Greenhouse: Part 1

In my Spring 2016 entry, I briefly mentioned that a greenhouse was in the works. As of writing this, construction has gotten to the point of being sealed from the weather – just in time for winter. There is still a lot of work to be done, including some siding, flashing and grading on the exterior, the construction of a deck over a small roof on the NW corner and all of the interior beds, shelving, plumbing, electricity, active ventilation and so forth. Still, it is probably worth pausing here and documenting what has taken place so far.

Greenhouse

The greenhouse approaching weather-tight, the morning’s snow melting down the front.

Genesis

Pool enclosure afterwards

The pool enclosure with the glass removed.

The greenhouse started taking shape when I responded to an ad on kijiji from a gentleman named Neil who was looking for someone to help him remove the enclosure around his pool. While nice in theory, the glass made it unbearably hot in the summer and the added evaporation from his pool because of that heat was requiring the constant filling of the pool. With a few days of work, we were able to tear the enclosure down to its structural steel beams and I carted all of the glass, aluminum, wood and vinyl siding to the property.

Design

With the glass in hand, it then became a matter of working on the design. It was decided early on to situate the greenhouse to the South of the garage. This provided one structurally sound and mostly insulated wall to the North as well as a windbreak. It also fit well with our overall property layout and kept it near enough the house to make frequent visits compatible with the daily flow of life.

Helical Pile

Helical piles are screwed down into the ground so that the broad disc falls below the frost line.

While the decision to use them came well into the design process, the next big decision that was made was to use helical piles for the foundation. Helical piles are meant to replace traditional piles as foundational support and consist of a metal post attached to a broad screw. The screw is driven into the ground until it is below the frost line and acts to distribute the load just like a traditional pile. Although we have clay soil that cannot bear immense weight and I had to use very conservative figures for my calculations, the up-side is that the clay is relatively uniform without many significant rocks. This made it easy to drive the piles to their required depth and makes me wonder why I haven’t been using them all along. The foundation itself was finished by affixing a heavy mesh and rigid foam insulation to the perimeter beams to help keep rodents out of the gardens and heat in the soil.

Beyond that, the construction is basically a variant of post-and-beam with the weight of the structure being borne by the one wall of the garage and ten piles. Some of the piles support posts and elevated beams, while three along the South wall support a massive beam at ground level. Beams were constructed with PT lumber, glued-and-screwed together. What was originally supposed to be a small sloped roof in the NW corner of the structure was replaced by a small section of metal roofing that will be covered with a deck. This change was made partly for the added utility of the structure but also as a means of introducing sheets of ¾” plywood above and below the roof joists as well as on the front and back facing of the central wall to the South of the roof. This helps add stability to the structure and, along with some diagonal members along the sloping South face, ensures that the structure doesn’t deform under wind loads. The glass we are using is impressively strong and flexible and is held in tracks by highly deformable butyl caulking – but deform the structure too far and the result will be explosions of glass.

The operable section of the greenhouse

Eight glass sections are operable, opening to just above horizontal for ventilation of the structure.

Speaking of glass, there are three main sections of it. The South face is at 32 degrees to vertical and consists of eight sections approximately 30” wide and 10’ tall. Above that are eight sections 30” wide and 5’ long. This section is operable, using the mechanism scavenged from the original pool enclosure. Eight rack-and-pinion setups are attached to a bar that is, in turn, attached to a geared system operated by a chain. On the original setup, the tops of the racks were attached to the far edge of the operable sections but in this setup there is a beam in the way and so a separate bar had to be attached to the underside of the framework to accommodate the mechanism. Fortunately the greenhouse is smaller than the original enclosure and so there was extra aluminum extrusion available for this purpose. When operated, the entire 22’ or so of frame and window will raise to just above horizontal, creating a massive source of ventilation at the top of the greenhouse. Behind the operable section is a large beam and then, behind that, one large slope of glass angled towards the East that bridges the South-facing greenhouse faces with the existing garage structure. There are also patio doors on both the East and West walls to provide additional light, ventilation, and access.

Operable section mechanism close-up

The geared mechanism for the operable section of the greenhouse.

One of the specifications handed to me by my wife is that the glass had to be dual layered. Anyone who has ever considered or tried to produce their own double-pane window should be cringing right now. Without access to the equipment and techniques used by those producing commercial sealed double-glazed windows, the need to seal both sides and remove the humidity from between the panes is a tall order. Not only is fogging a certainty if excess humidity remains between the panels but there is a chance that it may start to grow algae or mold. The solution in this case was to purchase desiccant crystals and put relatively large volumes in pouches between the glass layers when we were sealing them up. The long East-to-West roof actually isn’t sealed up yet at all and will have tubing at the top leading to a sealed container containing the desiccant crystals once it is sealed. I was certain from the off that some degree of leak chasing would be inevitable and that crystal pouches would need to be replaced but now I’m thinking that may have been a bit optimistic and that another solution entirely may need to be devised.

The last issue with glazing was that the design resulted in some triangular faces. The glass being tempered, cutting it is impossible. Rather than get into custom cut glass or finding and cutting our own non-tempered glass, I opted to purchase a few sheets of twin-wall polycarbonate. While not as clear as the glass, it does provide a reasonable insulative value as well as good light transmission. There was one long section of yellowed twin-wall polycarbonate from the pool enclosure that got used in a few locations but otherwise we used new, clear material.

Google Sketchup

The Greenhouse in Sketchup

The last working model of the greenhouse in Google Sketchup. Significant changes include the replacement of the west-sloping glass roof with a low-slope metal roof and deck as well as the exclusion of the vestibule on the West wall. Many of the glazing and sheathing details were only for design purposes and the finer points were designed as I went.

One incredibly useful tool in working out the design was Google Sketchup. Available for free, it is a 3D modelling program that is significantly less sophisticated than a proper CAD program but much easier to use. I generated a series of models in Google Sketchup and then used the final model when building the structure. The advantages of being able to go to the computer and take measurements between points to compare to the real-world structure or to be able to re-model sections to accommodate changing designs cannot be overstated.

Greenhouse shadows

Evaluating shadows in Google Sketchup. The Solar North plugin allows for the setting of a solar north axis within the model

Even better, when you first register for Google Sketchup you receive a month of access to the full-featured version before it reverts to the free version. This enabled me to install the Solar North plugin. This plugin allows you to designate a supplementary axis representing Solar North within your model, causing Sketchup’s shadowing feature to display shadows calculated for your actual location and orientation. This allowed me to evaluate the effects of shadowing from the opaque elements of the structure at various times of day throughout the year in order to evaluate the effect of changes on the overall performance.

As with most software that can do anything worthwhile, I recommend taking some time to learn it well and to investigate the keyboard shortcuts and other tricks. Being able to lay down arrays of evenly spaced objects with a few keystrokes makes things go much more quickly. Understanding the system of grouping, components, component libraries, inheritance and so forth also makes a big difference to how quickly the final project can come together.

Construction

So as not to unduly divide my focus during construction, I have been taking time-lapse video from a small camera enclosure I set up but have otherwise only been taking a few select photographs of the various stages or key features of construction. The early stages would likely make for a good demonstration of the correct use of batter boards, mason’s line and a plumb line to ensure that the initial elements were all level, square and plumb and matched the computer model. I may one day actually write that up, though it may have been rendered partly obsolete by the proliferation of laser levels and other layout tools.

Once laid out, the helical piles were sunk and a trench dug around the perimeter. The posts and beams were then assembled and hoisted into place and the rest of the structure built around them. The mesh and insulation was installed around the perimeter and weeping tile was placed around the South and East ends to help with drainage in these areas. The system currently terminates just outside of the perimeter but will eventually be connected to some piping already installed under one of our foot paths and leading to a lower area of the property. The trenches were then filled to below their final level to keep all the material in place but to still allow for the installation of vinyl siding to down below what will ultimately become grade.

.. and then everything ground to a halt. After years of fairly sporadic contract work doing web and media work for small companies, I landed a fairly significant contract with a multinational. Work weeks topping over 80 hours didn’t leave a lot of time for greenhouse construction and what time I did have my mind was too frazzled to accomplish much. After the first phase of the contract was fulfilled and released, things settled down nicely .. at which point I cut off my thumb. Okay, not my whole thumb .. just a little chunk of it which required some stitches and put a bit of a damper on the heavy lifting. Progress continued but was slow. Still, five months after I began tearing siding off of the wall of the garage, the greenhouse is keeping the rain and wind out.

Greenhouse - front

The front face of the greenhouse is 32 degrees off vertical to allow for better light penetration.

Next Steps

I am currently working on finishing up the various bits of trim, flashing, siding, insulation and so forth to truly seal up the structure. There will be some adjustable passive ventilation options and an active fan system installed at the peak of the long roof. After that it will come down to finishing the interior. In the last few days, my wife and I have decided to bury an Intermediate Bulk Container in the middle of the greenhouse to act as a heat sync. We have some evacuated tube solar panels on hand that we can use to create an active system as well if we want. After that, it is raised beds, shelves and possibly some trellising to take advantage of the height of the structure. There is a lot of work ahead of us, but I am optimistic that it will give us an immense boost in our food production over the coming years.

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