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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorlineweight
    • CommentTimeMar 26th 2019
    Can't seem to find a clear answer to this anywhere. If the seal on a double-glazing unit goes, how much does it actually affect its thermal performance? I know it will depend on what gas it was filled with and so on... but in broad terms what sort of change in U-value results?
    • CommentTimeMar 26th 2019 edited
    I think that the rdSAP calcs may have some values for 'good' vs 'old' DG...

    Here was my rdSAP some time ago, with old leaky DGL http://www.earth.org.uk/data/20090330_WW_pressure_test/SAP_calcs_and_Inputs.pdf


    If you look at the DG unit u value with air and then with the gas of your choice (typically argon) then the difference will be the additional heat loss.

    The increased heat loss between 2 layers of glass will arise from the different gas, if the original gas was air then I would suggest that there would be virtually no change in the u value.

    Whilst there would be a theoretical additional heat loss due to the flow of air through the DG unit this air flow through a blown unit will be so small as to be insignificant. (Given that you usually can't see where the unit edge seal has failed).

    A bigger problem will be condensation on the inside of the DG unit which can actually build up to a 'puddle' at the bottom of the unit. It is the condensation issue that causes people to change the units.
    • CommentAuthorTimSmall
    • CommentTimeMar 27th 2019
    Too many variables to give a single answer I think.

    As Peter said, it would depend on the gas originally used, the spacer width, but also the type of low-e coating (if any) that was used, and how much water is in the condensation is on the low-e coating.

    Any low-e coating with water sitting on it will no longer work, and soft-coat low-e coatings are degraded by water (including water vapour) in a matter of weeks.

    Best case might be very little additional heat loss (maybe a couple of % at a guess), for an originally air-filled non-low-e, or hard coat low-e unit with little condensation inside.

    At the other end of the scale, a krypton fill soft-coat with lots of water inside could end up with 150% of extra heat loss (i.e. drop to about 2.5x the original losses).

    I've seen a unit which was literally half filled with water, so I'd guess that was up above that 2.5x increase.
    • CommentAuthorlineweight
    • CommentTimeMar 27th 2019
    Posted By: DamonHDI think that the rdSAP calcs may have some values for 'good' vs 'old' DG...

    Here was my rdSAP some time ago, with old leaky DGLhttp://www.earth.org.uk/data/20090330_WW_pressure_test/SAP_calcs_and_Inputs.pdf" rel="nofollow" >http://www.earth.org.uk/data/20090330_WW_pressure_test/SAP_calcs_and_Inputs.pdf



    There are two 'windows' values there - one 2.44 and the other 4.03. Is 2.44 leaky DG and 4.03 SG?
    • CommentAuthorRobL
    • CommentTimeMar 27th 2019
    I've drilled a small hole in the outer pane of a DG unit years ago, which allows external air into the air gap and reduces the tendancy to fog. I think it was a 6mm dia hole using a tile cutter drill at the bottom alone that fixed it. Don't do it on a windows with a safety glass kitemark unless you're very brave!
    On the one I tried, it got rid of condensation, but the damage had left a few permanent streaky lines. As the glass was old (U=2.7), I replaced it with new much lower U value(U=1.1) DG glass a few weeks later, which is a much better fix.
    • CommentTimeMar 27th 2019
    @lineweight: possibly, we did have a little bit of SG left at that stage (front door and toilet IIRC). Can't check right now.
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