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

zoothornrollo_2

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 edges of each of the four 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?

leveller2911

zoothornrollo wrote: »

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.

Sounds as though you need very thin sealed units with a very narrow spacer bar. The issue with very thin units is they don't tend to have a long lifespan due to the fact that there is very little vulcanised rubber around the perimeter to keep the water compared to standard units. Some companies won't give more than 12 months warranty and those who do will charge a premium for the units. Two companies that do these units with a warranty is "Slim-lite" units (10yr warranty but very expensive) and "Timberlite" who are cheaper but give a 5yr warranty.

These units are typically 4-3-4, 4-4-4,4-6-4 etc etc generally Softcoat Low E glass filled with Krypton gas and a warm edge spacer as standard.A 4-4-4 Low E galss ,filled with Krypton will give a U value of 1.9 , These units will not comply with building regulations and are usually used for Grade II Listed properties..



If you are keeping the existing frames but replacing the sashes then you don't need to comply with thermal regulations of the Building regulations.If this is what you intend to do then you must ask them to fit draught strips to the windows, no point fitting DG if they are still draughty.

zoothornrollo_2

Leveller thanks a lot for that.
It is just a sash replacement (x2) not frame.
The sashes are 44mm deep - there will be a 6.4mm acoustic laminated pane and a 4mm low-e pane, with an 8mm gap.
There was no mention of this being more expensive because of the slim profile.
It is Manchester's Joinery Workshop who are doing them, c.£950 per window.
Think warranty is 10, but could be five.

leveller2911

zoothornrollo wrote: »

Leveller thanks a lot for that.
It is just a sash replacement (x2) not frame.
The sashes are 44mm deep - there will be a 6.4mm acoustic laminated pane and a 4mm low-e pane, with an 8mm gap.
There was no mention of this being more expensive because of the slim profile.
It is Manchester's Joinery Workshop who are doing them, c.£950 per window.
Think warranty is 10, but could be five.

Are there any glazing bars dividing the sashes ?. Doesn't sound like they are slimlite type units, Just standard units. The rebate is usually 15 high to allow for the spacer bar (perimeter bar).

With the sealed units you are looking to use there will be a lot of extra weight . Depending on the height of the window and the space in the sash weight pockets it can be a real job get the required amount of weight inside the boxes.

zoothornrollo_2

leveller2911 wrote: »

Are there any glazing bars dividing the sashes ?. Doesn't sound like they are slimlite type units, Just standard units. The rebate is usually 15 deep to allow for the spacer bar (perimeter bar).

Yes there are vertical glazing bars dividing each sash in two: they said that these will have to be aluminium, which means that if I want warm-edge spacers they can only be along 3 sides of each pane.

leveller2911

zoothornrollo wrote: »

Yes there are vertical glazing bars dividing each sash in two: they said that these will have to be aluminium, which means that if I want warm-edge spacers they can only be along 3 sides of each pane.

If the glazing bars are narrow (20mm-25mm) and the rebates are around 8-9 mm high then I think they are "slimlite" type units ,so a sash divided into 2 panes with have 2 units. If so then no reason why warm edge spacers all round can't be used.

Do they say what type of glazing system they are intending to use in the quote?.

zoothornrollo_2

leveller2911 wrote: »

Do they say what type of glazing system they are intending to use in the quote?.

Well we had a conversation regarding this, a while back and this is how it went. I said:

I came across this advice when researching replacement sash windows online –

Whichever option you are interested in, you should specify a dry glazing system, using double-sided adhesive tape rather than silicone or putty, a clear 5mm gap all round (especially between the bottom edge of the glass and the glazing platform), warm-edge spacers, and 20mm glazing rebates – to protect the edge seals from direct sunlight.
All of these measures are vital to prevent premature internal misting of the sealed glass units, and are specified in British Standard BS6262.

Is this correct / useful advice?

They replied -

I would suggest discussing this directly with the Surveyor.

Without going into too much detail I don't fully trust dry glazing in terms of stopping water getting through the seal as it's not great at coping with the natural expansion and contraction of timber - this is a far worse problem to have than premature misting.

In the last 10 years we have renovated in excess of 10,000 period windows. All refurbishment work is guaranteed for 5 years and all new 10 years, this includes premature misting - I can honestly say I can't recall a premature misting warranty claim, usually if the units do fail it is very quickly due to a manufacturing defect.

A real key factor is in the manufacturing process of the DGU's: ours are sealed in a vaccum chamber by an automated machine whereas some will have been done by hand on a worktop - this makes a huge difference in avoiding premature failure. My advice would be to stick with our tried and tested process but we can do whatever you want.

As I said, have a chat with the surveyor when he comes round.

leveller2911

zoothornrollo wrote: »

Well we had a conversation regarding this, a while back and this is how it went. I said:

I came across this advice when researching replacement sash windows online –

Whichever option you are interested in, you should specify a dry glazing system, using double-sided adhesive tape rather than silicone or putty, a clear 5mm gap all round (especially between the bottom edge of the glass and the glazing platform), warm-edge spacers, and 20mm glazing rebates – to protect the edge seals from direct sunlight.
All of these measures are vital to prevent premature internal misting of the sealed glass units, and are specified in British Standard BS6262.

Is this correct / useful advice?

We only use a dry glazing system similar to the above but 3mm gap around the perimeter. A 5mm gap around the perimeter will require an 18mm high rebate which means the sash stiles,rails etc would need to be a bit wider to compensate so they can look a little heavy ..

We use "security glazing tape" to be the units on which is a closed cell tape, fit plastic packers around the unit . Then we use glazing silicone around the perimter ,allow to dry and then fit timber glazing beads with security glazing tape which is pressed tightly onto the glass unit and them pinned in place with stainless steel pins. So very similar to the above system. I've used this system on properties down by the coast (100yds from the sea) and after 20yrs they are still going strong.

They replied -

I would suggest discussing this directly with the Surveyor.

Without going into too much detail I don't fully trust dry glazing in terms of stopping water getting through the seal as it's not great at coping with the natural expansion and contraction of timber - this is a far worse problem to have than premature misting.

I've used the dry glazing system for years ,out of 1,000's of units I've had around 9 fail prematurely. Some of them were down to "Thermal cracking" due to expansion/contraction., usually when using the Duplex system. Sounds like they are going to use a "wet" glazing system which is designed so that any water that gets behind the outside glazing bead ,runs down the face of the glass and there are "weep holes" in the bottom timber bead to allow the water to escape.

In the last 10 years we have renovated in excess of 10,000 period windows. All refurbishment work is guaranteed for 5 years and all new 10 years, this includes premature misting - I can honestly say I can't recall a premature misting warranty claim, usually if the units do fail it is very quickly due to a manufacturing defect.

Agree with this but some people don't pack the units up from the bottom of the rebate and use linseed oil putty alone to seal the sash. Linseed oil putty (if allowed) to come into contact with the vulcanised rubber will rot the rubber prematurely and the units fail.

A real key factor is in the manufacturing process of the DGU's: ours are sealed in a vaccum chamber by an automated machine whereas some will have been done by hand on a worktop - this makes a huge difference in avoiding premature failure. My advice would be to stick with our tried and tested process but we can do whatever you want.

But they don't say which system they intend to use. I'd be interested to know ........My supplier gives a 5yr warranty on units that are being fitted into timber windows and 10yrs in Upvc which I think is pretty standard.

zoothornrollo_2

Thanks for all your info Leveller.
I'm going with this company as I trust their workmanship and have already paid a deposit, been measured up etc etc.
I am just trying to decide whether to insist on warm-edge spacers and maybe krypton / xenon rather than argon - whether it's worth the bother.