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re: advice on design of new home
31 dec 1997
don kulha  wrote:

>- basically you provide egress to sunshine, soak it up with something
>(thermal mass such as a dark rock floor)...

egress? :-)

seems to me that won't work very well in a cloudy place like the uk...
a low-thermal-mass sunspace or air heater that provides warm air for
the house on a sunny day and gets cold at night, with an insulated wall
between sunspace and 24-hour living space, and most of the thermal mass 
in the house itself, is likely to be more efficient.

where i live, nrel says a square foot of south double-glazed window gains
750 btu/day in january, with an average of 1000 btu that falls on a south
wall and a 24 hour average outdoor temperature of about 30 f and an average
daily max temp of about 38 f. if the window has a us r-value of 2 (uk 0.35),
it loses 24h(70-30)1ft^2/r2 = 480 btu/day, for a net gain of 270 and an
effective solar collection efficiency of 27%. on a low-thermal-mass sunspace,
it might lose 6h(70-30)1ft^2/r2 = 120 btu, for a net gain of 630, ie more
than twice the effective solar collection efficiency. if the sunspace has
lots of thermal mass, or the glazing is over a trombe wall, the thermal
mass keeps the glazing warmish at night, and we are back to about 27%. 

a single layer of less expensive r1 sunspace glazing with higher solar
transmission might gain 900 btu and lose 6h(70-30)1ft^2/r1 = 240, for a
net gain of 660, ie a solar collection efficiency of 66%...

on a cloudy day, when the sunspace is cool, the house and sunspace glazing
lose little heat, unlike a house window or an uninsulated trombe wall.

happy new year,


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