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Green Building Bible, Fourth Edition
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    If the vacuum fails, as it will eventually, perhaps we could arrange to pump out the gas and restore it?
    • CommentAuthorAguillar
    • CommentTimeJul 6th 2007
    When our windows get grubby we clean them with a cloth rather than a vacuum cleaner...
    [Sorry - I just couldn't resist such a comment]

    Have heard a bit about vacuum windows, would be ideal if they can be made reliable! If an indicator could be built in to show when the vacuum is lost then it would be useful, then at least the householder would know.

    Suspect that trying to refurbish such windows would not be a goer as when the vacuum fails it would be due to perished seals and these would need replacing as well as pumping out the gas...
    • CommentAuthortony
    • CommentTimeJul 6th 2007
    Fail safe is the way to go and a terrestrial vacuum is definitely going to fail. the second it does you will have condensation between the glass.
    Their seals are made from glass, unlikely to perish.

    But gradual leakage may cause gradual failure, it depends on how good the seal is.

    Sudden failure is obviously possible too.

    The whole thing about sealed units is that they will fail, and we cannot reuse them easily at present, seems very unsustainable.

    That's why I like the idea of supply air windows ( http://www.biffvernon.freeserve.co.uk/air_flow_windows.htm) - if you make them with two single panes there is no sealed unit to fail. Therefore they may last for centuries, all the while giving effective u values as good as sealed units.

    It seems possible to make a vacuum unit that is robust and the only problem is the vacuum will gradually fail due to leakage, but that this can be made fixable by restoring the vacuum. Each Autumn, go round the house, plug a pump into the windows and pump out the air to restore for winter?
    • CommentAuthortony
    • CommentTimeJul 6th 2007
    Then the plug will let air in for sure -- its a non starter in my book.
    A plug can be made as airtight as a seal?
    But then if there is a problem, the plug would let you fix it, as long as it doesn't happen too often. Like car tyres but in reverse.
    I like the sound of the supply air windows also. Combined with passive ventilation, they seem to offer a viable alternative to very expensive, high performance triple glazing and ventilation systems for low energy houses.
    • CommentAuthorchuckey
    • CommentTimeJul 7th 2007
    One problem comes to mind is that the ambient air pressure is forcing the two sheets together, if the pane exceeds say 1m, the two panes will touch in the middle.
    Chuckey: The panes are held apart by stainless steel spacers on a 20mm grid.

    Biff: It's ok. The U value isn't particularly amazing. I think it's about 1.5 centre of pane. The edge effect will be massive if you just stick it in a wooden frame, as the edge is made of solder glass.

    There are some big problems with their manufacturing technique, in my view. For example, they don't make the stuff in an evacuated environment, so they have to have a pump-out tube, which I imagine is really ugly, and must be delicate despite their claims.

    They have to heat the glass during manufacture to off-gas it. This has two bad effects. Firstly, you can't have toughened glass. That probably rules it out for sash windows (the main potential market in the UK I imagine) unless you get some sort of dispensation. I'm sure you could get one, but would it be worth the effort?

    In addition, the high temperatures would destroy the coatings on soft-coat glass, hence they have to use hard-coat. This severely limits the insulation potential. Incidentally, somebody mentioned supply air windows. These can't use soft-coats either, giving rise to the same problem.

    One futher gripe is that the spacers are stainless steel. That's not very conductive for a metal, but some sort of high-strength glass would surely be much better.

    I imagine evacuated glazing probably is the future, but the first generation product isn't all that great.

    I'm sure other people would do a lot better with the idea. They know very little about insulation products in Japan - the main competition for this product is single-glazing. But of course, they hold the patents...

    There is a Dutch joinery manufacturer using this glass. Maybe you could talk to them: http://www.vanruysdael.com/en/glas/glassoorten.html
    By the way, this is now being sold in the UK as Pilkington energiKare Legacy.

    They seem to recommend the use of films in safety-glass situations. What effect this has on the thickness, I don't know.
    Posted By: Peter Clark

    That's why I like the idea of supply air windows (http://www.biffvernon.freeserve.co.uk/air_flow_windows.htm" >http://www.biffvernon.freeserve.co.uk/air_flow_windows.htm) - if you make them with two single panes there is no sealed unit to fail. Therefore they may last for centuries, all the while giving effective u values as good as sealed units.

    The only UK manufacturer - Howarth Windows - has stopped making them. Problems with unwanted draughts I believe. Experimenting with a new vent and unlikely to be avilable until next year at the earliest.

    Thermal bridging at the edge of the units is the major challenge it can really degrade the Uw.
    I've finally seen this in the flesh, here:


    It is great. The spacers are almost invisible even from close up, but the pump out tube is very visible even from the outside. So if you're going to use this in a retrofit, you might want to get the spacers located toward the edges of the window (I assume this is something you can specify, although how you say it in Japanese I don't know...)

    The windows of the house I saw are 3-over-3 and 2-over-2 (with true divided panes, which apparently were mandated by the conservation department). On this job the pump out caps were all in the bottom left as viewed from the inside (IIRC) which made them more visible in the upper sash and unpleasantly asymmetrical from the outside. If they had been in the margins, they would have been more of an attractive feature than a problem.

    The glass was being used in the lower sash of the windows, apparently without any building regulations safety issues, even though its annealed.

    I don't actually gives a better u-value than would single glazing with secondary gazing made of double glazed units, though, but I suppose it transmits more light.
    • CommentTimeSep 23rd 2012
    If you want to see how it works and some of the sums behind it have a look at this forum:

    If you double the distance between the walls (panes of glass) do you double the vacuum :wink:
    • CommentAuthortony
    • CommentTimeSep 23rd 2012
    In this situation I can only foresee the vacuum breaking down short, medium and long term and air getting in, leaks or diffusion, were moisture to get in then we all see the results.
    Has enyone (else) had a go at wide gap (order of 300mm) DG or 3G with inlet and outlet points for air?
    My thinking was to reduce air pressure at least (complete vacuum probably not realistic) and have the facility to "blow through" like a car demister should condensation ever appear.
    8 years in and no condensation yet ...
    Wish I had installed a visual guide for pressure now of course.
    • CommentAuthortony
    • CommentTimeOct 2nd 2012
    I built quadruple glazed 20 years ago with 2 dg units 300mm apart, no problems yet in the big gap, it was basically for sound insulation but saves a lot in heat losses too
    • CommentAuthorRobinB
    • CommentTimeOct 2nd 2012
    Posted By: tonyI built quadruple glazed 20 years ago with 2 dg units 300mm apart, no problems yet in the big gap, it was basically for sound insulation but saves a lot in heat losses too

    Interesting, I've been wondering if two sets of inexpensive DG windows one in front of the other might work better in certain situations than a single 3G window at five times the price. I was thinking to have the outer window very slightly vented to the outside.
    • CommentAuthortony
    • CommentTimeOct 2nd 2012
    I did no ventilating!
    • CommentAuthortony
    • CommentTimeOct 2nd 2012
    Also because they were all fixed panes/non opening they were little more than the cost of the glass and bits of wood, North elevation big overhanging eaves not even repainted yet
    • CommentTimeOct 2nd 2012
    Posted By: RobinBtwo sets of inexpensive DG windows
    Posted By: RobinBsingle 3G window at five times the price
    That's not right - there's little price difference now between 2G and 3G, from the right sources - some say 2G actually costs more because non-standard in Europe (where the best deals come from).
    • CommentAuthormatt-2052
    • CommentTimeOct 2nd 2012
    Tom, how did your quotes go for triple glazing? I had quote for hardwood 2G for 1 window that needs replacing at my parents and softwood 3G was almost the same.
    • CommentAuthorRobinB
    • CommentTimeOct 9th 2012 edited
    Posted By: fostertomThat's not right - there's little price difference now between 2G and 3G, from the right sources - some say 2G actually costs more because non-standard in Europe (where the best deals come from).

    Hopefully I'm out of date - sorry if I misled. Be interesting to see results of your latest "price survey"
    • CommentAuthorMikeRumney
    • CommentTimeJun 25th 2013
    ... of any interest?:
    Anyone know of a simple visual pressure check "gizmo" (like maybe a balloon or membrane based "widget")
    that could go between an outer (slate line) single pane and an inner DG unit (plasterboard line)
    Very interesting , not sure what he means by or uses as a 'Desiccant'
    • CommentAuthorSeret
    • CommentTimeJun 26th 2013
    Silica gel is the most common dessicant I've come across. It's that granular stuff you get in wee packets saying "Do not eat" inside packaging.
    and for some reason the "Do Not Eat" is always in quotes.
    Mike, I don't know whether you will read this now, but: sorry, reducing the pressure like that won't work at all.

    The conductivity isn't proportional to the pressure at normal pressures. In fact, it doesn't even change significantly with pressure.

    I used to know a proof of this (for an ideal gas), but have forgotten it.

    The pressure only starts to make a difference when it's so low that the mean free path (i.e. the distance a molecule of the gas travels, on average, before it hits another molecule) is near to (or below) the distance between the panes. The mean free path goes up as the pressure goes down, which is intuitive, but it doesn't get close to, say, 1mm till the pressure is really low.

    Also, even if you could reduce the pressure by, say, half (which would be pointless), without standoff spacers you would just break the glass.

    Actually, even the difference in atmospheric pressure at different altitudes can pose a problem. In Denver ("a mile high"), they have to either make the units nearby, or have each one fitted with a bladder on a capillary tube, to equalize the pressure by inflating, which they then seal off when the windows have been delivered.

    Gradual progress on vacuum glazing continues, however. Good list of links here (in the first comment by Dana Dorsett):

    I did not see SteamyTea's post.

    That forum thread explains it well.
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