Greenhouses part 5:
Footings and Foundations
The structural integrity of a greenhouse depends upon a solid foundation. Footers may be required for larger style greenhouses.
A dirt foundation is the cheapest and easiest but not suitable for most buildings. Like all other foundations a dirt foundation should be level. Only light weight and/ or small portable greenhouses should be placed directly on a packed dirt or grass base.
Added benefits for dirt foundation are the ability to plant directly into the ground and no permits may be needed.
I used a wooden frame foundation for my first greenhouse. Deciding which type of wood to use should depend on how your greenhouse will be used. When we first built our greenhouse I had planned to pour a concrete floor the following year. I knew I would not be planting directly into the ground so I was ok with using treated boards.
Our first greenhouse measured 10×12 feet so I purchased two 6x6x10’ boards and two 6x6x12’ boards and reserved the extra bits for another project.
I took my time and made sure the heavy wooden rectangle was perfectly level and square. Installing the foundation took most of a day since I was laying it myself; cutting the boards, fitting them together, and hand digging the soil on which it would be placed. Once finished, this foundation did a great job and held up well for the nearly 15 years I had my first hobby greenhouse.
Another wood foundation type is a deck. A deck is easy to construct and a cheaper alternative to masonry. Wooden decks are suitable for most greenhouses and are easy to clean. Heating and cooling may be an issue due to drafts between the boards.
Gravel bases are used to improve drainage. County building codes may also ban the building of any structures, besides the home, on a permanent foundation; in which case gravel may be allowed. Gravel foundations are less expensive and quicker to construct than most other foundation types. If the greenhouse structure is lightweight, like a hoop house, gravel works very well; however, heavy greenhouses may start to sink over time, and in random places, compromising its integrity and possibly causing glass breakage.
A raft or mat foundation made of concrete is a common base for greenhouses, sheds and other building types but may not be the best for hobby size greenhouses. The thermal mass of the concrete may make the greenhouse hotter than other foundation types during the summer. Concrete doesn’t drain well and may hold moisture which could freeze and crack in the winter or cause mold in the summer. It is also expensive for a hobby greenhouse.
Some county codes may require a permit since this is a type of permanent foundation.
Some pros for slab floors include: easy to sweep clean, rodents cannot dig up into the greenhouse, and it’s long lasting.
Stone foundations, or foundation walls, are strong with an old-time charm and fits well in historical areas.
We started building our second greenhouse during a time when the whole country was coming out of the pandemic shut down. When I called about having a slab of concrete poured, I was told it would be about 10 months “if” they could even do it then. I wasn’t willing to wait but we also didn’t have any money to pay someone to install a foundation.
My greenhouse kit was more than double the size of our first one – it was heavy, and built to last. I wanted the foundation to outlast it.
We looked around our yard and found a ton, no pun intended, of rocks. While others gathered rocks from around the yard, I went out and purchased some bags of mortar. Once I got home and as the rocks accumulated around me, I started mixing the mortar and carefully piecing together all the puzzle pieces to form a beautiful rock wall foundation.
The project was hard work but it cost me less than $200, and I loved the result! You can check it out at: https://www.queenbee1755.com/greenhouse-build/
Cinder blocks and Bricks
Cinder blocks and bricks are similar alternatives to a stone base. These options allow the blending of aesthetics to neighboring landscaping and structures. Masonry foundations give strength and lasting support to heavy and large greenhouses but can be very pricey and time consuming. These types of foundations may also require a permit.
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As a continuation of part 4 on heating a greenhouse, here is some information about a very popular heating method, the concept of which has been around for a long time.
An ondol is a uniquely Korean innovation with an elevated floor to allow wood smoke to pass under it. The name is derived from Chinese characters meaning “warm hollows.” A fire pit or hearth is placed at one end of a room, usually in an adjacent room or outside. A chimney is placed on the other side of the room connected by a hollow space or tunnel under the floor. This tunnel helps form a draft for the smoke to be drawn through. The floor is made of thick masonry, which absorbs the heat from the passing smoke and slowly releases the captured warmth into the room above.
In the early 1900s the famous American architect Frank Lloyd Wright visited a Japanese nobleman who had an ondol in his home. Wright fell in love with the warmth produced by this system. He invented radiant floor heating by modifying the concept to use pipes filled with circulating water.
Geothermal heating can be accessed in several ways.
Geothermal heat pumps circulate liquid, mainly well or surface water, through pipes. This system of heating is much more complex and not very cost effective for the average greenhouse.
The modern day GAHT system is most commonly created using two layers of perforated piping under the structure and a fan to move air through the pipes. A solar panel may suffice in running this system.
When temperatures are too hot during summer days a thermostat turns on a fan and pulls the hot air down into the ground. The heat exchange from the thermal mass below and moisture condensation percolating into the soil cools the air as it flows back into the greenhouse. During the winter, geothermal heat warms the cool air flowing through the pipes to warm the greenhouse.
For this system to work, all pipes must be at least several feet below the frost line; the frost line varies from location to location. Although the piping can be installed anywhere near the greenhouse, placing it under the greenhouse is the most efficient since there is an added layer of protection – think about warm air re-circulating in a car.
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