DIY Underground Greenhouse

Image from Neo-Farms.com
Walipini construction information provided by Benson Agriculture and Food Institute at Brigham Young University, 2002. Download the full instructions here.

The Walipini (underground or pit greenhouse) in this bulletin is designed specifically for the area of La Paz, Bolivia. However, the principles explained in the bulletin make it possible to build the Walipini in a wide variety of other geographic and climatic conditions. The word ‟Walipini” comes from the Aymara Indian language of this area of the world and means ‟place of warmth”. The Walipini utilizes nature’s resources to provide a warm, stable, well-lit environment for year-round vegetable production. Locating the growing area 6’- 8’ underground and capturing and storing daytime solar radiation are the most important principles in building a successful Walipini.

I. How the Walipini Works
Image from Peswiki.com
The Walipini, in simplest terms, is a rectangular hole in the ground 6 ‛ to 8’ deep covered by plastic sheeting. The longest area of the rectangle faces the winter sun -- to the north in the Southern Hemisphere and to the south in the Northern Hemisphere. A thick wall of rammed earth at the back of the building and a much lower wall at the front provide the needed angle for the plastic sheet roof. This roof seals the hole, provides an insulating airspace between the two layers of plastic (a sheet on the top and another on the bottom of the roof/poles) and allows the suns rays to penetrate creating a warm, stable environment for plant growth.

The Earth’s Natural Heat: Why dig in?
The earth’s center is a molten core of magma which heats the entire sphere. At approximately 4’ from the surface this heating process becomes apparent as the temperature on most of the planet at 4’ deep stays between 50 and 60º F. When the temperature above ground is cold, say 10º F with a cold wind, the soil temperature at 4’ deep in the earth will be at least fifty degrees in most places. By digging the Walipini into the ground, the tremendous flywheel of stable temperature called the ‟thermal constant” is tapped. Thus, the additional heat needed from the sun’s rays as they pass through the plastic and provide interior heat is much less in the Walipini than in the above ground greenhouse. Example: An underground temperature of 50º requires heating the Walipini’s interior only 30º to reach an ambient temperature of 80º. An above ground temperature of 10º requires heating a greenhouse 70º for an ambient temperature of 80º.

More Free Energy: The Sun
Energy and light from the sun enter the Walipini through the plastic covered roof and are reflected and absorbed throughout the underground structure. By using translucent material, plastic instead of glass, plant growth is improved as certain rays of the light spectrum that inhibit plant growth are filtered out. The sun’s rays provide both heat and light needed by plants. Heat is not only immediately provided as the light enters and heats the air, but heat is also stored as the mass of the entire building absorbs heat from the sun’s rays.

Heat Storage: Mass and the Flywheel EffectAs mass, (earth, stone, water -- dense matter) comes in contact with sunlight, it absorbs and stores heat. The more dense the mass (water is more dense than rock and rock is more dense than soil) the more energy can be stored in a given area. Mass of a darker color such as flat brown, green or black absorbs heat best. Light colors, such as white, reflect heat best. As the earthen walls of the Walipini absorb this heat they charge with heat much like a battery charges with electricity. This storing of the heat in the mass of the soil is often referred to as the ‟flywheel effect”, with the flywheel being charged in the day (storing heat/energy) and spinning down or discharging at night as heat/energy flows from the earthen walls out of the greenhouse up through the plastic glazing to the colder night air. The amount of heat stored in the mass is a critical factor in keeping crops from being frost bitten or frozen during the coldest nights of the winter. These critical nights are usually encountered around the time of the winter equinox (June 21 in the Southern Hemisphere and December 21 in the Northern Hemisphere). The Walipini is usually designed to absorb more of the sun’s rays/heat during the three coldest months of the winter than during any other time of the year. The key here is to have enough energy stored in the mass so that on the coldest nights, the plants are not damaged. In general, nighttime temperatures should not be allowed to drop below 45º. This minimum temperature is also dependent upon the types of crops being grown, as some are hardier than others and may require colder nighttime temperatures. An easy way to increase the mass is to put a few 55 gallon drums filled with water and painted flat black along the back wall of the Walipini. Some growing space will be lost, but the heated water will greatly enhance mass heat/energy storage and will provide preheated water for plant irrigation. Preheated water reduces plant shock, thus, assisting plant growth.

Cutting Down Heat Loss: Insulation

A double layer of plastic sheeting (glazing) should be used on the roof. This provides a form of insulation and slows down the escaping of heat during the nighttime. This sealed dead-air space between the plastic sheeting should be between 3/4” to 4” thick. Poles used to span the roof that are 3.5” to 4” in diameter provide the indicated thickness of dead air space when plastic sheeting is affixed to the outside and the inside of the roof’s structure. The inside sheeting also keeps the inside humidity from penetrating and rotting the wooden poles spanning the roof.

All above-ground walls should be bermed with as much soil as possible. This provides some extra mass, but provides much more insulation against above-ground cold temperature, winds and moisture penetration.

When nighttime temperatures are continuously well below freezing, insulated shutters made from foam insulation board or canvas sheets filled with straw or grass can be placed over the glazing. This requires more work and storage, and in many environments is unnecessary, such as is the case in the area of La Paz, Bolivia.

II. Location of the Walipini: 

The Danger of Water PenetrationWater penetration of the walls and/or floor of the Walipini is destructive. If water seeps through the walls, they will collapse. If water comes up through the floor, it will adversely affect plant growth and promote plant disease. Dig the Walipini in an area where its bottom is at least 5’ above the water table. When all of the above ground walls are bermed, a layer of water-proof clay, such as bentonite, or plastic sheeting, should be buried approximately 6” to 1’ under the berm surface. It should be slanted so that the water drains away from the Walipini to the drainage ditches. In some cases where the soil has a low permeability rate, the clay or plastic may not be necessary. Be sure to dig a shallow drainage ditch around the perimeter of the Walipini which leads run off water well away from the structure.
 
Digging into the Hillside
Walipinis can be dug into a hillside providing the soil is stable and not under downward pressure. Since the Walipini has no footing or foundation, a wall in unstable soil or under pressure will eventually collapse.

Size and Cost Considerations
The primary considerations in designing the Walipini are cost and year-round food production for the family. The minimum recommended size is 8’ x 12’. However, generally speaking, the larger the Walipini, the more cost effective per square foot the construction will be. A minimum of 94 sq. ft. of growing space per person is recommended for a year-round vegetable supply. Thus, for a family of seven people a 12’ x 66’ area = 792 sq. ft. Less 16% for access = 665 sq. ft. of growing space divided by 7 people = 94 sq. ft. per person in the La Paz model. Keeping the size of the Walipini manageable and its cost as low as possible are important design considerations.

The Walipini is designed to keep costs as low as possible using the following: 1) Free labor -- the builder’s and that of friend’s and neighbor’s; 2) Only unlined, inclined, interior earthen walls; 3) Traditional concrete footings and foundations are excluded because they are unnecessary, when the perimeter of the building is protected from water penetration; 4) Plastic ultraviolet (UV) protective sheeting on the top and underside of the roof instead of glass or corrugated fiberglass panels; 5) The most economical, durable materials found thus far for spanning the roof are 4” eucalyptus poles or PVC pipe; 6) The top soil from the dig is used at the bottom for the planting soil; 7) The rest of the soil from the dig is used for the rammed earth walls, berms and adobes; 8) Stones and any gravel from the dig are used in the planting area drainage system and sump-wells; and 9) Used materials are utilized where possible and practical such as used, cleaned 55 gallon oil drums, used doors, etc. It is assumed that only some of the materials will have a monetary cost and that labor will have none. The cost of materials will vary from location to location and will also vary according to what is available free of cost. Materials for the current La Paz models (20’ x 74’) are $250 to $300.

Water Collection Heating/Irrigation System
This system collects runoff from the roof at the front of the roof in a galvanized metal or PVC rain gutter. From the gutter water flows through a pipe into the 55-gallon barrel/drum system used for irrigation and mass heat storage.

Each of the barrels is connected by overflow piping at the top with the overflow pipe at the last barrel exiting at ground level under the back berm to the perimeter drainage ditch.
In case of a down-pour or continuous excessive rain, it would be wise to have a T pipe/valve at the bottom of the gutter so that the runoff can be diverted to an outside perimeter ditch instead of moving down to the already full barrel system. How much run off the system can handle in a given period of time will depend upon the size of the gutter and the diameter of the pipe used. The larger the diameter, the more volume of water can be handled. As previously indicated, this system provides not only preheated irrigation water, but a dense solar mass (water) in which additional heat is stored for the cold winter nights.
 
IV. Building the Walipini

Tool List
Hammers, shovels, picks, saws, wheelbarrows, crowbar, forms for rammed earth compaction (two 2 ‟ x 12” x 6’ planks held together by 2” x 4” or metal rods or many other type of forms can be made), 100’ and 25’ measuring tapes ( If 100’ tape is not available, measure out and mark 100’ of string or rope), levels, clear hose for corner leveling, cutting knives, hose, nozzle, hand compactors, adobe forms, drill, bits, stakes, nylon string, etc.

Materials List for a 20' x 74' WalipiniWater
20 -- 4” x 16’ poles or PVC pipes to span the roof
3 -- 3’ x 6’ hinged doors (one is for the 3’ x 5’ vent cover)
3 -- 3’ x 5’ door frames ( 2 if rear wall vent is not used)
2 -- 3’ x 6’ door lintels
1 -- 6’ x 3’ vent lintel or roof frame for vent, if used
1700 sq.’ of 200 micron agrofilm (polyethylene UV plastic)
640’ of 1” wood stripping to secure plastic sheeting to the poles
Shovels, tractor or ox drawn fresno plow to dig hole
30 cubic. yds. of gravel for the floor drainage system
1 cubic yds of gravel or stone to fill the 2 drain sumps
233 cubic yds of soil will come from the excavation
22 cubic yds of top soil for planting (8” x 66’ x 12’)
94 cubic yds. for the rammed earth walls
This will leave a remainder of 109 cubic yds. for wall berms.
2700 sq’ of plastic sheeting to bury for drainage, if needed
74 ‛ of drain gutter for the lower end of roof
100’ of overthrow/drain pipe from gutter through barrel system to perimeter drainage ditch
Nails
116 8” x 4” x 12” adobes for the perimeter to seal plastic roof edge

Interest piqued? Click here for the full instruction with illustrations:
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Nicole Caldwell

Nicole Caldwell is a self-taught environmentalist, green-living savant and sustainability educator with more than a decade of professional writing experience. She is also the co-founder of Better Farm and president of betterArts. Nicole’s work has been featured in Mother Earth News, Reader’s Digest, Time Out New York, and many other publications. Her first book, Better: The Everyday Art of Sustainable Living, is due out this July through New Society Publishers.