Grit Blogs > News From The Nest In Rural Pennsylvania

In Search of a Low-Cost Greenhouse

Lori DunnA greenhouse has always been on my list. You know the list I speak of, everyone has one. It is the mental list that we all keep of things we would like to have someday. That “someday” came for me when my husband suggested we go ahead and put up a greenhouse.

I was extremely excited with the thought of being able to get a jumpstart on our gardening, and experimenting with different plants and flowers, but I also knew we needed to be economical about it. We couldn’t afford to incur a large expense in the construction of the greenhouse, or in the heating of it later. This sent me on the path of the internet highway to do some research. As you can imagine, I found lots of information on all kinds of greenhouses. There is everything from prefabricated kits that come with everything you need to instructions on building your own greenhouse out of many different materials. What I found that caught my attention the most was information on a thermal mass greenhouse. This type of greenhouse uses energy from the sun to store heat, and then release that heat at night when it is needed. Energy from the sun = free, just the right price!

According to the information I found, many things can be used for the “mass” to collect the heat and energy from the sun. Anything that retains heat would probably work. Some of the items listed included soil in raised beds, wood, block or brick foundations, concrete, or even the floor of the greenhouse. But the most effective and least expensive thermal mass is water! Again, water = free! It didn’t take much for me to decide that a thermal mass greenhouse was the way to go for us. But how would we store the large amounts of water that would be needed? The recommended amount was about 2-3 gallons of water per square foot. Apparently, a common practice is to store the water in containers along the back wall of the greenhouse, or use 55 gallon drums filled with water to support the benches in the greenhouse. In one instance I read about, someone had used the method of water in barrels under their benches. They said it kept the temperatures in the greenhouse above freezing except on the very coldest nights. On those nights, they used a very small space heater as a supplement to keep the temperatures up.

My husband and I have a great setup to use this type of greenhouse. Our basement wall is underground on three sides. The fourth side is exposed and faces in a southeastern direction. We came up with a plan to use the face of the exposed basement wall as the back wall of our greenhouse. It would face the right way to capture the heat from the sun. The block wall in the back could store some of the sun’s energy, and we would also use barrels of water on both sides of the greenhouse with planking across the tops of the barrels as bench tops. The wall we wanted to put the greenhouse against has a window into the basement. I thought we could also use this to our advantage. We heat our home with a woodstove in our basement. This keeps our basement nice and warm too. If we built the greenhouse around that window, then on the coldest nights, we could open the window and use a small fan to pull some of the heat from the basement into the greenhouse as a supplemental heating source. I was excited to test all this and see if it would work!

Building a thermal mass greenhouse

We began construction in March. We decided to make the greenhouse 8 feet deep by 24 feet long. My husband was the brains behind all the measurements. He came up with all the figures and sizes for making the correct cuts, and attaching the greenhouse to our basement wall. With some help from our son, my greenhouse soon started to take shape. It wasn’t long till we had a frame in place.

Constructing a thermal mass greenhouse part 2

We knew that it was also important to have a way to let some of the heat escape when the temperatures got too hot, so we put a window in on the parallel wall to the door opening on the shorter 8-foot wall. This would correspond with the predominant wind flow here, so we could open the window and door and have air move through.

Constructing a thermal mass greenhouse part 3

Finally the framing was completed. Now, we were ready to wrap the greenhouse with plastic. This was the one thing that we would later realize we should have done differently. We wrapped the greenhouse, roof and sides, with the plastic. Dancing in the streets, laughter, and applause … MY GREENHOUSE WAS READY TO USE ... well, almost!

Constructing a thermal mass greenhouse part 4

We still had to get the “thermal mass.” This is where my wonderful son-in-law, Deron, comes into play. Where he works, he has access to large, 55 gallon black plastic barrels. The company he works for gets them full of alcohol for sterilization purposes. He was able to get the empty used barrels for me for free! I LOVE FREE! We set the barrels along both the outer and inner long, 24-foot walls, and filled them with water. We then used planking a crossed the tops of the barrels to act as my bench tops. Now, I was ready to start planting. Since we were already into March, I wasn’t sure if we could really accurately determine how the greenhouse would do in colder weather, and I thought I was getting a late start in getting seeds in the ground to be able to transplant for planting time. It turned out that timing for transplanting was perfectly fine by starting my seeds in March. For the most part, everything had plenty of time to sprout and grow to size for transplanting into the garden.

Thermal mass greenhouse

We installed one of those thermometers in the greenhouse that has an extra unit that we could put in our house. This way we could monitor the temperature of the greenhouse from inside our home.

Seedlings waiting for transplant to the garden

On average, the temperatures inside the greenhouse stayed a good ten degrees or more than the temperatures outside, overnight, with no extra heat source. As soon as the sun would come up, the temperature would raise substantially higher than outside. There were only a couple of nights that we opened our basement window to the greenhouse, to let in a little extra heat.

I consider this greenhouse almost a complete success. The one thing that was a problem for us would not be a problem for most people. When we bought the plastic to wrap our greenhouse, we did not take into account the fact that we have free roaming poultry! Here is an interesting little tidbit for you. Ducks and chickens LOVE to poke holes in plastic! Till the end of the growing season, the plastic on the greenhouse was pretty much done for! This is simply what I call a live and learn experience! We now need to rewrap the greenhouse with plastic. We will either use a higher grade of plastic, or use plastic sheeting. That will be determined by cost right now. Either way, we are planning a very large fenced poultry enclosure, so we shouldn’t have to worry about the same thing happening again.

All in all, I would recommend this type of greenhouse, but, beware of ducks and chickens!