Note to readers, attached is the complete final report for FNE99-234.
The goal of this project is to design and build a small commercial greenhouse (1000 sq. ft.) consistent with some green design goals of energy efficiency, longevity, affordability, and minimally polluting systems and materials. We will use technologies that show promise, but are not well established in the greenhouse industry.
The innovations we are testing include:
new frame design
airtight and affordable glazing system
heat recovery ventilation in heating season (passive ventilation in cooling season)
ventilation preheating using earthtubes
temperature moderation and heat storage with subsurface rock bed.
insulated, airtight shell
Our original plan for the greenhouse was to grow flowers for the fresh market. We have changed our plans, and are now concentrating on growing heaths and heathers and own-root hardy roses to sell both wholesale and at farmers’ markets. To date, we have started with rooting heath/heather cuttings in the fall of 2003, and will be experimenting with rose cuttings in spring/summer 2004. Our farm consists of about 55 acres, with 30 acres in hay and the rest wooded.
The Greenhouse Frame
Design Goals: The greenhouse frame had to be strong, withstand high winds, and shed snow well. It also should be optimized to capture the low winter light. We sought a design that was affordable and durable, and one that would require minimal maintenance. The frame should permit flexible use of the greenhouse space underneath. Finally, the frame should be compatible with the glazing system (i.e., it must have a flat outer surface at least 2 inches in width).
Structure: Our frame sections were welded locally from 2×3 inch tubular steel and 2 inch angle steel. The frame was then sent off to be galvanized in the Boston area. The frame sections are bolted together 6 feet on center with 2 inch angle purlins spaced 43 inches on center.
Plans: The plans (3 pages) used by the fabricators are included as Appendix A [Frame.pdf]. The frame drawings might make more sense if you looked first at a drawing of the whole greenhouse cross section (Appendix B) [eastelevationframe.pdf], viewed from the east.
Costs: Our frame consisted of 8 frame sections and 63 purlins, all predrilled, mounted with connector tabs, and galvanized. Total frame costs including materials, welding, shipping, and galvanization was $3,557. A large part of the cost was shipping the welded frame members to Boston (2.5 hours away) and back for galvanizing.
Design Goals: For the greenhouse to conserve heat, it needs to be airtight with controlled heat-recovery ventilation. Air infiltration around the glazing panels must be minimized. The two common alternatives for multi-wall polycarbonate glazing systems were problematic. The inexpensive aluminum extrusions that slip over the glazing panels are not truly airtight. Gasketed glazing systems that clamp the panels between beefier extrusions fitted with EPDM gaskets are airtight, but are exceedingly expensive. Our design seeks to provide a durable barrier to air infiltration without breaking the bank.
The Design: Our system uses 1/8 inch by 2inch aluminum bar stock as a cap to bolt the glazing panels to the frame below. Both the bar stock and the frame have 3/8 inch by 1/8 inch EPDM self-adhesive gaskets mounted to the outside edges. This gasketing is intended to seal out air infiltration without restricting seasonal movement of the glazing panels.
The Heat Storage System
Design Goals: For plants to thrive, temperatures during the heating season cannot be left to swing wildly. This temperature moderation should be separated from fresh air ventilation. Excess heat during the heating season should not be vented to the outside. It should be stored for later use.
The heat storage system should not reduce or restrict greenhouse floor space. It should be sized to handle the expected excess heat that might be produced on a sunny day during the heating season. Ideally, the heat should be stored below the root zone of the plants. This is to protect plants from shock that might occur when the greenhouse air temperature rises suddenly on a sunny morning, but the roots are still cold.
The Design: A 4 ft deep bed of 3 inch stone lies below the greenhouse floor with a plenum running the length of the greenhouse on one side. The plenum is constructed with pressure treated wood and welded wire to keep the stone in the bed and out of the plenum
Design Goals: The heat storage system does all the work to moderate the temperature on sunny winter days. The role of the ventilation system is simply to exchange stale air with fresh according to the needs of the plants. In the process, we want to recover as much of the heat in the air and embodied in the water vapor as possible.
Our Design: We will use a heat recovery ventilation system (HRV) running at approximately 500 CFM to ventilate our greenhouse. This was sized to handle the expected ventilation requirements of our 1000 sq. ft. greenhouse. The HRV will be turned on automatically by a humidistat. Fresh air from outside will be preheated in earthtubes. In the warmer weather, we will use manual ventilation, consisting of hopper windows installed along the bottom of the south facing wall and exterior doors installed sideways at the top of the north wall.
The Greenhouse Insulated Shell
Design Goals: The greenhouse shell should be long lasting and be able to withstand the high moisture levels of the greenhouse with little maintenance. The shell should be designed to conserve heat in the winter and ventilate passively in the summer. The appearance should be attractive. There should be storage for greenhouse supplies outside but adjacent to the greenhouse.
Walls – 2 inch x6 inch, insulated with dense pack cellulose
Exterior – plywood, covered with Tyvek, and finished with white cedar shingles
Interior – vapor barrier (6 mil poly), plywood, covered with 2-part epoxy paint
Roof – 2’x12’, insulated with dense pack cellulose, plywood covered with 30 lb. felt and metal roofing.
Design Goals: The role of the earthtubes is to preheat the fresh air supply to the ventilation system during the cold season. Two benefits are expected: the obvious benefit of replacing exhausted air with air that is warmer that ambient winter air, and the hope that incoming air will always be above freezing temperatures so that the heat recovery ventilator does not need a defrost cycle.
Mold problems and questionable effectiveness are a common theme of Internet discussions of earthtubes. Our system only uses earthtubes for heating air, not cooling, so condensation should not be an issue. Secondly, our tubes are smooth-walled and run continuously downhill, so any moisture that gets into the system should drain out. Thirdly, our tubes are not replacing a heat recovery ventilation system, but supplementing it. The function of the earthtubes is narrow, and specific to what they should do well.
Our Design: Air will be pulled through the earthtubes into our ventilation system at approximately 500 CFM. We used 351 feet of 6″ gasketed sewer pipe for the earthtubes. The piping was divided into three 117 foot runs starting from one point at the greenhouse about 3.5 feet below grade. The runs diverge to about 10 feet apart and dive to approximately 7 feet below grade. The pipes emerge above grade at the end of their runs. The whole run is pitched downhill; our sloped site made this easy.
There certainly are cheaper pipe options. For example, corrugated drainpipe would be a small fraction of the cost, and should be considered. We chose the SDR35 pipe because it was the safest option (i.e., no water accumulation in the pipes and can withstand 7′ of earth above) and because of the virtual impossibility of correcting any problems in the piping that might arise. We expect the earthtubes to be in service for a long time. Our focus is on the effectiveness of earthtubes in meeting our objectives. If they prove effective, future effort could be directed toward reducing costs and optimizing design.
The tabs on the frame members used to attach the purlins were, in some cases, welded so that the purlins would be slightly above the face of the frame. This would compromise the air sealing of the glazing system. To compensate, I affixed a 2″ wide strip of cured EPDM repair tape (Resource Conservation Technology sells a 6″ wide roll that is a EPDM and butyl sandwich with one adhesive side). The tape was thick enough for one layer to build up the frame so that the gaskets would make good contact with the glazing. Additionally, the tape will cover the steel frame and slow heat loss.
The cavity in the 2 inch x3 inch steel frame was filled with extruded polystyrene foam by cutting strips of the approximate size and ramming them up into the tube from the bottom with a hand sledge and an appropriately sized 2×4. I used two brands of foam that I had. One was stiffer, and consequently was easier to ram 18 feet worth in each frame.
sills: on top of the foundation wall, I put down a bead of Tremco acoustical sealant (available from EFI), a layer of the pink foam sill seal, and another bead of Tremco, before putting down the sills.
glazing system: the EPDM gaskets and glazing system made achieving an air seal easy, with one major exception. We used self-tapping screws to fasten the sandwich (consisting of 1/8 inch aluminum bar stock, 1/8 inch EPDM gasket, 8 mm polycarbonate glazing, 1/4 inch EPDM gasket, EPDM/Butyl repair tape, 3/16 inch thick wall of steel frame) together. The steel frame was too tough for the self-tapping screws, so we had to pre-drill the holes. All drill bits used except cobalt were useless after a couple holes. The cobalt drills lasted through the hundreds of holes but drilling was challenging, requiring about 20 seconds of pressure, usually in an awkward position. Perhaps pre-drilling the holes before the frame is erected will make this job much easier. Also, be sure there is enough grab in the screw to go through all the layers. Our screws, which were considered 1-1/2 inch had just enough grab when you subtract out the self-tapping end to hold everything together. Longer screws would be safer.
glazing: The glazing I used (Thermaglas from SPS) was perfect for my glazing system. My system squeezed the panel edges between a 2″ wide frame and 2 inch aluminum bar stock using 3/8 inch wide EPDM gaskets. Additionally, the steel greenhouse frame had rounded corners, further reducing the clamping surface area. Because the panels expand and contract, allowances for changing panel width must be accommodated by the glazing system. The integrity of the panel edges is vital to the success of this clamping system. The Thermaglas panels came sealed at the panel edges with a vertical rib. If the panels needed to be trimmed to width, it would have been impossible to get a good seal (unless it was trimmed to another vertical rib). Moreover, the vertical ribs in the Thermaglas panels were closer together at the panel edges, adding to the panel strength when clamped. If the panels did not have a clampable outer edge, a wider clamping system would be needed.
insulated walls: The steps we took to protect the walls from moisture, will be more than adequate to handle our air sealing requirements. From the inside out, our walls had 2 coats of epoxy paint, a sealing primer, plywood, 6 mil poly, 2×6 cavity with dense-packed cellulose, plywood, Tyvek, white cedar shingles.
After flip-flopping through many options for interior wall surface, we settled on plywood walls painted with an industrial quality paint. While we wanted to avoid wood inside when possible, the other options seemed too expensive or cumbersome. We used a 2-component water-based catalyzed epoxy paint (Sherwin Williams) over a high quality sealing primer. The literature described the epoxy in such a way that I was intimidated by the thought of applying it. It turned out to be very easy to work with. One coat of primer and two coats of epoxy provided a surface that seems like it will be up for the task. Time will tell.
Rock Heat Storage Bed:
We decided on 3 inch stone for the rock bed, based on tables for pressure drop and heat transfer. It was challenging to find consistently sized 3 inch stone in our area. All the quarries had smaller stone in consistent sizes, but the larger stone had excessive variability in size. Even stone called 3 inch stone might vary from 2 inch – 9 inch in size. We were able to get some stone re-screened to eliminate 2-1/4 inch stone and less, but the screening process does not seem precise, and there is still some variability to our stone. While I don’t know how perfect stone would behave, ours seems good enough, in that the fan we have does manage to blow air through the bed.
Do to a miscommunication (which means I blew it) we constructed our rock bed with only one plenum on the fan side of the rock bed. We should have had another block wall on the down-wind side so that the air would flow equally through the entire depth of the rock bed. In our system, the air will tend to flow more toward the top of the bed, compromising the efficiency of the rock bed. Our air returns on the south side of the greenhouse where the concrete slab was held back from the wall by 18 inchs. We will pull out as many of the stones in that strip as we can to improve the air flow at lower depths. The data in the performance chart is based on the system as is. Based on the temperature fluctuations in the bed, it looks like we would have been better off if we constructed the rock bed as the engineer intended. However, we haven’t yet pulled out the 18″ of stone from the return, and the greenhouse was not fully loaded with plants (and their resulting ventilations requirements and transpirational cooling), so next winter will be a better indication.
The fan is thermostatically controlled by a Dramm T42 2-stage thermostat to blow when the temperature exceeds a threshold (currently ~90 degrees) or falls below a threshold (currently ~45 degrees). This system seems to work perfectly.
The foundation and rock bed were insulated with 2″ of extruded polystyrene. Above grade, the foundation was covered by 24 inch brown aluminum flashing. The flashing was tucked under the sill and extends quite a ways below grade. On the east, west, and north sides, the sills extend out past the foundation wall to cover the foam. On the south side the frames cannot extend beyond the sill, so the foam was cut to taper to the sill, and the flashing was prebent to follow the foam’s profile. So far, this system was easy to install and seems to work well.
The decision to use cellulose in a greenhouse environment was slightly scary for me. We are big fans of cellulose insulation, however, so we decided to try it. We worked hard to keep moisture out of the walls, and to allow any moisture that gets in the walls to get out more easily than it got in. To get into the wall from the inside of the greenhouse, moisture would have to get through 2 coats of epoxy paint, a coat of primer, 1/2 inch plywood with joints sealed with silicone, and a 6 mil poly vapor barrier. Having constructed the greenhouse, it seems a safer bet now.
The performance data that follows provides the details of the outcome of our project. We collected data for two winters: 2001-2002 and 2002-2003. The first winter was fairly mild; the second was severe. In both cases, the circulating fan/rock bed heat sink system kept the greenhouse from freezing. We believe that the coming winter will show better temperature moderation, because of a 6? x 26? permanent soil bed built this summer which will provide additional thermal mass. The greenhouse was largely empty for most of the previous two winters with the exception of a small quantity of potted plants. We experimented with a variety of plants over the winter of 2002-2003, all of which survived, including tomatoes, broccoli, nasturtiums, new cuttings of heaths and heathers (taken in late October—they rooted in 60-90 days), and a few other tender perennials. The tomatoes produced flowers and fruit throughout the winter, but the fruit was small and did not ripen well. However, in the spring the quality of the fruit improved, so that each trip to the greenhouse included snacking on ripe cherry tomatoes.
The cost of building the greenhouse was more than we had anticipated, partly because of the cost of hiring labor to do work we had intended to do ourselves. We estimate that the total materials cost for this greenhouse ran about $29,000