In Greenhouse, Methods of Design & Patterns

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 blog post, to demonstrate how I would go about optimizing this innovative growing system.

What Works With The Original Design:

  1. 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.
  2. Because it is built underground, if the walls are buttressed properly, you can avoid the 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.
  3. Because it is underground you avoid the high energy loss associated with cold winds.

 

Suggested  Design Improvements:

  1.  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 blog, the walipini is glazed with poly which has almost no insulative value. Therefore, as the glazing loses 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?)
  2. 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 install on high tensile wire.
  3. Rescue rigid insulation from construction sites and use it to create a shallow insulation skirting all the way around the 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 space.
  4. 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 of a mass you are using.
  5. 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.
  6. Keep the greenhouse longer on the east – west axis and shorter on the north – south access to maximize solar gain.
  7. 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.
  8. On the low end of the shed-style roof, install an eaves trough to harvest the 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, send me some photos – I would love to see what you build!

Helpful Links:

Different Designs To Consider:

http://zombiehunters.org/forum/viewtopic.php?f=39&t=108526

http://www.inspirationgreen.com/pit-greenhouses.html

 

Resources on Rocket Mass Heaters:

https://www.youtube.com/watch?v=BoW-ICO83hM

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Showing 43 comments
  • es9836

    Hello,

    I am looking for a specialist in underground greenhouses to speak to my class about the subject. I have been randomly googling and have stumbled across this site. Rob would you be interested in doing something like this via video chat? You seem very knowledgable! We are doing a class project of making some mini versions of the underground greenhouses for our school. Please let me know as soon as you can!

    Thank you!

  • Brian Huntley

    Hello, I would like to know the answer to Joaquin’s question as well! I am in Nova Scotia and I am planning to build a Walipini this summer. Not sure if it will work, but I would like to combine an Aquaponics system as well. I am looking into different heating options as well. If I build this Walipini near our manure shed, I am wondering about circulating the heat from the manure into the Walipini for cheap heat. Any thoughts on this from anyone?

    Thanks

  • Kathy

    Rob – the link for the walipini.pdf took me to a totally different website – which totally confused me!! ??? Help please?? I sent an email as well so responding either way is appreciated.
    Thanks
    Kathy

  • Joaquin

    Hi there. I am very interested in your post, because I am planing to build a Walipini this days. I have read the manual of the Walipini of La Paz, Bolivia, which is very complete, and with your ideas mixed, would be perfect. But I have only one problem, the inclination od the roof. It will be build in Serbia, but the manual speaks of the souther hemisphere, and the formula is your latitude plus 23 degrees. I dont know if this applies for the north hemisphere. What do you think about this?
    Thanks, and looking forward for your answer.

  • Jay

    If interested in adding suplementary light you can consider hydro generators made of old washing machines check out google lots of info. Having a woodstove with fan system will help in northern geographic.
    Aswell building your retaining walls out of tires similar to earthship concepts will give you much more thermal mass. Having a thermal break on exterior of retaining walls underground and speration from outside ground temperatures will help with heat retention .As for the groud water situation a sump pump is a verry inexpensive soloution. Looking into trying this out myself.

    • Rob Avis

      Thanks Jay,

      Here are a couple of things to think about.

      1) Sump pumps fail and it is energy expensive to pump water.
      2) Earthship tire walls do not add any additional thermal mass as they are filled with the same parent materials. They may change the albedo which will change how much they absorb, not store.
      3) A wood stove is a good option as long as you have diurnal thermal insualtion for the glazing surface. Making these spaces work here is all about reducing night time thermal heat loss.

      Thanks for your comments.

  • Jamella

    Hi there, very inspiring work you’re doing–thanks for publishing on the web. I am intrigued by the walpini idea but I live in Whitehorse, Yukon and I’m concerned the ground may be too cold–especially as you mention the frost line in Cold Lake, AB may be 5-6 feet down. I will poll northern friends too but what do you think–is a walpini worth considering north of 60 or is it a better idea to build a passive-a-possible above-ground greenhouse with a stove? (Clearly either will need supplementary heat in the shoulder season. And in deep winter I suspect I won’t be growing anything as adding lights seems like maybe too much input).

  • Georgina

    Would a cement frost wall with earth floor be better or worse than all earth? Maybe with sunroom-type construction for the upper (glazing) part – but with the walipini shape/angle? I’m just wondering since they would be building the foundation for the house anyway. Trying to explore different options. My original idea was a traditional sunroom/greenhouse set on the frostwall with dirt floor – now trying to incorporate the walipini features for less power requirement and not over heating in the summer.

    • Rob Avis

      A dirt floor will be better for growing. Your excavation will need proper retaining walls to keep the soil from sluffing. Angle will be mostly chosen around snow load and the size of trusses you can afford.

  • Georgina

    Hi there and thanks for the awesome info. I am looking at building some sort of greenhouse attached to our house when its built. I want to incorporate veggies as well as flowers, decorative plants, etc and am even hoping to include a hot tub as well as other decorative features (waterfall, possibly a pond and stream) (basically a conservatory type thing – multi-use). I am finding the walipini idea intriguing as keeping costs minimal for both construction and maintenance are important. I live in Cold Lake, about 3 hrs NE of Edmonton in Alberta, Canada. What suggestions, etc could you provide for me? And is there a minimum required depth/size for the cold sinks? Thank you so much for your time.

    • Rob Avis

      Georgina. We offer consultancy in this area and I would recommend at the very least you talk to someone in construction from your area about this design. Be careful when attaching greenhouses to homes you can create a lot of mould issues if you don’t do it properly. Also the frost line in Cold lake is likely 5 or 6 feet down. So any excavation will be quite deep. Take account for ground water and if that is an issue you might want to look into alternative frost proof foundations like horizontal frost blanket foundations.

  • David

    I appreciate all the work you have done here. Thanks.I am in the planning stages of building a Walipini and I am sorry I still do not understand what you mean by “keeping the east-west access longer.” I read your response and I am quite challenged when it comes to building anything, and I still did not understand. Thank you for your time. David

    • Rob Avis

      In building you have a north south access and an east west access. Basically you want the building to be longer on the east west access and narrower on the north south access.
      Cheers.

      • Joe

        You mean AXIS,,, not access.

        • Rob Avis

          Thanks Joe.

  • vern

    How do you pollinate plants in the walipini that are normally pollinated by insects/bees during the winter months?

    • Rob Avis

      You would have to pollinate them by hand. If you are not adding additional lighting though, I suspect that you would only be growing greens which don’t need pollination. Plants only flower when there is an appropriate ammount of light.

  • Dan

    I recently built an underground greenhouse for an aquaponics project. It was a disused cinder block trench silo, 7 feet deep. My project is only recently up and running, but the temperatures have been very promising. This morning, outside temp was 13F and inside was 46F.  I have been cataloging my progress at Facebook.com/troutsalad. Feel free to check it out if you have time. I would love feedback from others, as this is my first project of this nature. If I install solar, I could run this off-grid and add productivity to an abandoned corner of my farm.   The greenhouse has been covered since early January, in this coldest winter in a generation in central PA. My lowest inside temp was 27F. I’m not certain how much photosynthetic quality light I’m getting, though. 

  • Denise Clarke

    HI Rob,

    WE have been working on our pit greenhouse (walipini) since mid-September, the outside is sealed to the elements and we have almost completed the rocket mass heater. Take a look at our website for pix … working with a slim budget, it has been quite an adventure! The goal is to grow vertically and use aquaponics for at least some of the crops.

    Located in Austria where the weather can get quite cold in the winter.

    If you have any questions on what we’ve done you can email me directly at:
    dclarke112@gmail.com

    Also have a group on facebook:
    https://www.facebook.com/groups/pitgreenhouse/

    Best Regards,

    Denise

    • Rob Avis

      Nice work!

  • Fae

    An idea you could incorporate for heating the space, that some aquaponics systems use, would be to install a solar panel which warms a holding tank of water, that then waters the garden on a timer. (Once installed, it would decrease man hours tending to the heater, and produces less pollution.)
    Dig your article. 🙂

    • Rob Avis

      Fae, good idea however these panels are not that effective at space heating in the dead of winter. It would however help in the shoulder season. Thanks for your comment. ~Rob

  • Mark Gillanders

    Awesome ideas on building a walipini.

    Mark
    NT

  • Ian

    Also, if installing just a poly roof, would one use something like Tuck Tape to fasten it to the wood and seal it so that it would retain the air you fanned into it?

    • Rob Avis

      I would use wood or plastic battons.

  • Ian

    Hi, thanks for the article. Just not sure what you meant by “keeping the east/west access longer”. Did you mean east/west axis? I.e., keeping the roof length longer than its height?

    • Rob Avis

      You want an aspect ratio of 1.5 to 3:1. Which means the east west axis is longer than the north sourth axis. It optimizes solar gain.

  • Hans @ Qberry Farm

    I suggest setting up a collection site for foam packaging to use between the North wall dirt and the cob mass wall for the rocket mass heater. I have watched their videos in the past and two problems they have is starting with flat land and having to make the elevation from the excavation alone.
    They have no way to store the excessive air heat during the day. If you could afford to install a furnace fan to draw the hot air from the peak and blow it through 4″ drainage pipes under the beds this has worked in other green houses.

  • Toña Luisa osher

    Great going. Keep going. Saludos del sur. Check out VIMEO, Video Espejo for De la Milpa a tu Mesa’s latest 10.5 min video about our corn harvest celebrations here in Morelos. We invite brother-sister projects and individuals to make bonds with us to defend small scale farmers maintaining native corn and original heirloom seeds, raising awareness thru vid showings in small villages and doing free workshops for farmers on natural bio-fertilizers. Will try to do the subtitles in English,soon…Love you guys! Toña, oshertona@gmail.com

    • Rob Avis

      Thanks Tona, we think of your often!

  • Nolan

    I remember thinking about trying to adapt my trampoline pit for this when Adrian posted a link a while back. However cold storage might be a better application.

  • Nolan

    Loose needs to be secured. Lose is the opposite of find. I’ll assume to instead of two was a typo. Great ideas Rob. Way to tweak.

  • kurtis

    Awesome adapations ser. Definitely something on the experimentation agenda.

  • Andrew Gillese

    Wow excellent! I am so excited to see these design recommendations! These optimizations outlined above are just great. It would be interesting to have a discussion on whether or not it would be feasible or beneficial to implement a ground heat sink (re: using seasonal thermal store) maybe behind the retaining wall

    • Rob Avis

      Sounds good Andrew!

  • farah

    One question, for the boardwalk cold storage. Would you have to worry about water seeping down into the lower lying areas, which would spoil the food? Do you have a solution for that? Maybe raise the cold stored things on pallets?

    • Rob Avis

      Farah, You could design the cold food storage under a roof and ensure that you control the drip line. So, yes, you need to be concerned about it. Also, put some gravel on the bottom of the pit so that if water does collect it has drainage.

  • Michael Moore

    Great work Rob! Good advice and thanks for pointing out the original design flaws.

    • Rob Avis

      Thanks Michael!

  • Dan Halsey

    Great work. I have been studying them for years and love seeing it tried out.

    Dan

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