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Window U-Values

  • 08-10-2008 7:31pm
    #1
    Independent
    Closed Accounts Posts: 55 ✭✭


    Hi, just wondering about window u-values, does the u-value depend on the size of the window, like does it vary with the size of window? thanks.


Comments

  • #2
    Danbo!
    Closed Accounts Posts: 3,942 ✭✭✭Danbo!


    In short, no. The U-value is the measured in Watts per metre squared Kelvin. I.e. the amount of heat which travels through a particular material per metre squared for every 1 degree temp difference between inside and out.

    Does that answer your question?


  • #3
    Juantorena
    Registered Users, Registered Users 2 Posts: 147 ✭✭Juantorena


    Not sure if that is absolutely correct, noblestee, particularly when talking about the whole window u-value.

    I stand to be corrected on this but generally a window (whole unit, i.e. glass and frame) u-value is calculated from testing on a standard size element (approx. 1.2x1.4m). As the window size goes up the proportion of (relatively poor insulating) frame goes down relative to the (relatively well insulating) glass. Hence the whole unit u-value should decrease slightly. Conversely, as the window gets smaller the proportion of frame goes up and with it the whole unit u-value.

    Highly insulating windows with insulated frames probably dilute this to a greater or lesser extent...


  • #4
    heinbloed
    Registered Users, Registered Users 2 Posts: 1,632 ✭✭✭heinbloed


    There are several U-values for a window to be looked at: the Uw-value covering the hole window, frame and glass, and this is indeed dependant on the size of the entire window.Then there is the Ug-value which covers the total glazing incl. the spacers but not the frame.And then there is the Uf-value for the frame only, without the glazing.Another inportant value is the g-value which represents the possible energy gain from (sun-)light.


  • #5
    Independent
    Closed Accounts Posts: 55 ✭✭Independent


    Conflicting views! so what do we think, does a larger window on the same elevation as a smaller window have a larger u-value? same glass, same frame etc.


  • #6
    sinnerboy
    Closed Accounts Posts: 5,433 ✭✭✭sinnerboy


    Glass technology is such that the double glazed unit ( DGU ) on its own tends to out perform the assembled window

    The now typically available un framed DGU will have a u value of 1.1

    But a 1.4m x 1.4m timber framed window , with that DGU may result in a whole window u value of 1.4

    A larger window ( more % glass ) will score a better u value 1.3 , 1.2 ...

    A smaller window ( more % frame ) will score a worse u value 1.6 , 1.7 ....

    The better window suppliers will issue a schedule of specifications to indicate each individual window size u value . If you let them know the oreientation some will also tell you the Ueff value - which takes account of solar gains . The exact same window , one placed facing North and one placed facing south will have the same whole window U value , but a different Ueff value


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