I ask because I could do with one on-hand for the geometry, but triple-glazed seems to have a long lead time. I've seen that there are some double glaze replacement sealed units available, which sort of suggests this swap might be true.

Yes absolutely, they sell kits for this which will come with the relevant butyl seal.

 

im sure if you can get the glass dimensions you could probably do it even more DIY for much less by ordering the glass (ensure correct spec) and a roll of butyl seal.

 

you’ll need to buy the same dimensions as it has currently, I think they did used to sell a triple glazed upgrade for the older windows but it only resulted in a 0.1Uw decrease so probably isn’t worth the extra cost

>>> but it only resulted in a 0.1Uw decrease so probably isn’t worth the extra cost

 

That's a very good point. I just did some quick variations in PHPP substituting different Velux glazing:

 

'66' triple glazed over '70' double glazed gave a 1.5% reduction in annual heating requirement and a 0.4% reduction in solar gain (worth about £25 a year) ... but at a 44% (£724) capital cost increment.

 

 

1 hour ago, Alan Ambrose said:

'66' triple glazed over '70' double glazed gave a 1.5% reduction in annual heating requirement and a 0.4% reduction in solar gain (worth about £25 a year) ... but at a 44% (£724) capital cost increment.

When buying my velux windows I was on a tight budget at the time and did a similar sum, and the energy saving on such small windows would never repay the extra cost in my lifetime so I chose doubles.