Sunday, December 11, 2011

Passive Solar and Other Hot Topics

Heating a greenhouse in the winter can be expensive.  I’m all for saving money, so I have been searching for ways to make heating my greenhouse more efficient.  This search has led me to consider using passive solar energy as well as insulation to help keep my greenhouse warm.

So, what is passive solar?  Passive solar is a method for collecting and using the suns energy without the use of a mechanical device.  Intriguing, right?

By following some basic greenhouse design principles, I started using passive solar techniques without even realizing it.  My greenhouse is located in full sun with the largest part of the roof facing south.  This allows my greenhouse to efficiently capture the suns energy.  On December 6th, when the outside temperature was 50 degrees, it was 81 degrees in my greenhouse.  The suns energy was doing all the work to keep the inside of the greenhouse toasty warm.

Greenhouse South Facing Roof
Capturing the suns energy and hanging on to it are 2 different things.  When the sun goes down, a greenhouse does not have any special properties that will allow it stay warmer than the outside.  Without a source of heat, a greenhouse will freeze if the outside temperatures are freezing.

Storing the suns energy requires something referred to as thermal mass.  The thermal mass, usually water, stone, brick or concrete, heat up during the day, and then, at night release the stored heat.  Water is the best source of thermal mass, so I installed two 55 gallon barrels filled with water in my greenhouse.  A 55 gallon barrel takes up a lot of space, so in a smaller application, you might try wine bottles filled with water like Jenny at Rock Rose.

To make the best use of my 55 gallon barrels I’ve placed them on the north wall where the southern sun will hit them directly through the roof.  My 55 gallon barrels are doing double duty as heat collectors and supports for a plant table.

55 Gallon Barrel
Plant Table Supported by Barrels
What kind of return will I get on my 55 gallon barrels?  Well, a gallon of water weighs 8.3 pounds, so a 55 gallon drum contains 456.5 pounds of water.  When the sun goes down and the temperatures start to fall, each gallon of water will release 1 BTU per degree of temperature dropped. So, let’s say the water made it up to 80 degrees and the temperatures are going to drop down to 50 degrees.  That’s a 30 degree temperature drop, or 30 X 456.5 pounds, which comes to 13,695 BTUs. 

To put this perspective, my 1500 watt electric heater puts out about 5,120 BTUs per hour.  To fully heat my greenhouse with water barrels, I would need a lot more water barrels, but hey, it’s a start, right?

If you want to take this a step further, check out this heat calculator from ACF Greenhouses (  This calculator will help determine how many BTU’s you will need to maintain your greenhouse temperature at your desired level. 

In order to hang on to the heat being generated by either passive solar or mechanical heaters, it’s important to seal up air leaks and insulate when possible.  I added 6 mil plastic sheeting to the north wall, plus I sealed up the air intakes and fan shutters, which I won’t be using during the coldest part of the winter.  It’s important not to cut off all ventilation, so I will leave 3 of the automatic vent openers active on the clerestory windows (see previous post)

Sealed Air Intake Vent
Sealed Exterior Fan Shutter
I have read that bubble wrap makes an excellent insulator, so my goal is to eventually insulate the north wall and north roof with bubble wrap.  Greenhouse supply stores sell special UV protected bubble wrap for this purpose.  Regular bubble wrap can also be used and is readily available at office supply stores, or better yet, get recycled bubble wrap if you can find it.

Another place to add insulation is the foundation.  Cold air can seep into the greenhouse from the surrounding soil.  I have temporarily packed the foundation with a hay, but my long-range plan is to install a foam insulation barrier.

Hay Foundation Insulation
Example of Foam Perimeter Insulation
Will all these measures help me save money?  I sure hope so.  I've already noticed that the greenhouse heaters have an easier time maintaining temperatures when it's freezing at night.  That's good news for my plants and my pocketbook.


  1. Wow! You really did the research on this one. My neighbor just delivered a box full of wine bottles. I have quite a collection around the edges.

  2. Passive solar is such a great idea for us Texans. Love the wine bottles. Definitely adds more panache than an ole' barrel :)