Bruised Musings: A Prototype Passive Solar hoop-frame greenhouse.

There is no better way to ensure you finish designing a new basket than to put all of your eggs in one.

This winter I have been building a 1000 square-foot (100m2) greenhouse to produce hardy native and rock plants and replace my backyard hoop nursery. I was unable to find anyone who has already tried to make a passive solar (self-heating and cooling) greenhouse but using traditional materials that nursery people are familiar with.  (Most passive solar greenhouses are serious things, framed in wood like a house or shed whereas most production greenhouses are metal frames wrapped in plastic film. 

I aimed to create a hybrid: something economical enough for a business- like my own- to reasonably use. This means lighter materials. (versus concrete floors, glass roofs, et al, of a long-term and serious greenhouse). There are also no utilities on site, so being off-grid is not solely to prove a point.

The dark truth and problem with greenhouses is twofold: They consume prodigious amounts of energy to heat and cool  (usually propane of natural gas). In America, the average greenhouse costs four times that of a house to heat/cool. Secondly, a traditional greenhouse is a needy, fussy, fragile bastard teetering on disaster all the time. A brief over/under-heating, usually from a power outage or vent catching or door left open, can result in damage or loss to everything in there. This shit kept me awake at night when I worked in commercial greenhouses. 

So the drive to create something better is both environmental and mental.

So we built it from scratch in the old-school way that the last generation of Colorado nursery-people did: bending fence rail into hoop ribs and building the ends by hand. A hoop-house on steroids kind of deal. It is inside of such things that my mentors have produced all of the cacti, native plants, and trees… where I worked for years and where for years later I have purchased the plants for my gardens.  These humble greenhouses are the workhorses behind all the things we love. 

Anyhow, I found myself inventing a wheel- a prototype and F.O.K: First of a kind. I have not finished and I don’t have the answers. Yet. But I want to share some main lessons so far.

Heating:

For economy and sanity, it’s not dug in. It’s powered only by water. IBC totes. I hadn’t seen anyone use those before so I’d like to take credit for that idea if nothing else and the rest of this folly ends in tears. 20 of them gives me 5500 gallons (21,000L) of water as a north wall, well above (50%) the average used in passives. (per square foot of growing space). 

Insulation:

I was going to make a straw-bale wall as the north side’s insulation but that grew more painfully more complex than useful. Instead, there are 2”-thick rigid styrofoam boards (salvaged) which would have cost $600 new, half that of using straw. Next winter I want to try hanging infrared reflective foil behind the water totes and measure the efficacy. 

One of the most heartening surprises has been how well the size, proportion, and two layers of film are working, getting me off the hook for further insulation. Usually, insulating the ground around the perimeter of a passive greenhouse is important, but the proportions of this one seem to be mitigating that. This is a huge win towards my goal of creating something that is not a complex pain for other growers to do.

The floor is simply black (heat absorbing) groundcloth on top of compacted gravel fines. Wherever possible, I’ve used salvaged, used, or free materials. While it’s an experiment, it needs to pay itself off some day.

Glazing:

For sanity the outside skin is one whole piece, as it will have to be replaced every 4-8 (or 10) years. The proportions are about 50x25, (15x7m) so the ends are theoretically big enough to vent it without side or peak vents. There is no electricity, so no extractor fan per se, but I will try solar-powered circulation fans which can be directed toward aiding the convection cooling. 

The Biggest pain in the Ass and one tentative regret

Two film layers. Traditionally these are laid together and a fan inflates them apart, operating perpetually for the whole life of the greenhouse, creating that critical insulating and structural airspace. Without electricity on site and a learned shyness to risk so much on electronics, I avoided that and had to build an interior wooden intra-structure to hang my second layer. It’s been a pretty flexible, easily adaptable system but a hell of a job to do: lots of hours and lots of exercise climbing ladders with a drill. I think if I were to do it over, it may have actually been easier to engineer a solar-charged battery-powered blower fan, rather than engineer the whole greenhouse.

The Regret?

At the moment, I feel that if I were to do it over, I’d use a kit greenhouse (without the heaters and motors) and retrofit one for ease of framing. But I’d shop around hard to see what brand or design would be sanest to retrofit. After all, more and more growers are not building their own from scratch but using modal kits. So far, materials have cost exactly what a kit greenhouse would cost of that size. ($6700) I’m taking that as a win. Hell, it’s bigger than our house. 

Snowload:

Most of the greenhouse sheds it fine, but there is one hoop whose curve isn’t right, and it accumulated a heavy snow in part also to an interior purlin acting as a dam. I’ve moved that purlin to solve my problem, but to any future builder I’d recommend going gothic shaped for any film structure wider than say 10’ (3m). 

-Ventilation is yet to be seen. I’m building these now, and their proving time is spring and summer. Stay tuned. 

-Cooling will be a multi-part thing. Wax-opener powered and convective Passive vents, shadecloth (on the outside of the greenhouse- this is critical) over the growing area, a shadow over the water wall, and we’ll see what evaporative cooling happens from plants and floors. 

Good news so far:

All the water and two layers of plastic are doing the trick. Outside temps around 5F (-15) made it freeze inside the unfinished greenhouse, sending a tomato plant, a “canary crop”, to its maker. Even when unfinished and before I sealed air leaks, the greenhouse’s coldest was 22F (-6C), which to its credit is 20F (10C) warmer than a 2F(-16C) outdoor temperature, and no sun for days. It’s sealed up snugly now.

I wonder if it would have frozen had it been fully glazed. The water tanks have never frozen, lingering in the 40-50 (4-10C) range so far. A tray of cilantro is growing like a chia pet even through the solstice. Hardy woodies and semi hardy agaves are all sleeping like beauties so far. The greenhouse refuses to go below 35F(1C) which is pretty ideal for sleeping hardy plants. Without ventilation it peaks at 75/80F (24/27C) on a very sunny 45F (7C) winter day.  

Cherries on top:

The greenhouse project became a village affair to finish in time for snow and deep cold; I had some back trouble that required friends to help out a day or two now and again, which led to having a thank-you solstice party with the aid of my friend Marla, which in turn led to a sort of a decorative Altar/shrine (after all, it is a plant-church!).

The greenhouse’s landlord insisted we re-use an old woodstove which had been sitting around gathering snow. While, for reasons of physics, a woodstove cannot actually heat a greenhouse in earnest, it will be useful to warm a corner of the greenhouse next to the potting bench on winter mornings (which might otherwise be 40F(4C) in there) to something comfortable for me to work sowing seeds and potting plants. It’s also just really nice. 

{Well, I say it’s no way to heat a greenhouse, but what organically came up in conversation and now sits in my mind like an inevitable, potent, explosive seed, is the concept of running a passive water radiator coil from the stove  and through the water tanks, which could hold the BTUs of heat from a single stove firing for weeks... Yet another passive system with high payoff that may be too fun not to try, even though the greenhouse will probably not need additional heat… stay tuned for that madness next winter}

I want to end with thanks: To my endlessly supportive greenhouse landlord. My ex-coworker did lots of tedious prep work on materials throughout her year working for me. I’ve also received generous advice on many occasions from folks including Kelly Grummons, Mikl Brawner, Jeff O, Dare Bohlander, Shane Smith, my neighbor orchid-Steve, John Stireman, the three musketeers of Minneapolis (you know who you are) and especially Tony Urschitz. My bible has been a gift from my other neighbor Steve: the cookbook for making passive greenhouses, Lindsey Schiller’s “The year-round Solar Greenhouse” Thanks to that second Steve, as well as Iain, Trina, JC, Hannah, Eric, and my endlessly patient partner Tori for lending a hand when I really needed it. I owe you all plants now.