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Solar Transmittance values for windows

  • 11-08-2009 2:36pm
    #1
    gears
    Registered Users Posts: 758 ✭✭✭


    In looking at different window companies I notice they show the transmittance values in different ways. Some use a scale between 0 and 1 (eg. 0.57) and some use a % (eg. 71%)

    So to compare windows is it simply a case of 0.57 equals 57% or is it not so simple as is often the case.


Comments

  • #2
    Mellor
    Registered Users, Registered Users 2 Posts: 39,565 ✭✭✭✭Mellor


    Yeah, they should be interchangeable. It’s called the G-Value and can be given as a % or as a value between 0 and 1.
    Bear in mind (just like everything else) how its calculated and what exactly the figure refers to is important. (eg do they refer to glass only or whole window unit?)


  • #3
    gears
    Registered Users Posts: 758 ✭✭✭gears


    Surely if it's solar transmittance, as in how mush light passes through it would only be for the glass, No?


  • #4
    Mellor
    Registered Users, Registered Users 2 Posts: 39,565 ✭✭✭✭Mellor


    gears wrote: »
    Surely if it's solar transmittance, as in how mush light passes through it would only be for the glass, No?
    Well obviously the frame has a 0% tranmittance.
    But this must be calculated into the overall average transmittance for the unit. (Just as with U-values)

    Consider a unit that is 1800x1200. With a 50mm frame. And the glazing has a G-Value of 65%.
    Now if company quote the only value as 65%, it is misleading as the glazing is less than 87% of the ope, and actualy tranmittance of the whole unit is 56%.

    The glazing value will remain constant accross the range but actually value will change depending on the size of the ope, frame and number of mullons etc. Again this is exactly the same as the U-value situation.

    Just make sure you know what they are quoting and compare like with like.


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