DIY Principles for a Passive Solar Greenhouse
By Bill Giebler | Jan 2, 2021
For 15 years now, I’ve had food year-round,” says Penn Parmenter. Standing in expansive productive gardens of her 8,000-foot-elevation Westcliffe, Colorado, homestead, we’re talking about the 6 feet of topsoil endemic to the fertile Wet Mountain Valley. We’re talking about high-elevation heirloom tomatoes, corn and squash, and we’re talking about passive solar greenhouses.
Penn and her husband, Cord Parmenter, moved onto this land on Christmas Eve 1991, with nothing but a camper and a woodstove. Here they’ve raised three sons and an ample four-season food supply in and around a makeshift 800-square-foot home.
That first spring, they built a raised garden bed. And then another. Eventually they’d built dozens of garden beds, an iron forging business (Cord’s) and a seed business (Penn’s). They never did get around to building a proper house. “We built a tiny addition around our camper, and then we tacked this little bedroom off the back.” Twenty-four years later she says, “This is supposed to be our little temporary house.”
Over the years they added two 100-percent passive solar greenhouses from which grew a greenhouse design and installation business. They’d read Bill Yanda and Rick Fisher’s, The Food and Heat Producing Solar Greenhouse, but had lent the book to someone. So when Cord built their first greenhouse of salvaged materials for $150, he did so from memory. He got many things right, got some wrong (“we didn’t even finish it out exactly and it didn’t have all the water it needed to have,” says Penn, “and it still grew food year-round and has been going over 15 years!”), and learned a whole lot in the process. By now, Cord has designed and installed more than a dozen greenhouses, and the Parmenters teach greenhouse design from Denver to Pueblo – and even in Wyoming and Florida.
GRIT has pulled together the Parmenter’s basic principles to set you on the path toward zero-energy-input, year-round food production just about anywhere.
Location and direction orientation
As you identify the greenhouse location, consider square-footage, sun orientation and building codes. “This is a real building with a real foundation,” says Penn. “Speak to your county. Find out if it’s an agricultural building, what square-footage you can build without a permit, which kinds of foundations have to be engineered and which ones don’t.” If you plan to have electricity or water, it will have to be inspected.
Now face south. In most cases, due south is where you’ll aim the unit. Minor modifications from due south will be based on any obstacles – trees, houses, mountains, etc. – that create what Cord calls a specific site’s “sunlight window.” If obstacles call for more than a 40-degree variation from south, though, you might be better off finding a new site.
“You want to figure out the place where you get your wintertime sun, and you want to point your glazing at that,” says Cord. Glazing is the glass, polycarbonate, etc.; the light- and heat-receiving side of the greenhouse. In the summer, you have plenty of light and heat. During winter it’s critical to get maximum heat gain.
“Easiest is to get out there on the winter solstice and see if there’s a shadow,” says Cord, “but you have to remember to look.” Other methods include creating a simple straw-sight (a drinking straw mounted to a protractor with a washer on a pendulum acting as a level). Find the azimuth of the winter sun by plugging your location into the Astronomical Applications Department site (http://1.usa.gov/1K9wmvy), set the protractor to that angle, and peer through the straw. Anything you see will cast a winter shadow. “Or,” Cord adds, “put a laser level on a compass at the proper angle and then shine it up and see if it hits the object or not.”
Now that you’ve found your winter-shadow-free greenhouse location, it’s time to figure out the glazing angle. Precise latitude will allow you to calculate the optimal angle for that south-facing wall. Bill Yanda’s formula is site latitude plus 35 degrees. “The reason we do that is because light travels through a glazed surface better if it hits it dead on” – allowing more heat gain in winter, and less in summer. Optimizing this angle typically calls for both south wall and roof glazing on two different angles, rather than a single glazed slope. Thus, the final greenhouse has four surfaces: the insulated vertical north wall, the glazed and appropriately angled south wall, and an asymmetrical A-frame roof consisting of an insulated north slope and a longer, glazed south slope.
Finding an insulated balance
“The biggest thing about how these work is balance,” explains Cord. “The greenhouses that are commercially available are generally all glazed, and the problem with that is it takes in more heat and light than you need and tends to overheat.” Then, as soon as the sun goes down it loses all of its heat. “What we try to do is achieve a balance that brings in enough light to grow the plants but also has enough insulated surfaces that you don’t lose as much heat.”
In other words, heavily insulate the north wall and parts of the east and west walls. The southern facing shell and the balance of the sides will be glazing – and that means double- or even triple-glazing, and never low-e (low emissivity) glass. Penn says, “Put that together in a 50/50 ratio – or as close as you can get – and you’ll have a great balance.”
In most situations, they recommend glazing a majority of the east wall to bring in the early morning sun, warming the greenhouse when it’s most needed. Again, this is based on your sunlight window and any obstacles that may be affecting it. If there’s no sun hitting the east wall, there’s no need for glazing. As for the west wall, they’ll typically put in some amount of glazing. “But,” warns Cord, “often by the time the sun has reached around to the west wall you’ve reached your heat capacity and you might even be venting.
“The whole thing with glazing-to-insulation ratio,” he continues, “is the more glazing you add, the more heat loss you have, so you want to really figure out the areas that receive optimum light, and those areas will be just glaze.” The other areas will be insulated.
As for glazing materials, use glass on the south-facing wall because glass allows in more light and heat. “You can use a clear polycarbonate or other materials,” says Cord, “but if you’re going to use glass, I like to put some in the south-facing wall, and sometimes to save money I’ll put it just in the center portion of that wall.”
Use diffused glazing – like polycarbonate, plastic greenhouse film or fiberglass – on the roof. “We’d never do glass on the roof,” Penn says, “because during summer when you have the most intense heat, it would be coming straight down through there. But if you diffuse the roof you scatter the light, there are no shadows, and it penetrates to the back wall throwing that light all over.”
The foundation needs to be insulated, too, on the outside, down to the frost line if possible. The Parmenters use foam board insulation in the soil right next to the foundation. “It’s like wrapping the foundation on the outside,” says Penn, “and now earth is never going to freeze in the greenhouse again. You’re golden because you’ve insulated the earth that’s under you and you’ve trapped the thermal mass of the foundation inside the greenhouse.”
Thermal mass water tanks
“Water is four times more efficient than earth or any other thermal mass,” Penn explains. “Using water is a perfectly available thing and it is actually quite sustainable because you only fill the water containers once for the life of that container.”
The Parmenters recommend using 2 to 5 gallons of water per square-foot of glazing, typically stored in 55-gallon steel or plastic black barrels. These are the batteries of the system, says Penn, and more mass means less temperature fluctuation. “If you want it to run cool in the winter and warm in summer – meaning grow some tomatoes in the summer and grow all your cool weather vegetables (greens and broccoli and all that good stuff that love the cool) in winter – then 2 gallons is enough. But if you want to stabilize that sucker on both ends, heat and cold, then go 5-plus gallons per square-foot of glazing.”
(Note: In addition to the perimeter foundation, pour a 2-foot concrete pad along the north edge. This will stabilize barrels up to three high, “which we only do with steel barrels, we don’t stack plastic three high – we don’t know how much they can load, and a barrel is 500-plus pounds when it’s full of water.”)
Some of this ratio can be made up for by the thermal mass of the earth and foundation. To get an approximate gallon equivalent, divide the mass of the masonry and earth (to 1-foot depth) by 0.536. For example, a 10-by-15-foot greenhouse would have 150-cubic-feet of mass. Divided by 0.536, that’s equivalent to 280 gallons of water.
Also, says Cord, “You want to calculate it so that your main thermal mass – your water barrels – is fully shaded in the peak of summer.” This means the shadow line a foot in front of the barrels. “As you go from the peak of summer, you’ll gradually get more sun exposure on the barrels until winter when it becomes less and less until summer again.”
“The water is cooling the greenhouse as much as it’s heating it,” adds Penn. “There’s an exchange going on, so when the greenhouse heats up, the heat travels into the water and the water takes in more and more of the heat getting it out of your way and cooling down the greenhouse.”
Natural convection ventilation
When it comes to keeping the greenhouse cool in the heat of the day, the Parmenters look to natural convection to get the air moving without fans. “Natural convection works when there are low vents and high vents in opposite positions and in equal size,” says Penn. “And then sealing the greenhouse up to within an inch of its life actually makes the natural convection work better than if you have a leaky greenhouse.”
“The lowest vents are as low as possible, the high vents are as high as possible,” adds Cord. Then there’s sizing, which can vary from 15 to 20 percent of floor space. In high elevations and cooler areas, 15 percent will suffice. Shoot for 20 percent in warmer areas. So, continues Cord, “In a 100-square-foot greenhouse, you may have 10-square-feet of low and 10-square-feet of high vent openings.”
Greenhouses under 200-square-feet may require only four vents, one low and one high on each east and west wall. The bigger the greenhouse, though, the more vents you’ll need in order to provide good airflow. “You don’t want to have any dead air zones,” says Cord. “When those vents are open, you want air moving freely throughout the greenhouse. Use common sense to picture how that air is going to flow to make sure all the areas of the greenhouse are getting ventilated.”
In those cases, there will typically be vents the full length of the greenhouse under the glazing in front, says Penn, “and then there’s a row of vents going out the back all along the top. And now you’ve got natural convection rockin’ and rollin’!”
While there are some vents available in greenhouse catalogs, the Parmenters have found them difficult to seal. Cord makes his own.
On the glazing side he uses polycarbonate. “And then,” he says, “FarmTek has these extruded aluminum frame pieces that come in 8-foot lengths. I use metal gussets that you can cut out of tin with tin snips and then pop-rivet them together with the polycarbonate.” In other words: make an aluminum frame, put it around the polycarbonate, and then use the gussets to secure the corners. “Then I get a piano hinge and pop-rivet that to the top of the vent.”
“On my upper vents,” Cord continues, “generally I put these on the insulated portion of the greenhouse and I make those out of 1-by-2s or boards that I rip down to a specific size and then inch-and-a-half foam board and quarter-inch plywood.”
For vent openers, Cord recommends passive openers like Univent and Gigavent that use paraffin-filled cylinders. As the temperature changes, the paraffin expands and contracts, opening and closing the vents.
“You should plan it so it will stay closed with gravity and then the opener pushes it open,” Cord says. “On my lighter vents, I’ll use a lighter weight vent opener, and oftentimes I only use one on those. (If you use two, sometimes one’ll be in the shade, and one’ll be in the sun, and it will try to open unevenly.) However, on my heavier vents that are out of direct sunlight I almost always put two vent openers.”
Cord also adds mechanical extensions to get the vent opener away from incoming air – similar to placing a thermostat in a home’s interior, not next to the window. “It also raises the vent opener about 4 feet so it’s up in a warmer area and will open sooner.” We won’t get into the extension design’s levers and joints here. Cord makes it sound easy, though: “I’ve just figured out how to mechanically transfer that energy.”
In winter, Cord notes, it’s wise to disable many of the vents. You just don’t need as much venting when it’s cold outside. “And I’ll use screen door springs on the remaining vents,” he adds. “By pulling that spring tighter, I can restrict that vent from opening as soon.”
Also, Cord cautions, “If your vent is placed in a way where snow can build up against it or slide off the roof and hit it, it can damage the vent opener. So you want to take steps to prevent that. Either disable the vent opener during the snow or position it differently.”
“I could probably go on for days about ventilation,” Cord says.
“It’s the biggest thing to overcome,” Penn chimes in, “because it’s easier to overheat the greenhouse than to freeze, and people think the opposite.”
Once you’ve built your structure according to sound building practices and the above principles, it’s time to plant. “Grow in the dang ground!” is Penn’s first recommendation, delivered with characteristic enthusiasm and maybe a pinch of hyperbole: “We grow straight into the earth because things grow a thousand times better that way.”
Penn starts seedlings on removable shelves and then transplants into the ground for optimal growing and space efficiency.
“Grow a lot of stuff!” is another Penn recommendation. “People tend to under-plant their greenhouses. I make a jungle and that actually helps with the cooling because the plants are transpiring and evaporating moisture so they’re cooling the air and shading you.” Tall tomatoes, corn, vining nasturtiums and even tropical fruit trees will help create that jungle.
That way, she continues, the greenhouse “breathes and has a life of its own. It creates an ecosystem and microclimates and air is moving and the seasons are changing.” Even an optimized greenhouse will have seasons, Penn reminds, “because it’s not artificially kept with a heater and a swamp cooler.” And that might be a final note: Embrace the seasons and enjoy the changing, year-round bounty!
Longmont, Colorado-based Bill Giebler loves to write about food and food systems. When he meets mountain homesteaders with great ideas, he likes to write about them, too. For more information on the Parmenters, please visit Penn’s and Cord’s website at Penn and Cord’s Garden.
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