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re: heat loss through skylight
28 dec 2005
z wrote:
>nicksans...@ece.villanova.edu wrote:
>> m ransley wrote:
>>
>> >if it could automaticly close with a foam insulating plug when shaded
>> >it could be a net gain.
>>
>> that's better, but any skylight is a net gain if more solar energy enters
>> during a day than heat energy leaves during a day, which can happen with
>> several layers of glazing. for instance, 3 layers of r1 glazing with 90%
>> solar transmission would gain 0.9x0.9x0.9x460 = 335 btu/ft^2 and lose
>> 24h(65-26.7)1ft^2/r3 = 306, for a net gain of 29 btu/ft^2 on an average
>> december day in chicago.
>>
>> by contrast, insulation just loses heat. an r100 ceiling would just lose
>> 24h(65-26.7)1ft^2/r100 = 9 btu/ft^2.
>>
>> would you rather gain 29 or lose 9 btu of heat per day in december?
daylight is also nice...
>might i point out that they call it a "greenhouse effect", after the
>greenhouses, where glass panes contribute to the retention of solar
>heat making the interior warmer than the exterior.
sure. glass is a high pass filter, passing high frequency solar energy with
wavelengths shorter than 3 microns and blocking low frequency heat energy,
eg 10 micron heat from an 80 f black body.
>provided of course they face south.
that's better, with 740 vs 460 btu/day of sun on an average december day
in chicago. the calc above was for a horizontal surface. a south window can
provide more light than a skylight, esp if it's near a reflective ceiling
with a lightshelf below the window.
>which in summer will make them heat up the interior more than you want.
south walls get less sun than horizontal surfaces in summertime, and
shading them from higher summer sun is easy with overhangs.
nick
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