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    • CommentAuthorArtiglio
    • CommentTimeJul 25th 2017
     
    I'm having 14 double hung sash timber windows made, in which the architect has specified glazing units should have a U value of 1.3 , however the joinery shop has been informed by their glazing supplier that they are now only able to achieve 1.4. As I understand it the windows are rated as a whole system rather than individual components, is this correct? However the windows are fairly large and as such the glazing is a major component, is the reduction of thermal efficiency likely to have a noticable loss of performance.
    The windows are replacing 20 year old (cheapest available at the time , upvc)
    Any info or suggested links would be appreciated.

    Thanks
    • CommentAuthorDarylP
    • CommentTimeJul 25th 2017
     
    Uw-values are the whole-window U-value inc. the frames.
    Ug-values are the centre-pane U-value , just the glass.....
    However, as IGUs get larger, the Uw-value should be calced properly as the frame bridging effect diminishes.
    'Normally', BFRC-standard window is used for the Uw-value calc.
    Cheers
    •  
      CommentAuthorfostertom
    • CommentTimeJul 25th 2017
     
    Yes, Ug of 1.3 or 1.4 is pathetic. Uw of 1.1 (minimum respectable performance these days) would require Ug of say 0.9. Russell Joinery Uw1.1 triple glazed windows are incredibly economical - £350/m2 fully glazed and finished. Plastic windows of better performance are cheaper still - but plastic ugh.
    •  
      CommentAuthordjh
    • CommentTimeJul 25th 2017 edited
     
    Thermix has a Uw calculator at https://www.thermix.de/winuw/

    Thermix make warm-edge spacers. It appears that changing from a standard aluminium spacer to one of their spacers in the example that the calculator is pre-loaded with changes its Uw from 1.4 to 1.3. So maybe that would also happen for your windows?

    Anyway, you can work out the actual Uw for different window sizes.

    Perhaps your joinery shop can find a better glazing supplier?

    Incidentally, and probably off-topic for this thread, I hadn't realized there was now a Part Q that applies to new buildings: "Security in dwellings". It affects glazing in doors and windows, but only for new dwellings.
  1.  
    Posted By: ArtiglioI'm having 14 double hung sash timber windows made, in which the architect has specified glazing units should have a U value of 1.3 , however the joinery shop has been informed by their glazing supplier that they are now only able to achieve 1.4.

    "the architect has specified glazing units should have a U value of 1.3" First you should get clarification from the architect about whether they mean Ug or Uw. (see posts above to see there is an important difference).

    Posted By: Artigliothe joinery shop has been informed by their glazing supplier that they are now only able to achieve 1.4.

    This implies that the joiners can only get a Ug unit of 1.4 from their glass supplier - a double glazed unit with lowE glass and argon (fairly standard DG today) should be Ug 1.1.

    I think you need to get clarification from both the architect and the joinery firm about what exactly they are talking about!!
    • CommentAuthorArtiglio
    • CommentTimeJul 25th 2017
     
    Again many thanks gents.

    A brief summary, the project is a front facade refurbishment of a building from around 1890 which i turned into flats 20 years ago, the area is undergoing a bit of a transformation part of which is a conservation area and a lottery heritage grant scheme. The original builder under bid my building and 3 others and walked away from them all, in order to complete the works within the priginal budget it has been agreed as the building owner i can do it. Not having been involved fully from the start in respect of specs , its been a bit of a struggle and must be said the architect is very hands off.
    Had a response from the building control officer which was basically along the lines of he'll accept what can be reasonably achieved as its a conservation area.

    Achieving maximum efficiency this late in the day is not going to happen, getting whole project over the line on an insufficient budget is currently more important. However I'd like to do the best I can.
    • CommentAuthorTimSmall
    • CommentTimeJul 25th 2017 edited
     
    If the pressure is for thinner units, then you'll get quite a bit better U value from Krypton fill. Not so many glazing unit manufacturers will supply it tho'.

    Do the joiners have a maximum glazing unit thickness which then can accommodate with the windows?

    U value vs. thickness vs. gas type (plus glass type) comes from:

    http://www.peterboroughwindowrepairs.co.uk/sealed-units/4578086771


    Whilst you can go all-out and end up with a pretty high performance (and very high price) sash window like this:

    http://www.jimmillerdesign.co.uk/blog/2017/05/24/loft-case-study-triple-glazed-sash/

    ... you can also get decent performance a lot cheaper too by borrowing a few ideas from it. The spring balances are a great idea (standard in North America on sash windows, I've been told and the same price or cheaper than normal balances, but not so leaky).
      4599221709.jpg
    • CommentAuthorTimSmall
    • CommentTimeJul 25th 2017
     
    n.b. I think the "Part L compliance" stuff is out of date in the chart above - that's for the old Part L. The U values are the thing you're looking at.

    For comparison (more performance than price) my Ug = 0.62 W/m²·K (Uw = 0.85) triple glazed PVCu windows were about £130 per m² inc VAT for a one-off small order (two windows) to go in my 1920s semi.
    • CommentAuthorGotanewlife
    • CommentTimeJul 25th 2017 edited
     
    Posted By: fostertomUw1.1 triple glazed windows are incredibly economical - £350/m2 fully glazed and finished.

    Posted By: TimSmallUg = 0.62 W/m²·K (Uw = 0.85) triple glazed PVCu windows were about £130 per m² inc VAT for a one-off small order

    Hmm so wood costs nearly 3 times more for substantially lower performance, what price
    Posted By: fostertombut plastic ugh.
    • CommentAuthorArtiglio
    • CommentTimeJul 26th 2017
     
    Good morning gents

    In reply to some of the above, the windows need to be of traditional construction, weights and pulleys etc , and retain original profiles this has given the following spec from architects drawings

    "Units fitted to be 'slimlite glass' 4-8-4 factory aealed units comprising 4 mm eclipse advantage clear (toughened) outer pane, 8mm krypton filled cavity and 4mm float glass inner pane laminated all to achieve minimum 1.3w/m2k U value.G-value:0.28 (tbc) "

    This was the spec the joiners priced on, the costs of which including fitting are £28k plus vat for the 14 windows. This is to have them made in sapele, and factory finished, paint applied by brush (to satisfy conservation officer).
    •  
      CommentAuthorfostertom
    • CommentTimeJul 26th 2017 edited
     
    Yes that is shocking, Gotanewlife - and almost incredible?
    • CommentAuthorTimSmall
    • CommentTimeJul 26th 2017 edited
     
    Hmm, haven't heard of conservation officers turning their nose up at those balances before - unlike some of the alternatives, they are fully concealed. If you have to go for traditional weights, then see if you can get them to seal the weight box at the outer edge of the frame, and also each weight channel from each other, otherwise you get a massive draft coming out of the inner pulley.

    "Pilkinton Eclipse Advantage Clear" is a solar control glass - (which gives quite poor U value) do the rooms get too hot at the moment? They'll also look quite dark (they cut out twice as much light as standard low-e soft coat) is that intended too?

    For the glass, the figures quoted seem to be wrong - "4 mm eclipse advantage clear (toughened) outer pane, 8mm krypton filled cavity and 4mm float glass inner pane" - if you stick that into Pilkington Spectrum it gives a U value of 1.66 (!) and a g value of 0.58. http://spectrum.pilkington.com/

    Here are are U values with one pane soft coat low-e (Pilkinton K Glass S), and one float glass krypton fill:

    1.26 W/m²·K = 4-8-4 (1x softcoat, 1x float) g value = 0.71 or 0.65 depending on which way around you have the two panes
    1.17 W/m²·K = 4-9-4 (1x softcoat, 1x float)
    1.12 W/m²·K = 4-10-4 (1x softcoat, 1x float)

    With two K-glass S panes instead of one:

    1.17 W/m²·K = 4-8-4 (2x softcoat) g value - 0.63
    1.08 W/m²·K = 4-9-4 (2x softcoat)
    1.03 W/m²·K = 4-10-4 (2x softcoat)

    With two Pilkington Optitherm S1 Plus:

    1.09 W/m²·K = 4-8-4 (2x softcoat) g value - 0.46
    1.00 W/m²·K = 4-9-4 (2x softcoat)
    0.94 W/m²·K = 4-10-4 (2x softcoat)

    Finally, with one eclipse advantage, and one K glass S:

    1.23 W/m²·K = 4-8-4 (1x softcoat 1x solar control) g value - 0.53


    All of the above can be toughened.

    So as you can see - if you do need a lower g value to avoid over heating, then better (and probably cheaper) to use two panes of low E glass instead.

    K Glass S (or equiv) is widely used and stocked - the Saint Gobain version is what's in my cheap-as-chips triple glazing.

    Also you can see the improvement from adding a mm or two to the IGU thickness - 9mm spacers are available but not everyone does them, cost of increasing the spacer width is usually zero, but 9mm spacers aren't available in all ranges I think.

    HTH!
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