Advice requested: fitting double-glazing into Georgian windows

[Pallet Fancier]

Hi folks,
It's been a while since I last posted. Hope everyone is well.

A relative has asked me to look into replacing the glass in a south-facing (and thus, often very hot) window. They want to put in double-glazing, but can't afford to get all new windows/frames installed. They can stretch to the price of a few panels of double-glazing.

The house is 1920's and has the original wooden Georgian windows, as shown in the attached image. The mechanism works fine. The frames seem solid and just need cosmetic repair. The glass, however, is a century old and now has the stripy residue of adhesive from sticking on a reflective sheet during last year's heatwave (note: it didn't make any difference).

The idea is to take out the six small panels in each frame and replace them with one large panel of double-glazing. This would be done for the two large sliding frames (apologies: I can't remember the correct terms). Yes, the beads have been painted in, but it looks like it'll be easy to free them. It's good, seasoned wood from back in the day when you could get that kind of stuff.

The big advantage here is that we have all the neighbour's old window frames from when they had plastic put in, a couple of years ago! These are identical to those in the picture, so the idea is that I would perform this proposed surgery on the neighbour's old frames, and then install them in place of the originals. So, if it all goes wrong, I haven't destroyed the existing windows with no way to replace them without spending £££, and it means I can get the installation done very quickly, in less than a day.

I would be very grateful for any wise words, recommendations for replacement glass, anything to be aware of that I might not have considered, etc. As always, please assume I am ignorant - it isn't very far from the truth! ;-)

Cheers

 

[MARK.B.]

Just a thought but I would think that replacing the original 6 panes of glass with a single double glazed unit would be big increase in the weight that the frames has to carry,also would the existing sash cord mechanism be able to handle the extra weight as the internal weights would have been just enough to open and close the single glazing in a controlled way.Adding a lot more weight could cause the window to come down with a considerable force without the correct counter balance. If my thinking is correct then i suppose you could overcome this by opening up the sides and adding larger weights but that would entail a lot more work.

 

[Jacob]

Less than a day! no chance!
Won't work anyway IMHO.
The vertical glazing bars are part of the sash structure and taking them out weakens it just as you are doubling the weight on it.
You'd also need new weights as @MARK.B. says and they'd have to be lead, and be difficult to move.
If the sticky residue is the problem I'd clean it off, one way or another, solvent, window cleaner stuff etc.
If too much sunlight is the problem then some nice old fashioned lace curtains, or light cotton etc.
If heat loss is the prob then thicker curtains, or new windows, or secondary glazing inside.
Also some careful draught proofing could be cheapest most cost effective remedy for heat loss.

 

[Ollie78]

You can do it by routering the existing sashes but it is a pain, creates a giant mess and weakens the joinery.
The proper way to do this is make new sashes with the same internal moulding but a deeper rebate then put in slim krypton filled low sight line units (6 individual) and glaze with heritage putty ( hybrid polymer ,not really putty ). or you could try vacuum double glazed units but I have no experience with those specifically. Various coatings can be applied to the glass.
As mentioned the weight will increase so replace the steel weights with lead or add lead accordingly if they are lead already.
Might as well completely strip it down replace the beads at the same time with new ones with a pile carrier so you can draught proof them, new pulleys if they are knackered and new cords.

you could look into window films, I know they do anti uv among others.

Ollie

 

[Pedronicus]

A further thought is that if the house is in a conservation area you may need planning approval to change the look of the windows from six panes per sash to one big one.

 

[Ollie78]

A further thought is that if the house is in a conservation area you may need planning approval to change the look of the windows from six panes per sash to one big one.

Only if you tell them.....
Usually you are ok because you are "repairing and improving" the existing window, because you are not removing the frame it is not classed as a replacement window but a restoration.

Ollie

 

[Pedronicus]

Only if you tell them.....
Usually you are ok because you are "repairing and improving" the existing window, because you are not removing the frame it is not classed as a replacement window but a restoration.

Ollie

Ah, but does 'restoration' not have to be an identical copy of the original?

 

[Jacob]

Ah, but does not 'restoration' not have to be an identical copy of the original?

They tend not to notice or be bothered at all unless it's a conservation area and glaringly badly done.

 

[Thingybob]

I used to make replacement frames for sash windows double glazing doubles the weight of glass it's usually 20kg per sq meter for 4mm glass so double that and the weights in the box would be too long to work I used to fit spring balances which can go way beyond what you would want , You can route a channel up the side of existing frame and fasten the balance to the side of the box that the window slides in have a look on the Mighton site there are explanations there on how to go about it plus all the materials you need you will need to deepen the rebate on the frame to take the thickness of d/g unit I used to have my units made to 20 mm thick 2x 4mm glass and a12mm air gap , Most double sash windows are 44mm thick so 12mm putty line 20mm d/g unit and 12mm ovolo bar so the rebate would have to be deepened to 32mm.

 

[Ollie78]

Ah, but does 'restoration' not have to be an identical copy of the original?

It varies between grades and conservation areas etc. In some cases I have had to submit a demo window to be approved before being allowed to start but that was a full on conservation grade 1 etc.
Mostly as long as it looks the same they don't mind.

Ollie

 

[Pedronicus]

It varies between grades and conservation areas etc. In some cases I have had to submit a demo window to be approved before being allowed to start but that was a full on conservation grade 1 etc.
Mostly as long as it looks the same they don't mind.

Ollie

I'm not too sure that replacing six small panels in each sash with one large d/g panel constitutes 'looking the same'.

 

[Ollie78]

I'm not too sure that replacing six small panels in each sash with one large d/g panel constitutes 'looking the same'.

I was not advocating that.

 

[sometimewoodworker]

The idea is to take out the six small panels in each frame and replace them with one large panel of double-glazing.

Since we live in the tropics we have considerable experience with heat protection.
1st) double glazing by itself does little to control heat
2nd) for heat control you need Low-E glass, this is impregnated with silver and comes in various colours. You can get a neutral grey, brown, blue or green the less heat transmission the darker grey the glass. This combined with double glazing may be enough but I doubt it.
3rd) silver backed blinds will drop the heat to virtually zero
4th) option is to get reflective glass, this is the least favourable option for looks but is the most effective glass option.

 

[sometimewoodworker]

I used to make replacement frames for sash windows double glazing doubles the weight of glass it's usually 20kg per sq meter for 4mm glass so double that and the weights in the box would be too long to work

from the detail of our windows. The weight of double glazing unit itself is quite a lot less than the numbers you have quoted
one unit is; 902mm x 1200mm the glass is 4mm clear + 1.52mm PVB + 4mm LowE + 8mm Ars + 5mm clear (Laminate C4+1.52+LowEC4+Ars8+C5) the weight of that unit is 35.18kg and it is 1.0824 Sq metres

So as a rule of thumb your weight probably includes the frame or is about 25%~50% + over the actual weight of the IGU

 

[Jacob]

from the detail of our windows. The weight of double glazing unit itself is quite a lot less than the numbers you have quoted
one unit is; 902mm x 1200mm the glass is 4mm clear + 1.52mm PVB + 4mm LowE + 8mm Ars + 5mm clear (Laminate C4+1.52+LowEC4+Ars8+C5) the weight of that unit is 35.18kg and it is 1.0824 Sq metres

So as a rule of thumb your weight probably includes the frame or is about 25%~50% + over the actual weight of the IGU

Older windows often have thinner glass. 3mm is common so two sheets of 4mm will more than double the weight of the glass.

 

[Agent_zed]

Something to bear in mind if you are routing new channels etc is whether lead paint was used. As they are quite old chances are high, so you probably don't want to breathe the dust or have it waft about the house.

 

[RobinBHM]

the only way to do it properly is to use slimlite individual units, which at around £25 each will cost £300 for the glass

and then there is the cost of lead weights, which could be say £30 each and you need 4.

by the time youve added in new parting bead and staff beads you will be looking at £500 for materials alone



you would need to see what space there is in the weight box as sometimes there isnt enough space to fit new bigger weights

 

[Thingybob]

from the detail of our windows. The weight of double glazing unit itself is quite a lot less than the numbers you have quoted
one unit is; 902mm x 1200mm the glass is 4mm clear + 1.52mm PVB + 4mm LowE + 8mm Ars + 5mm clear (Laminate C4+1.52+LowEC4+Ars8+C5) the weight of that unit is 35.18kg and it is 1.0824 Sq metres

So as a rule of thumb your weight probably includes the frame or is about 25%~50% + over the actual weight of the IGU

Your right memory not as good as it used to be 20kg was for d/g unit sq mtr not single pane

 

[sometimewoodworker]

Older windows often have thinner glass. 3mm is common so two sheets of 4mm will more than double the weight of the glass.

Of course they will. That statement is totally true and totally beside the point. It is equally true that one sheet of 2mm glass will weigh less than one sheet of 3mm glass

Did you actually bother to read my post!

The statement was that the weight of “glass it's usually 20kg per sq meter for 4mm” and “2 sheets are about double that”
my own 2 x 4mm + 1.52mm PVB + 1 x 5mm weighs in at under 40Kg for slightly over 1 sq meter. This means that the weight estimate is a bit over 38% high so the 2 x 4mm IGU would be under 25kg per sq metre

 

[sometimewoodworker]

Your right memory not as good as it used to be 20kg was for d/g unit sq mtr not single pane

That makes more sense and it is probably close enough, if not exact, I don’t know the weight of PVB and as you can see I can’t really work out the exact weight given that I have a smaller argon fill and 6 more sheets of material making up my IGUs (the PVB is actually 5 layers)