Suggestions Appreciated- Solar Earth-Sheltered Greenhouse Plans in New England

Hey all,

I’m a longtime lurker, but first time posting because I finally have something to contribute and I need your input.
I’m in Zone 7A, costal Connecticut, and I’m planning on building semi-earth sheltered greenhouse, hopefully breaking ground this fall. I put together my design from Mike Oehler’s Earth Sheltered Greenhouse book, The Year Round Solar Greenhouse, and various internet sources (permies forum included).

I’d really appreciate some input from you on if it looks like it would work, and if you have any suggestions for improvement. I’ve included some drawings that are approximately to scale, for reference.

My main goals for this greenhouse are:
1) winter housing for our quail covey (about 12-15 pharoah coturnix)  
2) extending the growing season (not much space for seed starting in a tiny house)
3) winter composting/compost heater in the cold-sink

and some stretch goals:
1) overwintering dwarf citrus and fig trees
2) some year-round vegetable production (even if it’s just lettuce)
3) possible rabbit warren in cold-sink area

To get around zoning regulations that would require a permit, it has to be a separate structure, less than 120 sq ft of floor space, and 12’ at its highest point. (The way the regulation is written, it’s unclear if I can count height from the base of the structure or from ground level, but I probably don’t need 16’ of growing height anyways.)  So, I’m planning on about 8’ x 15’ x 12’ roof peak. I chose a saltbox style roof to maximize Southern exposure and  headspace. Only partially submerged to make use of the brand new triple-paned windows I got for free (!!!) on freecycle, and because I want something nice to look at in the yard.

Siting:
Our yard is naturally sloped, so the north face of the greenhouse will be a little deeper and have a bit of natural berm already. We have several large deciduous trees on our property that won’t block sun in winter but will provide some shade when our summer temps get in the 90s+. At our prospective site, the greenhouse should be in full sun from 8a-330p during winter (earliest sunset is ~4p), and closer to partial shade in the summer. Prevailing winds from the SE are mostly blocked by our house and garage.

Construction:
Frost line in my area is 42”, so planning on excavating to 4’ for the floor level and an additional 3-4’ for the cold sink. Walls for the submerged portion of the greenhouse will be cinder block construction on cement footings (I want to avoid pouring a full slab if possible), with foam board insulation on both sides. Installing French drains for the rest of the floor space.  Above-ground is stick framing to accommodate my differently-sized windows (my main reason for avoiding PSP construction). Roof will be corrugated plastic. I will most likely have electrical run from the house in the form of a buried cable, but I’ll leave installation of that to an actual electrician.  

Heating/Cooling:
Glazing will consist of triple-paned glass windows on South wall, clear corrugated plastic roofing on South Side, and maybe a glass door on the East face (depending on if I can find a nice one at the Re-store, otherwise a solid door). All other walls will be insulated. Vents with automatic vent openers will be on East and West faces, both at roof peak and just above snow level, and I may add more venting capabilities on North side of roof. I plan on keeping water barrels inside on the North wall for passive heat collection and storage, and I have access to plenty of wood chips and poultry litter to keep a compost furnace going in the cold sink.  Our winters are fairly mild. We had maybe one week of negative temps this year, but otherwise averages of 20s-30s F. I’m hoping to only use extra heating during freak storms.


So my questions to you is- does this look feasible? Am I incorporating design elements appropriately to meet my goals? Or is there anything that you’ve found would work better? Any constructive feedback is appreciated. The last thing I want to do is have a pit in the backyard that I can’t use for its intended purpose.

Thanks for reading

Hi Elle, welcome to the posting side of Permies

The only thing that caught my attention was insulating both sides of the foundation.  I think it's recommended to just insulate the outside so that the foundation can become part of your thermal mass.  But maybe I'm off base if it's for a cold sink?  

Would you be insulating below the floor of the cold sink or trying to harness the natural temps down deep in the soil?

With an insulated foundation, insulated walls and a corrugated plastic roof, you'll probably lose 90+% of your heat through the roof.  This might be a case where it's worth upgrading to twin wall polycarbonate for a better R value.

Looks like a fun project!

I thought I remembered reading in one of the books that they recommended foam board on either side to keep the warm interior environment you’re creating from trying to exchange with the cooler exterior. Though that may have been me reading into the ICF building section too much.

The cold sink would be uninsulated to promote heat exchange for the colder air that should collect there.

And the roof is definitely the weak point of my design right now. Looking into roofing with a better r-value is on my list. I am worried about not enough light transmission, though, given that my wall is triple-paned glass.


Edited to add: year-round solar greenhouse does indeed suggest two layers of rigid foam board insulation, BOTH on the outside. Oops.

We're always trying to find the balance between maximum light and maximum heat retention.  Most of the greenhouse books I've been reading say to try to keep light transmission up around 80%.  I think twinwall polycarbonate is in that range...

Nice design! I like it! It's VERY feasible.
I'm envious of your triple glazed windows. Wow! What a score!

I'm with Mike on using double walled polycarbonate for the roof, as far as I can tell, it's the best balance of insulation, light transmission, and budget. One thing I would suggest is only use polycarb on the south facing part of the roof, make your north roof something solid. North face of the roof is both where you get little/no light, but lose a lot of heat. You can insulate it much easier, and make it light enough that it opens for venting easily (could hinge the whole thing in sections so you can open as much as needed?) My second choice, if budget is bad (check your math first though, a couple panels of good polycarb might be cheaper than you think when you factor in the insulative value, you don't really have a lot of roof to cover) would be a double layer of corrugated fiberglass panels, with an air space in between (be sure to cap all the edges, so it's not just a breezeway in between.)

A question is what kind of truss/roof framing are you looking at? If you are using corrugated, the panels might do better (not sure of your climate) to run up and down so they shed water well, polycarb you can easily run lengthwise, might make for less framing required. If you run corrugated lengthwise be sure to check how close you need to put your supports, it's not very structural, snow might cave it in.  Polycarb will also seal to your walls easier than corrugated which takes those wavy spacers, be sure to math those in between wall and roof and center connection, and some decent sealant on both sides of the spacers to keep the draft out.  Polycarb still would need sealant, but much less of it, as there would be fewer gaps.

And perhaps I'm not familiar with the term, but to me "compost furnace" implies you are going to want to be digging the cooked compost out to use elsewhere at the end of the year, do you have provision for how to do that easily, instead of a lot of lifting up into a wheelbarrow, that will have no place to park, because you are digging out the floor? Worth considering if not...  I'm not into excess manual labor if it can be designed against.

Nice design! I think you'd have some happy quail! I keep chickens, never done quail, will they be allowed to root around in the floor mulch? I know my chickens would LOVE that! If so, be sure to make provision for bird proofing your starter trays, I lost 3 trays of not cheap seedlings to my chickens last year when a guest moved the trays out of their way, and the chickens could reach them. Yay! Baby plant snacks! Dig them up! Oh no....

VERY feasible! And we want pics!! Keep us updated on it....
Awesome first post, welcome to Permies!

Couple of notes. I worked for many years in solar heating, both installation and design, including some passive, and many solariums (Northen California). Definitely do not insulate inside the foundation, there is no point. Good thermal mass, as another noted. Roof, insulate the north side, 8" - 12".

I viisted a home-made greenhouse in northern Massachusetts that met your needs. He was growing cirtrus. Don't recall the name, but Eric Toensmeier (sp?) took us there during a Permaculture event.

This man made his own trusses, brilliant. He made a form from stakes on the ground,  in a quarter-cricle; then bent a 1x4 pine board and clamped it. Then he glued and screwed 2x6 blocks ( I would recommend 2x3 on the south, 2x8 or more on the north), then bent another 1x4 over those, glued and screwed. Instant curved truss.

Concrete floor and foundation, insulated. (Btw, as one option, insulation is as effective laid flat, just underdroud, out from the perimeter.)

He used a center beam, end to end, with the trusses from beam to foundation. On the south side he just stapled plastic inside and outside of the truss, instant double-glazing. On the north, he covered the outside with this plywood, then asphalt shingles. Inside on north I don't recall the finish, might have just been plastic, or thin ply again.

End walls were insultated.

Even though your structure is different, with your windows, the same technique could be used.

Good luck!

Christian Sweningsen

Thanks Christian and welcome to Permies!    One question/concern/intriguement...  Would the wood trusses between the layers of plastic be likely to rot quickly due to condensation and trapped air around them?  

An interesting question. What I can report is that it had operated for decades and showed no signs of rot. I would imagine that any condensation would run down the plastic to the sill. Sill, on top of conrete, should be rot-resistant in any case. We used redwood in Cali, imagine cedar or locust, maybe white oak out here, but it would be standard practice (though "*standard* practice" would be pressure treated!)

On the north side you would want a vapor barrier on the under side.

Christian

Thanks!  And I should have worded my question to be about how the Massachusetts greenhouse worked under those conditions.  It's good to hear that it is holding up over such a long period of time.  I'm planning a similar truss construction on my greenhouse so it caught my attention.

Some Serious Synchronicity. I'm in the midst of organizing a decade of chaos in office/study, and just found notes from the Toensmeier workshop, including the greenhouse!

It is "Tripple Brook Farm" (not a typo: http://www.tripplebrookfarm.com

He used 1x2, not 1x4 on the trusses, and 2x4 blocks on the south. The inside insulation was held in place with homosote blocks, screwed, with fender washers, 2 ft on center. Also had added a 5/4x3 brace from base to maybe halfway up the truss, with a 5/4x2 stringer to catch the brace.

Had figs, pomegranate, qumquats, guava, loquats, dwarf avocado growing . . .

-- Christian

Observation from my greenhouse: The south sloping glazed roof is useless. It collects too much heat in the summer and looses to much heat in the winter. I would be better off with 8' above ground on the south side and an insulated roof sloping north to the top of the berm. The low winter sun shines all the way to the north wall and there is plenty of light in the planting bed in front of the south wall during the summer.
Ideally I think the roof angle should be in line with the sun angle at the spring equinox. That should provide the maximum sun penetration in the winter and shade in the summer.
The fiberglas resin roofing has deteriorated to the point it needs to be replaced so my plan is to reverse the roof. It is a pole building and the poles on the north have deteriorated on the bottom so I will jack it up enough to remove the north poles,shorten them and reset them with better ground protection. I will simply have to remove the windows and plywood on the east and west walls and replace it plumb when I lower the north side of the roof. This will also put my rain water collection on the north side where it is not blocking the sun and will provide more heat sink. My glazing is 12 sliding glass door panels in aluminum frames.  Withthe roof supported by the pole structure the panels have proved to be self supporting just screwed to a 2x6 on the top and bottom.

Hi just wanted to talk about solar gain and thermal mass. If the front window is tilted in at the top at an angle based on your latitude for efficient solar gain. It should then have an overhanging sun shade or awning to keep the sun off in the summer time. It should be insulated above the ground I dont think it would hurt to insolate a small amount into the ground but not all the way down. The berm, faundation, and even the water stored in the back will absorb the heat in the day and put it off at night. Some buried air lines could be used help move the heat around similar to goethermal.

Hi, Ive long been interested in a pit greenhouse idea on my smallholding.

The design Im working on is a very simple pit 60' long by 25' wide and about 6' down from ground level, but making a large bund maybe 10' high on the north side with all the earth that comes out. So trees can be about 16' tall on the north side.

A cold sink trench on the south side, and a plastic bottle roof about 45 degree angel to the top of the bund.

So simple, nothing complicated. Just dig the hole and put the plastic bottle roof over it. Keep the topsoil separate and put that back in after.

My hope is that if I go deep enough that the cold sink trench will hold some or a lot of water for much of the year, I can use this to water the plants and maybe do a bit of aquaponics. Maybe tilapia and fresh water crayfish.

The main plant I want to grow is moringa, they grow very fast and can be very hardy even in poor conditions. They have a long tap root that can go down as far as needed to find water even in dry times like now. But they need warmth and they wont survive the winter here in the UK unless in a pitgreenhouse like the one im building.

My inspiration came from Mikes book, but Im not going to bother whit any frame at all, just dig the hole and put the plastic bottle roof over it. With a door and ladder going down.