Published on July 16th, 2020 | by Scott Cooney0
Truly passive greenhouses – possible?
by Paul Wheaton
Once upon a time at a kitchen table in Montana, four people talked about installing a new sink in an off-grid cabin. And then there was the idea of where to route the drain. This is paraphrased, of course, but the convo went something like this.
“Let’s put in a greywater system.”
“But the plants in your greywater system will be dormant in winter.”
“We could put the greywater system in a small greenhouse.”
“That will need to be heated.”
“Not if we use Oehler’s techniques.”
“But his stuff didn’t make it through the whole winter.”
“But we have a lot of experiments we’ve done here that could augment Oehler’s techniques.”
We mentioned the idea in a podcast and people encouraged us to do a Kickstarter so they can prove to us, with monies, how much they want us to do these experiments. If this works as well as we think it will, it could be the foundation for a new home design. Something that will be comfortable in a Montana winter with zero carbon footprint.
The Rough Design
Let’s start with a typical greenhouse. When the sun hits it, it can get so hot that all the plants die. And in winter, at night, it can get so cold that all the plants die. Conventional greenhouses have heaters and need human discipline to vent them on sunny days. But what if we could store the excess heat and use it later? The attempts to date are either in mild climates or still use electricity or heat augmentation. And we have some tricks that have not been tried yet.
Mike Oehler’s greenhouse design (as documented in his The Earth Sheltered Solar Greenhouse Book) grew tomatoes in December, off-grid, with no heat of any kind in northern Idaho. The genius of his design includes three improvements over a traditional greenhouse.
Improvement #1: reduce the size of the air space with a trench
a basic greenhouse with a trench
Improvement #2: an earth berm on the north side (thus, some added insulation and less glass)
an earth berm greenhouse
Improvement #3: most importantly, the five foot deep trench under the walkway
I have visited a dozen greenhouses that claimed to follow the Oehler design, only to discover that they were missing at least two of these elements. I think that the trench adds a big dose of superpowers that most people overlook. If the deep ground temperature is a constant 50 degrees, then whenever the temperature in the greenhouse drops below 50, the 50 degree air from the trench will rise up, and the colder air will sink down.
Before he died, I talked to Mike about another possible improvement:
Improvement #4: What if a one-inch copper pipe was blackened and then shaped to follow the glass vertically and drop down to near the bottom of the trench. When the sun hits the greenhouse, the top of the pipe would heat up and act like a straw to pull cold air from the bottom of the trench. Eventually, the hot air at the top of the greenhouse would be pulled into the trench and warm it. I called it a destratification pipe. Mike agreed that this would probably help a lot!
Improvement #5: A common farmer trick is to put a 20 foot piece of well casing into the ground — so it makes a sort of 20 foot deep hole that is six inches wide. Then put a watering trough on top of that. When the outdoor temperature is 25 degrees below zero (F), the water does not freeze. When the water gets cold, it cools the air at the top of the hole. Cold air sinks and warm air rises. So the 50 degree air from underground rises to the top and the colder air right below the trough sinks. There is so much air exchange that the water remains warm enough not to freeze. So with this trick added to the previous improvements, maybe we can circulate enough heated air from the greenhouse through the pipe to get the temperature at the bottom of the hole closer to 70 degrees instead of 50!
destratification greenhouse with well casing
Improvement #6: Our wofati house design attempts to use the heat from the summer to heat a home through the winter. We do this by berming dry earth against and on top of the structure — and we keep it dry with a membrane we call “the umbrella.” We are still in the experimental phase with these structures, but so far they have had no freeze, even in our Montana winters. So let’s add dry dirt to the roof, the west side, and the east side of the greenhouse — and an umbrella to keep that thermal mass dry.
Improvement #7: Our original mission was to design a year-round greywater system for a cold climate. In this case, “the problem is the solution”: greywater tends to be rather warm. Further, water is one of the best substances on earth for holding heat. So if part of the greenhouse is dedicated to accepting greywater, then there is a constant stream of warmth pouring into the greenhouse.
Here is an idea of what the final structure might look like, as rendered by our engineer:
We have plenty of experience with roundwood timber framing, natural building, and appropriate technology here at Wheaton Labs, and we’re bonkers about experiments, so building this thing isn’t too much of a stretch. Rearranging our priorities and coming up with the coin, on the other hand, is more of a challenge.
We started the Kickstarter and the whole thing was funded in three hours! We were still emailing people to announce it when the word came. Wacky! And now a list of wonderful people are offering their cool stuff as stretch goals! We’re already thinking of more experiments we can do!
This is brand new territory in the world of homesteading — so we are going to do this experiment and film it so we can share it with the world. Now we are lining up lots of amazing stretch goals so that we can give even more goodies away to people who support our Kickstarter. If you’d like to support our innovations, see how the experiment turns out, and get a whole bunch of cool stuff, please visit our Kickstarter!