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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorlineweight
    • CommentTimeFeb 11th 2019
     
    Am I right in thinking that there is often some confusion about what a Uw value applies to - that is, to a notional 'standard' window of a certain size/frame configuration, or to the specific window that is under consideration?

    If so, what's the correct terminology to distinguish between the two?
    •  
      CommentAuthordjh
    • CommentTimeFeb 11th 2019
     
    There are various different parts of a window that a U value can apply to, yes.

    There's the U-value of the glazing. (sometimes Ug)
    There's the U-value of the frame. (sometimes (Uf)
    There's the U-value of the whole window. (sometimes Uw)
    There's the U-value of the whole window, installed.
    Or sometimes the installation is dealt with separately by a psi value,
    as can be the edge effect where the glazing meets the frame.
    These can all be applied to each actual window and will be different for each because of size
    or they can be applied to sizes that are nominated by particular standards.

    There's no one correct terminology that comes to my mind immediately, but there are certainly manufacturers and others who write in a clear explicit way whilst others use a more vague and ambiguous way.

    In particular, it is always worth requesting the certificate for a particular window or set of windows acccording to whatever standard you're interested in and checking it very carefully to make sure it is what is claimed and does apply to your particular windows.
    • CommentAuthorlineweight
    • CommentTimeFeb 11th 2019
     
    Sure, I understand the difference between Ug, Uf, Uw and so on.

    Specifically, the question is about Uw because when I get quotes it's often ambiguous whether the Uw they give me applies to my particular configuration, or if it's a general Uw for that particular system based on, as you say, a size nominated by a particular standard.

    If there were an unambiguous way to request one or other of those two values then that would save quite a bit of to-and-fro and misunderstanding I think.

    Also it would be good to know if they are *supposed* to be giving me one version rather than the other when I simply ask for the Uw value, or whether it's basically up to them. This information seems to be hard to come by.
    • CommentAuthorDarylP
    • CommentTimeFeb 11th 2019
     
    there is the BFRC 'std' window, for which a Uw-value is given. this could be taken as a 'benchmark' to compare different systems...?
    •  
      CommentAuthordjh
    • CommentTimeFeb 11th 2019
     
    Posted By: lineweightIf there were an unambiguous way to request one or other of those two values then that would save quite a bit of to-and-fro and misunderstanding I think.

    Well you can certainly be unambiguous when requesting. Just ask for exactly what you want!

    Also it would be good to know if they are *supposed* to be giving me one version rather than the other when I simply ask for the Uw value, or whether it's basically up to them. This information seems to be hard to come by.

    I don't think there's any presupposition. If you make ambiguous requests then you can't blame them for responding with whatever number suits them or is easiest to find. It can indeed be quite hard to get the numbers you want even when you ask unambiguously as many people asking for numbers suitable for PHPP have found. My architect struggled.
    •  
      CommentAuthordjh
    • CommentTimeFeb 11th 2019
     
    Posted By: DarylPthere is the BFRC 'std' window, for which a Uw-value is given. this could be taken as a 'benchmark' to compare different systems

    The problem is that different measurement systems have different 'standard' window sizes, so there's no easy way to compare. Leave alone the different measurement and calculation methods etc etc.
    • CommentAuthorlineweight
    • CommentTimeFeb 12th 2019
     
    Posted By: djh
    Posted By: lineweightIf there were an unambiguous way to request one or other of those two values then that would save quite a bit of to-and-fro and misunderstanding I think.

    Well you can certainly be unambiguous when requesting. Just ask for exactly what you want!

    Also it would be good to know if they are *supposed* to be giving me one version rather than the other when I simply ask for the Uw value, or whether it's basically up to them. This information seems to be hard to come by.

    I don't think there's any presupposition. If you make ambiguous requests then you can't blame them for responding with whatever number suits them or is easiest to find. It can indeed be quite hard to get the numbers you want even when you ask unambiguously as many people asking for numbers suitable for PHPP have found. My architect struggled.


    The point of this thread was to aid me in making unambiguous requests. I was hoping there might be a widely used industry standard, that all or most suppliers would use or recognise, so I could simply ask for that. It sounds a bit as if there isn't though.
    •  
      CommentAuthorDamonHD
    • CommentTime7 days ago
     
    Maybe I was just being silly, but I asked for whole-unit U values for window openings that I specified. In some cases IIRC my requests for that Uw were ignored, and those suppliers will have dropped out of my list. I think that these notes are relevant:

    http://www.earth.org.uk/superinsulating-our-living-room.html#glazing

    Of course all this was slightly undermined by the fitters leaving a 1cm unfilled gap down the side of at least a couple of the units, discovered some time later during other work... %-P

    Rgds

    Damon
    • CommentAuthorgyrogear
    • CommentTime7 days ago edited
     
    I am trying to sympathise as it does sound complicated...

    I guess windows must be a bit like aircraft or motor cars: lots of industry-wide (and international) standards that apply, but not all apply to a given aircraft, and when you are buying an aircraft, you have to check the boxes that you are looking for, so I guess it's the same with windows.

    I expect that aircraft-purchasers make unambiguous requests !

    (I expect that the same reasoning applies to houses also...).

    In a common market, with free movement of goods and persons, windows "ought" to be like-for-like and fit for purpose in any scenario. A metre is a metre and a kelvin is a kelvin. The only truth that applies is "caveat emptor".

    gg
    • CommentAuthorDarylP
    • CommentTime7 days ago
     
    @lineweight, the BFRC std window IS a standard size. in the UK.
    EU-wide ask for Uw-value to EN ISO 10077-1 or -2.
  1.  
    Posted By: DarylP@lineweight, the BFRC std window IS a standard size. in the UK.
    EU-wide ask for Uw-value to EN ISO 10077-1 or -2.


    Cheers. I guess the question then becomes, which of these is the best bet to ask for: or both.

    I know some will say, ask for it all, and if suppliers don't provide exactly what you want, drop them from your shortlist. Suppliers who willingly provide good info do get moved towards the top of my shortlists. But dropping anyone who doesn't, isn't always practical. It may end up with only one supplier, whose product costs ten times as much as the others, and/or is not entirely suitable for other reasons.

    Sometimes, the information is there, but the person you're dealing with doesn't know how to find it. They may be baffled or intimidated by overly technical demands. If you deal with larger operations where you can benefit from economies of scale, then you just don't always get to speak to the person in the company who has all the detailed technical knowledge. Knowing what to ask for, in terms the person you're speaking to will recognise is often useful in these situations.
    •  
      CommentAuthorDamonHD
    • CommentTime7 days ago
     
    Since the energy performance of the window IS the reason for multi-glazing (even if sometimes being bought less numerically to be "more comfortable") then not answering such questions is a bad bad sign.

    As you'll see in the end I wasn't happy with any of the responses I got and I waited quite a time until my local glazier that I'd used for other jobs went out and did the leg-work for me and contacted me out of the blue to say they were ready. I was their first 3G install.

    Rgds

    Damon
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    Here are a couple of sensible basic pages:
    https://www.inoutic.de/u-value-for-windows
    https://www.greenbuildingstore.co.uk/understanding-windows-u-values-psi-values-g-values/

    Plus point for BFRC is that some of their certificates can include Uw, and the solar factor and the airtightness. Negative points are that other certificates include none of those just an overall letter rating, plus there are competing rating systems so there might not be one at all, plus it is only available for the standard size in a standard orientation so your particular installed window almost certainly has different values.

    There is no magic bullet to get quick and accurate answers. Being clear about what you're looking for plus a hefty dose of persistence is about the best you're going to do.
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