Rob’s Modified Walipini

When a friend of mine told me that they wanted to build a walipini, my first reaction was, of course, “what the heck is a walipini?” The answer was simple: a walipini is an underground passive solar style greenhouse. It’s an innovative and potentially inexpensive way to grow food in our cold northern climate. When I checked out the pinterest photo though,  I noticed some major flaws in the design that would minimize its potential and efficiency. So I went about writing this post to demonstrate how I would go about optimizing this innovative growing system.


What Works With The Original Design:

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  • The walipini is underground so it benefits from the low energy geothermal heat that we get at about 6 ft (or 2 meters) below ground.  The earth down there is about 6 degrees C so it will help keep the space warm.
  • Because it is built underground, if the walls are buttressed properly, you can avoid most of the cost of building materials. Really the only cost for this greenhouse is the truss system on top, the glazing material and the excavation.
  • Because it is underground you avoid the high energy loss associated with cold winds.


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Suggested Design Improvements:

  • The underground nature of the walipini means that cold air will collect in the bottom of the structure. Unfortunately this is where the majority of your produce is growing. In the video above, the walipini is glazed with poly, which has almost no insulation value. Therefore, as the glazing dissipates heat from the growing space, cold air will migrate down to the ground and create a frost pocket around your vegetables. To resolve this, I would raise my garden beds up and dig below the paths to create a cold sink. Basically wherever your pathways are, dig them deeper and then build a board walk over them. The cold air will then sink to these lowest points, instead of around the plants. (And since the space below the board walk is valuable, why not use it for something useful, like cold storage?)
  • I would build my walipini into a south facing slope and use a shed style roof instead of a gable style roof as depicted in the photo. Shed roofs are easy to build, cheap, and take advantage of the southern aspect of the sun. These roofs are also easier to glaze (you can cover them in polycarbonate), and if the right slope is chosen the snow will shed on its own. In addition, the shed style roof has less corners. The more corners we have in a structure, the more heat loss and infiltration we have to deal with. The glazing surface is where the majority of heat loss is going to occur. Finally, with a shed style roof, it’s much easier to deploy an insulative blanket at night than with a gable roof. This is important as using an insulating blanket at night can reduce your heat loss by up to 75%! For your insulation cover, you can just use an insulated construction tarp. They are cheap and have eyelets which make them easy to secure with high tensile wire.
  • Rescue rigid insulation from construction sites and use it to create a shallow insulation skirting all the way around the inside perimeter of the greenhouse. Even though it’s underground, heat is still going to migrate out of the space until the pit is in equilibrium with the soil around it. This insulative barrier will prevent frost from migrating into the inner wall space.
  • I would build a rocket mass heater with a giant thermal storage bank within the space. These heaters are not perfect, but they are inexpensive and will burn whatever you give them. They store heat in their thermal mass, allowing them to burn only once every 12 – 24 hours, depending on how well your space is insulated, how cold it is outside and how big a mass you are using.
  • On the entrance, I would borrow a design strategy from indigenous building traditions: Make the entrance go much further down into the ground than the growing surface inside the greenhouse. In essence you have to go down and then climb back up again. Cold air sinks and this depression in the ground will be your airlock. It will prevent all the warm air inside from leaving the building.
  • Keep the greenhouse longer on the east-west access and shorter on the north-south access, to maximize solar gain.
  • Use a polycarbonate panel if you can afford it. They last a lot longer and they are more efficient. If you can’t afford it, use two layers of poly — one on the outside and one on the inside. Then take a bathroom fan and inflate the glazing wall for maximum insulation.
  • On the low end of the shed-style roof, install a gutter to harvest rain water. Store the rainwater inside the structure for additional thermal mass, and on the north wall of the space to capture maximum solar gain.

Overall I think this design is pretty awesome. If you take these recommendations, please share your photos — I would love to see what you build!

Helpful Links:

Different Designs To Consider:

Resources on Rocket Mass Heaters:

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10 thoughts on “Rob’s Modified Walipini

  1. Thanks Rob – lots of practical tips there. I’m curious – what’s to stop the internal space from flooding?

  2. I suspect that a unit like this would be built in hill country. If this was the case you would build in drainage similar to a walk out style basment with weeping tile.

    Thanks for comments.

    rob

  3. Would a hoop house roof work for this?
    The bent pipes might be stronger and allow for a shallower depth and allow for opening to the sky in summer. It might “catch” more light into the pit. Just a thought.

    Dan

  4. Excellent improvements, Rob. Would love to see some of these implemented. Like the cold storage idea, too, but I’m curious if the temperature swings during the day due to the greenhouse effect would negate their night time benefits?

  5. How about some ground tubes in the bottom? Go down another 10 feet with a culvert pipe and tap deeper heat and frost sink. Doubles as a French drain or well depending on water table.

  6. I have been working on this in my mind for some time. Aire tubes built into the north bank could draw hot air from the top as the soil cools the air and causes it to drop and then draw cold aria at the bottom at night as the soil warms the air and causes it to rise, The tubes could be installed in the north wall as the planting beads are built. with a plenum along the peak of the shed roof connecting the top of the tubes the air could circulate through the tubes or the top of it opened to vent heat when over heating drawing air in through the entrance trenches on the east and west ends. I would like to have the south wall lined with water barrels and collect the rain from the outside and condensation on the roof panels on cold nights. Maby some solar panels on the south slope to pump water and LED lights.

  7. Another source of heat could be a compost pile in a trench along the northside. Airducts through the compost could deliver warm air inside the greenhouse. I will build one next year in about 6500 ft elavation on a southwest orientated site in the swiss alpes. The northeastern wall will be a stone wall to store sun warmth from the day. in addition I will install a waterbasin inside the greenhouse to level out temperature changes between day and night and to grow fish inside the greenhouse.

  8. I am curious what your weather and location are where you live. I am in Colorado and live at about 8000 feet elevation. I have a very short growing season and would love to extending my growing season.

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