Before starting on this project I’d built or substantially remodeled a few homes so I was relatively familiar with the kinds of decisions I would need to make and also the order and general pace of the architect’s and builder’s activities. Once I started to consider building a greenhouse-enclosed home, I wanted to know what might be different in the design and construction process for a greenhouse-enclosed home versus a standard home. Although I searched pretty thoroughly, I did not find where any of the owners, designers, or builders of the existing greenhouse-enclosed homes made that information available. Which is one reason I started this blog; to leave a ‘road map’ for those who come after me. Previous posts have addressed some of the design considerations like controlling for pests, for mold, and for heat. This post describes some of the construction differences.

Who Erects the Greenhouse?

Some construction differences flow from the choice of greenhouse. I found a greenhouse company in Europe that designs incredibly beautiful bespoke greenhouses. They were not only outside my budget, and too far away, but a bespoke greenhouse package limits who can erect it and how long it will take to erect. By contrast, I purchased my greenhouse from a major agricultural/commercial greenhouse manufacturer by selecting among a few options for one of their standard models. It comes pre-cut with all the pieces and fasteners. Assembly is about the same for every greenhouse of this make and model. There are builders who specialize in erecting this manufacturer’s greenhouses who haved gained the efficiency that comes from assembling more than one of these.

Home builders are familiar with engaging sub-contractors for the parts of the construction they normally sub out but they’ve probably never had a large greenhouse as part of the build. We got the contact info for the greenhouse building team from the greenhouse manufacturer’s salesperson when I placed my order. That provided some weeks to make contact; compare schedules for their availability versus my package’s arrival and my builder’s estimate of when my site would be ready; then to get a bid, and to secure a slot in that team’s schedule.

Greenhouse or House First?

Once we had a slot in the greenhouse builder’s schedule, we needed to be ready for them to start or, possibly, lose our slot as they move on to the next job who is ready for them.

What constitutes ‘ready’ is different for a project where the house is built after the greenhouse than it is for a project where the house is built before the greenhouse. In my case, it’s sort of a hybrid situation. My permit was approved in two parts – the first approval included the house, retaining wall, and the level area for the greenhouse, the second approval was for the greenhouse structure. It took so long to get the first approval that we began that work as soon as the excavation contractor was available. Lead times for the greenhouse manufacturer were running so long we planned to press ahead building the house and erect the greenhouse over it when the permit was approved and the package arrived. But then schedules started to fall into line that the greenhouse package would arrive around the time between when the foundation was done and before framing started so we shifted to erecting the greenhouse first.

There are a few reasons to erect the greenhouse first and then the house inside. First, the greenhouse arrived before the house framing started so it didn’t make sense to have the greenhouse material taking up valuable space and have to be protected for months while building the house. Next, but maybe more importantly, is it allows the house to be protected from the elements during construction. I’m building over the winter in the Pacific Northwest of the United States. That means we expect rain every day and any day that doesn’t rain is a bonus. Building the house inside the greenhouse means less time and expense to dry it out before finishing the interior, less chance of materials cupping or warping, and less chance of mold growth. Finally, it will be more comfortable for everyone working on the house. Although we don’t get snow and freezing temperatures like most of the United States, every day is wet and the average temperature may be in the mid-30s to mid-40s. Not only will people and materials be dry inside the greenhouse but we’re hoping for a 5 to 10 degree temperature gain. Folks perform better, and work more safely, when they’re not wet and freezing. Finally, as crazy busy as all construction trades are these days, I’m not above using the promise of working dry to entice workers to my job over others vying for their time.

Getting Ready

Erecting the greenhouse first means that no one will be able to use a machine taller than fits in the double doors and under the trusses (approx. 12’) when operating. This is changing the order that a builder would normally do some tasks, or at least takes away the flexibility of scheduling some tasks later based on personnel or material availability; tasks like backfilling the foundation, setting grade, and laying pipes and conduit for utilities. Between the pre-existing structures on my property (2 cabins, an RV pad, 2 driveways and parking areas, and a 3-part septic system) plus mountains of dirt from excavating for the house, pool, and retaining wall, there was little room to maneuver a backhoe, much less receive the greenhouse package. So each task toward getting ready was like dominoes where one task had to be done to make room to do the next and so on.

Drainage, Utilities, and Backfill

Often a house is framed before the foundation drain and backfill is done. In this case, however, we needed to put in the footing drain for the retaining wall and backfill that to make room to receive the greenhouse package in case it didn’t fit in the ‘upper parking’ area. Also to use up enough of one of the dirt piles to be able to move the backhoe down to the house and greenhouse level. Then we installed the drainage around the house to be able to finish backfilling the house. Also trenched to stub out conduit or pipe for all utilities from the house to outside the perimeter of the greenhouse and then closed all those trenches.

The greenhouse wall will be at the edge of the pool – too close to drill the 24” diameter post holes as will be done for the other 3 sides. So 24″ sonotubes were put into place before the north/pool foundation was backfilled. But to make room for the backhoe on the north/pool side we first had to clear some tall brush and a couple storm-downed trees.

Final Grade

My greenhouse is 35’ wide by 60’ long. That means I needed a level area about 45’ wide by about 80’ long. But my property has about a 4’ change in elevation from one end of the greenhouse area to the other end. We’d planned to lower the high end about 2’ and raise the low end by about 2’ but ran into rock in the upper corner. Rather than incur the delay and added expense to bring in additional equipment to break out the rock, we raised the lower end about 3’. That meant a lot of dirt moving and leveling that certainly would not have been done this early in standard construction where grading the yard around the house is generally considered landscaping. It was, however, very satisfying to watch the mountains of dirt disappear and to see final grade.

Air-to-Ground Heat Exchange

It’s critical to provide cooling in the greenhouse and one of the least expensive and easiest to install is an air-to-ground heat exchange system. These are comprised of buried pipes, connected to manifolds, connected to risers. In my case, there will be 2 air-to-ground heat exchange systems (A2G); one under the pool and one under the food garden area. The bottom of the pool will be just a rubber liner laid over a cushion layer laid over washed rock. That A2G system has the riser in the greenhouse’s northeast corner that will draw hot air down to a single layer of pipes run under the length of the pool where the cool water will cool the air that will exhaust back into the greenhouse via a riser on the northwest end of the pool. This needed to be installed before the greenhouse so that the backhoe could bring in all the rock laid under and over the pipes and to level the pool floor.

The A2G system under the food garden is a double layer of pipes; the first layer at about 4’, and the second layer at about 2’ below the surface. These pipes are backfilled with dirt, rather than rock like the one under the pool. But again this was installed much earlier in the process than a heat exchange system would have been for a standard home build.

Post Holes

Final step to be ready was to drill the greenhouse post holes. For safety reasons my builder didn’t want to have twenty-two 24” diameter holes opened too far ahead of the greenhouse installer’s arrival. But once again the schedules aligned pretty well that all the other tasks completed only a few days before the installers would arrive to pour the concrete and stand up the posts. Except for the occasional rock to fish out of the holes, the drilling process seemed to go smoothly. Oh wait! Let’s not forget about nicking the new septic line from the trailer to the waste tank. I swear my builder can’t catch a break on this project; everywhere he digs hits something. At least the next couple weeks of work, except for pouring the concrete and standing up the posts, will be above ground.

We’re now ready for the greenhouse installers!

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