Passive solar greenhouses are overbuilt and too expensive

I'm looking to build a large scale(30x90) passive solar greenhouse in zone 5b, capable of being considered a forest garden greenhouse and growing small tropical trees, I'd like to jump 2 or 3 zones to zone 7 or 8. I've read the forest garden greenhouse and it seems that most of the greenhouses that are of large scale end up going with expensive concrete foundations with huge metal rafters in order to have a really large span, the cost of these heavy duty structures is too much.

I'm going to pose some questions here and answer them myself. Please feel free to jump in and provide your own answers or thoughts.

Why do we need so much concrete to build a passive solar greenhouse? I think we could do it with almost 0 concrete or at least just concrete posts in a few spots instead of a full foundation wall.
Can we use a much cheaper structure, such as a hoop house? I think we can use a cheaper structure like a large hoop house that is built to withstand snow load(https://www.rimolgreenhouses.com/greenhouse-series/noreaster to example hoophouse). On the north wall and roof, forgo glazing and insulate with spray foam sandwiched between two billboard tarps, one fixed to the interior of the north wall and one as a cap over the spray foam on the exterior.

I would also add frost insulation in the ground around the perimeter of the greenhouse, have insulated east and west end walls as well as a climate battery (GAHT,SCHS).

Passive grreen houes
- catch and hold the suns heat
- have mass masonry or concrete to hold and release stored heat
WIDE CLEAR SPANS
- enable movement anywhere in a commercial greenhouse with no restrictions as to the type of equipment
Frost insulation is admirable but you need heat storage banks also.

Try you ideas with the billboard tarps and let us know how it works.

The Rimolgreenhouses  look great and serve a purpose well.
- They are not passive
- they use gas heaters

Kevin Swanson wrote:I'm looking to build a large scale(30x90) passive solar greenhouse in zone 5b, capable of being considered a forest garden greenhouse and growing small tropical trees, I'd like to jump 2 or 3 zones to zone 7 or 8. I've read the forest garden greenhouse and it seems that most of the greenhouses that are of large scale end up going with expensive concrete foundations with huge metal rafters in order to have a really large span, the cost of these heavy duty structures is too much.

I'm going to pose some questions here and answer them myself. Please feel free to jump in and provide your own answers or thoughts.

Why do we need so much concrete to build a passive solar greenhouse? I think we could do it with almost 0 concrete or at least just concrete posts in a few spots instead of a full foundation wall.
Can we use a much cheaper structure, such as a hoop house? I think we can use a cheaper structure like a large hoop house that is built to withstand snow load(https://www.rimolgreenhouses.com/greenhouse-series/noreaster to example hoophouse). On the north wall and roof, forgo glazing and insulate with spray foam sandwiched between two billboard tarps, one fixed to the interior of the north wall and one as a cap over the spray foam on the exterior.

I would also add frost insulation in the ground around the perimeter of the greenhouse, have insulated east and west end walls as well as a climate battery (GAHT,SCHS).

I think you could skip the concrete completely, or just use some for pads supporting the arch bases. If you need thermal mass, it might as well be soil that is growing your food. I think your idea about insulation around the perimeter is good, assuming you meant laying it flat and extending it out from the walls. That would/will be a huge greenhouse.  

I have hoop houses.  They get very warm in the sun, but as soon as the sun goes down, the drop right back to outside temperatures.  With water stored in them, you can retain enough heat to keep plants from freezing in the spring and fall when the temps drops below freezing, but not much more than that.  The work well as season extenders, but I don't think you can achieve a zone 7 or 8 greenhouse year round without considerable expense.  With heat tubes, you can get much closer, ala "citrus in the snow", but he still uses extra heat, concrete, etc.

I think it depends on a lot of things. Walipini style would be relatively cheap. Built into a hill, dug into the ground, two or three layers of plastic for extra insulation. If you did a hoop house inside a hoop house that would add extra insulation as well. I could imagine a dome greenhouse set into the ground an additional four feet under the protection of a hoop house set into a hill in order to create that extra 1 to 2 zones difference.

My tiny passive greenhouse seldom gets down below 25, and the only time I've seen it hit 20 it was 4 degrees outside. It's set back against a hill and has a waterwall. When we built it we repurposed cement blocks from some garden boxes we tore down. My friend dug hers (also passive) into the ground and I've never seen the ground freeze. She has some borderline tropicals in there, and while the tops died they came back from the root. I see no reason that with a little more insulation it couldn't stay above freezing year round.

One thing you may want to consider is that swimming pool cover, the kind that looks like bubble wrap. It's designed to catch the sun and concentrate the heat back into the water. I see no reason it wouldn't do the same to a greenhouse.

The One Yard Revolution YouTube channel uses multiple layers of cover to good effect.

Mine wasn't cheap but I think it was under $12k.  20x40.  But I spent a LOT of time on it.  Minimal concrete.  I'm in zone 4a and it gets me to 8a.  Not the tropical temps I was hoping for but in a slightly warmer place with some sun in November/December it would do just fine.  No GAHT due to the lack of sun for half the winter.  Mike's passive solar greenhouse

This guy did it pretty smart imo

      Howdy Kevin and everyone else,


      This is an interesting conversation and I'd like to add my thoughts because I'm getting ready to create a greenhouse on the land I take care of. However, as an aside and still germane to this prospect I'd like to broach two points. I have always appreciated Mollison's teachings; they are always design experimentation, to see what works and share that, as in were all in this together, here are my results. This is what, as I see it, Permaculture Institutes do. Secondly, since there are many different people and situations we can and should build what we want, what suits our goals, climate, personality, income etc. best. To that end, people being creative, build many different styles of greenhouses, buildings, cultures etc. For me, usually I think this is rather a good thing.
       The greenhouse I will create, with a little help from my friends, no scratch that, a lot of help from my friends, will be 24' by 12' on a hillside slope to the South infilled with strawbales on the sides and back on grade. Then we're going to cover it with 6mil clear plastic. We'll see how much this simple, inexpensive greenhouse will perform. A last point I would like to note is, changing one's climate several zone's, usually means more expense  and complex inputs, however I have seen a basic "heat pump" idea that seems to me on the face of it gets you there, "Nebraska retiree uses earths's heat to grow oranges in snow" on YouTube; It's worth the time. Last point, I like small gravel on the floor of greenhouses. Best of luck with your project.


     Thomas,

      Mitama farm

Mike Haasl wrote:Mine wasn't cheap but I think it was under $12k.  20x40.  But I spent a LOT of time on it.  Minimal concrete.  I'm in zone 4a and it gets me to 8a.  Not the tropical temps I was hoping for but in a slightly warmer place with some sun in November/December it would do just fine.  No GAHT due to the lack of sun for half the winter.  Mike's passive solar greenhouse

Mike, I looked at your greenhouse thread. Pretty cool, what you built. Also, your jump from 4a to 8a seems like the kind of encouragement the OP needs.

Trace Oswald wrote:I have hoop houses.  They get very warm in the sun, but as soon as the sun goes down, the drop right back to outside temperatures.  With water stored in them, you can retain enough heat to keep plants from freezing in the spring and fall when the temps drops below freezing, but not much more than that...

Trace, how large of a container do you use? My multiple 2 liter bottles experiment have failed me.

The problem I have with Ground Air Heat Transfer Systems, and using the ground as thermal mass inside a greenhouse in general is the digging.
Unless you dig down deep to where earth temperatures are steady year round and/or dig down and install insulation, you will be losing a lot of heat to the surrounding soil.
My (under construction...)greenhouse has some perforated black pipe buried under it, but they are only a few feet deep and  the soil volume around them is not insulated.
Because of this the greenhouse has repurposed refrigerators as raised wicking beds and a water heater tank as a tempering tank.
With such a small volume of mass to work with, seasonal thermal storage is out of the question, but daily storage is not.
I might use air to earth or air to water transfer to move the heat into the beds, I'm not sure yet.
Each bed will have its own layer of glazing, something like a low tunnel and I will hang clear plastic from the same rods that hold the shade  curtains during the summer.


As for construction, the wooden posts I used were set into free draining buckets of rock and gravel.
The buckets are pinned to the earth with metal stakes, and the combination seems to be quite stable, horizontally and vertically, as long as the posts buttressed to each other as part of a structure.


The other greenhouse I'm planning will also have insulated beds and might also have an insulated floor.

I have wondered, would a heavily insulated windowless building  equipped with PV solar ,an electric heat pump/AC and grow lights, make for a more effective winter growing space , even if it had NO batteries?

William Bronson wrote:I have wondered, would a heavily insulated windowless building  equipped with PV solar ,an electric heat pump/AC and grow lights, make for a more effective winter growing space , even if it had NO batteries?

William, there is a much better way to insulate and at the same time does not sacrifice for transparency. That is to introduce water or better still liquid soap bubble foam into the greenhouse envelope. Check what I am doing which is cost effective in two ways: much cheaper to build and much cheaper to operate.

Joylynn Hardesty wrote:

Trace Oswald wrote:I have hoop houses.  They get very warm in the sun, but as soon as the sun goes down, the drop right back to outside temperatures.  With water stored in them, you can retain enough heat to keep plants from freezing in the spring and fall when the temps drops below freezing, but not much more than that...

Trace, how large of a container do you use? My multiple 2 liter bottles experiment have failed me.

I use gallon milk jugs. I have a waterwall at the back, two 50 gallon tanks, and water jugs around the entire base. Probably about 200 gallons altogether for a 9x11 interior space.

I have never seen this word before
Quote
"Germane" meaning relevant to a subject under consideration

Where is it in common use please?

Here's a link to Paul's passive greenhouse.

John, that word is not exactly common but it is used here in the USA.

Aubrey Zhang, I've followed some of your other work, very inspiring.
Just scrolling through the greenhouse build it looks like you're setting up a GAHTS using gravel.

I will definitely look into your information on retrofitting a bubble system to an existing structure.

Of course a GAHTS or bubble insulation are the opposite of passive solar.
Storing more of the solar heat in the thermal mass is why we turn to active systems
To adresss the original point of this thread, I think eliminating the concrete foundation is a good idea.
Use water/and or your growing medium as your thermal mass and insulate it from the ground.
Add layers of some kind of glazing,  for insulation, and hopefully it will be enough to work as a passive system.
Presumably standard greenhouse vents are acceptable as a passive solution to overheating.



A side note, glass is heavy and can be expensive but it doesn't decay from sunlight.
Plastic is the opposite.
For these reasons, I'm putting layers of plastic under a single layer of glass, in hopes of splitting the difference.

Joylynn Hardesty wrote:

Trace Oswald wrote:I have hoop houses.  They get very warm in the sun, but as soon as the sun goes down, the drop right back to outside temperatures.  With water stored in them, you can retain enough heat to keep plants from freezing in the spring and fall when the temps drops below freezing, but not much more than that...

Trace, how large of a container do you use? My multiple 2 liter bottles experiment have failed me.

I have 4 55 gal drums.