We’ve been planning for a greenhouse in the vegetable garden area for some time.  Initially we had expected to construct a shed in this part of the garden, but instead we decided that a greenhouse would prove to be a much more efficient use of the space, and be much more useful throughout the seasons for seed starting, plant propagation,  and occasional sheltering of cold sensitive plants.

A greenhouse will help to extend the growing season, and provide much needed seed starting space in the garden

We seriously considered designing, and constructing, our own greenhouse, but with so many other projects still awaiting completion here we thought we’d first assess our kit-greenhouse options.

After some sleuthing, we found one greenhouse kit that caught our eye in a catalogue, but not having seen one in person, we were reluctant to purchase it sight unseen.  However, as luck would have it, the manufacturer of that kit was on the exhibit floor of the San Francisco Flower and Garden Show this last March, with an example on display of the exact model we were considering.

We were pleasantly surprised to find this greenhouse was very sturdy (unlike most we’d seen up to that point).  This kit is constructed from Pacific Coast Redwood, and twin-wall polycarbonate panels, instead of glass.

The greenhouse is comprised of preassembled redwood and polycarbonate panels

Although most traditional greenhouses are constructed of glass, polycarbonate has a number of advantages.  Single pane glass is heavy, and doesn’t insulate against the cold very well.  Polycarbonate panels are light weight, and the airspace between the walls of double or triple-wall polycarbonate helps to prevent rapid heat loss and gain.

Twinwall polycarbonate has an airspace between the walls to help moderate heat loss and gain

Glass also is potentially hazardous if it breaks (especially if the glass isn’t tempered), which was a concern here as we are surrounded by very tall trees that occasionally fall over, or drop limbs without warning.

Most importantly though, glass doesn’t diffuse the sunlight entering the greenhouse, and can actually intensify the sunlight as is passes through the pane, which can lead to scorching, and burning of tender young plants.

These polycarbonate panels help to diffuse sunlight as it enters the greenhouse to prevent scorching of tender plants

So although glass greenhouses are beautiful, polycarbonate is generally more practical, and isn’t prone to the opacification that some other types of plastics are when exposed to UV light.

After considering numerous options, we felt this was the best kit we could find locally, so we ordered the 8 x 12 foot greenhouse, which arrived in early May.  Unfortunately, other projects had priority at the time, and we couldn’t install the greenhouse without first constructing a retaining wall in the garden.  The recent completion of the garden wall though, finally paved the way for the greenhouse’s installation this weekend.

We began the installation late on Saturday afternoon.  The first task was leveling the base for the foundation, which was fairly straightforward.  Although no special base is required for this kit, we did add a few inches of base rock (with fines), both to improve stability and drainage, and compacted and leveled the area.

Greenhouse Foundation

The foundation of the greenhouse is made from a recycled plastic to resist rot, and was easily assembled.  We then chose to add some 1/4″ drain rock inside the base, as once the walls were up, it would be much more challenging to add the gravel later.

Once the base was prepared the pre assembled end-wall panels were installed.

First two back wall corner panels installed

The light was quickly fading as we completed the end-wall installation.

Corner panels, including the pre-hung door, in position

We managed to get the center roof truss together, but as much as we wanted to finish, the rest would have to wait until the morning.

In just a few hours the foundation and end walls were complete, and the center roof truss installed

Sunday morning we resumed construction with the completion of the remaining roof trusses.

The remaining roof trusses were installed early the next morning

The greenhouse is 10 feet tall at the peak of the roof, providing plenty of head space.

Completed Roof Trusses

Now it was time for the sidewall installation.  Each side wall required the installation of 3 pre assembled 4-foot wide panels, which were easily maneuvered into position with two people, and secured to the roof beams.

The side wall installation went quickly

The next task was to install the rear vented panels on the back wall.  In my experience, all kits have shortcomings, either in the quality of instructions, or the components.  Although we didn’t have an issue with the quality of materials in this kit, we did have an occasional issue with the instructions, or lack thereof.  It wasn’t clearly stated either in the instructions, or the instruction video that was included with the kit, that the rear base plate should be installed with the bottom support tie facing inward (those ties face outward on the other three lengths of the foundation plate).  We had suspected earlier that something might be amiss, and now we realized we’d installed that plate backwards.

With the post support plate facing outward, the vent can’t close completely

Fortunately it was easy to rectify by simply removing, and reversing the support plate.  Except for the lizards watching us from the retaining wall, I don’t think anyone else would have noticed.

As predicted, the lizards in the garden are finding the retaining wall next to the greenhouse to be a welcome addition to the garden, and like to hide under the cap stones

After that slight adjustment, with the walls complete, it was time for the roof installation.  In addition to the two rear wall vent panels, this model also has two independently operated vented roof panels, which can be installed on either side of the roof depending on local prevailing wind conditions.

There are two panels in the roof that open to vent hot air from the peak of the greenhouse

This was the part of the installation that we thought might prove to be too challenging for just two people, and had the greenhouse contained glass panels, it may have been, but the twinwall polycarbonate made the weight of the panels very manageable.

The vented side of the roof was installed first

With the roof installed, the core of the greenhouse was complete.  Next we installed automatic solar powered vent openers on each of the two roof, and two rear-wall vented panels.

The paraffin in the handle expands in warm weather, causing the vents to open

The black handles contain a paraffin wax that expands in the heat, and contracts in cold.  Cooler air is drawn in through the vents close to the floor, as warm air escapes through the roof panels on warm days.  If necessary during winter weather, the spring mechanism can be decoupled to prevent opening, and conserve heat.

In cool weather the vents close as the parafin contracts

The greenhouse installation wasn’t quite complete though.  As we will be using the greenhouse almost exclusively for seed starting and propagation, we’d need ample bench space for seed trays.  We had ordered two optional 12 foot bench kits, and a rear wall bench kit at the time we ordered the greenhouse.

The bench kits were a little awkward to install single-handed, as they’re constructed entirely in place.

The benches are constructed in place, and it took some time to install each of the redwood bench-top planks

After spending most of yesterday sweltering inside the greenhouse with a drill, a level, and a framer’s square though, they turned out beautifully, and are very strong.

Overall, it took two people a day and a half to construct the greenhouse, and I spent most of a day yesterday getting the benches installed.  The end result is a tremendously sturdy, very spacious, and efficient structure, that far exceeded our expectations for a do-it-yourself kit, that we hope will serve us well for many years to come.

With the benches complete there is ample space for seedlings to grow and thrive

Before the heat mats and grow lights are installed though, we’re going to use a clear sealer on all the wood components.  Although redwood is naturally rot-resistant, its life will be improved by occasionally sealing the wood.

We’re so happy to finally have the wall and greenhouse in position in the garden, as now we can finish building the last of the raised garden beds in this area.  Not to mention there’ll be no more potting soil in the kitchen!  Now we just have to decide which seeds we’re going to sow first…