Duplex casement windows vs 'traditional'?

[vicky whitworth]

Our builder has just installed our new wooden windows in the extension today, and my neighbour said, "oh , couldn't you afford plastic ones then?" !!! Vicky

 

[biffvernon]

Figures for just how much difference dg makes? Of course - U-values (or at least reasonable estimations) are easy to come by. How much effect a change in u-value of a building componant has is a function of area and temperature difference so obviously with a tiny window the effect is, er, tiny, but with a large window it is large.

Personally, I'm not keen on the stick on fake but thin glazing bars. Because they are fake. I'd rather make double glazed windows with thicker glazing bars. Some of the mass produced stuff is very chunky and wider than it needs to be. Mine are usually about 32mm - a good bit wider than the 20mm that was common in the 19th century when not painted carelessly - but slimmer than some. Think Queen Anne.

Actually I don't think one should get too hung up about glazing bar thickness. For sure, thin glazing bars, fashionable in late Georgian and Victorian times are beautiful to behold. So are open fires, horse carriages and steam trains, but we're in the 21st century now. Preserve our cultural heritage by all means but when it come to new windows let's use well designed 21st century ones.

As it happens, I was wandering round one of the Cambridge colleges yesterday, looking at windows. There was a fabulous set of buildings with seriously heavy glazing bars, Jesus College I think, that had been extended in the late 18th century with windows of the new-fangled modern whispy thin bar design. I prefer the old stuff.

 

[biffvernon]

PS I really don't like the 13-15 mm wide strips that Oldcollier mentioned. They have no historic precedent and no funtion.

 

[Ian Bond]

If we're to consider that thin glazing bars are an anachronism along with horse carriages etc, would it be better to dispense with the bars completely, if 21st century technology allows each sash to be glazed with a single sealed unit?

I'm thinking here of the potential problems with windows consisting of 16, 20 or more individual sealed units, and what happens in the decades ahead when they start to fail.

Why bother with thicker glazing bars (or stick on bars) at all, particularly where they would be historically incorrect, or fail to match existing windows?

 

[Penners]

what the upvc trade calls "astrical" bars (presumably originating from "astragel")

Or even "astragal". :wink:

Personally, I'm not keen on the stick on fake but thin glazing bars. Because they are fake.

I'm with Biff 100%. I'd much rather see a slightly thicker timber glazing bar. Although I confess that, for me, nothing really matches the fine glazing bars beloved of the Georgians for elegance and beauty.

 

[biffvernon]

If we're to consider that thin glazing bars are an anachronism along with horse carriages etc, would it be better to dispense with the bars completely, if 21st century technology allows each sash to be glazed with a single sealed unit?
I'm thinking here of the potential problems with windows consisting of 16, 20 or more individual sealed units, and what happens in the decades ahead when they start to fail.
Why bother with thicker glazing bars (or stick on bars) at all, particularly where they would be historically incorrect, or fail to match existing windows?

Interesting questions, Ian. There's a design and aesthetics thing. When you have a building with a complex texture, like most old buildings possess, then large blank sheets of glass don't harmonize well. Single pane windows seem more in harmony with their building in 20th century architecture where plain textured surfaces and continuous simple lines are characteristic.

And there's the view out to consider. Glazing bars interrupt and partially obscure the view, block light and, in the case of double glazing, reduce insulation since the bar has a higher U-value than the mid pane value of the uninterrupted unit. On the other hand, glazing bars frame the view, enhancing appreciation of landscape. Last week I was looking out of the window of John Ruskin's bedroom at Brantwood (as one does). I noticed the view framed by one pane included the little jetty and the Coniston Launch just pulling out, it's smoke drifting across the water, while the pane above framed the snow-capped peak of The Old Man. Perfect.

 

[Penners]

it's smoke drifting across the water

http://uk.youtube.com/watch?v=SvfvX9Py5is

 

[Nemesis]

Gosh - I was at Brantwood week before last ... and on Gondola too...

(Fine tearooms at Brantwood also.)

 

[Ian Bond]

A lovely image indeed, Biff.

No wonder they made such a feature of the windows at Brantwood, with views such as that.

Returning to double glazing, my concern remains that having 16+ individual units per window will make future maintenance more time-consuming and costly.

I suppose one would be paying a premium for the aesthetics, and therefore accepting the additional expense?

But I keep returning to the same question. Is the additional cost of double glazing, aesthetics aside, worthwhile over the lifetime of the window?

Or putting another way - how long must a double glazed unit last in order to make it's higher cost (compared with single glazing) worthwhile?

Or how long's a piece of string, given all the variables of climatic exposure, construction, fuel costs, occupants' heating requirements, etc.

 

[biffvernon]

it's smoke drifting across the water

http://uk.youtube.com/watch?v=SvfvX9Py5is

No no, Penners. Much more like http://uk.youtube.com/watch?v=zzhT2kgIhk4

 

[Nemesis]

Actually it was taken out of service for quite a number of days in October because of bad weather, thankfully it sailed again for my birthday!

Now out of service for the winter.

 

[AMc]

A lovely image indeed, Biff.
Returning to double glazing, my concern remains that having 16+ individual units per window will make future maintenance more time-consuming and costly.

I suppose one would be paying a premium for the aesthetics, and therefore accepting the additional expense?

But I keep returning to the same question. Is the additional cost of double glazing, aesthetics aside, worthwhile over the lifetime of the window?

Or putting another way - how long must a double glazed unit last in order to make it's higher cost (compared with single glazing) worthwhile?

Or how long's a piece of string, given all the variables of climatic exposure, construction, fuel costs, occupants' heating requirements, etc.

I've been considering some of this with our vinery. 8m x 4m with a roof line rising from 2-4m in glass- a lot of sealed units. We'd hoped to save the original frame and reglaze with DG, it but as we took it down we discovered more rot than wood a the key points and in the end we couldn't.

From an energy efficiency point of view DG is twice as efficient as single glazing - as most people in unlisted houses must comply with building regs , the single/double debate is null and void once you've decided you need a new window and can't repair (as we have).

There is a trade off between larger units with plant on bars and spacers vs. many smaller units. My understanding is that there is an energy efficiency advantage to larger units with spacers over multiple smaller units - up to a point where you get problems of rigidity with very large units.

Our main house has older replacement sashes 6 over 6 with individual units - some failed after 12-14 years - all in the middle panes of the most sun exposed windows on the East and West walls. Given that sunlight is what breaks down the seals, having one seal recessed into the frame around instead of 12 units into multiple glazing bars seems like a safer option.

On a purely odds basis you might argue that having 12 sealed units you were more likely to experience a failure than if you have one unit with a single seal.

Even if you don't assume a single seal as a point of risk in a 2x3 + 2x3 grid of units there is more than twice the length of seal in to fail compared with a single large unit which again must make the risk higher.

In the event that a unit fails it still retains most of its insulating ability, but if it mists you'll want to replace it. If you need to replace one larger unit it will cost more for the unit, but I'm imagining that the labour removing, replacing and redecorating will be broadly similar in either case. I can't imagine I'll get away with only repainting around the failed units when I do, I'll have to repaint the whole window. I'd also wonder how cleanly I could extract one unit from the middle of my windows without damaging the wood holding the surrounding ones.

What really swung me to larger units with plant on bars and spacers was going to see one of our builders sites. Looking at a replacement window with panes the same size as our multiple units I commented on how the glazing bars could be so thin and keep the seals protected - the builder looked at me a bit funny and told me the whole window was one unit with spacers and plant-ons but 'done properly' as he put it. Then he pointed out how much further he'd recessed the perimeter into the frame compared with ours. I really couldn't tell it was one unit, it looked like our windows but with more glass and less wood and more in keeping with a house of a very similar age. It was clearly double glazed, but I don't think there is a way around that visual difference.

So I've made up my mind for our new windows - larger units with spacers and bars.

 

[Penners]

Luvverly! Has it got double-glazed windows?

 

[Ian Bond]

Yes, but as an owner of an unlisted period building I'm not prepared to just roll over and accept that double glazing is mandatory if in the long run I will be paying a lot more to maintain it.

12 - 14 years isn't a good enough lifespan for what should be one of the most long lasting building elements - glass.

It's very unlucky if you're windows are in such a bad state that total replacement is necessary though.

However I do accept that many people want double glazing, and even get permission to have it in listed buildings in some cases.

It's interesting to see some good argument both for and against the planted on glazing bars.

 

[biffvernon]

I'm not convinced that modern sealed units need have such a short life - but only time will tell.

 

[Feltwell]

Absolutely no idea how well it works, but this company have been advertising widely around here that they can repair failed DG units in-situ:-

http://www.apple-services.co.uk/index3.html

Mind you, the same site advertises sandblasting services for beams.....

 

[biffvernon]

Sounds too good to be true.

The several errors of fact within the text do not inspire confidence.

 

[AMc]

12 - 14 years isn't a good enough lifespan for what should be one of the most long lasting building elements - glass.

Perhaps my rambling response was a bit impenetrable

I think my units have misted because they put a lot of small units in glazing bars that are 'only' 30mm across.
The units in the middle of the row of E & W windows have gone, none of the North facing units have failed.
I believe the seals have caught the sun and degraded.

I also have no information on the windows and now FENSA certs for them so they could have been made by anyone to any standard

Using larger units and plant on bars I should be able to recess the units further into the frame to minimise their exposure and gain shadowing from the brick surround. Along with having fewer seals and less length of seal to fail.

I won't reglaze my existing single glazed windows with double glazing. They are getting secondary glazing which will perform well and retain the historic fabric including the old glass as untouched as it can be. I wouldn't recommend anyone replace a decent single glazed window with a new double glazed one unless the original is out of keeping (like my 1970's casements) or rotten beyond all repair at which point you have very little choice.

I can recommend reading the document Nemesis linked in this thread which covers some interesting tests - esp how wooden shutters can perform as well as double glazing when they're closed of course! Skip to page 17 for the good bit.
http://www.periodproperty.co.uk/forum/viewtopic.php?f=1&t=9891

 

[Taff in Glaws]

MdB I finally found my photobucket password with my keys! This is the DG timber framed window I replaced.
IMG]http://i171.photobucket.com/albums/u293/taffinglaws/helloween08062.jpg[/IMG]

 

[Taff in Glaws]

oops there you go