What to think about when building a greenhouse
May 15, 2015 12:25 AM   Subscribe

We're going to build a greenhouse from a kit in the garden, around 16 sq meters (around 170 sq feet) in size, probably rectangular and with a regular peaked roof. It will be used mostly for herbs and vegetables, and starting the garden plants in the spring. We haven't owned a greenhouse before, so we need some assistance with the different design decisions:

We're thinking of one of the kits that mount on top of a low foundation wall, like in this picture.

Layout: We're thinking about making the wall from LECA or lightweigh concrete blocks, and building two raised beds inside the greenhouse from the same material along the long axis walls of the greenhouse. This will of course be a rather permanent layout, compared to the more flexible approach of a flat greenhouse floor and benches with containers on them for the plants. Any toughts on the advantages of one over the other? Or maybe a hybrid of the two?

Should we spring for a water connection to the greenhouse? My wife says it's no problem to carry water in, but I suspect that the initial investment of water on tap will be repaid in increased comfort rather quickly.

The size of the foundation: I'm thinking that we should make the foundation a bit larger than the footprint of the greenhouse, and put down some patio slabs or other covering in an area in front of the doors, to have some solid surface to work on when it comes to planting. My wife thinks the lawn will suffice, but I think we will basically kill it with traffic and turn it into mud. Any thoughts?

And please hit us with anything else you think we should consider. This will be a major investment, and we'd like to get it as right as possible.
posted by Harald74 to Home & Garden (9 answers total) 10 users marked this as a favorite
The best greenhouses I've seen consider, like any structure, the resources they need to run efficiently and involve them as part of the structure / have them on hand nearby.

Consider the placement of the structure on your plot. Most people put a greenhouse out of sight, but the south wall of your house catches the warmest rays and acts as an overnight heat store, so putting it there would be great if allowable. Additional bonus being easy access from your living space - makes it all the easier to get in there for propagation in the colder months.

Depending on where you are you might also want to consider how you will regulate the temperature. Making sure you can relieve heat with vents and I've even seen a wood burner in a greenhouse for additional winter warmth.

Finally, water - you can't have too much, but you too little is easy, so use the greenhouse as a rain water collection unit. Either into a water butt or even, straight into the planting via drip irrigation.
posted by mjlondon at 1:45 AM on May 15, 2015

I agree that you should consider attaching the greenhouse to the south face of your house. You might also look into building it with a heat sink.
posted by mareli at 5:22 AM on May 15, 2015 [1 favorite]

Having the water connection would allow you to set up automated irrigation, which is what will allow you to go out of town during hot weather.

Personally I'd go for flexibility (shelves) over permanent beds, but I've never built a greenhouse, only used already-built ones, so that was never a decision I have had to make. The shelves also allows you to use the space underneath for storage, and greenhouses always seem to accumulate thousands of pots and other planting supplies.
posted by Dip Flash at 5:27 AM on May 15, 2015

Hauling water is not fun. If having running water in the greenhouse is at all possible, do it.
posted by sciencegeek at 5:45 AM on May 15, 2015 [2 favorites]

I built a 160 sq ft. greenhouse two years ago, very similar to the picture you linked to, but not from a kit.

You'll want water in the greenhouse if possible. Not only to eliminate any water hauling, but also to facilitate an automatic watering system. I built a simple drip-gravity system that runs off a pump which runs for 2 minutes, twice a day. I plan to convert it to solar eventually. Here's a picture of the 500gal "trough" I built to hold water and serve as a heat sink, temperature regulator and a source of water for irrigating.

Automating the watering made a huge difference in my enjoyment of the greenhouse. The first Summer I didn't have it, and would regularly go into the greenhouse and find the temp over 100 degrees F and the plants stressed out. The automatic watering system made the plants healthier, which (I think) in turn kept the greenhouse cooler (more shade from healthy leaves).

Greenhouses get very hot. You'll have to consider how you're going to keep it cool, or even how to harness that warmth. I put a passive solar heat sink beneath one of the beds in my greenhouse. It's basically about 1,000 punds of rocks that I blow warm air into when the sun is shining. The air is pulled from the peak of the greenhouse by a small fan. This is totally automatic, and runs off a small solar panel and a 12v fan. Here's a picture where you can see the piping used to move the air. Another picture. And here's a picture of what it looked like after loading the rocks and preparing the bed for soil. In the Summer I can revers the fan and pull cool air through the rocks and blow it onto the peak of the greenhouse. Here's a picture of the mechanical parts of the system.

About your foundation... Having a pad in front of the door will be helpful, as would a door wide enough to accommodate a wheelbarrow. You probably don't need to go any wider along the sides of the greenhouse.

If I was to do it over..... I'd go bigger. 10 ft by 16 ft seems large, but in hindsight I should have built it 14 x 20. I think a bigger greenhouse wouldn't have the severe fluctuations in temperature that mine has, plus -- more room for plants!

PM me if you have any other specific questions, or want to elaborate on anything posted here.

gratuitous pictures
posted by bricksNmortar at 7:01 AM on May 15, 2015 [3 favorites]

Response by poster: Thanks for the replies so far! Having a heat sink like mareli and bricksNmortar recommend hadn't even crossed our minds!
posted by Harald74 at 9:13 AM on May 15, 2015

Kill the grass with traffic and turn it to mud, then you can fix the problem.
posted by aniola at 10:32 AM on May 15, 2015

The more water you store inside the greenhouse, the more stable the internal temperature will be. If you can design space under your raised benches for five thousand litres of water, you won't need an active heatsink. If you can arrange for the containers that the water is stored in to be dark in colour and exposed to winter sun but shaded from summer sun, so much the better.
posted by flabdablet at 11:28 AM on May 15, 2015

Water has about twice the heat storage (and hence temperature regulating) capacity, volume for volume, as granite.
posted by flabdablet at 11:31 AM on May 15, 2015 [1 favorite]

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