Replacement wooden double-glaze sash windows - spacers and gas filling?

[ajlowery]

We are getting some replacement sash windows made in wood with double-glazing.
Due to space limitation within the sash, the gap between the panes will be much less than most double glazing - something like 6-8mm.

This made me think warm-edge spacers might be worth using.
The only place we get condensation on the (uPVC) double glazing elsewhere in the house is round the edges of the panes.
But the sash window company are trying to dissuade me - apparently the glazers will only use aluminium for the vertical centre-bars that split each of the sashes in two.
So if I did have warm-edge spacers they would only be along three out of four edges in each of the four pane-sections, and they would not match in colour the aluminium down the centre.

The narrow cavity also made me think of asking for krypton or xenon instead of argon, which apparently performs very poorly as an insulator in narrow cavities. The firm has yet to give me an answer on this.

Am I tinkering at the margins here with things that will make hardly any difference given that we are moving from single-glazing to double?
Or should I stand my ground and have them done as optimally as possible?

 

[chappers]

They should be able to get you a bigger gap than that, I have recently had some made with 4-16-4 glazing, they rebate the sashes around the boxes slightly to increase the profile width of the sashes to accommodate the glass. The overall box thickness is 134mm these are for weighted sashes.
If your interested then I can give you their details. I have been using these guys for years and have yet to find anyone to compete on price. as an example a box weighted sash with furniture and factory finished in white was £941+ VAT

 

[Buxus]

I'm not sure warm edge spacers will work properly in a gap that narrow but as chappers said, look into having thicker units. I've almost finished diying 12 replacement wood sashes, the thickest unit I could fit was 4/12/4 but couldn't justify the ultra thin units, they're expensive to buy and less future proof if the gas leaks out. Which ever glazing unit you decide on, make sure the frames are drained and vented, there should be drainage holes visible at the bottom of the frame.

 

[ajlowery]

Thanks both. Sash depth is 44mm: going to have a 6.4 AL outer pane, 8mm gap, 4mm low-e inner pane. There was no talk of this being 'too slim'.
Think they are using wet-glaze system.
I'd've thought warm-edge spacer would make more difference in a narrow cavity as aluminium would just conduct the cold directly across such a small gap?
Price is £950 per window.

 

[chappers]

ditch the 6.4 mm and up the gap, that's the easiest way to increase the efficiency. Go 4-10-4 if that's all the space you've got.
But as previously said you can get a thicker unit in, the sash depth on the ones I use are 58 mm.
Just had a look at the nominal u-values for sealed units these are all with 4mm glass low-e inner
4-16-4 1.5
6-8-4 2.1 what you have
4-10-4 1.8 what you could have in same width sashes
6-8-4 krypton 1.6
4-10-4 krypton 1.4

I have no ideas on the price differences between argon and krypton, but I had heard that krypton leaches out far more readily than argon.

Can I ask where those windows are from and are they fully factory painted and finished and what size that was for @£950 and was that including VAT

 

[Gazman16]

I shouldn't worry about which type of gas as it will have all leaked out in about 5 years anyway.
Warm edge spacers are much better than metal ones and I always get them with the same spacers in the duplex units too. I don't know why they are saying they cant do that unless its something to do with the slim units

 

[chappers]

But as I said previously solve the problem by getting them to make the sashes thicker and using a thicker SGU or find a joiner who can.
These are the guys I use together with their spec 4-16-4 SGUs

 

[ajlowery]

Thanks for replies - been away.
The firm is Joinery Workshop.
The problem re. the spacers is that they are saying the glaziers will only fit a vertical bar in aluminium, so the other spacers can be warm-edge but the centre bar has to be aluminium (i.e., one side of each of the four panes).
I want to keep the acoustic laminated 6.4mm pane as noise is one of the main issues. That and condensation. Heat / thermal insulation not so much as it is not a cold room and out heating bills are v low.
Don't have measurements handy sorry but the total bill including painting and VAT is £2k.