Passive Solar Greenhouse

figure1Aren’t all greenhouses solar?

Yes, but a Passive Solar Greenhouse does not use an artificial heat source such as propane but rather utilizes the sun to heat water, concrete, or some other heat holding material.

What are the uses for a Passive Solar Greenhouse?

  • Extend the Growing Season and/or grow plants year around
  • Provide a Greenhouse for home use that is economical
  • Provide an economical source of heat


Before I start constructing the greenhouse what do I need to know?


The length:width:height ratio of the greenhouse must be 2:1:1 (In this case 24 ft x 12 ft x 12 ft)

Slope is south facing and as a rule of thumb should be the latitude plus 10°
In central Missouri that would be 38.9+10=49°. Just to make things easy we made our 45°.

figure4Heat Source:

Since this is a passive system the heat source are black 55 gallons barrels filled with water.The rule of thumb is 2.5 gallons/ft2 of glazing for season extension or 5 gallons/ft2 for all season.

What does Season Extension Mean?

This would mean that all you want to do is grow plants those extra few months after the first frost in the fall and before the last frost in the spring thus you are extending the growing season. Whereas, full season means that you want to grow plants in the greenhouse throughout the winter months.

So how many barrels do we need in the 24 ft x 12 ft x 12 ft greenhouse?

The area of the plastic is 24 ft x 12 ft=288 ft2
For Season Extension that would be: 288 ft2 x 2.5=720 gallons
For Full Season that would be: 288 ft2 x 5-1440 gallons

We have 20-55 gallons barrels or 1100 gallons.
For true full season we would need an additional 300 gallons or 6 more barrels.

How Many BTU’s Will The 1100 Gallons of Water Release?


1100 gallons of water weighs 9130 lbs (1100 x 8.3 lbs/gallon). A BTU is the energy to raise 1 lb of water 1 degree F. So a drop of one degree per pound of water would be a release of 1 BTU.

For each degree of drop in water temperature at night then 9130 BTUs are released. During the winter in 2007 we observed a 10-20 degree drop in greenhouse water temperature. A 10 degree drop would equal 91,300 BTUs released and a 20 degree drop would equal 182,600 BTUs released. Typically a home furnace is rated at 80,000-100,000 BTU’s per hour.

figure6Other Heat Saving Ideas:

The plastic covering is a double layer of 6 mil plastic. A 60 cfm squirrel cage fan pushes in outside air into the double layer. This extra insulation created by the 4 inch air gap adds about 10 degrees to the inside air temperature on a cold day.

figure7Exterior Walls and Insulation:

The greenhouse is built with 6 inch walls that is then wrapped in plastic and insulated with R-19 insulation.

Interior Wall Surface

figure8The interior wall need needs to be reflective but also water proof. We shopped around and found some material used in bathrooms that has a glaze on the outside.

How To Keep It Cool

Even in the middle of winter it can get quite warm in the greenhouse so getting rid of the excess heat is extremely important. Since warm air rises we installed an Exhaust Fan in the top eave.

figure9Fans are rated on their CFM (or cubic feet per minute of air flow) and as a generally rule you need one CFM for each ft3 of greenhouse space: -ft2 x peak height or 24 ft x 12 ft x 12 ft=3456 cfm

They sell standard sizes and ours is 3200 cfm.

You also need an inlet shutter which is tied together with the exhaust fan so that it will open when the exhaust fan comes on -ours is a 3000 cfm 27 inch shutter.

figure10Fan Control

A thermostat controls when the fan comes on. It is important not to set the thermostat too low in the winter or the water in the barrels will not get warm enough.

Additional Idea That Allows the Greenhouse To Be Useful Year Around

figure11Typically greenhouses are not used from the late spring to mid fall because the fans just can not keep all of the excessive heat out. So we, installed a plastic roll up side on the south sip wall. This can either be left down or raised and lowered each day. During the winter it is sealed down.

An added bonus to this system is that since the summer sun does not directly shine into the greenhouse it stays within reason in the summer with the side rolled up.

Year Round Uses

Tropical plants survived and bloomed in January.

Building A Passive Solar Greenhouse

Laying the foundation

figure15The 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.

Framing the Greenhouse

figure16The greenhouse was framed with 2 x 6’s throughout and then the entire outside walls were wrapped in plastic. R-19 insulation was put in the walls and roof.

Getting The Right Angles

figure17aIt is extremely important to cut the correct angles where the glazing supports line up with the hip wall and roof.

Metal Siding Was Added To The Outside As Well As An Insulated Door


The Finished Product


What Did it Cost?

  • Lumber, fasteners, hardware, door, insulation, etc-$1619
  • Exhaust Fan, Shutter, Thermostat, plastic, etc.-$786
  • Concrete-$190
  • Electric-$490
  • Water-$190
  • Total-$3,275

This was in 2005 so costs may have risen slightly since.

What Would We Have Done Differently?

We should have left a six inch space between the back wall and the barrels. This would have added some extra insulation. Also, before we added weed mat and gravel to the interior floor we should have added some foam insulation. When we poured the concrete pad that the barrels are setting on we should have wrapped it on plastic before pouring.

Sample Plans