Super-thin double glazed units (for retrofitting in period windows)

[d000hg]

I've been looking into things like http://www.slimliteglass.co.uk/ as a retrofit option when renovating Victorian sash windows.
They appear great on paper and I see they've been featured in the likes of Grand Designs and are approved for listed buildings in Edinburgh.

However none of the renovating companies locally (Durham) appear to offer this service - they will renovate the existing sashes or build new timber ones with slim (4-6-4) panels.

The chap I saw today was very dismissive of the idea, and told me that it's been proved anything under a 5mm gap doesn't work, that they have been shown to fail, and that they look awful when the existing sash is routed out.

Of course you often see experienced experts quite set on whichever way they've always done things so I'm looking for more professional wisdom to get a balanced view. Are these 3-3-3, 3-4-3, 4-4-4 panels worth considering to get the best balance of modern and traditional, or are they a liability?

 

[ronniecabers]

We've fitted 4-4-4 units for customers and I have to say they don't really look much different to 4-6-4 units when installed . I would say that yes the thermal qualities aren't as good as modern 4-20-4 or 4-16-4 units but they certainly won't be that much lower than 4-6-4 units ( after all the airgap is only 2mm difference ). They will certainly be better than single glazing! I don't think they fail any quicker , there are a lot of variables as to why sealed units fail , but I've never known the size of the airgap be one of them. One thing that won't work very well at all , is the use of a 'Low e ' glass ( such as Pilkington K or Planithem ) due to the size of the air gap.
I can see how an old sash routed out could look wrong , and if putting into sliding sashes you will need to take into account the weights , as the sliding may not work unless you also alter the sash weights

 

[Mr Chibs]

I've got slimline double glazed panels in my old french doors, from memory, they are 4-6-4.

They don't condensate on the inside like single glazing, and at the time or writing this, 4 of the 8 need replacing, as they are very slightly misty inside, so yes, maybe they do fail earlier... that said they've been there for over 15 years, so not bad really.

FWIW, I had a local glazing place give me a price for replacement units, they are around £15, for 400 x 300mm.

 

[ronniecabers]

4-6-4 units are a very common size in old wooden double glazing and 15 years is good for wooden windows!