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Passive Solar Greenhouses

greenhouse illustration

Aren't all greenhouses solar?

Yes, all greehouses are solar but a passive solar greenhouse does not use an artificial heat source. Water, concrete or some other heat holding material is used instead.

A passive solar greenhouse extends the growing season and/or provides the ability to grow plants year around. Passive solar greenhouses can be used for personal use and provide an economical source of heat.

greenhouse construction

Need to Know

  • Size-ratio of 2:1:1
    • Length, Width, Height
    • 24 ft x 12 ft x 12 ft
  • Slope
    • Latitude plus 10° (Central Missouri:
      38.9 + 10= 49°)
greenhouse slope

Heat Source

  • Black 55 gallon barrels filled with water.
  • Rule of Thumb: 2.5 gallons/ft² of glazing for season extension or 5 gallons/ft² for all season.
  • How much do we need then?
    • 12 ft x 24 ft = 288 ft²
    • 288 x 2.5 = 720 gallons
    • 288 x 5 = 1440 gallons
    • We have 20 x 55 = 1100 gallons
greenhouse heat source barrels

BTUs Released

  • 1100 gallons of water weighs 9,130 lbs.
  • For each degree of drop at night, 9,130 BTUs are released.
    • At the Southwest Center, approximately 8 to 9 degree change is occuring so there would be 73,000 - 82,000 BTUs released.

Temperature Over Time

greenhouse temperature chart

Heat Saving Ideas

greenhouse heat saving
  • Double layer of plastic glazing
    • 6 mil four-year-plastic
    • Should add about 10 degrees to the inside on a cold day.
  • 60 cfm squirrel cage fan - outside air!

Walls and Insulation

  • 6-inch walls
  • Wrapped in plastic
  • R19 insulation

Interior Wall Material

  • Needs to be reflective.
  • Bathroom material that is glazed on the outside.

Keeping It Cool

greenhouse fan
  • Exhaust fan
    • Rule of Thumb for CFM required - ft2 x peak height or 24 x 12 x 12 = 3,456 cfm.
    • Standard sizes are sold. (Bradford's is 3,200 cfm.)
    • Also need an inlet shutter that matches the exhaust fan. (Bradford's is a 3,000 cfm, 27-inch fan.)
greenhouse fan control

Fan Control

  • Set temperature when exhaust fan comes on.
  • Be careful not to keep it too cool.

Additional Ideas

greenhouse rolled up
  • South wall plastic being able to be rolled up and down.
    • Allow for cooling in the warmer weather.
    • Can be raised or lowered each day.
    • Sealed down during the winter.
  • Roll up side
    • Allows for year around use.
    • Ssun does not directly shine into the greenhouse therefore it stays within reason in the summer with the roll up side.
    sun path

    Building a Passive Solar Greenhouse

    greenhouse layout
    • The first thing to do was to lay out the foundation. We used 4 x 6 inch treated lumber setting on two foot concrete piers at the corners and in the middle of each post. Each post was wrapped in plastic.
    • Two x six walls were used throughout.
    • The entire greenhouse was wrapped in plastic.
    • 6-inch insulation was used in the walls.
    • Cutting the right angles was extremely important.
    • Metal siding was applied to the outside.
    greenhouse angles

    Cost

    • Lumber, fasteners, hardware, door, insulation, etc. - $1,619
    • Exhaust fan, shutter, thermostat, plastic, etc. - $786
    • Concrete - $190
    • Electric - $490
    • Water - $190
    • Total - $3,275

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