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Glenn's passive solar greenhouse build

 
Posts: 15
Location: New Brunswick, Canada
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After much research and observation, I have started my dream of building a passive solar greenhouse. I’ve decided to use Permies as the medium to share my project. I love this site.
I drew inspiration from many sources but the core design came from uMinn’s Deep Winter Greenhouse project. The exact plans are self-designed and so far construction has been just me, but definitely influenced by what I’ve learned from other threads on Permies. I welcome questions and comments.

Some quick points:
• I’m in New Brunswick, Canada. Zone 4.
• Growing goal is to extend my season from March and to November (outdoors the growing season is only June to September)
• Dimensions: 32’W x 16’Dx  14.5’H (I tried to hit 2-1-1 ratios). Oriented so the long side faces South.
• 2x6 wood construction, steel roof/siding, insulation TBD (narrowed down to mineral wool or fiberglass)
• It will have a Climate Battery/GAHT (Ground to Air Heat Transfer).
• Glazed with SolaWrap

Climate Battery:
The GAHT was a huge factor in the overall design and cost, but I felt it very much inline with my goals. I could have built several hoophouses for the cost of just this system but I want to grow in March, not 4x the size in May when I’m already busy outside. I also don’t want to rely on supplemental heating. I’m going with the full rock bed style like uMinn instead of layers of drainage pipe surrounded by native soil. I live very close to a quarry that has suitable rock and eliminating hundreds of feet of pipe seems very appealing from a construction standpoint.

Foundation:
The foundation is on concrete piers, as inspired by some of the Ceres Greenhouse photos I saw online. I did not want any wood touching soil, but also didn’t want a full concrete “basement”. This option seemed the perfect hybrid. The pit was excavated down 4’, I went with 6ft tall 8” piers on 19” footings. I’ll be landscaping the perimeter so the new grade is a foot higher the originally, sloping away. Doing the math that leaves a foot of clearance above the ground before any wood construction. The entire perimeter of the foundation is going to be wrapped in R10 styrofoam insulation. The foot that is above ground will be covered with flashing.

Glazing:
For the glazing wall, originally I was going to go with a straight wall at 60 degrees, glazed with inflated poly. Much like the Bradford Research Center greenhouse. I ultimately opted for a gambrel style roof instead. Partly because a straight wall calculated to 13’ and I figured if I’m going to splice, why not use as much of both 8’ pieces as I can and can some head space by putting them at an angle. Partly because I feared the plastic would “bounce” a lot on a flat surface whereas having a change in angle would keep things tighter. Partly because I really like the look of Penn and Cord’s greenhouses (though structurally very different). Although I haven’t purchased the glazing yet, my current plan is go with with SolaWrap. My quotes have it at double the price of poly over a 12 year timeframe, but that doesn’t account for the labour savings of replacing poly every 4 years or the blower to inflate it. Polycarbonate is a dream; it’s way outside my initial budget. R-values seem comparable on all 3.

This build greenhouse is more of a starting point for personal growth than just a one time project to grow more food. I have many future projects in mind for the interior, often things that I can’t do in the house because it would void my home insurance. For starters, a rocket mass heater. Secondly, interior rolldown bubblewrap as an insulated curtain. Even with an insulated glazing surface, that wall is a relative gaping hole when compared to the rest of the structure.
I’m late in getting this thread started, as the build has already begun! So here are a couple pictures to get things going. One of the profile of the greenhouse (not to perfectly to scale), and another showing the inaugural ground breaking.

Plenty more to come of course. Since it’s not entirely built yet, still chances to change as opportunities are discovered.
Greenhouse-dimensions.png
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Profile and exterior dimensions
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Breaking ground
 
Glenn Van Agten
Posts: 15
Location: New Brunswick, Canada
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Here is a picture of where things currently stand.

The concrete work is done. I have 6x6 cedar beams forming a perimeter frame, except for the north side where I made a 3 ply laminated 6x8 to handle the roof weight).

The greenhouse is going to have a 4.5x11.5 ridge beam (3 ply 2x12, already built just waiting to go up), supported on the end walls and by 2 piers to in the middle of the greenhouse. Originally I was going to do 2 ply 2x12 with the center piers on 8' spacings (as per the uMinn DWG design), but I had a beautiful Spruce tree topple over in a windstorm this past winter and want to use that on the piers instead of cut lumber. They are much larger than the 4x6 called for in the plans and I felt it would make the greenhouse seem too cramped so I changed things to have just 2 supports and spread them out. I welded custom concrete holds for the logs as per a design I saw on log cabin construction.

I've started adding in the rock for the climate battery. Plans for call for "1.5 inch+, no fines", and most of the DWG prototypes having using river rock. I'm using 3"-5" limestone sourced locally. So far I have about a foot of rock dumped in. I'm using my tractor to dump it in, one bucket at a time.  I had originally naively thought the dumptruck could just dump a whole load in, but no can do for a few reasons:
- I need the perimeter posts in place to support the concrete until the rockbed is full, this gets in the way
- Most obviously from the picture, the North wall is already in place (construction done out of order to do availability of some help)
- I have to build the rock level up slowly, balancing it with dirt on the exterior of the styrofoam so that it doesn't break.
It's actually going well it just slower.

For ducting, I'm using 12" forestry grade culverts for both below grade and above grade inside the greenhouse. The plans called for 10" diameter, but 12" forestry grade is cheaper than smaller diameters (for which only heavier DOT grade was available) so a nice bonus. Most surprisingly, it's cheaper than regular HVAC ducting so I'll be using it above grade inside the greenhouse too.  For the below grade ducting, I drilled 1/2" holes through the pipe. I wasn't sure how many I'd need, so I decided to drill enough so that the cross sectional area of the all holes equaled the cross section of the duct itself (12"), and then added a buffer. So I drilled 4 holes in every rib to hit at least 576 holes over the course of 30' (times 2, one for the inlet and one for the outlet). It ended being very quick and easy to do.

Keen eyes may notice an extra concrete pillar in the front left of the photo, only spaced 4' instead of the usual 8'. This is where the door will go. I added a second version of the photo with mockups for clarity.  I felt like I was breaking with convention with this part. Typically with concrete pier construction, the floor is above the top of the piers. But in my case they are 1.5' below! That meant cutting right through perimeter beams, but then what braces the door and takes vertical load? So I added another pier, the door will be sandwiched right between them. I plan to use concrete screws to anchor the jack studs to the concrete piers.
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proposed-door.jpg
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Hi Glenn, welcome to Permies!  Awesome build you have going on there!  I did a bunch of research as I built my own passive solar greenhouse so if you have any questions, I might have been through it already.  Here's My Thread.

A couple things spring to mind:

1.  How much ventilation do you have built into the design?  I see the 1' vent at the bottom of the south wall, do you have an upper vent(s) as well?
2.  Do you think the snow will shed off the north roof well enough?  Maybe it's fine if it stays on there as added insulation.  
3.  Have you figured out the summer and winter sun angles and how much of the interior will be illuminated?  I'm guessing the summer sun will not hit the 5' closest to the north wall.  Don't forget the ridge beam when figuring the sun.  It's not too late to make it a bit taller
4.  With a high roof and low plants, it could be fun to add an attic or loft to just hang around on in the winter.
5.  How cloudy is your area in December?  What are the minimum temps in late Dec and in the depth of winter?

With R20 insulation and a climate battery, I wouldn't be surprised if it would stay near or above freezing all winter.  You may be able to grow some interesting trees like figs or avocados.
 
Glenn Van Agten
Posts: 15
Location: New Brunswick, Canada
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Mike, thanks for your insightful comments!

Ventilation. I've gone through numerous iterations and am still not fully settled. The base design I’m following has 2 awning windows up high on each side wall; 2’x4’ each. I’m cutting costs and not using windows. Instead I plan to build removable insulated plugs. I’ll monitor temps next summer and might upgrade one of them to louvered fan. Combined with the venting on the front, this puts total ventilation space at 10% of glazing which I know is on the low side but it also doesn’t account the climate battery.

I don’t think the snow will shed off the roof but I have accounted for snow load in the design. I’m actually going with a steeper roof pitch than the original design. I’m not sure if it’ll offer any insulation value though, because there will be a 1” air gap under the steel for ventilation to ensure no moisture ever rusts the steel roofing. I still find this crazy, but the insulation in the roofing is actually going to be sealed in from both sides; plastic vapor barrier on the interior and synthetic roof underlayment on the exterior  (there will be small vents along the ridge to permit venting of any moisture that does find a way in).

Great question about sun angles, I had not calculated that yet! I looked into this last night and based on my latitude, the sun will stop 2.4ft shy of the north wall. So you weren’t far off I’m quite comfortable with this number, as the long term plan is to have a rocket mass heater with the mass bench running along the north wall. I would not want to have that soaking up sunlight in the summer; so this number works quite perfectly for passive solar design.

I am quite interested in adding a row of 55gal barrels along the back for thermal mass in the winter. My worry is the weight, technically they’ll be directly above the underground 12” climate battery. I’m not sure how much 3’ of rock and soil depth helps to disperse the weight.

December is a cloudier than January and February but generally is reasonably clear. It’s much warmer than the depth of winter; December lows are 25F with an average of 34F, January lows are 20F and average of 29F. There isn’t much reprieve from the cold, without heating it just permeates everything. Long term I do hope to keep temps above freezing year-round, but initially I plan on leaving the greenhouse dormant for those months and will be satisfied simply with the ability to overwinter some warmer climate perennials such as a tea plant as well as perennials seedlings started the spring prior.

Funny you mention a loft to hang around; while I haven’t thought of that do I am planning to have a 10’x10’ space designed as a multi-purpose area. While the greenhouse will otherwise have a dirt floor, this space will have patio stones. The wall will have a fold down table for coffee/tea or small meals. A hammock can be strung between one of the center log posts and the wall.  The ceiling above it will have a set of tracks from which greens will grow in gutters; they can be simply be pushed out of the way to clear the space for other temporary uses like pressing apple cider.

I've attached current draft of the interior layout. I'm planning on growing in raised beds on the ground.  It's pretty earlier in the build so I expect to be adapting this part as a I go.
Interior-layout.jpg
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Mike Haasl
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Good point on ventilation with the climate battery.  That could ease your needs significantly.  My greenhouse doesn't have a battery and it keeps ventilated on hot 90 degree days with about 46 square feet of low intake and 26 square feet of upper exhaust (passive) for a 40' long greenhouse.  But I have 17' of chimney effect to help the air move.

I bet the snow will still help even with the air gap.  The air gap just adds to the R value.  Better to have 45 degree snow sitting on the metal than 25 degree air.

I find that enclosed insulation idea a bit worrisome too.  I'm assuming that comes from the DWG plans?  I'd build in a drain at the bottom to let collected moisture out as well.

Wow, your winter temps are balmy!  With my greenhouse, without a climate battery and without heat, on a -29F morning, the inside temp dropped to +20F.  So if my greenhouse could passively keep a 49 degree delta T on the coldest night, I'm guessing yours can easily stay above freezing.  You may be able to keep it closer to 40F all winter.  This should be interesting...

Hopefully you get it built in time to see how it works this winter.  Then your plans can evolve for what you grow.  Who knows, you may have avocados in there a year from now
 
Glenn Van Agten
Posts: 15
Location: New Brunswick, Canada
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I converted the winter temps incorrectly from Celcius. Winter temps will definitely hit lows of -25C (-13F), possibly -30C (-22F). Very different from the +20F I accidentally stated.

With regards to combating those temps, the second phase of the build (which is likely to happen around 2021) will include 2 key things:
1. A rocket mass heater
2. Water thermal storage system. One part mimicking that of another Permie's greenhouse, David Maxwell. It will consist of an automotive radiator at the peak to capture heat, and water lines running through the grow beds to heat the soil (with the soil then acting as additional mass as well). The second part is to run pex lines through the Cob mass of the rocket mass heater to siphon away wood heat. At this point I have no interest in trying to capture heat from the RMH barrel itself and prefer to stick with sub-boiling temps.
 
Mike Haasl
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Ahh, that makes more sense.  

Regarding the RMH, there is an alternate design for greenhouses.  It's often so damp in them that cob melts away.  I think it uses gravel and rocks as the mass.  Just something to think about.

I'm planning out my own heat storage system using barrels of water and a radiator.  I'll duct hot air off the ceiling down to the radiator.  Then store the heat in a series of drums.  Then when the system is in "heat" mode, that warm air will be down low by the plants.  Maybe more of an issue for my tall greenhouse?  I'm also thinking of incorporating phase change material in with the barrels as a second round of experiments.
 
I hired a bunch of ninjas. The fridge is empty, but I can't find them to tell them the mission.
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https://permies.com/t/143286/trade-huckleberry-pie-Kickstarter-promotion
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