New thermal double glazing -- possible?

[atlasfever]

I just did a few searches on this site, but didn't quite see a similar question: Glazing and a Period property

Background: Grade II listed building

An architect suggested that it's feasible to install new thermal/double glazing panes into my double sash windows, using the existing glazing bars. I was rather surprised to hear this, as I assumed that one could never 'touch' the original windows (with the exception of maybe fixing the wood carefully, or I suppose, replacing a broken pane with a single glazing pane.

Could this be true? Even if the new glazing technology is super slim and makes it technically and visually applicable, I am surprised one could get consent for this, especially on the front of a building. Perhaps different on the rear. By the way, I have double sash windows, with 12 panes of glass each (1st floor of building, so very tall windows).

Additional question: I know this is a long standing debate and I don't want to be too philosophical, but I would really like to educate myself: In today's world of learning to be enviro friendly, I'm a bit surprised that there isn't more of a push to use double glazing (assuming the panes are thin, attractive, etc). It is the belief that adding a secondary glazing inside the flat is enough to sort it out, and so you should keep the original outside, but then install double glazing panels inside? Ie. It doesn't matter if it's unattractive on the inside, but the outside should remain attractive? That's all I can figure out, b/c the tremendous waste of resources (money or heat) is rather sad. I guess I can understand the value of keeping crown glass, but otherwise, maybe the technology of the original window is in fact outdated for good reason?? Just thoughts -- Open to more opinion. Cheers

 

[Moo]

The experts will no doubt be along soon, with sensible suggestions, but if I were to double-glaze my sash windows (the same 3x4 as yours) I'd go for secondary double glazing. Not necessarily unsightly, very efficient, and the beautiful originals would be untouched.
Remember the mantra when dealing with old buildings - don't do anything that's irreversible.

 

[atlasfever]

Right, ok.

It's just that I don't think I could use my shutters if I put the secondary glazing... but I could be wrong. FYI, the glass panes in my windows is not original.
thanks

 

[Moo]

You've got shutters??? !!
Sorry about all the mad punctuation, but I'm green with envy. The room below our piano nobile has shutters, and we regard them as a very efficient form of double-glazing (and security).

 

[Andrew Bodenham]

Atlasfever,

Fitting double glazed units to existing sash frames is not easy, and in my opinion unlikely to be successful. Amongst other things there is the additional weight to be accomodated - the thicker glass and second pane means that the glazing weight will be more than doubled. Even if your box stiles could accomodate larger balance weights the sashes themselves may not be strong enough in section or joints.

A better option (if geometry allows) would be to remove the original sliding sashes and have new double glazed ones made to go in their place, still utilising the existing primary frame. The originals would be preserved intact to allow refitting by some future owner if desired.

In terms of heat loss some recent research by, I think, Glasgow university showed that mechanical repair and effective draught stripping could make a significant improvement in traditional sash performance. If combined with shutters and / or thick curtains the overall performance was greatly improved to the point of being close to compliance with current regulations. Heat loss through windows is only really a problem in the winter months, and the best double glazing systems achieve about half the thermal transmision of single glazing, so the performance difference is not that great. Certainly in financial terms the payback time is likely to be decades rather than years, by which time the replacement glazing may need replacement again - see the life expectancy of upvc glazing for example!

There are other benefits to d.g., of course, such as reduced condensation, and if your windows require replacement due to mechanical / materials failure then new d.g. would be an obvious choice. But if the existing windows have heritage value and can be repaired (and they usually can) then perhaps secondary glazing would be a better option.

 

[AMc]

As others have noted - secondary glazing offers almost the same benefits as double glazing the sash and is reversible and non destructive to the original windows. I dearly mourn the original 8 over 8 windows that were replaced by wrong looking 6 over 6 double glazed sashes during a previous renovation. It was particularly gauling to find one left in the coachhouse showing it was in good nick

I fitted secondary glazing to the existing period windows as I needed to prove Building Regulations compliance in conjunction with other work.
A sealed double glazed unit as secondary glazing offers the best thermal performance but it's unlikely to be able to fit that in many cases.

It may be possible to fit secondary glazing that doesn't interfere with your shutters, I would look at this as the second thing to do.
The first is to make sure the sashes themselves are well fitted with good draughtproofing - ventilation heat loss is likely to be much more significant than radiant heat loss. Brush strip parting beads and staff beads are available and make a significant difference.
There is a company called Ventrolla (sp?) who refurbish sash windows, they get a favourable mention on here and my neighbour used our local franchise to excellent results.

This is a good science backed read on the subject of what does and doesn't work with original windows.
http://www.english-heritage.org.uk/professional/research/buildings/energy-efficiency/thermal-performance-of-traditional-windows/

 

[MatthewC]

There is a company called Ventrolla (sp?) who refurbish sash windows, they get a favourable mention on here and my neighbour used our local franchise to excellent results.

My neighbour and I both had quotes from Ventrolla - he went ahead and I waited to see the results. The trouble (IMO) is that they do not even attempt to keep any old glass but all panes are replaced regardless, so if you've got wavy glass then you can wave goodbye to it.

Personally I didn't think they were cheap either.

 

[Feltwell]

I've heard several bad reports about Ventrolla round here, and they are very expensive. Like any franchise operaton, it depends upon who you're dealing with, some will be good, some not so good. I've heard they use a flappy plastic parting bead which doesn't look as good as timber with a brush strip - but that may be just a particular franchisee and it's just my opinion, others think they look OK.

As ever, the best route is a local recommendation and one where you can see the results for yourself.

Having done a few sash refurbishments now, my advice to anyone who is a half decent DIYer is do it yourself - it's really not difficult at all. If you've enough confidence to tackle laying a timber floor or hanging a new internal door, you could tackle refurbishing a sash I reckon.

This is a reasnoble quick guide:-

http://www.sashwindowspecialist.com/diyguide.html

One thing that guide doesn't seem to mention is routing a groove in the meeting rail and in the top of the top sash to fit carriers for extra brush strips - if you don't, those areas will still be draughty as the parting/staff beads don't cover them. I bought an extra-small router cutter just to do this. Even if you ignored these areas though, you'd still get a big overall improvement.

All the bits can be bought from various places, the site above, Reddiseals or Mightons. Last time I checked Reddiseals was the cheapest, but that was some time ago.
It's well worth it - I've really noticed the difference, especially on our high up windows, not just in the reduction in draughts but in how much easier the windows are to open and close, and that they don't rattle any more. For anyone stuck on top of a hill (Joce!) I'd say it's an essential job.

 

[Flyfisher]

I'm with Moo on this. We have a number of lovely big sash windows but even if suitable DG units were available, I seriously doubt that the window frames would support the extra weight, such is the thinness of the glazing bars. And that's without the awful thought of replacing the original wobbly glass.

I wish we had shutters but in their absence I will be considering secondary DG when we move into the decorating stage. This will leave the original fabric untouched and the secondary DG would be completely reversible. Indeed, with the right design, it may be completely removable during the warmer months, arguably giving us the best of both worlds because there is nothing quite like a fresh summer breeze wafting through wide open sash windows.

 

[JoceAndChris]

The necessary thickness of the glazing bars make double glazing a no-no in my opinion, especially for a Georgian house where the symmetrical arrangement and delicacy of the sashes is really critical to the overall readability, if I can use such a word, of the house. I've seen a lot of Georgian properties with the wrong windows, roof and doors and suddenly it doesn't look Georgian any more.


I've often wondered if I could fit secondary glazing during winter months in the temporary manner Moo discussed some time ago, but there are shutters throughout, giving just one inch of wood ( with a curved profile) on which to screw the the Perspex. If you screwed it onto the front face of this skinny bit of wood the shutters wouldn't close, and I can't see how you'd screw it to the side. There might be a way of making little rests on the Perspex - oh, hard to explain without diagrams.

It's all very well to say shutters and thick curtains eliminate drafts, they really do, but only when closed of course. Winter 2010 I lived by electric light some of the time, this winter was OK.

You get used to the cold and then others' houses seem far too hot. And drafts are so brilliant for eliminating damp.

 

[worms]

I bought an extra-small router cutter just to do this.

Out of interest, can you remember what size of cutter it needs? I hope to attend to a couple of windows this summer and not sure whether this is really a job for the full-size router, the wee router or the Dremel!

 

[Feltwell]

I wouldn't use a Dremel, not unless you've got some sort of guide.

I use an 8mm straight cutter, which enables the self-adhesive carrier with pile from Reddiseals to go in.

http://www.reddiseals.com/sash-window/carrier-with-pile.html

I've also got a extra-tiny 2mm straight cutter, which gets used for the "leg" of the non-self adhesive type carrier, or for the "bubble" type seals as per below - I had to get this in order to be able to seal the top of a curved sash, which the brushpile type wouldn't fit.

http://www.reddiseals.com/seals/silplex-timber-seals.html

I use even the tiny 2mm cutter in a large 1/2" router with no problem, colleted down to 1/4". I tend to use the bubble seal for the top of the top sash - unlike the brush seal it's designed for compression, and because I had to buy a big reel of it to do the curved sash!

 

[worms]

That's interesting, thanks. I've never routed at 2mm nor routed with my Dremel, so I am interested that you find that the 1/2 inch router is ok for this. Hopefully, though, I can stick with the self-adhesive (if you pardon the pun!) and the 8mm channel.

 

[atlasfever]

Hi,

Update:

I met with an architect who after seeing that my panes are non-original and rather cheap, and my bars are not original either, suggested what an above poster recommended: Replacing the entire top and bottom sash with a double glazed unit that would have matching thin bars, and keeping the original frame (perimeter, which is fixed to the bldg). But, I still can't figure out if that's not allowed as it's a Grade II listed building.

Does anyone else in London have any experience?

Unfortunately, repeated attempts to ask the council for feedback have proven useless. I keep getting a regular planning officer (not listed consent) who just seems rather bored by my question and provides the vaguest of answers. I know that I have to formally ask for consent, but understanding whether it's even possible to change my windows given non-orig current windows, would go far in planning/architects/budgeting. I recognize they are busy and constrained, but so am I. Excuse the frustration...

Thanks for Feedback.

 

[atlasfever]

PS - Oh, and by the way: Secondary glazing just can't work given the architraves, and don't feel the use of curtains or orig shutters works for me either.

 

[Feltwell]

PS - Oh, and by the way: Secondary glazing just can't work given the architraves

Are you sure? See the picture below - ignore the window being a horrible yellow, I've just decorated the rest of the room and haven't restored the sash yet, when I do it will be off-white like the architrave. The architrave has steel tape on it so that I can use magnetic secondary glazing - I've blocked out a piece in red so you can see where it is. Not invisible but it is very subtle and if you didn't know it was there, I think you'd think the edge lines were just part of the architrave profile.

The secondary glazing itself will be acrylic sheet with a self-adhesive white magnetic tape stuck to it's inside, round the edge - the tape backing is a white foam so it blends in quite well to an off-white architrave, and you take it down completely for the summer. This does of course mean you've got to have somewhere to store it. I've not ordered the sheet yet for this window, but I've done a few others now.

No secondary glazing is invisible, but it can be discreet.