re: direct gain passive solar glazing confusion--which ratio to floor area???
5 feb 2002
>i'm working on the design of a small (1200 sq ft), direct gain passive
>solar home... i want most of the heat provided by the solar.
you are probably barking up the wrong tree. mazria's book was helpful
when written, but it contains more rules of thumb than basic heatflow
arithmetic. direct gain houses are seldom more than 60% solar-heated.
for 80 or 90 or 100%, you might fill the space between two layers of
south glazing with tiny soap bubbles at night, or use a low-thermal-mass
sunspace with insulation between the living space and the sunspace.
circulate warm air between living and sunspace during the day, and
let the sunspace get cold at night and on cloudy days.
put lots of insulation outside the concrete: if the house cools
from 75 to 65 f over 5 cloudy 25 f days, with no internal heat gain,
65 = 25+(75-25)exp(-5x24/(rc)), so rc = 5x24/ln((65-25)/(75-25))
= 538 hours... c = 16.7 btu/f-ft^2 for 8" of concrete makes r = 32,
about 6" of styrofoam.
you might use less thermal mass at a higher temperature with r48 sips...
how many kwh/month of electrical energy consumption do you need to keep
a 1200 ft^2 r48 house with 100 ft^2 of r4 windows 65 f on a 25 f night?