ABSTRACT
When selecting a backyard greenhouse consider first the size� The size of greenhouse best suited for your needs depends upon what you wish to grow and your weekly consumption� From past experience with manufacturing greenhouses, I found that a 10½ ft by 12 ft greenhouse could easily supply a family of four to six persons� Most clients soon started using their greenhouse for rejuvenation of their indoor house plants so took some of the greenhouse space for them� Our 8 ft by 12 ft greenhouse had about one-third less growing space since only two beds would fit instead of three with the 10½ ft wide model� In terms of cost and growing space, we found that the 10½ ft by 12 ft model was the best value for the hobbyist�
Hobby greenhouses may be free-standing or lean-to structures (Figure 19�1)� The choice here depends upon the location and space available in your backyard� A lean-to structure can be attached to a building wall such as your garage or onto a tall wooden fence� Attaching it to a building has the advantage of getting some insulation from the building� Keep in mind when locating such a lean-to structure that it must receive adequate natural light so the location must not be in a shaded area or on the north side of a building that would prevent natural sunlight from entering� On a fence do not locate it on a south-side fence where the fence would restrict direct light to the greenhouse�
The best location in your backyard is where the greenhouse can receive sunlight most of the day (Figure 19�2)� To achieve this locate it in the middle or at one side where no shade is cast by fences, buildings, or trees� If your backyard is fully treed or large trees in neighboring yards shade the light entering your yard, the trees will have to be thinned out by removing lower branches or take out the trees� If the shade is cast by neighboring trees you will need cooperation from the adjacent property owners to prune their trees or perhaps remove some of them� You may be able to convince them to do so if you are willing to share some of your vegetables with them�
A free-standing type of greenhouse is most efficient in receiving sunlight from the east, south, and west cardinal directions� It also gives you the greatest growing area for its size� If possible, orient the greenhouse north-south� That is, the ridge of the structure is oriented north-south� This will give the best light for vine crops as light will enter the rows of plants also oriented north-south� If oriented east-west, there will be mutual shading of plant rows with the south facing row receiving most light as it will cast
shadows on the other rows, especially during the fall, winter, and early spring months when the sun’s angle of incidence is much lower than during the summer months� Pick the spot in your backyard that has best sunlight exposure to maximize crop production�
Whether the greenhouse is a lean-to or a free-standing structure, there are two basic shapes� The traditional gable with straight eaves where the sides meet the roof and the gothic shape with curved eaves where the sides bend to form the roof (Figures 19�3 and 19�4)� While you may construct a backyard greenhouse with a polyethylene covering, it lacks the beautifying appearance for your backyard compared to the clarity of glass or polycarbonate� Greenhouses with curved eaves use polycarbonate to enable forming the bend at the eave location� Glass is perfect for the traditional gable-shaped greenhouses, but polycarbonate is also suitable� Polycarbonate has the advantage of higher insulation� If you wish to grow year-round in your greenhouse it must withstand wind, snow loads, and resist ultraviolet (UV) breakdown of the covering� Polyethylene has a limited life expectancy due to UV light turning it brittle as the polymers are damaged� The last thing you want in your backyard is an eye-sore structure that turns milky to a yellowish color with age of the covering� Also, if the covering has to be replaced in the cold winter months you risk losing all of your plants� For these reasons, invest a little more in a permanent structure that has a nice appearance and will withstand the weather all year�
Some do-it-yourself (DIY) papers suggest that you construct a greenhouse using plastic polyvinyl chloride (PVC) pipe� It combined with wooden end-framing allows you to construct a Quonset style house, but this type of structure will not withstand snow loads or strong winds� These types of houses are okay as cold frames using polyethylene covering� They are really only cold frames suited to starting your bedding plants early in the spring� They are not greenhouses with heating and cooling systems that permit you to grow year-round�
You may construct the greenhouse of treated wood framing, but that is not as permanent as aluminum framework� With wood framing you could cover it with polycarbonate, but not easily with glass as that would require a lot of framing for the glass panels� In the end, you may not save much money building it yourself� I recommend that you purchase a hobby greenhouse in the market as there are many sizes and shapes available to fit your budget and also most are constructed of aluminum framework so are permanent for many years� In addition, the aluminum frame comes in white, green, or brown so you can pick a color that adds to the overall appearance of your home and backyard� Galvanized steel structures are fine, but may develop rust spots over time� I do not think that they are much cheaper than aluminum structures� The aluminum will retain its beauty over time so will always be an asset to the surroundings of your home�
The frame of the greenhouse must be strong enough to support your plants whether they are hanging baskets or vine crops supported by string to an overhead cable attached to the greenhouse superstructure� It must also support the covering and equipment such as heaters and fans� If it is wood, the structural members or rafters, as in your house, must be sufficiently close together to prevent any sagging of the covering material, especially in the winter under snow loads� You may need to attach cross members to the rafters in several locations to strengthen the support of the covering material, such as glass or polycarbonate� Additionally, wooden structures intercept light causing a lot of shading, especially in the winter when you need all the sunlight available� If you use wood, be sure to paint it with white paint, preferably an oil-based paint to resist the constant moisture present in the greenhouse� Aluminum framing naturally reflects light and at the same time the dimensions of the ribs and purlins (cross members) are much smaller so do not cast as much shadow as wood framing�
There are many sizes of both lean-to and free-standing greenhouses� The most common dimensions for lean-to greenhouses are in widths of 6 ft, 8 ft, and 10 ft with
lengths of 6 ft, 8 ft, 10 ft, and 12 ft, with increments of 2 ft to 20 ft or longer� For the best utilization of the growing space choose a 10 ft width with 12 ft in length or more� Prices for these houses range from $3000 to $6000� Prices also depend upon the covering material� For example, double tempered glass is the most expensive, elevating prices to $15,000 for the larger structures� Most come with two roof vents operated by solar vent openers that do not require electricity� These vents open as sunlight builds up heat around a cylinder mechanism that pushes open the vent�
Free-standing greenhouses come in widths of 8 ft, 10 ft, and 12 ft and in 2-ft increments to 20 ft� Similarly, lengths are available in 2-ft increments starting at 10 ft up to 20 ft and longer� I feel the most common size for a family of four to six members is 10 ft by 12 ft or 10 ft by 16 ft� That will produce sufficient vegetables to please the entire family� Prices for these greenhouses range from $4000 for an 8 ft by 12 ft to $6000 for a 10 ft by 12 ft and higher� The covering material also influences prices as mentioned earlier�
The more light let through (transmissibility) the covering, the better for plant growth� Coverings include glass (single or double tempered); rigid plastic (polycarbonate)- single, double wall, triple wall, five wall; and polyethylene-single or double layer with dead air space between the layers� The covering type influences not only the price, but most importantly the insulation capability�
Glass is the favorite covering for gable style greenhouses� It is very attractive, showing plants growing visible from the outside and offers lots of light, especially during the winter� Single and double tempered, unbreakable glass is available� The double glass retains heat better than the single layer� If you purchase a single layer glass greenhouse you can insulate it with a layer of polyethylene on the inside during the winter to retain heat� This, of course, will reduce some of the incoming light and make the greenhouse less transparent so it may lack the visual appeal of the glass alone� I do not like this method to retain heat as the inside poly layer will also cause more condensation that can drip on your plants and promote potential diseases� It is far superior to purchase the tempered double glass covering� One thing that can be done to reduce some heat loss is to install 2″ thick Styrofoam on the greenhouse sides below the bench level� In most cases the hydroponic systems for lettuce nutrient film technique (NFT) and vine crops use raised trays so that they are at least 2 ft above the floor� Thus, the Styrofoam will not significantly reduce light at the crop level if it is fitted to 2 ft high around the inside of the greenhouse walls� Regardless of the covering, the use of the Styrofoam along the base walls will save additional energy loss�
Polycarbonate is my choice of covering as it is available in different wall thickness to reduce heat loss� Polycarbonate is available in 4 mm, 6 mm, and 10 mm twin wall and 8 mm and 16 mm triple wall thicknesses (Figure 19�5)� A 16 mm 5-wall polycarbonate gives the best insulation, but its light transmission is reduced to 62%� This would not be good for regions having very dark winters with little natural sunlight� For example, in the rainy areas of the West Coast where minimum winter temperatures are not that low, use the twin-wall polycarbonate� In Central regions and the East Coast where extremely cold periods occur during the winter, but in most cases under sunny conditions, the thicker polycarbonates of triple wall and five walls would be okay�
Light transmission of polycarbonate is diffused and twin-wall polycarbonate light transmission is 10% less than that of glass� The effectiveness of thermal insulation of a covering is measured as an R-value the same as insulation in your home� The higher the R value the greater the heating and cooling efficiency� Here are a few R-values for different greenhouse coverings:
¾"
⅜"
⅜"
¼"
Another aesthetic plus for polycarbonate is its ability to be bent to form the curved eaves making the gothic shape of a greenhouse� This has a very attractive modern look�
The greenhouse door should be of aluminum and tempered glass window with a screened opening that allows ventilation during the hot weather� The sidewall height should be at least 6 ft to allow the training of vine crops up to the roof height� Additional height is another advantage of the curved eaves of the gothic shape� The ridge height should be about 8-9 ft� To raise the greenhouse height you may place it on top of a brick or masonry concrete block foundation� This can raise the greenhouse up from 16″ to 24″� Just realize that the door must then be positioned this same height lower than the greenhouse base so that there is not a step into the greenhouse�
The greenhouse should be easily accessible and be located within reasonable distance from a source of water and electricity� A PVC water line will have to be buried below the expected frost depth of winter months, usually at least 3 ft� A 1″ diameter main will be adequate� When digging the trench add an underground buried cable that will carry a 220 volt, 80-100-amp service from your house to the greenhouse� These will be stubbed at a location on the back wall of the greenhouse, so a good plan should be made to get the exact location of the greenhouse� Once the utilities are in place, lay a landscape weed mat over the entire area that the greenhouse will occupy and extend it several feet beyond the perimeter of the greenhouse� This is to prevent weed growth within and immediately adjacent to the greenhouse�
In the center of the weed mat construct a treated 2″ × 4″ wooden frame having the edges of the wood laying on top of the weed mat (Figure 19�6)� For example, for a 10 ft × 12 ft greenhouse make a wooden frame 12 ft × 14 ft� Attach the corners of the frame with metal brackets screwing them into the wood with stainless steel screws� Level and secure the frame in place with 1″ × 2″ × 18″ treated wood stakes or rebar at the corners and along the sides� Be sure that the frame is square by measuring the diagonals to make them equal length� Place 1″ thick Styrofoam on the bottom for insulation� Then fill the entire framed area with pea gravel to give the greenhouse a good base with free drainage� The greenhouse will then be set on top of the pea gravel� If you wish to get a very nice clean look, install stepping stones in the greenhouse and as a path to the greenhouse from your home�
Some references recommend that the top ridge of the greenhouse be oriented east to west� This will provide maximum sunlight exposure during the winter when the angle of incidence of the sun is very low; however, this applies only to low-profile crops and potted ornamental house plants� For vine-crops, orient the greenhouse ridge north-south so that the sun will shine down the rows of plants and not just maximize light on the south row as it will shade the others�
When erecting the greenhouse, fasten its base to 6″ × 6″ treated wooden timbers or make a foundation with several layers of 8″ × 8″ × 16″ concrete blocks� The blocks must sit directly on the weed mat, not on top of the pea gravel fill� The use of blocks will raise the height of the greenhouse� If you use treated wooden timbers for the base install a 10 mil polyethylene barrier between the base and the aluminum greenhouse sill to prevent corrosion of the aluminum�
Do not use old railway ties for the base as they are treated with creosote that gives off fumes, especially under warm temperatures, that are toxic to plants� Pentachlorophenol was an early wood preservative, but is now restricted in its use and is not approved for residential application� It gives off damaging fumes so do not use it for greenhouse wood preservation as it acts as an herbicide� Cuprinol, copper naphthenate, is probably the most commonly used wood preservatives used by gardeners� It is recommended for use on sheds, fences, garden buildings, decking, and garden furniture as a wood preservative� It is approved and safe to use on these structures as well as for greenhouses� I would, however, recommend painting the treated wood to seal it and get good light reflection�
The greenhouse frame consists of ribs set onto a sill and purlins and girts (horizontal bars) that bolt to the ribs (Figure 19�7)� If this is a prefabricated kit, the sills will screw or bolt into the base you prepared of timber or concrete blocks� Start with the end sides (traditional) or ribs (gothic) and bolt them with the purlins at the eave height (traditional) or ridge in the case of the gothic style� Then attach the rest of the cross members (purlins) to the gable ends� Square the structure and tighten the bolts of this outside frame before filling in the rest of the studs (traditional) or ribs (gothic)�
Most structures will have knee braces that keep the house from folding in accordion motion when strong winds push on the gable ends� They attach to the gable end ribs and go on an angle to the sill� They will also keep the house square while attaching the remaining ribs and purlins� With the traditional form, the rafters are attached to the studs at the eave height and are secured at the top with the ridge purlin� With the gothic style, the ribs are both the studs and rafters in one piece and so are attached with the ridge purlin and others (usually two to three) in the roof area� Once you complete the basic frame with the entire purlins, frame the gable ends with the studs� If you plan on using an exhaust fan ventilation system, frame the area where the fan is to be installed on the gable opposite the door, and on the other gable, where the door is located, frame two openings for the automatic shutters� The exhaust-fan frame should be about 5-6 ft above the floor (at the eave level) and the automatic shutters at about 4 ft high� Some backyard greenhouse builders may pre-assemble the gable ends� That will save lots of time in the erection of the house� If the greenhouse has roof vents assemble the framework for them�
The next step is to install the polycarbonate or glass covering� The following description is for the installation of polycarbonate as glass is more specialized and requires that you follow the installation instructions exactly as provided by the greenhouse manufacturer� In any case, if you purchase a pre-fabricated greenhouse, it will come with full step-by-step instructions for its assembly� Complete all the supporting structure (superstructure) and painting before installing the covering� Orient the sheets with the ribs (cells) parallel to the rain flow (slope) and the outside face is the side of the panels marked “UV side out” (Figure 19�8)� This orientation applies to multi-walled and corrugated polycarbonate� Polycarbonate can be cut with a jigsaw
with a fine-toothed blade� Remove any dust in the flutes (channels) with an air compressor or vacuum cleaner� Remove the protective film a few inches back from the edges where you are working with its installation� After installation is complete remove all the protective film� Seal the upper edge of the sheets with a special aluminum tape and the bottom edge with a polycarbonate vent tape that allows moisture to exit but prevents debris from entering into the flutes (Figure 19�9)�
Polycarbonate expands and contracts with temperature, so when fastening it to the framework use slightly larger holes than the diameter of the fastener to allow
some movement� Set the first sheet on square and screw every fourth corrugation to each purlin of the greenhouse structure� Before attaching the next sheet run a bead of caulking compound, such as silicone, along the last corrugation where one corrugation of lap takes place with the next sheet to seal the laps� At the sill insert a closure strip (foam having configuration of the corrugation) as you screw through the corrugated sheet into the sill� Likewise, at the roof peak insert a closure strip as the sheets are held in place with the ridge bar (Figure 19�10)� If the greenhouse is of a traditional shape, closure strips will need to be placed at each end of the panels� At the sill, purlin where the sidewall meets the roof (eave), the lower end of the panel of the roof where it overlaps the sidewall and at the peak use closure strips to seal the panel ends� Allow at least 2″ of overlap of the roof panels beyond the sidewall panels at the eave to make water flow past the sides of the greenhouse� All fasteners should be stainless steel screws or teck (self-drilling) screws (Figure 19�11) that have neoprene washers under the heads to seal the screw holes as they are tightened to the polycarbonate� Be careful not to tighten the screws excessively causing the polycarbonate to become indented underneath�
If the greenhouse is a beamed structure having 2″ × 4″ rafters without purlins, such as in the case of a wooden structure, spacer blockings will have to be fastened between the rafters (Figure 19�12)� The spacing of the rafters and spacer blocks vary with the polycarbonate sheet thickness and the snow load, for example, using 8 mm thick polycarbonate panels, space rafters every 2 ft with blocking spacers at every 6 ft going across the rafters for a roof load of 60 lb per sq ft� With 16 mm thick polycarbonate sheets, rafters may be spaced at 4 ft with blocking every 2�5 ft for a 60 lb load� If the rafters are at 2-ft centers with 16 mm thick panels no blocking is needed� The panel width is 4 ft� Keep the spacer blocks 3/8″ below the rafter face to permit condensate movement past the blocks�
If the greenhouse superstructure is made without purlins, use specific fastener bars to join the polycarbonate sheets (Figure 19�13)� These aluminum base fastener bars are attached with self-drilling screws (teck screws) about every foot to the center of the rafters� Work one sheet at a time going across the structure� Slide the sheet under the first fastener bar and then go to the next rafter and slide the fastener base under the sheet and attach it to the rafter� Repeat this with all sheets until complete; then attach the caps to the base fastener bars using a rubber mallet� The caps lock the panels into the fastener bars� Alternatively, you may use just a cap and screw it directly into the rafters without a base bar� Attach the panels to the spacer blocks between the rafters with a screw, washer, and 3 8″ thick by 1″ diameter neoprene spacer� This permits the condensate moisture to flow on the inside of the polycarbonate sheet without it touching the cross blocking so drip will not occur within the greenhouse� To seal the ridge use a ridge cap formed from aluminum� Seal the panels
into the ridge cap using a foam gasket similar to the closure strips for the corrugated material, but this weather seal is square without corrugations�
If the greenhouse is of aluminum or other metal framework, purlins will be bolted across the ribs� The polycarbonate sheets are attached to the purlins using base and cap bars (Figure 19�14)� With self-drilling stainless steel screws fasten the
base connecting bar to the purlins underneath the sheet edge� Place these bars at spacing equal to the width of the polycarbonate sheets� Attach the base connecting bars working across the greenhouse as the polycarbonate panels are added to get the correct spacing of the bars� Continue adding all of the sheets and the caps for the connecting bars as you proceed� Underneath the base bar use a shock-absorbing support such as a wooden block as the cap is connected to the base using a rubber mallet when the panel is in position� Remove the protective film from the polycarbonate
and drill screws with metal and neoprene washers to further fasten the panels to the eave purlin at the bottom edge of the panels if it is a gable shape greenhouse� Use a no-drip spacer neoprene washer between the sheet and purlin to allow condensate drainage� The connecting base bars have slightly raised edges to keep the panels from touching the purlins and thus permitting movement of condensation moisture� Once the gable ends, sides, and roof have been constructed caulk the edges of the gable ends where they meet the sidewall sheets�
The various fastening bars, caps, and ridge cap will be supplied by the greenhouse manufacturer; otherwise, if doing it yourself these bars should be available where you purchase the polycarbonate panels� Similarly, foam closure strips, all hardware come with the greenhouse kit or purchase it with the polycarbonate sheets�
When constructing a gothic style house, where the polycarbonate is bent from the walls to the roof, keep in mind that the polycarbonate sheets have a minimum radius that they can be bent without buckling� Table 19�1 shows the smallest cold bending radius for various panel thicknesses�
Here I am presenting an example of the construction of an 8 ft × 12 ft backyard greenhouse� These greenhouses were constructed of aluminum framing and corrugated fiberglass panels as at the time polycarbonate panels were not common and very expensive� That was in the mid-1980s� The procedures, nonetheless, are the same as when they were constructed of corrugated polycarbonate� For twin-wall or multi-wall polycarbonate, the general procedure is the same with some modifications as to the use of the aluminum base bars and caps that attach to the purlins, which was explained earlier� The use of these fastener bars makes the installation simpler than previously as we did in the past with Resh Greenhouses Ltd� using pop-rivets and tech screws onto the purlins directly�
We built our greenhouses with prefabricated finished gable ends to make the erection simpler� However, if the gable ends are not completely finished with cladding, a door, and so on, this can be done after the framework is assembled as was explained earlier� Today, use the twin-or multi-walled polycarbonate panels to cover the entire greenhouse-sides, walls, roof, and end-wall gables� With the base pad ready prepared (Figure 19�6) as discussed earlier, position the treated wooden sills, level and square them, then fasten them with aluminum brackets at the corners� Assemble the ribs, purlins, ridge cap, and gable-end framing, square the entire structure, and tighten all bolts (Figure 19�15)� Next, place all of the polycarbonate
TABLE 19.1 Bending Radii of Polycarbonate Sheets
panels, starting by pushing them under the ridge cap and bending them down over the purlins and attaching them to the sills (Figure 19�16)� Use closure strips at the ridge cap and inside on the sills� Secure the panels to the sill with stainless steel wood screws having washers with neoprene seals underneath them� Screw into the wooden sill through the closure strip on the inside at 6″ spacing across the panels� With completion of the gable end panels caulk the joints between the gable ends and sidewalls using silicone or other outside weather-resistant caulking (Figure 19�17)� The next steps include installing the glass and aluminum door, fan(s), and inlet shutter(s) for ventilation if doing this yourself and not a purchased pre-assembled gable� The completed greenhouse with hydroponic beds, fan, and inlet shutter is shown in Figure 19�18�
The greenhouse environment must be kept optimum to maximize plant yields� The two most important factors to control are temperature and light� Temperature control includes heating, cooling, ventilation, and air circulation� The heating requirements depend upon the crop grown, location of the greenhouse, and its type of construction materials� The nature of the covering material and its R-value determines the amount of insulation capacity of the structure� R-values were discussed earlier� The following example demonstrates the calculation of heat loss in a typical greenhouse�
The following example is to calculate the heat loss of a backyard greenhouse 10 ft wide (W) by 12 ft long (L) by 9 ft to the roof peak (H) having 6 ft high sidewalls (S)� It is constructed of aluminum framing with 16 mm thick triple-wall polycarbonate cladding situated in a climate where the minimum extreme winter temperature would be about 0°F (−18°C)� If under the extreme minimum temperature we wish to maintain 50°F (10°C), the temperature differential is 50 F (28°C) degrees (T)� So, the heating system must be capable of raising the temperature of all the air volume within the greenhouse 50°F (28°C)� The next step is to calculate the total exposed surface area (A) of the greenhouse� Refer to the diagram (Figure 19�19) for the dimensions of the greenhouse� The following websites give a simplified explanation for calculating heat loss for various shapes of greenhouses and have calculators that will do all the mathematics for you:
www�gothicarchgreenhouse�com/heating�htm www�gothicarchgreenhouse�com/Greenhouse-Heater-Calculator�htm www�gothicarchgreenhouse�com/Greenhouse-Surface-Calculator�htm www�littlegreenhouse�com/heat-calc�shtml www�littlegreenhouse�com/area-calc�shtml www�sherrysgreenhouse�com/oldsite/GHheating�html
Use the following equation:
1� Exposed Surface Area (A) = Area of Gables + Area of Walls + Area of Roof
Area of Two Gables: This is a traditional gable-shaped greenhouse� To calculate the area of the gables, we must calculate the bottom rectangular surface plus the gable-shaped component� For that calculate the area by use of triangles� Each upper portion of the gable is made up of two triangles (Figure 19�19)�
The area of a triangle is: A = ½ base × height = ½ × 5 ft (triangle base is ½ of the gable width) × 3 ft = 7�5 sq ft
For the two triangles: 2 × 7�5 = 15 sq ft Add the bottom portion: 10 ft × 6 ft = 60 sq ft For two gables: 2 × (15 + 60) = 150 sq ft Area of Walls (2): 2 × (6 ft × 12 ft) = 144 sq ft
Area of Roofs (2): For this we use the triangle to find the length of the hypotenuse (longest side) using the Pythagoras theorem equation: a2 + b2 = c2; where c = the length of the hypotenuse (see diagram in Figure 19�19)�
c2 = a2 + b2 ⇒ c2 = 32 + 52 = 9 + 25 = 34: Therefore, c is the square root of 34: 5�8 ft� You may use a calculator to get the square root of a number� The longest side is 5�8 ft, which is the end width of the roof (R)�
The total roof area is (two sides): 2 × L × R = 12 ft × 5�8 ft = 139 sq ft Total Exposed Surface Area (A): 150 sq ft + 144 sq ft + 139 sq ft = 433 sq ft
2� Determine the greenhouse construction factor (C):
C = 1/R-value The R-values were given in the table earlier for the various covering materials�
For 16 mm triple polycarbonate R = 2�5� Therefore, C = 1/2�50 = 0�40
3� Determine the wind correction factor (W): Refer to the following table�
For 30 mph use: W = 1�12
4� Determine heat loss in British Thermal Units (BTU):
BTU = T × A × C × W = 50 × 433 × 0�40 × 1�12 = 9700 BTU
There are BTU calculators on the Internet websites listed earlier� You can simply plug in the dimensions of the greenhouse, the R-value for the covering, and the temperature differential and they will calculate the total BTU loss� To determine the projected electrical cost for heating convert the BTU to Watts as follows:
Watts/h = BTU/3�413� Therefore: 9700 BTU/3�413 = 2842 watts or 2�84 KWH (kilowatts/hour)
Cost of operating under the most extreme cold conditions (assume cost of power is $0�10/KWH):
2�84 KWH × $0�10 = $0�28
However, note that this would be under the most extreme minimum temperature� Normally, this period would be only a few hours during the coldest day of the year� To project an annual cost of heating the greenhouse over a year, you will need to visit the website of the National Weather Service and look up the weather data for your specific location� The website is www�ncdc�noaa�gov (National Climatic Data Center)� Click on “Most Popular Data” and go to “4� 1971-2000 US Climatic
Normals Products;” then go to “1971-2000 Normals Products Page” and finally to “Monthly Station Climate Summaries (CLIM20)�” Then type in your state and city for your area records�
While you must design your heating requirements on the minimum extreme temperatures, when calculating the monthly predicted costs adjust the calculations according to the climate data of your area� The following example is for Seattle-Tacoma (airport), Washington (Climatic Div� WA 3) from the weather data charts (Table 19�2)� The minimum extreme (using records from 1971 to 2000) for January was 0°F (−18°C), the minimum monthly mean extreme temperature was 34�8°F (1�6°C), and the daily minimum for January was 35�9°F (2�2°C) over the period�
To calculate the projected heating cost, use the daily mean for each month and add all of the months or use the annual mean and the temperature differential needed to heat the greenhouse to 70°F (21°C) (65°F or 18°C night, 75°F or 24°C day)� For example, for January using the daily mean of 41°F (5°C) the temperature difference is 70°F-41°F = 29 F°(16 C°)�
Then, multiply the previously calculated BTU by the fraction of monthly temperature difference divided by the extreme temperature difference�
For example, for January: 9700 BTUH × 29/50 = 5626 BTUH The cost of electricity for the month of January: 5626/3�413 = 1648 Watts/h or
1�65 KWH For 24 h: 24 × 1�65 = 39�6 KWH or about 40 KWH Cost per day at $0�10/KWH: 40 × $0�10 = $4�00 Total cost for the month of January: 31 × $4�00 = $124�00
Do this for the rest of the months of the year or use the annual daily mean (52�3°F) as follows:
TABLE 19.2 Recorded Temperatures over the Period 1971-2000 for Seattle, Washington
Factor to multiply the extreme temperature BTUH by is:
Temperature difference of 70°F-52�3°F = 17�7 F degrees� 9700 BTUH × 17�7/50 = 3434 BTUH WH is 3434/3�413 = 1006 watts/h or 1 KWH KWH per day: 24 × 1 = 24 KWH Cost per day: 24 KWH × $0�10 = $2�40 Annual Projected Heating Cost: 365 × $2�40 = $876�00
Solar radiation is the force causing transpiration in plants� As the sunlight shines on the leaves of the plants, their temperature rises causing the stomates to open and transpiration to occur� As the water evaporates from the leaves the leaves are cooled�
Only about 70% of the light passes through the greenhouse roof cover, the other 30% is reflected� Plants absorb about 70% of this available light and re-radiate the other 30%� Therefore, plants absorb about 50% (70% of 70%) of the incoming sunlight using this for transpiration as shown in Figure 19�20� With a mature large crop, only 20% (30% of 70%) of the solar energy is left to heat the air� When there is no crop in the greenhouse, about 70% of the available total solar radiation is available to heat the greenhouse air�
During high solar light from spring through early fall, temperatures will rise in the greenhouse due to the “greenhouse effect�” This effect is from the wavelength of the light changing as it enters the greenhouse through the covering and the heat component cannot escape� As a result, the air temperature in the greenhouse rises�
It soon exceeds the optimum maximum temperature range for the plants causing the stomates to close and restrict photosynthesis, which reduces growth and yields� To reduce excessive heat build-up, the air in the greenhouse must be forced out and be replaced by cooler outside air� Natural ventilation with roof vents will assist in the air exchange� This is the least expensive method of cooling as the vents can be operated without electricity using solar vent openers� A black cylinder filled with paraffin wax expands with heat and pushes a piston that opens the vents automatically with metal arm linkage as the temperature changes (Figure 19�21)� These vent openers can operate roof and side vents� There are a number of different sizes of vent openers (about $50) that are capable of opening vents 12-15″ with vents weighing from 15 to 25 lbs and more� Operating temperatures to start opening can be adjusted from 60°F to 78°F (15�5-25�5°C) and maximum opening at 86-90°F (30-32°C)�
Side louvers to permit cool fresh air to enter near the base of the greenhouse can also be operated by the automatic solar vent openers� With installation of both roof vents and inlet louvers, the movement of air will flow from the lower cooler air entering below and rising as it is heated within the greenhouse to escape through the roof vents (Figure 19�22)� Wind passing over the open roof vents also creates a pressure difference that acts like a vacuum and sucks the heated air out of the vents� This movement of air with the temperature gradients exchanges the air within the greenhouse adding carbon dioxide as well as maintaining optimum temperatures�
If you live in a region where summer temperatures are very high, you may install an exhaust fan and inlet shutters to force the air out of the greenhouse (Figure 19�23)� This forced ventilation should exchange the entire air volume of the greenhouse within 1 min to minimize temperature gradients from one end to the other� If the
exhaust fan is operating, roof vents must be closed to prevent the air from short circuiting down from the roof vents directly outside the fan without being forced through the crop� To determine the size of the exhaust fan and shutters needed, you must calculate the total air volume of the greenhouse and express it as cubic feet of air per minute (CFM)� The total volume of air is the width × length × height of the greenhouse� The industry uses a factor of 10 ft times the width × length or 12 ft times the width × length for more southerly locations� Using our example earlier for heating, the greenhouse was 10 ft × 12 ft so the total CFM calculation is: 10 ft × 12 ft × 12 ft = 1440 CFM� Or we can take the total area of the gable end of the greenhouse
and multiply it by the length (12 ft) as follows: The gable end area was 150 sq ft� Multiply that by the 12-ft length: 150 × 12 = 1800 CFM�
Now refer to charts of various sizes of fans and shutters to determine the model of exhaust fan needed� Then, match the total CFM output of your selected exhaust fan to the CFM ratings for the shutters� There will be one exhaust fan and two inlet shutters� Shutters are square in dimension starting at the smallest ones 12″� The next are 16″, 18″, 20″, 24″, and up to 54″� A 16″ shutter has a capacity of close to 900 CFM, so two of those would be adequate� The next size, 18″, is rated at 1125 CFM� When purchasing an exhaust fan, it is best to have a two-speed motor activated by a two-stage thermostat so that as the temperature rises in the greenhouse the first thermostat setting operates the slower motor speed and as temperature continues to rise the second setting would initiate the faster motor speed� Set the two ranges at least 5°F (2�8°C) apart�
Another method of cooling is to prevent some of the direct sunlight from entering the greenhouse by use of a shade cloth over the top of the greenhouse� Usually, 35% shade is adequate� This would only be used during the hottest time of the year when most sunlight occurs� You can also white wash the outside of the covering with a special white paint that can be easily removed by washing� It generally has a life expectancy of 3-4 months so that it is easily removed by late fall when the sunlight is no longer intense and when you then need as much light as possible as the season progresses into winter�
Evaporative cooling pads are also effective in lowering very high temperatures under low ambient relative humidity (RH)� Evaporative cooling pads would be positioned on one end of the greenhouse opposite the exhaust fans� In that case you would need two exhaust fans near the door end and have the cooling pad on the opposite gable end� Still another system is to use high-pressure fogging� The fogging misters are mounted within the greenhouse� Their efficiency, like the evaporative pads, depends upon low ambient RH� I do not recommend these evaporative cooling systems for small greenhouses as they are costly�
An article on greenhouse ventilation by the University of Connecticut states that fan ventilation can consume from 0�5 to 1 kilowatt hour (KWH) per square foot of greenhouse area per year� So the potential annual cost for the 120 sq ft hobby greenhouse example would be: 120 KWH × $0�10 = $12�00�
For more accurate calculation of a given location, use the number of days of sunshine obtained from the weather data� During those sunny days use about 8 h per day as that is when the light would be most intense� This would be only for the months from May through September when day length is longest and temperatures highest�
For our example greenhouse, a 16-18″ exhaust fan would give adequate ventilation of 1200 CFM� This type of fan has a 1/10th HP (horsepower) motor that uses 110/120 volts (V) and draws 1�5 amperes (I) of current� The power law is: P (watts) = Voltage (V) × Current (I)� Therefore, the power utilized per hour of fan operation is: P = 110 × 1�5 = 165 watts per hour�
To operate this fan for 8 hours it would consume: 8 × 165 watts = 1320 watts or 1�32 kilowatts per day or 1�32 KWH per day� If the cost of power is $0�10 per KWH, the daily cost would be 1�32 × $0�10 = $0�13� If the number of full sunlight days is 20 days per month over this period May through September (5 months), the total power consumption for the period would be 5 months × 20 days × 1�32 KWH = 132 KWH� The cost based on $0�10 per KWH would be $0�10 × 132 KWH = $13�20�
A more effective cooling system for a backyard greenhouse is a greenhouse evaporative swamp cooler (Figure 19�24)� This combines the exhaust fan and cooling pad within one unit� It draws hot outside air through evaporative cooling pads within the unit using a blower� As the water evaporates it takes the heat out of the air, resulting in pushing cool air into the greenhouse� This is a positive pressure system whereby the cooler pushes air into the greenhouse and allows it to exit through roof vents or exhaust shutters (Figures 19�25 and 19�26)� When using an exhaust fan, it sucks air into the greenhouse through inlet vents and expels the hot air out� The positive pressure method is much superior as it also assists in preventing insects from entering the greenhouse�
Note that the evaporative cooler is mounted close to the base of the greenhouse to bring in the coolest air possible and the exhaust shutters are mounted high near the height of the eaves to expel hot air (Figure 19�26)� The exhaust fan, on the other hand, is installed high near the eaves height (Figure 19�23) with its intake shutters mounted low within a few feet of the ground to bring in cool air� The swamp cooler pushes out heated air via the exhaust shutters or roof vents (Figures 19�25 and 19�26) while the exhaust fan sucks in cool air at the base of the greenhouse and exhausts it higher at the eaves height (Figure 19�23)�
The amount of cooling provided through evaporation is a function of the ambient RH� The lower the outside RH, the more cooling capacity is available by evaporation� When the RH is lowest during the hottest times of the day, the air temperatures can be reduced significantly within the greenhouse� For example, if the outside temperature is 88°F (31°C) and the RH is 54%, the cooled air temperature entering the greenhouse
will be 78°F (25�5°C)� Similarly, with an outside temperature of 106°F (41°C), RH of 38% the air temperature after cooling would be 78°F (25�5°C)� However, with a combination of high temperatures and RH, the cooling capability of the evaporative cooler is reduced since little water will evaporate as the ambient RH approaches 85%�
The size of evaporative cooler needed for the example hobby greenhouse is determined by the total CFM as for the earlier exhaust fan� The evaporative cooler comes with a two-speed fan, and can hence service from 1400 to 2800 CFM� A model to satisfy this need has one-eighth HP, is 110/120 volts, and draws 5�4 amperes� As a result, this unit will consume more power than an exhaust fan system� The power consumption is: P = VI; P = 110 × 5�4 = 594 watts� Cost per 8 h of operation is 594 × 8 = 4752 watts or 4�75 KW� That is 4�75 KWH per day� For the 5-month cooling period of the year, the total power usage is 100 sunny days × 4�75 KW = 475 KWH� The cost would be $0�10 × 475 = $47�50�
The intake shutters or exhaust shutters can operate on the pressure differences from the fans or they may be equipped with a small motor to open and close them
in coordination with the operation of the exhaust fan or evaporative cooler blower� The shutter motors are very small drawing about 0�17 amperes at 120 volts for some models� The operational cost of two shutters is much less than the exhaust fan� Projected power and costs are as follows: P = VI; P = 120 × 0�17 = 20 watts� For two shutters that is 40 watts� Total power consumption for the 5-month period is 100 × 8 × 40 = 32 KWH� The cost would be $0�10 × 16 = $3�20 for the season�
Heating: $876�00 Cooling: $13�20 (for an exhaust fan and automatic shutters not motorized) Cooling: $47�50 (for an evaporative cooler and non-motorized shutters) Motorized shutters: $3�20
The total cost is either $926�70 for an evaporative cooler with motorized shutters or $889�20 for an exhaust fan with automatic shutters (not motorized)� Even these two systems are not significantly different in the annual cost for temperature control�
Overall, the cost of heating and cooling would be covered by the value of the vegetables� The expected annual yields from the greenhouse production of vegetables are dependent upon the amount of sunlight the plants will receive� The following yields are based upon commercial greenhouse production in the Vancouver, BC area, which is very similar to Seattle, WA, the location of our earlier example� Table 19�3 gives annual production per crop and in the last two columns are projections for a combination of crops in a 10 ft × 12 ft backyard greenhouse� Herbs
TABLE 19.3 Potential Annual Production in a 10 ft × 12 ft Greenhouse
should be grown in plant towers to increase the production per square foot of greenhouse area�
In a 10 ft wide by 12 ft long greenhouse, there will be three beds� There is one on each side and one in the center of the greenhouse� The two side beds are 12 ft long and the center one is 9 ft long to permit entrance from the door that swings outward� A 2-ft wide aisle is allowed for access between the beds� Vine crops are supported vertically from support cables attached by eye hooks and turn buckles along the roof frame� Bolt the eye hooks and turnbuckles into the aluminum ribs of the greenhouse� Plastic vine twine supports the vine crops from the overhead cable with special hooks with additional string called “Tomahooks�”
In a 10 ft × 12 ft greenhouse, we may grow 12 tomatoes on one side in either bato buckets or rockwool/coco coir slabs� Details of the hydroponic crops and their growing systems are given in Chapter 20� The central bed contains two European cucumbers, eight peppers, and two eggplants� Due to the large size of eggplants, grow one plant per pot� The other area of 2 ft × 12 ft has two plant towers with herbs in the one, and basil and arugula in the second one� The remaining 2 ft × 6 ft is for a floating raft-culture system� It is used for lettuce� The raft system can fit 48 head of lettuce� Lettuce, arugula, and basil may be grown in either NFT or raft culture� Alternatively to the plant tower, the arugula and basil could be grown in the raft culture system in place of some lettuce� The production for the greenhouse is summarized in Table 19�3� The projections use plant towers for herbs, basil, and arugula� A plan for these crops is given in the diagram (Figure 19�27)�
Cut and paint a piece of ¾″ thick plywood for a backing for all of the controls as shown in the diagram (Figure 19�28)� The length must be sufficient to span across two of the greenhouse vertical frame bars along the back end wall near the nutrient tank� Make it at least 2 ft wide to fit the components� Locate it opposite one of the aisles for easy access and at a height just above the irrigation header� The electrical cable from your residence enters at the back wall of the greenhouse behind the nutrient tank and is connected to a breaker panel� The breaker needs a 220 volt, 30-amp circuit for the electric heater, four 110 volt, 15-amp circuits for outlet socket boxes, and two time-clocks� From the breaker panel, using electrical conduit or other approved water-proof cable, place three dual socket outlets for the exhaust fan, and three 8-ft, dual tube, high output fluorescent lights� One time-clock operates the lights and the other time-clock is wired directly to another circuit that has a dual outlet box for the pump� The outlet for the 220-volt heater is wired directly into the 220 volt, 30-amp circuit of the breaker panel� The heater has its own built-in thermostat� The exhaust fan is operated by a thermostat hung in the middle of the greenhouse� The control panel will keep all the wiring neat and all controls centralized for easy operation� This arrangement of components for a backyard greenhouse is shown in Figure 19�29�
As mentioned earlier under “Site Preparation,” lay the approved underground cable before starting the preparation of the greenhouse base� If a propane or natural gas heater is used instead of the electrical heater, the electrical circuit for the greenhouse can be reduced to 60 amps� If natural gas is to be used, it will have to be installed according to local codes by an approved gas installation company� The trench for the placement of the electrical line may be able to also contain a water line for the greenhouse� Again, local codes dictate the depth of these trenches and what utilities may be placed together� A water line of ¾″ black poly tubing approved
for the pressure of your water source would be adequate for the greenhouse usage� Natural gas heating would be the most efficient and lowest cost form of heating�
Combining the projected annual production in Table 19�3 with an expected price to purchase the product in a supermarket, the overall revenues are given in Table 19�4�
The revenues, of course, make the assumption that you would consume that amount of salad crops a year� Most of these products could be consumed by a family of four to six people providing they are much focused on eating vegetables as an important part of their diet� The lettuce is more than would be needed, so the arugula and basils could be grown in the raft system with the lettuce and the second plant tower freed up for more herbs, bok choy, chard, spinach, and even some flowers such as viola, nasturtiums, petunias, marigolds, and so on, that normally do well in hanging baskets� The bok choy and chard could also be grown in the raft system replacing some of the lettuce� If this is still too much produce, share it with relatives and/or neighbors� You might even sell some to them at a discounted price from that of the supermarket�
TABLE 19.4 Projected Annual Revenues in a 10 ft × 12 ft Greenhouse
