[Greenbuilding] R20 window insulation?

[Bob Waldrop]

----- Original Message ----- 
From: nick pine

>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.

On a cloudy day, or at night, those windows would be covered with our 
insulating panels.

>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.

The utility room was added onto the house at some date after its 
construction, but before I moved here in 1999.  The south wall of the 
kitchen (the next room inwards from the utility room/sunporch conversion) 
has two windows opening into the area, and two doors. The windows actually 
open and close.   Nice for ventilation, not as nice for having a cooled down 
space at night, which we have experimented with a bit.

I like the idea of turning the insulating panels into air heaters.   How big 
should the slot be for the heater air to enter the room in a 4' x 8' panel? 
And where?  Close to the top but still open to the glass?  (The top part of 
the panels hits the wall above the windows).

Bob Waldrop, OKC