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Green Building Bible, Fourth Edition
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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
    • CommentTimeFeb 12th 2019
     
    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
    • CommentTimeFeb 12th 2019 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
    • CommentTimeFeb 12th 2019
     
    @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.
    • CommentAuthorlineweight
    • CommentTimeFeb 12th 2019
     
    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
    • CommentTimeFeb 12th 2019
     
    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
    • CommentTimeFeb 12th 2019
     
    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.
  1.  
    So...over the past few weeks have been going through the process of getting window quotes and this time round have been a bit more persistent than usual in asking for configuration-specific Uw values.

    Similar to other comments above, the response has been varied amongst suppliers.

    Some simply gave me a Uw number without any document to back it up.

    One claimed that Uw remains the same regardless of window size and configuration (they were swiftly removed from the shortlist)

    One company (smaller UK manufacturer), I actually spoke to their main technical guy, who was refreshingly honest about the situation and told me that they didn't have a means of giving me an objective Uw value calculated for a specific arrangement. He told me that he was also frustrated by this and that it was something they were looking into. He agreed there was a need for a more standardised system so that people knew what they were comparing. I think though, that ultimately there's just not much commercial incentive because most people don't care - all they want is an email containing a number they can give to a building control inspector who will question it no further. The chap I spoke to, I think, would be pleased if more people asked. Maybe that's why he seemed willing to speak to me directly.

    The two systems that we have ended up choosing between each gave me a calculation sheet for the unit Uw. This at least gave me confidence that the values relate in some way to the specific configuration, but I can't be sure that they were both calculated by the same method. Neither mention EN ISO 10077. One doesn't say anything about the method, the other one gives some details about a particular method, used by the 'IFT Rosenheim'.

    I guess I could push either of those further, but come to a point where I don't know what I'm actually asking for. For example does the EN ISO 10077 standard apply to unit-specific calculations, or is that only a system for standardising generic measurements based on a standard frame size?

    Where I did find some consistency was in a few of the product data sheets where they would mention the generic U value with reference to EN ISO 10077. Where this was given, and I then got a unit-specific value, it seemed generally to be the case that they were proportional - in other words if the generic value of one system was better than another then the unit-specific value seemed also to come out better by a similar-ish degree. (For this particular application, the unit-specific value was always lower than the generic one by something like 0.3 to 0.5)
    •  
      CommentAuthorfostertom
    • CommentTimeMar 7th 2019 edited
     
    Free https://windows.lbl.gov/software/window calcs window U values.
  2.  
    Posted By: fostertomFreehttps://windows.lbl.gov/software/window" rel="nofollow" >https://windows.lbl.gov/software/windowcalcs window U values.


    Hm. Maybe I should pass that to the technical guy who said there was no easy way to come up with an objective U value for any configuration of their system.

    But maybe his issue is that to set up a custom configuration in there is pretty time consuming?
    •  
      CommentAuthordjh
    • CommentTimeMar 7th 2019
     
    There's another document that might be relevant at https://www.ggf.org.uk/wp-content/uploads/2018/02/Data_Sheet_2.2_Nov_14.pdf
  3.  
    Posted By: djhThere's another document that might be relevant athttps://www.ggf.org.uk/wp-content/uploads/2018/02/Data_Sheet_2.2_Nov_14.pdf" rel="nofollow" >https://www.ggf.org.uk/wp-content/uploads/2018/02/Data_Sheet_2.2_Nov_14.pdf


    When I look at what they call 'simplified methods of calculation' (rather than simulation) I can see how that can work for relatively straightforward configurations but it must start to fall apart when you are looking at things like complex sliding systems, where panels might meet each other in a number of different ways that would be hard to capture under standard assumptions. Then it would seem to me that simulation type modelling can be the only way to arrive at a realistic figure.
    •  
      CommentAuthordjh
    • CommentTimeMar 7th 2019
     
    Posted By: lineweightThen it would seem to me that simulation type modelling can be the only way to arrive at a realistic figure.

    Well, presumably in a complex situation you use the full standard ISO 15099 instead of the simplified ISO 10077. But yes, a simulation will be more flexible ultimately.
    • CommentAuthorgoodevans
    • CommentTimeMar 8th 2019
     
    Another factor to consider is the amount of light the window lets in. As the glass is getting heavier with security requirements and triple glazing the frames and sashes are getting wider and wider. In addition extra glass pains and coatings cut out more light.

    I thick we could assume the main reason for a window is light - with Purge ventilation and escape a close second.

    Given that a well insulated wall may have a U value of .1 to .13 perhaps - but good 3G window can only achieve the 0.7 mark - windows are bad news if you ignore solar gain (double edged sword in summer).

    It would be interesting to see a heat loss for a standard window expressed value per m2 of glass rather than the glass and frame combined. It would then encourage thinner frame design to let in more light allowing smaller frames and reducing the size of the hole in the well insulated wall.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 8th 2019
     
    With that increasing timber cross-section tendency, the difference between the German and the Scandinavian approach becomes significant.

    Those parts of Europe or those manufacturers following the 'German' approach produce windows that are relatively thin, from inside to out, therefore must fatten up the timber such that there's less glass area and more frontal timber.

    Whereas those following 'Scandinavian' go the other way - frame deepness from inside to out but still slim frontally.

    For example Russell Timbertech's PH windows fixed frame sections are 105 deep x 56 wide, the opening sections 95x56, and when assembled 105 deep x 95 wide overall - not bad, quite 'slim' frontally, unusually so amongst PH windows. Munster tend that way too.
    • CommentAuthorlineweight
    • CommentTimeMar 16th 2019
     
    Posted By: goodevansAnother factor to consider is the amount of light the window lets in. As the glass is getting heavier with security requirements and triple glazing the frames and sashes are getting wider and wider. In addition extra glass pains and coatings cut out more light.

    I thick we could assume the main reason for a window is light - with Purge ventilation and escape a close second.

    Given that a well insulated wall may have a U value of .1 to .13 perhaps - but good 3G window can only achieve the 0.7 mark - windows are bad news if you ignore solar gain (double edged sword in summer).

    It would be interesting to see a heat loss for a standard window expressed value per m2 of glass rather than the glass and frame combined. It would then encourage thinner frame design to let in more light allowing smaller frames and reducing the size of the hole in the well insulated wall.


    Yes indeed. It would be interesting to see how aluminium and timber-framed windows would measure up against each other on this basis, especially when looking at bifolds, sliders or anything with a lot of sub-division of the basic wall opening.

    The types of window where you can bury the perimeter frame behind wall insulation gets round some of the problem you describe. Then it doesn't matter so much how chunky the frame is - it might mean quite a large structural opening relative to glazed area but not such a large 'insulation opening'.
  4.  
    It is not the frame that has to be wide to support the weight of the glass but rather the opening light, that is the bit that hinges out/in. The frame could always be buried in the wall or insulation leaving just enough to open the window. I have some old window frames with opening lights where the lights have frames made out of 4cm x 4cm, try getting away with that with 3G

    Also with the old 1G windows the glass was puttied in and as the putty went hard this held the glass and frame together, thus the glass helped the frame stay square. Modern windows have a gap between glass and frame so the glass gives no support the the frame which has to be sufficient to take the weight of the glass without turning into a parallelogram.
    • CommentAuthorgyrogear
    • CommentTimeMar 16th 2019
     
    Posted By: Peter_in_Hungarybut rather the opening light, that is the bit that hinges out/in


    whence the advantage of the sliding chassis...
    particularly in windy (read coastal...) locations

    gg
    •  
      CommentAuthordjh
    • CommentTimeMar 16th 2019
     
    Posted By: gyrogearwhence the advantage of the sliding chassis

    Yes but FWIW, we found our lifting, sliding door to be the most fussy when it came to installation levels etc.
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