The real "300$" walipini (pit greenhouse)

Intro:
I've started my winter greenhouse project and have my first update picture. I'm digging a pit greenhouse with three big black water barrels and lots of stones for thermal mass and a small stove to heat the water for super cold nights. I live in Ontario in the Ottawa Valley, so it gets to - 20c often enough and colder still. My goal is to have a grow area for the entire winter, mostly heated by passive solar heat. The size is 8ft wide by 10ft deep with a sloped front and back wall and straight side walls. This design is made for future expansion to the sides. There will be windows in the front, angled at about 25-30 degrees for winter solar angle in the area.

Bear in mind, I have a lot of resources available to me since I'm a carpenter but most people could scrounge around or barter for a lot of this stuff.


That being said, I bought 40 bales of straw at 5$ a piece for 200$.

I bought two pieces of 2'x8' 2" foam for the floor for 40$.

The rest is recycled or extras. I cut tamarack poles from the property, I dug a hole in hard clay sand at the top of a hill by hand (far from the water table or any natural drainage). I'm going to re use old steel roofing from a burnt garage and just tar the holes with some leftover fibrous tar I have in a can. I got three double pane patio doors for the front, for free. The house wrap is extra, the 6mil poly is extra and so is the tuck tape.

If you bought all the vapor barriers, sheet metal and wood, it'd be more like the 1000$ walipini.

Will post more as the project progresses.

I'm debating going with a sod roof but I've never done it before and the ground is freezing.

I'm a dirty liar, it's now the 500$ walipini. The 5" Galvanized ducting was about 185$

As much as I love digging with a shovel, the freezing weather is coming and my brother stopped by with his backhoe. He saved me a ton of work and dug me a 45 ft trench for my greenhouse air intake.
The idea is to warm the air from well below freezing to above freezing by slowly pulling it through a metal pipe, 5-6ft underground.

I also finished framing the structure and started lining the side walls with 1-2" trees so that I can backfill the sides with the tractor and have it bermed on 3 sides.

Pics

Pics

Hadez411 said:

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This all seems super rad! Hopefully it works out for you.

How are you going to back fill in on the pipe without crushing it?

rikdabrick said:

This all seems super rad! Hopefully it works out for you.

How are you going to back fill in on the pipe without crushing it?

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It's clay/sand so I just hand shoveled some of the less chunky stuff until the pipes were covered and supported on all sides. The trick being to toss the sand wide and sprinkle it down lightly over a large area as opposed to plopping big shovels full.
I don't mind if it gets a bit lopsided though, so long as it's not folded flat to the point of reducing the volume of flow.
Although, assuming the dirt is fairly evenly packed around it, it should be fairly strong given that it's a circle and the joints are strong because they're doubled up and ribbed.

I'll backfill half tomorrow with the tractor. The rest is awaiting more ducting from the hardware store.on Friday.

If your not trying to stealth the intake, build a box and paint it black to warm your intake, or build a passive solar with beer cans sprayed black

Coloradoclear said:

If your not trying to stealth the intake, build a box and paint it black to warm your intake, or build a passive solar with beer cans sprayed black

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I like the idea. I even have a half completed pop can heater project. I'll see if it's necessary though, as the inside of my greenhouse will basically be doing the same thing during the day. It'll be filled with black rock and three black water barrels, with the option of heating them via woodfire.
The biggest benefit to the underground air heating system will be at night time. The earth will provide a near infinite amount of temperature heat sinking to bring the air up from - 20c and be less of a drain on my stored heat in my rocks and water.

I think I'm going to use my pop cans to make a heat exchanger tube for my exhausted air to recover heat. It will be rather inefficient if my air coming in is already sufficiently warmed by the dirt tube, but if by chance I start pulling in cold air then all of a sudden my heat exchanger will become useful and act as somewhat of a fail safe. That or if my ground tube is over taxed I can draw some air straight from outside through the heat exchanger and reduce demand on the ground tube.

Also, the sun is being used solely to heat the water and rocks. My plants will have grow lights, as the sun we receive here is too little to grow with. But it can be used to heat your space nearly for free.

Filled in the sides with big sticks/little trees and threw the branches on top for extra dirt catching action. Didn't want it all sifting through.
Back filled with the tractor but it was too dark for pics after that. Updates tomorrow on the steel roof and the sides being bermed.

..interesting, following along, good luck!

A few shots before and after berming the sides and putting the metal roof on.
Also a shot of the trench but the lighting doesn't show the ducts sticking out. I'm awaiting the rest of the ducting from home hardware to backfill.
The tube already convects slightly warmer air through it and it's only half buried.

Some things I noticed are that at noon my roof angle is just ever so slightly under angled for this time of year (about 12 inches of shadow on the back wall), but more or less it's OK and will be better and better as winter gets deeper. My slope was dictated partially by my patio door glass height.
I was entertaining the idea of reflecting ceiling light down for more heat, but that would require a very steeply pitched roof which was more trouble than I wanted for a rushed project. Ideally a super pointy structure would work well for both snow shed and light reflection, but there's a balance to be found between the amount of glass (heat loss) and the amount of insulated area and heat sink material.I'm going with super insulated to begin with and I'll get more bold from there on out if heating it is manageable.

Also, I noticed that the sun is only covering all corners of my greenhouse for a brief moment, so it will have to have some thermal mass wrapped around the side walls to get the most out of morning and evening angles that only shine on one half or the other. That or make the building less wide in the back and more wide in the front for a wider angle of sun catching from east to west. It'd increase your glass area significantly though and that's where your heat loss is. Perhaps an insulated door on each side of a three sided bay window would work. You'd just open the east side in the morning then close it and open the west side after noon. The plot thickens.

I put the windows in temporarily to stave off the upcoming snow storm. At -10 and with no insulation, the snow was melting off the inside of my windows. Promising.

I also finished burying my ground heat pipe. Had a minor issue where a cave in crushed the pipe but it's all fixed.

This is such a cool project! I'm going to follow along if you do not mind?

SoMe_EfFin_MasS_HoLe said:

This is such a cool project! I'm going to follow along if you do not mind?

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By all means, ask questions and give advice as you please.

How many plants are you planning?

pinner420 said:

How many plants are you planning?

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4 per residence until I complete my application to grow my own medical cannabis prescription. In Ontario, 2 grams/day prescription works out to 10 plants you can grow at any one time, so that's what I'm aiming for eventually.

The space will be good enough for 4 plants this winter and the sides will be dug out and widened in the spring/fall if expanding it seems wise.

I stuck my hand inside the ground pipe today and it was noticeably warm It is capped though so there's no airflow and no real challenge to it, but it was still really neat to have a tangible amount of heat at the pipe's exit. Also, it proved to me that it works and I just need more pipes and less air flow if it's not getting warm enough.
I also put up some Typar on the inside. Why the inside? Well, I wanted something to protect the straw from direct contact with dirt and rain moisture. Ideally I'd have liked to put it on the outside of my structure but I rushed to bury it and I'm not sure if it'd function so well in direct contact with the dirt anyhow.
This way I'm leaving my sticks exposed to the dirt but I'm not too worried about it for this version of the greenhouse as those side walls will be expanded and made out of better materials in the future. My finished wall composition should look like this:

Hadez411 said:

By all means, ask questions and give advice as you please.

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Thank you.