Converting SG windows to DG

[Feltwell]

Here at Feltwell Towers - which is not listed - most windows are 2 over 1 vertical box sashes, all largely original, all single glazed. I have no intention to alter these, too many problems with conversion to double glazed units plus I'm not convinced on the benefits, so it's been restoration and draught proofing as I've posted about before.

However - the bathrooms are a different matter. Here we have small windows with a fixed large bottom pane and a smaller centre pivoting top opener, original to the house - but all require attention, all have some rot, and in one bathroom there is later obscure glass fixed directly to the inside of earlier outer plain glass, with lots of grot inbetween - nice!

Being prone to condensation for obvious reasons, even with extractor fans in the rooms, having double or secondary glazing seems a good idea. No room for secondary unless I went for fixed units - not an option, I want to be able to open the top windows - so I was looking at conversion to DG as a possible solution. I think you can do it without too much visual impact to the window.

I've heard you can get stepped units, where the outer sheet of glass is sized as per the original SG sheet, but the perimeter seal & inner sheet is smaller so it sits inside the frame, intruding into the room slightly. I understand that most DG unit failures are caused by the seal going round the edge due to trapped moisture - so - rather than fully bedding a unit on sealant, which would always make it prone to trapped moisture, my thought was to install said stepped unit with the outer face bedded and faced into the frame as per the original SG sheet - but with a modern putty replacement - but then with plastic blocks around the perimeter seal and inner sheet of the DG unit to space it away from the frame slightly, and beading round the inner edge to hide it all up - which should avoid any trapped moisture.

Heresy? Not really - the external appearance will be the same, plus these are high up on the 2nd floor - and internally the rooms are nothing like they were originally anyway, one wasn't even a bathroom, it was just a box room. The internal appearance will be different but should be acceptable if done well.

Anyone successfully done or seen such a conversion?

 

[Kearn]

I can’t vouch for these, I didn’t even get as far as doing any due diligence, as we just lost out on an early Victorian project where everything was single glazed sashes and I was going to investigate these further as recommended from another site … a bit of a faff that you have to remove the whole panel to open the windows, but I suppose in the less cold months you could store all of them away somewhere instead of popping them in/out. I thought they were very discrete when in place.

https://www.extraglaze.co.uk/extraglaze-for-sash-windows.html

our current home has the hideous plastic internal sliding secondary glazing - I haven’t got round to removing it and it does a half decent job, but Christ I’d never install it!

 

[Feltwell]

I thought much the same thing Kearn, so much so that I went down the route of perspex secondary glazing on the sash windows, held on with magnetic strips. Alas it didn't work out and I abandoned it after a few windows.

• I didn't see the point of SG on the sashes themselves, better on the surrounding box frame, but that gives very large sheets of perspex (8' x 4' for some of ours!) to handle.

• The perspex sheets would rattle in high winds and pop off the magnetic strip at one point

• The sheets were a right pain to handle & store when you wanted to open the windows - big, flexible & floppy, easily damaged things

• They never looked completely clean - not as good optically as glass

• The steel tape on the window frame used to rust on it's edges and went tatty quite quickly, even though it was painted over

So I decided to stick with SG and just draught proof the sashes properly - that alone makes a big difference, and makes the sashes silky smooth to open and close. Done well and it's hardly noticeable.

 

[malcolm]

I understand that most DG unit failures are caused by the seal going round the edge due to trapped moisture - so - rather than fully bedding a unit on sealant, which would always make it prone to trapped moisture, my thought was to install said stepped unit with the outer face bedded and faced into the frame as per the original SG sheet - but with a modern putty replacement - but then with plastic blocks around the perimeter seal and inner sheet of the DG unit to space it away from the frame slightly, and beading round the inner edge to hide it all up - which should avoid any trapped moisture.

Just a thought about how to stop double glazed units from misting. In the photo below the edges of the double glazed units sit on the wood and the wood near the seal is routered away with holes drilled to the outside for ventilation.

Can't suggest much else. I've got some double glazed sash windows but they are thicker than the windows they replaced and stick out into the room. Neatest fix will probably be to bung a little insulation on the walls and replaster to match. I've been making secondary glazing for the awkward windows I don't want to replace. Just wood, a table saw and a router to keep with the theme.

 

[Feltwell]

Thanks Malcolm. I think keeping that space beneath the seal vented is key.

I took the paint off one of the windows last night, it needs more repair than I first thought. Just for a change! :roll:

 

[eezageeza]

For what its worth, I've just replaced a dozen sealed units, in timber sliding sashes made about 20 years ago, all of which had the drain slot and vent in the bottom of the frames. Most of the units had misted up (not quite all of them, but I thought I might as well replace the whole lot whilst I was at it).

Interestingly, or perhaps the word is annoyingly, all the drain holes were bunged up with insect detritus, so probably not venting at all...so maybe it needs to be part of routine maintenance to poke the drain holes clear once a year or something?

Having said that,I wonder how long DG units are supposed to last - is 20 years good enough? They do seem to vary a lot - most of the other units in other windows on the same side of the house (all much the same vintage) seem fine!

 

[88v8]

My FIL had stepped units put in the front of his house, they were there 20+ years without failure. They were clumsy looking and visually obtrusive but they did the job.

I would route out the rebates to allow space for conventional units. The thicker the better of course but even a 6mm gap will help.

Ensure they are backed and bedded and fronted with a putty-replacement approved by the makers. The edges of units must be totally isolated from moisture. No beading!!!!
Fussily and carefully use plastic back spacers and side spacers to ensure you get a full all-round bed.

The sixteen units I installed in 1982 were bedded in Arbo 1081 and by 2012 when we sold, none had failed.
Arbo is not 'the right stuff' as it won't take paint, but it proved the merit of full bedding.

In contrast, the bead-glazed units in our 1990 conservatory - installed 'professionally' - maybe half had failed.
Beading is a botch method that guarantees premature failure.

The 31 units I installed here were bedded and fronted in Kawo Elastokitt, but post-Brexit that was hard to find last time I looked.
We previously tried Hodgsons Heritage Putty, but it's horrible to work, when fronting it drags on the knife. Try one tube of something before you commit to a larger quantity.

Ivor

 

[Feltwell]

Thanks Everyone.

Stripped the paint off a few nights ago, which shows how bad the window is - with the glass out, running a router round the inside would be possible, the corners could be finished off with a multi-tool. It would help take some of the rot out in fact, so maybe stepped units are a bad idea.

Ivor, sounds like you went for the fully bedded, seal out any moisture approach rather than the leaving a breathing gap around the seal approach.

 

[88v8]

Ivor, sounds like you went for the fully bedded, seal out any moisture approach rather than the leaving a breathing gap around the seal approach.

Yes.
As if one were puttying a normal window.

These are the units I used, from Holloseal https://www.thinsealedunits.co.uk/ (Original Glass Co).

This is their bedding method. https://www.thinsealedunits.co.uk/heritage-glass/specifications/#ensure
Even when beaded, the actual unit must be fully encapsulated in sealant.
They use the horrible Hodgsons.

I also used some Histoglass units, which actually came from Germany. They specify the Kawo sealant, and can probably supply it to you. Much easier to face-work than the horrible Hodgsons. https://histoglass.co.uk/wp-content/uploads/2020/04/Histoglass-Brochure-Installation-Instructions-2020.pdf

Ivor