Attached Solar Greenhouses

In colder areas, the greenhouse should have between 0.65 and 1.5 square feet of south-facing double glass for each square foot of adjacent building floor area to be warmed. In warmer areas of Virginia, use 0.33 to 0.9 square feet of glass for each square foot of adjacent floor area. This area of glazing will collect enough heat during a clear winter day to keep both the greenhouse and adjoining space at an average temperature of 60 to 70F during the day.

However, it is also important to utilize enough thermal mass to absorb direct sunlight and dampen interior temperature fluctuations. A thermal-mass wall for storing collected solar energy should be located between the greenhouse and the adjacent space with a large surface area of thermal wall exposed to direct sunlight. The wall can be constructed with several different materials at recommended thicknesses (Table 1).

Table 1. Thermal-Mass Walls

Material	Recommended Thicknesses
Solid Masonry	8 - 12 inches
Concrete	12 - 18 inches
Water in Containers	1 cu. ft. (7.5 gal.) for each sq.ft. of south facing glass

For maximum heat retention, the surface of the wall should be a medium or dark color and care should be taken to not block direct sunlight from reaching it. Small vents or operable windows should be located in the wall to allow heat from the greenhouse directly into the building during the daytime. Operable exterior vents and shading devices to prevent a heat buildup in the greenhouse in the summer are required.

An alternative to thermal-mass storage of heat is to actively take heat from the greenhouse during the day and store it in the building for use at night. Heat can be taken from the greenhouse by a fan and stored in a rock bed located in the crawl space under the floor of the building. The advantage of this system is that space is not lost in the greenhouse to a thermal-mass wall. The greenhouse should receive enough heat back from the building at night through the common wall and glass to keep it at a temperature average between the indoor heated space and outdoor temperature. However, it is important to use operable windows or a door between the greenhouse and building to ensure that during periods of extremely cold weather the greenhouse can receive direct heat from the building to keep freezing temperatures from affecting the plants.

For adequate passive heat transfer from the rock bed to the adjacent space to be heated, 50 to 75 percent of the floor must act as a heat transfer area. The system moves warm air to the rock bed and returns cool air to the greenhouse from the bottom of the rock bed. In temperate climates, 3/4 to 1-1/2 cubic feet of fist-sized rock for each square foot of south-facing greenhouse glass should be used.

Solar greenhouses in some areas require double glazing to prevent undue heat loss. Insulating glass may be used as well as plastic products such as fiberglass or polycarbonate sheets. Plastic films are best used as the second layer of glazing inside the glass or rigid plastic glazing. The cost of glazing may account for nearly half the total cost of the greenhouse structure. The average cost of a solar greenhouse is, in most cases, one third that of standard construction.

When the primary function of the greenhouse is to heat an adjacent building, taking heat from the greenhouse by mechanical means and storing it for use in the building will increase the efficiency of the system. However, the greenhouse will drop in temperature to about 40 to 45F at night in temperate and cool climates and considerably lower in very cold climates. Additional sources of heat will be necessary if plants are to be healthy and productive. Most tropical plants will drop their leaves if subjected to long periods below 60F, and few vegetables will produce a yield at low temperature.