Firm Foundation for a Productive Greenhouse

A solid foundation will lead to a solid building that will last for years. Originally I was thinking about going with a Permanent Wood Foundation (PWF). Although sub grade lumber is accessible here, it is quite expensive. Water proofing and insulating a below ground wooden structure would also be quite challenging so I decided against it. Instead I decided that a concrete foundation on top of concrete footings would be a better idea and although I am not able to do the work myself, the money spent will lead to a foundation that will last for years. Interestingly enough, if you factor in the cost of lumber and labor, they probably cost about the same according to my estimates. Besides, my previous estimates showed that there would be enough thermal mass in the concrete to mitigate temperature swings without the use of larger water barrels taking up floor space.

I contacted John Randell (Matchless Foundations) and he gladly took on the job. John and his crew have completed 3 jobs (including this one) for me now over the past three years. They completed the foundation for my house/garage , my cellar, and now my greenhouse. John has yet to disappoint me with his service. He listens to what you have to say and you see a result that is exactly as you specify. I wanted to try something different this time. I decided that I would pour the sill plate with anchor bolts already installed into the pressure treated sill rather than pouring them directly into concrete. I kept them 1' from each corner and not more than 6' apart as code states. Picture 1 shows a couple of the installed bolts. Luckily Saturday (May 25) was an glorious sunny day with open clear sky; a great day to tar the foundation. Knowing that it was going to be sunny on saturday, I did some preparatory work the evening before. When the wall forms are removed the tie rods need to be removed. A swift lateral blow with a hammer quickly cracks off the end flush to the wall. However, there is often a blob of concrete left attached to the wall. These knobs get in the way of installing exterior foam so need to be chipped off. I have found in the past that a good masonry hammer does the trick. A quick blow to the inside corner of the knob and it cracks off at the face of the wall. A few more cracks with the hammer takes care of the left over pieces. (Picture 2,3).

The tie rod locations create a crevice around the rod that can allow water to enter through the concrete to the inside. A good blob of plastic cement trowelled on with a piece scrap wood does the job. You just need to make sure that the holes are clean and free of debris so the plastic cement sticks to the wall. Work it into the tie rod hole and flare out the patch so it makes good contact with the concrete wall. Once it sets, it remains flexible and will expand and contract with the concrete. It is cheap and easy to do it this way and it is likely that this will last beyond the above ground part of the structure.

Although fancy dimple mats and waterproofing membranes are available, I have found that a good thick coat of foundation coating, rolled on with a coarse roller, does a fine job of protecting a foundation. My cellar was treated the same, (completely subterranean) and there were no issues at all with water infiltration or dampness. The air inside is humid, mainly from the earth floor but the walls are not damp. I roll it on thick and make sure that it seeps into any air holes in the surface that formed after pouring the concrete. I saturated the connection between the foundation and footing and then coated the footing also (Picture 4)

By Sunday, the foundation coating was fairly dry. Dry enough to start insulating. I typically choose EPS type foam for my projects. Although EPS has less R-value per inch when compared to other foam insulation board, it is more environmentally friendly. The manufacturing process uses gases with less global warming potential than other foams. Cutting EPS foam sheeting is simple. A good utility knife and a strait edge are all you need. I marked out 6" strips, snapped a chalk line at each mark, and used my utility knife and strait edge to cut the strips (Picture 5). Each strip was glued to the footing using EnerBond SF (Picture 6.) I have used this product many times. It is great for fastening foam to a foundation. You just need some way to hold the foam till the glue cures. Luckily there are enough rocks around the building site to take care of this job! (Picture 7.)

After two hours or so, the rocks were removed and I started laying the air intake pipes. Picture 7 shows the two duct manifold with the vertical riser where outside air will enter (bottom right in picture 7). The corrugated drain pipe had to be ran around the foundation and connected to the air intake on the back side of the building. Each run is about 50-60' (Pictures 8,9). Experts recommend 2-3 duct diameters between earth tubes so there is minimal thermal short circuiting between them. I laid them a little more than 3 diameters apart and held them in place with rocks. Thank goodness for rocks! I needed a lot of them! The perimeter around the greenhouse is shorter than that recommended length for an earth tube, but the effectiveness increases with decreasing air velocity. Splitting the run into two separate runs achieves two things: Firstly, air can now move inside of two pipes instead of one which doubles the heat exchanger surface area. Secondly, the air volume in each pipe is half of the total so it means that the velocity of the air is half of that experienced in a single pipe. This increases the heat exchange effectiveness and decreases static pressure which means that air can be transported through the pipes more effectively using a smaller fan. Smaller fans draw less power so this will increase the coefficient of performance (COP) of the system. Based on simulated estimates, the payback on energy will be under 5 years. Payback aside, the overall benefit of growing fresh greens and vegetables throughout the year and the food security alone provide a payback well beyond the energy savings.