Solar heating design for the Garden

Those that know me well, know that I am a big fan of solar. The idea that we can harness the sun for energy and other similar uses is great, in my mind! In particular, one area of solar design that has caught and held my attention for  a while is solar heating- harnessing the sun to heat houses and greenhouses, in the most efficient way possible.

In essence, as the diagram that I found on the web shows above , the solar heater principle is extremely simple in design- a slanted surface captures the sun’s heat and as it heats, it is vented into the house. Simple and effective.

However, with any solar heating project, regardless of the application or type of sunlight collector, there are two keys to success that will make or break any project:

  1. Adequate insulation
  2. Thermal mass

Usually, collecting heat and distributing it into a room is not incredibly difficult. What is more difficult is making it efficient. You want to retain the heat generated as long as possible, especially after the sun goes down.

Insulation is the first line of defense (and believe me, most houses , especially older one are lacking in this area, as are most greenhouse designs). It keeps cold air out, and keeps the warm air in. The more of it, and the greater the R value, the better.

Thermal mass is what helps maintain the air temperature after the sun goes away, and slows the natural cooling process. Basically you want a material that can absorb the heat from the sun, hold it in, and slowly release that heat energy back into the air after the air around it cools down. Black jugs of water and solid blocks of concrete or other masonry material are both good thermal mass materials.

These two keys are, from my study, essential to having an off-the grid greenhouse. Most traditional greenhouses require a heat source whether electric, wood or gas to maintain the heat levels after dark, which is a bit of extra energy to use. If these two principles are followed adequately, they will eliminate the need for a heater on the cool to cooler nights, and make the coldest nights less expensive in fuel costs.

For my garden, since we live in a northern, cooler climate, our growing season is not as warm or long especially when it comes time in early spring or late winter to start seeds for the summer and fall harvest. Since space inside is not available, I had to have a set-up outside to start my tomatoes, squash, brussel sprouts, etc. A common solution is a hot box, or as some call it, a cold frame. In essence, a wood structure with large windows at an angle, to create a mini greenhouse.

So in keeping with the above principles, this spring, I built two hotboxes that utilize these principles:


The backs of both are extended a bit from a traditional hot box design to allow room for the thermal mass, which in the case of the closest box is 4 gal buckets of water painted black. I also stirred a couple of heaping handfuls of rock salt in each bucket as an anti-freeze precaution.


The buckets sit and absorb the sun’s rays all day, and then release the heat back into the insulated box. The walls are wood framed,with foam insulation board and a waterproof draft block board, making the total width of a given wall around 3-4″ thick

IMG_20160406_082424202I have checked on them after dark briefly a couple of times, and a few times first thing in the morning and have been pleased to see that my design has worked well- the heat stays in the box adequately. The seeds I planted so far are thriving, and likely will be ready to plant in the garden soon. Now that it is starting to get warmer in the day, I need to prop the windows open most of the day, so the plants are not scorched, and close them up a few hours before dark, so the heat is still retained.

The materials for this project is primarily pallet wood, and some old, free windows I was able to get from a friend. So far, this excursion in solar heating has worked out well for the garden… I might build a green house in the future, using these same principles. If I design it right, I might be able to have fresh vegetables all winter long, which will be a major plus!


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