Greenhouse Sizing

Hi All,

Zone 6, north aspect, 1000' above sea level on an island.

I will be building a 4 season greenhouse come fall.
Translucent hard polycarbonate. Dirt foundation.
I hope to insulate the north wall from the outside and use water in black tanks to store heat.

The greenhouse long wall will be facing ESE on a slope.

I'm waffling about size and shape. 1. 12'x24' with 10.4' height. Or 20'x21' 10.4' height. This is to feed two people. Is it too small?

Thoughts?
Thanks
Jess

Hi Jess,
Welcome to Permies!

I have never heard anyone ever say "my greenhouse is too big". So my short answer, would be to build it as big as you can afford.

The more nuanced answer would require knowing how you are planning to use the greenhouse. Will you be planting in the ground? Will you be using pots? Will you be doing seed starting on part of it? How much of your food are you hoping to grow inside? With 10ft ceilings, could you create a tiered shelf system to get more plants in the space?

I agree with Matt.  Bigger is better.

But how do you plan to use it?  

If you plan to grow bananas and citrus you may want more height and more square footage.

If you plan to only grow greens the smaller size is probably big enough.

I have a winter greenhouse that is about 16x 9.  It is barely big enough to use it to start our plants a couple months early to transplant into the garden.

Do you plan to add heat to the greenhouse in the winter?  If so maybe bigger ISN'T better.  It will take less fuel/electricity to heat a smaller area than a bigger area.

Consider snow load and the ability to shed snow.  I have raised beds along each side of my 12 ft wide high tunnel.   It is more than adequate for salads for 2 people.

J Hillman wrote:Do you plan to add heat to the greenhouse in the winter?  If so maybe bigger ISN'T better.  It will take less fuel/electricity to heat a smaller area than a bigger area.

I was thinking before I read this, that being able to divide the large greenhouse into zones, would be something I would try if I had a large greenhouse. A Greenhouse within a greenhouse for cold intolerant plants, surrounded by areas for plants like kale that can even take some freezing, might give you extra options.

I read a book from someone in France with a high green house who had a low area used as a chicken roost with a flat roof so they could start seeds above the chickens. The chickens had a pop-door that led to outside paddocks, but also had areas within the greenhouse where they could be contained for pest control and fertilizing services.

So much depends on your vision!

If you have a style already in mind, I would say for two people start out with 8 to 10 feet wide, and 20 feet long in whatever configuration works, and expand as you go.

I built five 10x19 foot quonset hut style greenhouses 25 years ago, and they have evolved to two of them holding fruit trees with just chicken wire covering and some fabric in the winter, and the other three has vegetables.  The reason I say 19 feet long instead of 20 is that the greenhouse plastic comes in 20-foot lengths, and you need 6 inches on each end to roll around the ends to seal it.  (As an example, the ends will be flapping open if you put a 20 foot piece of plastic on exactly a 20 foot frame.)

The trees are frost sensitive, banana, avocado and tangerine, lemon.  

A quonset hut is 20-foot rebar, covered in 1/2" PVC pipe,  bent in an arch, ends 10 feet apart.  Ends inserted into the ground, but the ground doesn't freeze here, so that extra depth beyond frozen soil would be part of the calculation.   It is about 71/2  feet high, arches are about 3 to 4 feet apart.  There are wooden supports going down the middle because the center of the rebar, over time, will start to sink, especially with the weight of plastic or panels or shade cloth on it.  

This style is really good in bad winds.  We had what they call a microblast, approx. 80-90 MPH winds for 1-2 steady minutes, not a gust, everything shook, but the greenhouse had very little damage.  It can be added onto if you want.  It's DIY, so it's not as expensive as a kit.   I put a chicken wire wall between two 19-foot sections so if a packrat gets into one section, it probably can't get into the other section, and I can get rid of it before too much damage is done.  The door is wide enough to accommodate a wheelbarrow.  

Not sure why people want really high greenhouses.  I want the heat down around the plants, not up at the 8 or 10 foot height.  I also don't want to have to get on a ladder to repair them.  I don't want them up in the wind.  Greenhouse plastic comes in specific widths, and a 20-foot width of plastic is what covers an 8-foot wide arched greenhouse.   I put 2-foot wide, 8-foot long patio panels around the bottom and the plastic comes down over the patio panels.  It keeps the animals from seeing all the green in there and trying to get to it.  Deer will try if there's a drought and they are low on food.  Foxes, and rabbits, too, they love tomatoes and fruit.  

Exactly what I was thinking. Thanks for that reinforcement. My preference is off grid but if I want to heat it in the winter there will be energy demands. A larger GH is so much more appealing but the reality is it will use far more energy.
Thanks!

J Hillman wrote:Do you plan to add heat to the greenhouse in the winter?  If so maybe bigger ISN'T better.  It will take less fuel/electricity to heat a smaller area than a bigger area.

I really appreciate everyone's input so far 🙏
Don't stop if you have more, I appreciate every bit ❤ Thank you!

I hope to have a greenhouse someday with a rocket mass heater running either down the center or on one side, using the mass as a heated shelf for potted plants and seed starts in addition to heating the space on the coldest days/nights here in Western West Virginia.

I also hope it can be big enough to host a lemon and/or tangerine.

My greenhouse is quite small. I live where I can grow year round outside, so I saw it as an indulgence. It's about 6 1/2' X 5 1/5'.  I already wish it was bigger. I filled it up when I was starting seeds. Mainly I wish it was big enough for a table, or potting bench. Because I didn't realize how wonderful it is on a cold rainy day to spend time in the greenhouse. It's just a great place to spend time.
I'm short, and I didn't want to waste all the space I could not reach. I built a very simple pulley system, and now I can use the upper space and still have access.
I'm not necessarily a bigger  is better type of person. Bigger holds more, but it also cost more to build, heat, cool, and even consider the space it will take up.   Maybe decide what you want to do with it. Try to estimate how much space you need, Then maybe build a little larger, for the unforseen. Good luck, please post pictures.