Planning and Building a Greenhouse (How to Build a Greenhouse)
As with any major purchase, a few key decisions must be made at the beginning of the buying process. Of course, the first decision to be made is the budget you are willing and able to spend on your greenhouse. Next, you should decide just how you are going to use it. Will it be to over winter plants, start vegetables, or grow orchids year round? Next you should decide what greenhouse will function the best for your needs. Do you need a portable structure, or a more permanent one? Do you want a maintenance free frame, or are you willing to take the extra care needed for a wood structure? Is tempered glass or polycarbonate the correct covering for you? What is the proper size greenhouse for you? Finally, looks should be considered. Will your subdivision or permitting agency have special requirements? Once you have gone through this thought process, you are ready to proceed. Of course we are always available to help guide you through this process.
The greenhouse should be located where it gets maximum sunlight. The first choice of location is the south or southeast side of a building or shade trees. Sunlight all day is best, but morning sunlight is most desirable because it allows the plant’s food production process to begin early; thus growth is maximized. An east side location captures the most November to February sunlight. The next best sites are southwest and west of major structures, where plants receive sunlight later in the day. North of major structures is the least desirable location and is good only for plants that require little light.
Deciduous trees, such as maple and oak, can effectively shade the greenhouse from the intense late afternoon summer sun; however, they should not shade the greenhouse in the morning. Deciduous trees also allow maximum exposure to the winter sun because they shed their leaves in the fall. Evergreen trees that have foliage year round should not be located where they will shade the greenhouse because they will block the less intense winter sun. You should aim to maximize winter sun exposure, particularly if the greenhouse is used all year. Remember that the sun is lower in the southern sky in the winter causing long shadows to be cast by buildings and evergreens.
Good drainage is another requirement for the site. When necessary, build the greenhouse above the surrounding ground so rainwater and irrigation water will drain away. Other site considerations include the light requirements of the plants to be grown; locations of sources of heat, water, and electricity; and shelter from winter wind. Access to the greenhouse should be convenient for both people and utilities. A workplace for potting plants and a storage area for supplies should be nearby.
Foundations and Floors
Some of the greenhouses either come with or have a base available to build them on. The others can be built on treated lumber bases, but we prefer a more permanent foundation such as a poured concrete, brick, or block similar to those in residential houses. The base is only referring to the perimeter of the greenhouse.
Permanent flooring is not recommended because it may stay wet and slippery from soil mix media. A concrete, gravel, or stone walkway 24 to 36 inches wide can be built for easy access to the plants. The rest of the floor should be covered by several inches of gravel for drainage of excess water. Water also can be sprayed on the gravel to produce humidity in the greenhouse. If a permanent floor is desired it should be provided with good drainage and an anti-slip surface.
These are only representative figures for the light transmission and U values. The actual values of the sheets will be dependent on the manufacturers of the glazing materials. Polycarbonate typically has a ten year warranty against yellowing and getting brittle, and against hail damage. We have seen sheets looking good at 15 years, and have had reports that 20 year old polycarbonate is still functioning. Polycarbonate is a multiwall material that will give you added insulation over single glass. The only down side that we have found to polycarbonate is the clarity of the material. The rib which runs through the center of the sheets will distort your view. You will see color of the leaves and flowers, but not clearly enough to distinguish which type of flower you are seeing.
Product Light Transmission / U- Value
6mm Polycarbonate Clear 80 / 0.645
8mm Polycarbonate Clear 80 /0.635
16mm Polycarbonate Clear 78 /0.545
Single clear glass 90 / 1.11
Double clear glass 82 /.55
Double clear glass W/ low E 75 /.35
The heating requirements of a greenhouse depend on the desired temperature for plants grown, the location and construction of a greenhouse, and the total outside exposed area of the structure. As much as 25 percent of the daily heat requirement may come from the sun, but a lightly insulated greenhouse structure will need a great deal of heat on a cold winter night. The heating system must be adequate to maintain the desired day or night temperature.
Solar-heater greenhouses were popular briefly during the energy crisis, but did not prove to be economical to use. Separate solar collection and storage systems are large and require much space. However, greenhouse owners can experiment with heat-collecting methods to reduce fossil-fuel consumption. One method is to paint containers black to attract heat, and fill them with water to retain it. However, because the greenhouse air temperature must be kept at plant growing temperatures, the greenhouse itself is not a good solar heat collector.
Calculating heating system
Heating systems are rated in British thermal units (Btu) per hour (h). The Btu of a heating system, Q, can be estimated using three factors;
1. A is the total exposed (outside) area of a greenhouse sides, ends, and roof are one unit; on a curved eave, the sides and roof are one unit; measure the length of curved support beam and multiply by the length of the house, on a free standing curve multiply by two. The curve ends area is 2 (ends) X 2/3 X width. Add the sum of the first calculation with that of the second.
2. U is the heat loss factor that quantifies the rate at which heat energy flows out of the greenhouse. For example, a single cover of glass has a value of 1.2 Btu/h x (ft x ft) x degree F (heat loss in Btu’s per hour per each square foot of area per degree in Fahrenheit). The values allow for some air infiltration but are based on the assumption that the greenhouse is fairly airtight.
3. (Ti – To) is the maximum difference between the lowest outside temperature (To) in your region and the temperature to be maintained in the greenhouse. For example, the maximum difference will occur in the early morning with the occurrence of a 0 to -5 degree Fahrenheit outside temperature while a 60-degree inside temperature is maintained. Plan for a temperature differential of 60 to 65 degree’s. The following equation summarizes this description Q = A x U x (Ti – To)
This discussion is a bit technical, but these factors must be considered when choosing a greenhouse. Note the effect of each value on the outcome. When different materials are used in the construction to the walls or roof, heat loss must be calculated for each. For electrical heating convert Btu/h to kilowatts by dividing Btu/h by 3,413. When using wood, gas, or oil in the greenhouse a fresh air inlet is recommended. Unvented heaters are not recommended.
Installing circulating fans in your greenhouse is a good investment. During the winter when the greenhouse is heated, you need to maintain air circulation so that temperatures remain uniform throughout the greenhouse. Without air mixing fans, the warm air rises to the top and the cool air settles around the plants on the floor.
Small fans with a cubic foot per minute air moving capacity equal to one quarter of the air volume of the greenhouse are sufficient. For small greenhouses, (less than 60 feet long) place the fans in diagonally opposite corners, but out from the sides and ends. The goal is to develop a circular (oval) pattern of air movement. Turn the fans off during the summer when the greenhouse will be needed to be ventilated.
The fan in a forced air heating system can sometimes be used to provide continuous air circulation. The fan must be wired to an on/off switch so it can run continuously, separate from the thermostatically controlled burner.
Ventilation is important even in cold weather, a greenhouse can get too warm on bright, sunny days. Ventilation is the exchange of inside air for outside air to control temperature, remove moisture, or replenish carbon dioxide. Several ventilation systems can be used. Be careful when mixing parts of two systems.
Natural ventilation uses roof vents on the ridgeline with side inlet vents (louvers). Warm air rises on convection currents to escape through the top, drawing cool air in through the sides.
Mechanical ventilation uses an exhaust fan to move air out one end of the greenhouse while outside air enters the other end through motorized inlet louvers. Exhaust fans should be sized to exchange the total volume of air in the greenhouse each minute.
The total volume of air in a medium to large greenhouse can be estimated by multiplying the floor area times 8 (the average height of a greenhouse). A small greenhouse (less than 5000 cubic feet in air volume) should have an exhaust fan capacity estimated by multiplying the floor area by 12.
The capacity of the exhaust fan should be selected at one-eighth of an inch static water pressure. The static pressure rating accounts for air resistance through the louvers, fans, and greenhouse and is usually shown in the fan selection chart.
Ventilation requirements vary with the weather and season. One must decide how much the greenhouse will be used. In the summer, 1 to 1.5 air volume changes per minute are needed. Small greenhouses need larger amount. In the winter, 20-30 percent of one air volume exchange per minute is sufficient for mixing in cool air without chilling the plants.
One single speed fan cannot meet this criterion. Two speed fans are better. A combination of a single speed fan and a two speed fan allows three ventilation rates that best satisfy year round needs. A single stage and two-stage thermostat are needed to control the operation.
A two-speed motor on low speed delivers about 70 percent of its full capacity. If two fans have the same capacity rating then the low speed fan delivers about 35 percent of the combined total. This rate ventilation is reasonable for the winter. In the spring, the fan operates on high speed. In the summer, both fans operate on high speed.
Some greenhouses are sold with a manual vent or ridge vent. The manual system can be a backup system, but it does not take the place of a motorized louver. Do not take short cuts in developing an automatic control system.
Shade Cloth is one of the most important elements in keeping a greenhouse cool in the summer. It may be installed inside hung from wires or on the outside either fastened on grommets on the edge of cloth with ropes or using a roll up and down system. It comes in 30- 90 percent light transmission. The most effective in heat reduction when installed on the outside but here it interferes with the vents. There are also blinds that can be installed with the same problem of interfering with the vents as with shade cloth on the outside. The blinds on the inside can be adjustable. Used with a properly designed ventilation system temperature inside the greenhouse should be within 3 to 4 degrees of the outside temperature.
Misting systems there are many systems available that may be used for cooling and or raising the relative humidity in the greenhouse. Relative humidity (RH) is a measure of how much water is dissolved in the air at a particular temperature expressed as a percentage. Generally, growth of many plants is relatively unaffected by RH between 45 percent and 85 percent. Plants growing at RH below 45 percent may grow slowly, have smaller leaves, require water more frequently, or develop burned leaf margins or leaf tips. Plants growing at RH above 85 percent are susceptible to fungal pathogens, especially if water condenses on the foliage.
Several conditions can occur in a greenhouse that result in problems caused by high or low RH. During the summer, high light, high temperature, and rapid air movement from fans can reduce RH to unacceptable levels. Shading to reduce light and temperature and using misters or evaporative cooling are the best the best solutions. It is also advisable to keep the greenhouse full of plants because plants generate a lot of RH.
Evaporative cooling systems are available in two configurations, small package evaporative coolers and fan and pad. The small package coolers have a fan and pad in one box to evaporate water, which cools air and increases humidity. Heat is removed from the air to change water from liquid to a vapor. Moist, cooler air enters the greenhouse while heated air passes out through roof vents or exhaust louvers. The alternative system is used in commercial greenhouses, places the pads on the air inlets at one end of the greenhouse and uses exhaust fans at the other of the greenhouse to pull air through the house. The evaporative cooler works best when the humidity of the outside air is low. On a hot sunny day, the moisture content of the atmosphere remains virtually constant. This means that the RH is lowest in the afternoon when the temperature is at its highest. And the lower the humidity, the better the evaporative cooling effect. In other words, the cooling effect is best when you need it the most. Size the evaporative cooler capacity at 1.0 to 1.5 times the volume of the greenhouse. Temperature inside the house can be as much as 10 to 15 degrees cooler than outdoors temperature with a properly designed system.
Automatic control is essential to maintain a reasonable environment in the greenhouse. On a winter day with varying amounts of sunlight and clouds, the temperature could fluctuate greatly; close supervision would be required if a manual ventilation system were in use. Therefore, unless close monitoring is possible, both hobbyists and commercial operators should have automated systems with thermostats and or other sensors. An alarm system is also very important if a system should fail.
Thermostats can be used to control individual units, or a central controller with one temperature sensor can be used. In either case, the sensor or sensors should be shaded from the sun, located about plant height away from sidewalls, and have constant airflow over them. An aspirated box is suggested; the box houses each sensor and has a small fan that moves greenhouse air through the box and over the sensor (s). The box should be painted so it will reflect solar heat and allow accurate readings of the air temperature.
Planning and Building a Greenhouse is an enjoyable process. Just be sure to do your homework.