Double glazed window

[Giff]

I have been asked by my neighbour to make a replacement window frame for the one shown here. I have made window frames before for my house,but single glazed as the original period ones. He has asked for this to be double glazed and his builder will fit and glaze it. He has said the DG unit will be 20mm thick. The question is how are these DG units normally bedded / fitted. Should the rebate depth be 20mm + for bedding in and sealing or should it have a wooden bead. How are the DG units held in. Thanks Geoff

 

[Max Power]

Make beads to suit and either use double sided tape to bed it into the rebate or butyl compound (Do Not use linseed oil putty ) and the same to bed the beads to the glass

 

[Steve Maskery]

Units should idally be 24mm (4-16-4) but it does get rather tight cutting a rebate for them in standard sized window sections.
I've not use the DS tape but I believe it works well. I have used the butyl and glazing silicone. If you use the silicone, and your beading is external, just make sur you have squeeze-out all along, especially on the bottom strip. If you don't, water and dirt will get in, won't be able to get out and you will see it from inside and the beads will rot.
S

 

[graduate_owner]

Hi,
I'm not sure about this but I was told that wooden windows have thinner section double glazed units fitted because the wood is less stable than uPVC, which is where the thicker glazed units are used. I don't know if the fact that needing such a thick rebate is anything to do with it, or if the wider d/g units break more easily if the wood twists, but the d/g units in my own hardwood windows are certainly much thinner than my neighbour's uPVC ones. One of my own d/g units has come adrift - when the wind blows rain in the right direction I get a 2" layer of water between the outer and inner panes.

I'm no builder - just passing on what I was told, but it might be worthwhile checking up before you start chopping away at the rebates.

K

 

[Steve Maskery]

K
Standards for DG units have gone up over the years. The first I made had 4-6-4 units and were the standard for the day. I don't see how wood is so much more unstable that it can make that difference. Maybe I'm wrong, but I don't see it myself.
But just do the maths. If you have a 12mm lip from a 45mm section, you have just 33mm left to accommodate sealant, DG unit (inc its silver tape) and retaining bead. Now you can reduce the beads a little but it's still tight. The alternative is to make your windows from ex-62mm stock, rather than ex 2", but that requires other accommodations in terms of plaster, clearance angles, etc.

 

[Giff]

Steve do you mean the d/g units are bedded on silicone and then silicone and beading ? Geoff

 

[Steve Maskery]

Yes.
There should also be an air gap all the way round. This means making the bottom strip breathable and sitting the DG unit on packers - usually those blue pads that come stuck on them. There used to be a very helpful piece by a bloke called Hislop on the TRADA site. Sadly I think it's now sold rather than free. A good read if you can find it though.
S

 

[Giff]

Thanks Steve
Really helpful. Geoff

 

[Lons]

All good advice especially about not using putty which breaks down the seal between the glass panes causing premature failure.

I'm not at all sure about Steves suggestion to use the blue foam spaces as packers. I would never do that as they would compress over time and the glass would slip to the bottom of the frame. The correct packers to use IMO are easily available from any glazier or even screwfix and the like. I have hundreds so if you needed any I'll post a few.

Bob