Designing and engineering a passive solar greenhouse

Hello, I want to build a structure, a passive solar greenhouse, about 12x20 feet in area with possibly 1.5 or 2 floors. I will be storing hundreds of gallons of water for heating, so the load of the structure will be quite hefty. The ground is a 30 degree slope in a forest. I need help identifying my soil type and its load capacity. I believe my soil has high clay content as I can roll up the soil to a pencil size. Ive included some pictures of rocks, can anybody help me identify them? I had the idea of creating pyramid size piers with a 16.5 inches square base, about 7.5 inches tall, with a a 5.5 inch square top to fit a "6x6" post. The piers would be reinforced with 1/2 inch rebar. 10 inch L shaped threaded rods would stick out of the top halfway to connect to the posts. I thought to just set the piers on a few inches of gravel, but a friend of mine recommended that I dig 18 inches to below frostline, and fill with rocks and concrete and rebar sticking out, and then attach the piers at a later date.

More info is required to help.
- Are you planning to have the water storage sitting on the ground or the structure?
- can you have benches cut in the land?
- what are the walls to be made from?

Can a greenhouse be passive solar?  Seems to me a greenhouse has too much glass too be passive solar ...

Rather than passive solar the question might be how to build a greenhouse on total rock?

C Oakes wrote:Hello, I want to build a structure, a passive solar greenhouse, about 12x20 feet in area with possibly 1.5 or 2 floors. ...

How exciting! I wish I could help more, but I'm not particularly good with rocks. They look like nice building rocks to me, but the cliff face is a bit fractured. Have you tried looking at a geological map of your area?
Are you looking at yearly thermal inertia? I found a couple of references online for specific heat capacity, and it does look like water can store 4 x more heat than stone. Can you make it completely earth sheltered at the North side and use the rocks at the back as a support for the roof, or is that too much rock moving !

I've looked at a lot of them on various sites.  The ones that make the most sense to me are the greenhouses designed by Cord Parmenter in Colorado.  I'm going to use his ideas when I get to build mine in about a year.  They have classes every year at the Denver Botanical Gardens.  If you want to see some examples of his designs they are at pennandcordsgarden.com

It would be nice to know what part of the world you are in C Oakes.  It might help you in getting better ideas and answers in the future.  

Exactly, it would help to know your conditions and goals in order to give advice.
-- What are the typical winter night time minimum temperatures? (e.g. US hardiness zones)
-- What is your latitude? Further north (or poleward) there is significantly less sunlight in the middle of winter, but by March 21 it's the same as everyone else.
-- What is your sun exposure on the site? Major shading obstacles? How close to south-facing are you able to site it? (Go by true south, not magnetic south)

Goals:
-- Mid-winter fresh food? (Leafy greens and herbs are easy. Fruits much less so)
-- Tropical plants surviving a cold winter? (Difficult without heating and pest control)
-- "zone pushing"? That means growing things that require just a bit warmer winters than your location. (This is easy.)
-- Some greenery and flowers and a pleasant place to sit throughout the winter? (Very easy)
-- Starting seedlings in spring? (Easy)

I have been using a passive solar greenhouse in the high desert for some 25 years. I love it!

-- It is attached to my house and actually heats the house, being the sole source of central heating for the house. I did this at an offgrid school for 20 years, and at my (on-grid) private house with a few hours of electric back-up on winter evenings (mattress pad and occasional space heaters). This works great in the high desert with earthen thermal mass walls. Maybe not suitable in a stick-frame wooden house but idk. The house also moderates the night time lows in the greenhouse.

-- Mine is at 34N latitude, high altitude and desert, so the winter sun gives a LOT more heat than it would in someplace further north such as northern US, or anyplace in Canada or Europe. The typical outdoor night time lows in the area are -21C, a little below 0F. As a benchmark, the local pond hockey season is 6 to 8 weeks long.

-- Because these conditions are so ideal for solar gain even in winter, ours are very cheap and easy and low tech, just a sheet of UV resistant greenhouse plastic attached in October and removed and packed away in May. The plastic lasts about 7 years in my experience. Overheating is a major concern and plants would be roasted dead if left under glazing in summer in this location, so removing the plastic is great for my situation, also reducing overheating of the house in summer.

-- My greenhouse allows leafy greens and herbs all winter, which outdoors get frozen and die off in mid-Oct.

-- It keeps year-round plants that need just a bit warmer winters than outdoors, such as rosemary.

-- The temperature in the greenhouse actually goes below freezing on many winter nights, from Dec through Feb, but I think it's the protection from wind that allows the plants to stay alive. Lettuce, arugula, mustard greens, claytonia, mache, dill, parsley, cilantro, and several more stay green and good to eat all winter. Often they are frozen hard and dark green in the early morning but if I wait till the greenhouse warms up, these particular plants are good to go. Because we still get good sunlight all winter, these plants will grow all winter and produce, especially if sown in September and allowed to get established before winter.

-- The small asparagus bed in the greenhouse, and the tazetta daffodils, bloom in Feb, whereas they'd start in late April outdoors. This is a really nice emotional boost.

-- Aphids are a problem in my greenhouse, and sometimes mites or other things. I think it's due to the humidity and limited ventilation. For most of the winter it's bearable, and has been getting better as the permanent wood chunk mulch and its spider and lizard population have become established. But in mid-Feb, March and April when the greenhouse tends to overheat, the aphids get so bad, especially those cabbage aphids, that I pull out the arugula and mustard, and sometimes other things too. Ugh.

-- It's great for starting and hardening off seedlings. I start my tomatoes and squash etc indoors in a warm spot, then start hauling them out to the greenhouse for the daytimes and back indoors for nights, and then as temps in the greenhouse stay over freezing at night, I just leave them in the greenhouse. Works great!

-- For tropical plants, my greenhouse is not sufficient. I've tried leaving a lemon tree in a pot and a curry leaf plant in the greenhouse over the winter. They quickly turn black and drop their leaves in November. Then in spring they do leaf back out (so far) but are not as healthy as the previous season. They also have suffered from mites and aphids, etc. So I have to keep these in pots and haul them indoors for the middle of winter and back out to the greenhouse for shoulder season. Both would prefer to be in the ground or in bigger pots that I wouldn't be able to haul around.

Bryan Elliott wrote:It would be nice to know what part of the world you are in C Oakes.  It might help you in getting better ideas and answers in the future.  

Hello! I am in eastern TN at 36 degrees N

John C Daley wrote:More info is required to help.
- Are you planning to have the water storage sitting on the ground or the structure?
- can you have benches cut in the land?
- what are the walls to be made from?

I am planning to use 55 gal steel drums to store water on the inside of the north wall. The walls are going to be made of wood. I was also considering digging out earth to create the north wall, but considering i am doing everything by hand, it would be a lot of work. By "benches" do you mean like steps?

I drew a rough sketch of the structure that I would like to build. The dimensions of the passive solar Greenhouse Are 12 ft wide 19.2 ft long And about 22 ft or so high from the middle of the width to the very top. The angle of the roof is about 60° on the south facing side. This will allow the optimal amount of light to pass through on the winter solstice. Since I am at 36 degrees N latitude, the angle of the sun from the horizon at solar noon on the winter solstice would be 90-(36+23.5) = 31.5 degrees. This is a steep roof, perhaps it would be better to angle the roof to optimize light collection on the equinoxes? Or perhaps the average between the winter solstice and the equinox angles. For maximum collection on the equinoxes the roof should be angled at 36 degrees. For maximum collection halfway between the equinoxes and the winter solstice I should angle the roof at (36+59.5)/2 =48.25°.

Rebecca Norman wrote:Exactly, it would help to know your conditions and goals in order to give advice.
-- What are the typical winter night time minimum temperatures? (e.g. US hardiness zones)
-- What is your latitude? Further north (or poleward) there is significantly less sunlight in the middle of winter, but by March 21 it's the same as everyone else.
-- What is your sun exposure on the site? Major shading obstacles? How close to south-facing are you able to site it? (Go by true south, not magnetic south)

Goals:
-- Mid-winter fresh food? (Leafy greens and herbs are easy. Fruits much less so)
-- Tropical plants surviving a cold winter? (Difficult without heating and pest control)
-- "zone pushing"? That means growing things that require just a bit warmer winters than your location. (This is easy.)
-- Some greenery and flowers and a pleasant place to sit throughout the winter? (Very easy)
-- Starting seedlings in spring? (Easy)

I have been using a passive solar greenhouse in the high desert for some 25 years. I love it!

-- It is attached to my house and actually heats the house, being the sole source of central heating for the house. I did this at an offgrid school for 20 years, and at my (on-grid) private house with a few hours of electric back-up on winter evenings (mattress pad and occasional space heaters). This works great in the high desert with earthen thermal mass walls. Maybe not suitable in a stick-frame wooden house but idk. The house also moderates the night time lows in the greenhouse.

-- Mine is at 34N latitude, high altitude and desert, so the winter sun gives a LOT more heat than it would in someplace further north such as northern US, or anyplace in Canada or Europe. The typical outdoor night time lows in the area are -21C, a little below 0F. As a benchmark, the local pond hockey season is 6 to 8 weeks long.

-- Because these conditions are so ideal for solar gain even in winter, ours are very cheap and easy and low tech, just a sheet of UV resistant greenhouse plastic attached in October and removed and packed away in May. The plastic lasts about 7 years in my experience. Overheating is a major concern and plants would be roasted dead if left under glazing in summer in this location, so removing the plastic is great for my situation, also reducing overheating of the house in summer.

-- My greenhouse allows leafy greens and herbs all winter, which outdoors get frozen and die off in mid-Oct.

-- It keeps year-round plants that need just a bit warmer winters than outdoors, such as rosemary.

-- The temperature in the greenhouse actually goes below freezing on many winter nights, from Dec through Feb, but I think it's the protection from wind that allows the plants to stay alive. Lettuce, arugula, mustard greens, claytonia, mache, dill, parsley, cilantro, and several more stay green and good to eat all winter. Often they are frozen hard and dark green in the early morning but if I wait till the greenhouse warms up, these particular plants are good to go. Because we still get good sunlight all winter, these plants will grow all winter and produce, especially if sown in September and allowed to get established before winter.

-- The small asparagus bed in the greenhouse, and the tazetta daffodils, bloom in Feb, whereas they'd start in late April outdoors. This is a really nice emotional boost.

-- Aphids are a problem in my greenhouse, and sometimes mites or other things. I think it's due to the humidity and limited ventilation. For most of the winter it's bearable, and has been getting better as the permanent wood chunk mulch and its spider and lizard population have become established. But in mid-Feb, March and April when the greenhouse tends to overheat, the aphids get so bad, especially those cabbage aphids, that I pull out the arugula and mustard, and sometimes other things too. Ugh.

-- It's great for starting and hardening off seedlings. I start my tomatoes and squash etc indoors in a warm spot, then start hauling them out to the greenhouse for the daytimes and back indoors for nights, and then as temps in the greenhouse stay over freezing at night, I just leave them in the greenhouse. Works great!

-- For tropical plants, my greenhouse is not sufficient. I've tried leaving a lemon tree in a pot and a curry leaf plant in the greenhouse over the winter. They quickly turn black and drop their leaves in November. Then in spring they do leaf back out (so far) but are not as healthy as the previous season. They also have suffered from mites and aphids, etc. So I have to keep these in pots and haul them indoors for the middle of winter and back out to the greenhouse for shoulder season. Both would prefer to be in the ground or in bigger pots that I wouldn't be able to haul around.

Thank you for a thorough response!
I am in zone 7b. I dont know the average night time temperatures during the winter but I think it can get down to -5 or -10 degrees farenheit. Some of my goals with the passive solar greenhouse are to stay warm during the winter, grow some leafy greens, but also tropical plants like cold hardy bananas and maybe some others like citrus, avocado, or mango. The site is more or less a directly south facing slope, the are some trees obstructing my view, but if I had to guess I have about a 60 degree opening. There are also very tall trees maybe a few hundred feet away. But when I used my clinometer app on my phone, I measured the angle to the top of one of the trees at the very edge of the opening to be 12 degrees above the horizon. So I believe I should get maybe 5 hours of direct winter sun. I am only guessing as I have not experienced the wintet solstice on this site yet.

This is my latest design for the passive solar greenhouse, which is now earth sheltered. I decided to forego raising the structure on posts because it is less thermally effecient than building an earth sheltered structure. I made the roof 'curved', it is composed of multiple 4 ft segments, which decreases by 15° from the south facing vertical wall to the horizontal roof. Having a roof that is more curved will enclose a greater amount of space. The south facing side is still at least 18 feet. I have to figure out how to frame the windows as this is the determining factor for the length. The width, or east and west facing sides, are at least 17 feet in length and is determined by the geometry of the roof. The floor is composed of steps that are dug into the earth. Each step is at least 8.5 feet wide and about 5 feet deep. Apparently the earth stays around 50 something degrees farenheit 5 feet down, so i figure the earth is a better thermal mass than steel drums with water. I have to figure out what to do for the walls that are below grade. I want to do something like rammed earth or cob, but I don't know how those would hold up to exposed water. Perhaps I can waterproof the exterior somehow?

  What are you planning on using for the glassing?  
That design has many many joints.  To keep warm in the winter you must minimize air leaks. With all those joints you will have much higher air leakage.  Earth sheltered should work every good in zone 7.  Your ground temperature should ~50f and design temp can be 35f. This means the soil will heat the space on cold nights.  Make sure you insulate the foundation.  
   When I built mine I found the 55g barrels on the north wall very effective.  The other thing I learned was you need a lot of openable vents. The suggestion is 20% of glassing area.  Tha means 10% high and 10% low.  Without enough vents you will find it kills, bakes, everything in it in July.  I ended up with solar powered fans because I did not have enough vents. Tom

Thomas Michael wrote:   What are you planning on using for the glassing?  
That design has many many joints.  To keep warm in the winter you must minimize air leaks. With all those joints you will have much higher air leakage.  Earth sheltered should work every good in zone 7.  Your ground temperature should ~50f and design temp can be 35f. This means the soil will heat the space on cold nights.  Make sure you insulate the foundation.  
   When I built mine I found the 55g barrels on the north wall very effective.  The other thing I learned was you need a lot of openable vents. The suggestion is 20% of glassing area.  Tha means 10% high and 10% low.  Without enough vents you will find it kills, bakes, everything in it in July.  I ended up with solar powered fans because I did not have enough vents. Tom

I have 6 panels of 4' by 6' by 1/4" single pane tempered glass that I would use for the bottom two rows of windows. Most books that I have read recommend double paned glass. Maybe I can add another layer of thin window glass on the inside? I would also have removable insulated covers for them in the winter.

I've been reading some earth sheltered books and most of them recommend to use concrete and add petroleum based insulation to the exterior of the foundations. I was thinking of doing a more natural approach, like rammed earth walls that are underground. I had the thought of waterproofing them by adding a layer of clay to the exterior and then firing and glazing it. I watched a bushcraft video where some guy made a rainwater collection storage by this approach. Has anybody else tried doing this?

Our permies expert was Mike Oehler's his underground greenhouse book is worth a look.
https://permies.com/wiki/23444/Earth-Sheltered-Solar-Greenhouse-Book

Mike is where Paul got the 3 layers of poly earth roof used in the wofati.  From what Ive found on line dry dirt insulates very well.  Using 3 layers of normal construction poly that all slopes away from the building gives a large chance of dry dirt around the building.  Paul has brain-stormed natral waterproofing.  The most likly is a 6" layer of clay.  It would need to be a multi level reduntant system. Somthing like 2" of clay plaster directly on the wall and roof, 12" of sand/dirt on the roof plus walls bermed to 45° and then 6" of clay, plus 18" soil.  I dont think anyone has confirmed it works.  Plastic works and is much easier to spread.  

Researching portland cement the problem is it wicks water.  Your walls would be wet and that lowers the insulation effect. But it is strong even wet so it is the safe answer.  Another safe answer is an 18" thick wall above ground with large roof overhangs.  In zone 4 i don't think it will work, in your zone 7 it should be fine.

I have a been planing an underground greenhouse and ramed earth sounds like the wall I will use.  Stablized with type-K quick lime and protected by a 3 layer plastic earth roof Mike Oehler style.  Everyone says dont bury a ramed earth wall but if Mike can bury pine logs and not rot for 50 years, i dont see why i cant have dry walls with stablized ramed earth.  Plan is two 12'x12' rooms to suport the back wall in the center with an interior wall, flying butress.
I'm @ 43° North and planning on building something like this next summer.  Without glass in the roof it will not have full sun in summer, but I'm gardening outside then.