[Greenbuilding] R20 window insulation?

[nick pine]

"Bob Waldrop" writes

>>... we came up with the present plan (which included replacing the south 
>> wall of our utility room with glass to add some passive solar)
> 
>>Have you considered turning this 24/7 heat-losing passive solar glass into 
>>an air heater with little heat loss at night >and on cloudy days?
> 
> I have been reading this forum for some time and your frequent contributions 
> on the subject, but the math generally escapes me

Windows only lose heat during times when there's a temperature difference 
between one side and the other, so solar air heaters or windows on a low-mass
sunspace that gets cold at night lose no heat at night. (If a sunspace contains
lots of thermal mass, it doesn't get very cold at night, so it loses lots of heat
at night.)

NREL says 1340 Btu/ft^2 of sun falls on a south wall on an average 35.9 F January
day with a 46.7 high in Oklahoma City. A 1 ft^2 U0.25 window with 50% solar transmission
on a 65 F living space would gain 0.5x1340 = 670 Btu and lose 24h(65-35.9)1ft^2xU0.25
= 175, for a net gain of 670-175 = 495 Btu/day. A less-expensive U0.5 window with 80%
solar transmission on a low-mass sunspace that's 80 F for 6 hours per day would gain
0.8x1340 = 1072 Btu and lose 6h(80-35.9)1ft^2xU0.5  = 132, for a net gain of 940 Btu/day.
On a cloudy day, the living-space window loses 175 Btu. The sunspace window loses 0 Btu. 

> it hasn't been clear exactly what you would advocate in this situation.  It is
> a space about 20' long, and 6' wide, with doors at either end (no east or west
> windows).  The windows are stock double paned patio doors, plus two double hung
> windows for summer ventilation.  I am always interested in anything that will 
> help us stay warm in the winter for less effort.

Sounds like you might a) use the utility room as a low-mass sunspace and circulate
warm air through the house living spaces during the day and let the utility room 
get cold at night, or b) turn these utility room windows into passive solar air
heaters in wintertime by darkening the south side of the R20 window insulation and
allowing solar-warmed air to flow out of the slot during the day, automatically,
with no need to move the insulation twice a day.

Patio doors and conventional windows are OK for sunspace glazing, but they cost
more and have more wood and less glass with lower solar transmission than some
other forms of glazing, eg 4'x8' 10 mil HP92W polycarbonate glazing panels with
2x4 frames, slightly inflated with argon, with a tire valve. I've made about 20
of these with butyl rubber tape seals, but I just bought one of these $400 band
sealing machines http://www.simplesealers.com/fr900-horizontal-heat-sealer.html,
which should make the job a lot easier.

Nick   
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