Can I double glaze an existing sash?

[Anonymous]

Sorry if this has been discussed in the past (I couldn't spot the thread if it has) but I'm trying to find out whether it's possible to put double glazing into an Edwardian sash window.

I know it's possible to have replacement windows put in, but I'd prefer to use the original frame if I can. I can see there may be problems with the mechanics of the window, caused by the extra weight of the additional glass.

Maybe someone out there has tried this?

 

[Anonymous]

It really depends on the section of your window as sealed double glazing units now have to be a minimum thickness of 24mm (outer glass surface to inner glass surface) as part of the FENSA regulations, anything below this and you won't get building regulations signoff which could cause you a problem if /when you sell your house.

 

[Anonymous]

Wou;ld it be possible to fit secondary glazing?

 

[Anonymous]

Secondary glazing is an option, but it's the idea of using the existing sash that fascinates me.

I guess the aesthetics of secondary glazing are my main problem, closely followed by the extra bother of opening two windows in the summer.

 

[Anonymous]

> It really depends on the section of your window...

I'll measure that and see what I've got.

> ...sealed double glazing units now have to be a
> minimum thickness of 24mm... anything below
> this and you won't get building regulations
> signoff...

That's an interesting thought. Couldn't one argue that replacing the old glass only is simply a repair? That would mean no building inspector involvement (if I understand correctly).

I should add that my aim isn't to side-step building regulations. I'm really just interested in the prospect of keeping an original window, but improving insulation.

 

[Anonymous]

You don't need FENSA certification or Building Regs if your only replacing the glass. This is even the case if your replacing a double glazed unit in a modern PVC window.

Alan

 

[Anonymous]

Simon

I've fitted stepped double glazed units to ordinary timber windows in the past (the outer leaf of the unit sits in the rebate and the slightly smaller inner leaf sits on top of the rebate on the inside).

The glazing wouldn't be the problem. You would double the weight of the sashes and probably wouldn't be able to add enough lead to your existing weights to counter balance them.

Alan

 

[Anonymous]

Certainly doesn't involve building regulations. (FENSA is just the organizations that allows window makers to self-certificate instead of having to call out the Building Control Officer each time.)
The 24mm dg unit with a 16mm gap is best for insulation. Narrower and heat conducts across, wider and convection starts to happen within the gap, transferring heat. Such units use 4mm glass but if yours is an old window it may be closer to 3mm so the weight of the new unit will be almost three times as much as the old. Converting iron weights to lead will go a long way to compensate. Secondary glazing may be easier to fit and is good for sound insulation but not quite as effective thermally.
All in all it's not an easy problem to solve.

 

[Anonymous]

Why not use those lovely spiral balances ;-)

 

[Anonymous]

I notice you're winking but spiral balances have their place. They're not appropriate where you laready have a box sash. Swapping irons for leads is more sensible. The spirals need a solid style with a c.18mm groove cut in them. They have real advantages in a new window designed for them.

 

[Anonymous]

The answer to your question, can you double an existing sash window is YES!

However there are some constraints.

1. The ideal double glazed unit is 4mm Pilkington "K" glass on the inside an 8mm super spacer, with a 4 mm float glass external pane. Giving a 16mm DGU.

2. The thickness of the sash section needs to be minimum of 38mm. This measurement must not be taken at the meeting rails, because they overlap the rest of the frame section.

3. The meeting rails need to be greater that 30mm tall. If they are not there will be insufficient timber to allow you to fix a catch the bottom sash.

4. By changing from cast iron to lead weights you double the density of the material used. Therefore the weights and sash will balance. The only exception to this is where the sashes are wider than the height where you may lose some movement.

There are absolutley NO building regs issues, because you are just changing the glass. The reason I have stated this so strongly is that I have done or overseen this being done to over 1000 pairs of sashes, including bowed sashes.

Forget secondary double glazing just do it the way you obviously want to and good luck with your project.

Jerry

 

[Anonymous]

You will, of course lose the historic glass, which is probably lovely wobbly stuff which adds greatly to the charm of the building.

Flat stuff is simply no substitute. One it's gone it's gone. I wouldn't part with mine.

 

[Anonymous]

If this really is a concern then use wobbly glass as the external pane instead of the float glass.

Don't forget that the heat in you house that you have paid for "Once its gone its gone"!

 

[Anonymous]

Well, at night use thick curtains. Ensure windows are draught proofed.

I actually wonder how much energy and fuel is used making new glass, DG units, transport, the factories to make them, energy to heat and light such factories, junking and disposing of the genuine historic stuff... which is very expensive to buy a replica of...

 

[Anonymous]

Do you use the curtains during the day? If you do why have the window in the first place?

Little Johnny with his football has disposed of hundreds of acres of historic glass free of charge

 

[Anonymous]

Of course we don't use curtains during the day! At night, however, shutters and curtains are used.

Thankfully, Little Johnny hasn't disposed of our historic glass, It's still intact, as is much else of the historic fabric of our home.

It's why we like living in an old building. I have no doubt so much could be 'updated' - we could dry line and insulate the walls for example - but we may as well then consider moving to a Noddy box.

Repair not replace, conserve not destroy.

 

[Anonymous]

By double glazing existing sash windows the main structure of the box frame and sashes is being retained

 

[Anonymous]

Draughtproof, stick secondary glazing in for the winter.

Sealing up an old house isn't a great idea anyhow!

 

[Anonymous]

Jerry,

Do they make up these units specially ? Are they vacuum 'filled' or inert gas ?

Do you fit them using putty as you would a standard pane of glass?

I have a couple of windows that were originally sash and a previous owner had upvc put in, it looks as if the new windows have been put into the old boxes and next year I intend to try reinstating sashes. Double glazed would make a lot of sense.

thanks

Mark

 

[Anonymous]

You can get the double glazed units made to any size you like. They do not have a vacuum, but may be air or, for a lower U-value, argon filled. Some smaller glaziers don't have the equipment for argon fill so go to a company that does this as routine - it shouldn't cost any more. Don't use putty, use 'glzing tape' and beading. You probably need a roll of 12mm wide 3mm thick tape. It is very sticky on both sides. It has a backing sheet and is very easy to handle. It doesn't stick to the fingers if gently handled but once it is pressed to the glass and wood it will absolutely not come off. So get it in the right place first time. For added waterproofing run a bead of silicone along the top edge of the tape. They key to ensuring dg units don't mist up is to keep the edges dry. They must never be allowed to sit in a puddle.