’s more likely you’ll want to go with retrofitting the original sash window double glazing but if you prefer a more sleek and new look then new double glazed sash into the original frame is far more likely to interest you.