the DIY Greenhouse – Instructions for Home Handypersons
When I posted this view of our little greenhouse, it was to emphasize how too-small it is for major seed starting. But sharp-eyed and perhaps hopeful Melinda asked about the brickwork; I passed the comment along to Bill (who built the whole thing) and he promised he’d describe building it, just as he has described building our wood-burning clay oven.
Short version: Adding this greenhouse was neither difficult nor expensive. For longer version, with instructions
BUILDING A GREENHOUSE ADDITION
Before this one, I had built another greenhouse at my previous home, following well established rules laid out in the happy hippie homebuilder sources I consulted at the time. I learned a lot building that one, perhaps the most important being that one can vary quite a bit from the perfect orientation and layout and still accomplish huge solar gains.
Construction of this greenhouse therefore represented a series of compromises and tradeoffs which all seem to have worked out wonderfully well. It all began when Leslie decided to move the kitchen of this old house from the north side where it was situated in a cold dark entry room right off the blacktopped parking lot to the warm bright south side where a closed-in porch looked out over the greenery of a side yard.
The first compromise was the decision to keep the greenhouse small enough so that views of the side yard from the eventual kitchen would be preserved. We decided upon the eastern corner of the south wall and here is where I laid the approximately 10 X 10 foot foundation. A modest adjustment in size was made when we followed the footprint of an existing garden foundation that was laid out by one of the previous owners.
The three eastern-most windows (on the right in this image) were removed, two to make way for a sliding glass door, and the third to become a storage cupboard for the kitchen.
The dirt inside the foundation was removed to fill and build up the soil line outside the foundation. Four specific things were accomplished by this. The built-up soil allowed a smooth landscaping transition around the perimeter of the greenhouse; added to the insulation of the foundation itself; allowed the introduction of bank-run and pea-gravel inside the greenhouse to increase the thermal mass and provide superior drainage; and permitted the floor of the greenhouse to be well below that of the kitchen. More about this point later.
So the major trade-off at this point was to sacrifice a very large solar gain and greenhouse workspace (running the entire length of the south wall) for one that would be smaller , in order to preserve the views from the kitchen-to-be.
SETTING THE ANGLES OF THE SOUTH FACING WALLS
The next set of design decisions was to set the angles of three slanting south-facing surfaces. The lower two are transparent glazing, the lowest is set to the angle of the winter sun; the next to the angle of the spring and autumn sun. The topmost surface is opaque, a standard, well insulated tin roof set to the angle of the strong, overhead summer sun.
These angles do not have to be exact. In fact, I understand they can be off as much as 45 degrees without altering the efficiency to any appreciable degree. We therefore set up a trio of 2X4’s, fit our topmost angle to the slope of the existing roof and moved the lower angles around a bit until we had a pleasing configuration and slope for all three surfaces.
This led to the innermost wall being about 9 feet in height, the break near the door about 8 feet, and the break at the lower knee wall about 5 feet. If I could remember my high-school geometry I could calculate the inner volume, but suffice it to say there is plenty of elbow room inside. So the second trade-off was in adjusting the angles of the glass to allow for a more pleasing attachment to the existing structure.
Although it has a small footprint, this 10X10 floor plan balloons upward to enclose much usable space.
Pressure treated wood used was for the 2×6 sill plate, but this is the only place where it was used. All 4X4 exposed structural members are Cedar, as is the siding.
Two headers, one 2X10 bolted into the house and a 2X12 running along the major East-West axis through the center of the greenhouse are standard construction grade lumber.
All glass panels in the South, East and West facing walls were made to order. They are double walled insulated panels made from quarter inch tempered plate glass. This was done to minimize the probability of having them broken by accident, say an errant stone thrown up by a lawn mower.
An important glazing consideration is to make sure the butyl seal of the double paned windows does not come into contact with any silicone sealant as this would degrade the butyl seal and allow the windows to leak.
We were fortunate enough to find commercially made doors which came with full length insulated glass panels.
THERMAL MASS CONSIDERATIONS
The below-grade cement block walls go down to footers below the frost line, capturing the ambient 50 degree temperature of the earth, and both the cinder blocks and interior space have been filled with bank-run and pea-gravel to promote good drainage and to increase thermal mass.
After the gravel floor was leveled, it was covered with recycled red clay bricks from remains of the Hudson River brick industry. A set of stairs into the house was also built from these bricks. They are not only attractive and authentic, but also capture and retain the sun’s heat for release in the nighttime.
AND A FINAL TRADE-OFF
A conscious decision was made to share the heat captured by the greenhouse with the living space of the house. As you can see the floor of the greenhouse lies below that of the kitchen floor. Consequently when the sliding glass door to the house is opened a natural thermal convection loop is established: Cool air flows along the kitchen floor and falls into the greenhouse while the warm/hot air from the greenhouse rises to come into the kitchen and is naturally distributed to the remainder of the house.
Heat gain is at its greatest during the clear cold days of February into the longer days of March and April. This trade-off does require some extra heat at night for tender plants. If one wanted to have only cold-tolerant Geraniums, or heat-loving tropical plants, the amount of heat shared with the living space could easily be altered by adjusting the door opening.
Of course, the joys of working in this greenhouse make us yearn for a bigger one, but that is a wish akin to never being thin or rich enough. In such cases, “Modesty in all things” is a mantra worth repeating.
If we had it to do all over again, I think both of us agree that a larger entry door to the outside would allow the larger plants to be moved in and out with greater ease and with greater security to the plant itself.