The Passive Solar Greenhouse

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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 been insulated.

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