Retrofit double glazing onto old single glazed sash windows

[groovybug]

Hello everyone,

Has anyone tried to retrofit double glazing onto old single glazed sash?

The previous owners of my house has fitted a second layer of glass onto the windows (see photo). It looks pretty good however it is obviously not 100% air tight, so I wonder how effective that is. I'm thinking of ordering the proper double glaze glass to fit and would like to hear your experience of how to do this properly.

Many thanks.

 

[Mr Chibs]

It’s the wooden sashes that are leaking the air.

Changing to a double glazed window, will not yield you much.

Would be better to guys some seals to the window sashes.

Do the windows open, or painted shut?

 

[groovybug]

@Mr Chibs The upper sash is sealed shut, the lower can open. So you reckon draught-proofing the windows is more effective than double glazing them?

 

[Tigercubrider]

Yes.
The glass in sash windows can be replaced in some cases with special thin DG units. But they also change the balance of the opening as the weight changes.

A mate has a house in a conservation area - he had the windows routed and seals fitted.
He said it was very effective.

You need to seal the existing window before adding secondary DG. Otherwise the system won't really work. You could have one massive sheet of glass on a track to cover a window/ some kind of sliding system or make period style timber frames to fit the openings like shutters but glazed.

 

[Deleted member 174758]

Another thing to do is check the mastic sealing around the sash frame. If this is cracked or broken you can get a fearsome draught through the edges of the sash weight box

 

[opps]

I agree that fitting new staff and parting beads with brushes would be the better option.

If you fit double glazed units in them you will massively increase the weight of the sashes. Those sashes are counterbalanced by weights. It is unlikely that unless you purchase depleted uranium, you will be able to counter balance them.

 

[groovybug]

Thanks everyone, I will look into sealing/draught proofing properly first.

I know I can get retrofit at a window joinery, however looking for some DIY advice. Is it possible to just order the double glazing ourselves and do it?

 

[groovybug]

I agree that fitting new staff and parting beads with brushes would be the better option.

If you fit double glazed units in them you will massively increase the weight of the sashes. Those sashes are counterbalanced by weights. It is unlikely that unless you purchase depleted uranium, you will be able to counter balance them.

Sorry, I still do not fully understand the point about counterbalancing. Surely if I replace the glass for both upper and lower window (and they are roughly the same size) then they will still be balanced?

 

[opps]

Thanks everyone, I will look into sealing/draught proofing properly first.

I know I can get retrofit at a window joinery, however looking for some DIY advice. Is it possible to just order the double glazing ourselves and do it?

Other than the weight issue, your windows will not accept anything other than the thinnest double glazed units. Even then you may need to use a router to increase the maximum possible depth.

 

[groovybug]

Other than the weight issue, your windows will not accept anything other than the thinnest double glazed units. Even then you may need to use a router to increase the maximum possible depth.

It's hard to see from the photo, but if you look carefully, my window has glass panels on both side (the inside panel added on later on by previously, mimicking double glazing) and the gap between them is about 18-20mm. Is that good?

 

[opps]

Sorry, I still do not fully understand the point about counterbalancing. Surely if I replace the glass for both upper and lower window (and they are roughly the same size) then they will still be balanced?

The weights are in "pockets" that are only accessible when the lower sash is lifted. If the lower sash weighs 24 pounds, you will have 2 cast iron weights that are 12 pounds each. as you lift the lower sash, those weights stop it sliding down.

 

[groovybug]

The weights are in "pockets" that are only accessible when the lower sash is lifted. If the lower sash weighs 24 pounds, you will have 2 cast iron weights that are 12 pounds each. as you lift the lower sash, those weights stop it sliding down.

Oh I understand. Thank you I will look into this. I know that this can be done, just wondering whether I can DIY it.

 

[JohnD]

Sorry, I still do not fully understand the point about counterbalancing. Surely if I replace the glass for both upper and lower window (and they are roughly the same size) then they will still be balanced?

no, the upper and lower sashes do not balance each other.

Each sash has its own pulley, and a weight on each side, hidden behind a strip of wood (if old, will have many coats of paint on and be difficult to open without breaking).

In some cases the weights are individual lead blocks, maybe an inch thick, strung together, which are easier to add or remove.

 

[opps]

The beading nearest you as you look at the windows is the staff bead. Someone has put yours on back to front (not a biggie). You need to remove that beading first and then replace it with one that has brushes in it. the next moulding is the staff beading. Again you want to replace it with one that has brushes. To remove it, you need to take the top sash out.

You have no idea if the top sash still has cords connected to the weights. I would recommend driving a couple of screws in to the sash box just below the the underside of the top sash. It may be the case that someone had screw the top sash closed, look for raised filler ot a slight dent.

With the top sash removed you will be able to remove the staff bead. With the staff bead removed, you will be able to spike the cords for the lower sash, remove it and then access the pockets.

You then have the freedom to replace both moldings with ones with brushes. That said, you buy the moldings and brush piles separately
. The people that I know that restore sash windows purchase their rolls of brush piles in 50m rolls. they have 3 or 4 different depths of pile though.

You may also need to purchase nylon "gliders". They are z shaped nylon profiles that will help to centre the sashes as they go up and down.

I have no idea how competent you are. If you are looking at quite a few windows and own (and know how to use a planer and router.) Then it might make sense to diy it (an extra pair of hands helps a lot). If it is only 2 or 3 windows it is probably cheaper to get someone else to do it.

If you want to look at the cost of materials, have a butchers at Mightons

https://www.mightonproducts.com/

It is definitely something that a reasonably competent DIYer can do with advice. Unfortunately, minimum purchase quantities might become prohibitive.

I still think that installing double glazed units may be a bad idea, not only because of the issues related to the increase in weight but also because the units don't have the same freedom to expand and contract as they would in a uPVC frame.

A couple of years down the line you may find that each of the units has "blown"(ie become cloudy).

On the up side, removing the sashes, means that you can paint your windows without needing a ladder.

 

[JohnD]

the brush pile is very good for blocking draughts in a sliding sash window. It accomodates movement and a modest variation in gap size, and the sashes slide smoothly against it. It prevents them rattling in the wind. The overlap of top and bottom sash is not successfully sealed with original technology.

I also have some with a soft pile like a synthetic fur.

You can buy cut lengths of it if you don't need a whole roll, also glazing tape and modern mastic (I learned to putty in windows long ago but the modern products are better if you do not seek historical accuracy).

My BiL has a historic house in Australia; the sashes do not have counterweights but a sort of cogged ratchet to hold them in place. He is a self-taught joiner and can make replicas of such things.

I'd suggest making a dummy window out of ply (you can paint it black, with a dummy window in white if you want) to fill the gap for security and warmth while you are working. It may take some time...