The Passive Solar Greenhouse
B. Avoid unnecessary glass via the optimum inclination
of windows: Maximum winter "insolation" (the receipt
of heat providing sunlight) is received by the windows being tilted
at a 60 degree angle from the sun. This enables perpendicular sunlight
penetration minimising any loss due to reflection. By contrast,
vertical and horizontal glass surfaces reflect from 30 to 60% of
their winter "insolation" due to reflection. With 60 degree
glass inclination, the passive solar greenhouse receives as much
insolation during the winter as any other combination of greenhouse
configurations possible. Read my lips: NO UNNECESSARY GLASS. Extra
glass is impermissible because unnecessary glass enables the unnecessary
escape of an irrecoverable amount of night-time heat. For comparison
purposes, a 4' x 4' sheet of single pane window glass allows the
escape of the same amount of heat as an 8' tall by 40' long wall!
Imagine the heat loss from an entire ceiling of unnecessary glass.
C. The depth of a passive solar greenhouse must
not exceed 16 feet: Wide and shallow, not wide and deep. If
a greenhouse exceeds more that 16 feet in depth, it will be difficult
to obtain "abundant" winter heat. Abundant winter heat
is necessary to exceed the heating needs of the greenhouse and thereby
provide for proper ventilation and air circulation as well. If the
greenhouse needs to be larger, make it wider, not deeper. Palms
like heat the year around and they like heat even on cloudy days.
In areas such as the North Mediterranean and the Pacific Northwest,
this requirement is frequently thwarted by cloudy, overcast days.
In general, cloudy days will heat a greenhouse only about 10% as
well as a sunny day. If a shallow greenhouse requires only 1/5 as
much sunlight to be adequately heated, it will still receive a substantial
benefit on a cloudy day.
D. Optimum insulation: Any surface of the
passive solar greenhouse that is not essential for sunlight collection
must be insulated instead. The north wall is of no benefit for solar
collection and insulating the north wall is essential. The east
and west walls are of minimal solar benefit, and should also be
insulated. Finally, if the front wall is tilted back at 60 degrees,
winter, overhead sun exposure is of no additional benefit. Therefore,
the ceiling can be insulated. In this situation, nearly 4/5 of the
previous heat loss has been restricted merely by insulating the
top, back and sides of the greenhouse. If this greenhouse previously
lost its heat 1 hour after sunset, it will now require about 4 to
5 hours to lose that same heat. A substantial improvement.
Double pane glass or plastic: Double pane
glass reduces insolation about 13%. Glass has an R (insulation)
value of about R-0.6. This is equivalent to about 1/2" of wood.
However, with a slight air space, the R value of double pane glass
increases nearly threefold to R-1.6. The equivalent in wood requires
about 1 1/2". Compared to standard R-11 wall insulation, double
pane glass is only about 1/7th as effective. But in the example
above, with double pane glass, the heat, reduced by only 13%, would
now remain in the greenhouse for 10 to 12 hours instead of 4 to
5. A further, substantial improvement. Double pane glass can be
quite beneficial if non-essential, solar collecting surfaces have
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