Notes from Windward: #67
The Propagation Greenhouse
One of our goals for this summer is to get the Propagation Greenhouse up and running. This 8'x12' greenhouse may be small in size, but the role we look for it to play is large. For example, we'd like to have a large vineyard of concord grapes, but the per vine cost of that would be significant--better to root cuttings and grow our own.
Another goal is to facilitate the propagation of Black Soldier Flies which need special conditions in order to breed. In order to pupate and breed, BSFs need temps in excess of 85° F, high humidity and sunlight. Our goal is to be able to build a stockpile of BSF eggs such that we can hatch out a large number of larva whenever they're needed to deal with food processing waste or offal. For most of the year, it's just too cold here for BSF to maintain a wild population sufficient to meet our needs and run off the houseflies. Our hope is that the Propagation Greenhouse will solve that problem.
The heart of this greenhouse is an insulated 1,250 gallon septic tank buried beneath it. A thermal solar panel will be used to heat water during the day, and then that heat will rise to warm the greenhouse at night in much the same way that a hot water bottle will keep your feet warm on a cold night.
The first step was to dig a level trench around the tank and fill it with bond-beam cinder blocks. These blocks come with a notch in the top of each end such that you can lay a stick of rebar from block to block. When filled with concrete, the blocks unite to form a solid footing.
Now that the retaining wall is filled in with concrete, it's time to top it with a sill made with treated lumber since untreated wood in contact with cement will rot away--and that's under ordinary conditions. The interior of the propagation greenhouse is going to be kept at a high humidity, so all the wood involved needs to be selected with that in mind.
The first step was to clear away the forms, and then cut the treated 2x8's to run the full length of each side.
Instead of butt joints, we put in the extra effort to make half-laps. Makes for a better foundation for the greenhouse, and besides it's fun to use the great slick.
Once the half-lap joints were cut, the final step was to use Red-Heads to secure the sills to the foundation. Red-Heads are a sort of hardened nail that is driven into the concrete by a 22 caliber powder charge-lots of noise, much fun.
Then came the challenge of erecting the greenhouse sides, bolting them together and securing them to the sills. As ever, many hands make lite work.
The roof is on, but not yet fully secured--lots of detail work left to do.
Once the roof was secure, then next task involved bringing home a yard and a half of volcanic cinder to fill in between the retaining call and the cement tank. We went with volcanic cinder because it has good insulation properties, and readily soaks up water.
The ability of the flooring to handle water without becoming muddy is important since this structure will be used to (1) propagate plants such as lavender and concord grapes by rooting cuttings, and (2) facilitate the breeding of Black Soldier Flies. The interior will need to be kept at around 85% relative humidity--since cuttings lack a root structure, they need the humid environment so that they don't dry out, and since the BSFs lack a mouth with which to drink, they too need a humid environment in order to live long enough to mate and deposit their eggs.
Since light intensity isn't a critical factor in propagation, but heat is, we elected to remove the polycarbonate coating from the north wall and replace it with plywood and foam insulation covered in a reflective coating. That way light entering through the southern half of the roof will be reflected off the back wall's aluminum surface onto the plants. insulation on the inside. The north wall of a structure such as this will recieve about 15% of the total light input, but lose about 30% of the heat, so having clear paneling on that wall results in a net loss of heat.
Here's a pic showing the interior of the propagation greenhouse with its newly installed cinder floor and the insulated north wall on the right.
The propagation greenhouse is holding heat so well that we're seeing inside temps in the afternoon in excess of 115°F so we're going to have to make modifications. The heat was such that even down on the ground, it was too much for our BSF larva to deal with.
We could rely on a fan to exhaust the hot air, but we'd rather go with a more passive solution. And the first step in that direction involves removing the polycarbonate roof from the northern side.
The decking for the new north roof goes on.
The propagation greenhouse is so well insulated that we're seeing afternoon temps in excess of 115 °F. Prop (as we affectionally call it) does have a thermostat controled fan but we try for passive solutions whenever possible, preferring to reserve the fan for back-up.
The solution we're going with involves mounting three vents in the north side of Prop's roof. That will allow natural convection to exhaust most of the excess heat during the summer, and we'll use 2" thick foam to plug the vents in the winter.
After cutting three 8" holes, the next step is to put down a layer of roofing felt on top of the plywood. This is probably the last of the hot afternoons, so we're taking time away from Octangle to get this done since the afternoon sun makes the felt and roofing tar easier to apply and get a good bond.
One way that working here at Windward is different than working in town is that when a project runs out of some necessary item, we shift to another project (there's no shortage of worthwhile things to work on) instead of spending the fuel to run to town for just one thing. In this case, it was a shortage of roofing felt that delayed work on Prop's new roof.
As it turned out, Todd had the remainder of a roll tucked away in one of the shipping containers, so today saw the remainder of the roof tarred and felted. Then the three roof vents were tarred and screwed into place.
With Prop just about ready to go, I've ordered in a new batch of Black Soldier Fly larva in hopes that we'll be able to try another shot at getting them to propagate before winter drives them into dormancy--which is a bit ironic in that we failed at our most recent attempt at propagating them because Prop got too hot.
With the new roof vents, we'll be able to keep that from happening, but that won't be a problem again until next summer. Which is a good example of how our research flows in more of a circle than in anything like a linear process--it's often a toss up as to whether what we're working on is too late for this year, or too early for the next. Either way, it works out over time--which is much of what sustainability is all about.
Notes From Windward - Index - Vol. 67