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Green Building Bible, Fourth Edition
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Housing - New Build: Heavyweight verses lightweight construction

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    <blockquote><cite>Posted By: fostertom</cite><blockquote><cite>Posted By: Paul in Montreal</cite>very cold parts of the US compared to the UK - even Canada is very hot in the summer compared to the UK</blockquote>So, it's hard to believe that simple reflective, anti-overheating glass predominates, is all that's used; and low-e glass as we know it in Europe (welcomes solar gain, resists radiant heat loss) isn't a vital part of the picture - are you sure about that,

    Low-e glass/film is uni-directional and but can be fitted facing either way, one way for predominately heating climate or other way a predominately cooling climate!
    • CommentAuthortony
    • CommentTimeSep 19th 2007
     
    Sounds to me like it might be a good idea to turn the glass round inside out every spring and autumn?
    •  
      CommentAuthorfostertom
    • CommentTimeSep 19th 2007 edited
     
    Posted By: Jeff Norton (NZ)Low-e glass/film is uni-directional and but can be fitted facing either way, one way for predominately heating climate or other way a predominately cooling climate!
    I'm sceptical about that - no way outgoing radiation could be at the high frequency of sun-originated radiation, which gets it through the coating unimpeded.
  4.  
    I stand corrected, there appears to be different types of low-e, designed for different climates. I orignally thought it was on which pane it was placed and which way it pointed!

    http://www.efficientwindows.org/lowe.cfm
    •  
      CommentAuthorfostertom
    • CommentTimeSep 20th 2007 edited
     
    Though this American site says "Low-E glass product designed to reduce heat loss but admit solar gain", it doesn't explain properly how that trick happens. The 'reduce heat loss' bit gets no importance - all interest is in the 'solar gain' bit - maybe that is indeed, as Paul in Montreal suggests, all that North America is aware of. Four different glass types/levels of solar gain resistance are mentioned, but no indication of how these affect the 'reduce heat loss' performance.

    But why are we asking the glass to resist solar gain anyway, in whatever climate? OK, some buildings where that's the only way, but for any newbuild or C21 refurb, there should be external canopies or louvres designed to keep the sun off the glass while it's at high elevation i.e. in summer and/or at mid day, and allow the sun to strike and penetrate the glass when the sun's at low elevation, i.e. in autumn/winter/spring and/or nearer dawn or dusk, when solar gain would be welcome.
  6.  
    Tom siad "But why are we asking the glass to resist solar gain anyway, in whatever climate?"

    Offices have much larger internal heat loads. Glazing with a low g-value/SHGC is benefitial in this environment, particularly during the summer. Reversable windows are being developed so that a greater amount of long wave radiation can be excluded (reflected anyway) in the summer and then they can be flipped around so that long wave radiation can be reflected back to the interior.
    When the low-e is placed on plane 2 (rather than the more typical plane 3 of double g) then the windows reject a greater amount of long wave raditaion and thus have reduced solar gain. This is due to the fact that low-e effectively de-couples one side of the the glazing unit with the other and influences the radiant and conductive aspects of the glazing system. Hope this helps.

    Mark
    •  
      CommentAuthorfostertom
    • CommentTimeSep 20th 2007
     
    Mark, could you decode?
    Remind me what is g-value/SHGC?
    Are we still talking about resistance to summer solar gain, when sun hits glass? Or is it now about dispersing internal summer heat? That sounds interesting.
    Do you mean a coating that, when in summer it's on an outward facing glass face (plane 2), encourages or has low resistance to outgoing long-wave (low frequency) radiation originating from warm internal things like lights, computers, bodies? And when in winter it's flipped to face inward (plane 3), it resists same outgoing long-wave (low frequency) radiation? I'm amazed if that's more than a marginal effect - am I wrong?
    How does rejection of a greater amount of long-wave (low frequency) radiation reduce solar gain, when the latter (because it originates from a hot hot body) is short-wave (high frequency)?
    What in detail is meant by 'low-e effectively de-couples one side of the the glazing unit with the other and influences the radiant and conductive aspects of the glazing system'?
  8.  
    SHGC = Solar heat gain coefficient.
    >>> Are we still talking about resistance to summer solar gain, when sun hits glass?

    Yes. In the summer, when the low-e is on plane 2 the coating reflects a greater %age of solar long wave radiation. Still reflects internal long wave from walls, appliances etc.

    When flipped in for winter use the SHGC/g-value is greater thus allowing more solar gain when it is most needed. I've read about this in a Swedish paper. I don't recall if it cited the actual performance, but it is certainly an are of strong academic interest (if it is not in production in some more adventurous, some would say enlightened, countries).

    Short wave solar radiation is absorbed by a body and re-radiated as long wave radiation. Low-e is resistance to long wave (though, as I understand it, the low-e "mix" can be tuned to suite various conditions).

    Decoupling: a DG unit with out low-e has relatively uniform thermal transmission. A low-e coating changes that and make the performance eccentric i.e. off center. This eccentricity is what I referred to as decoupling.

    Hope that answer the question/s.

    Mark

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