I have been brainstorming a bit. Initially, I was thinking of building a coldframe. As I thought about it, it morphed into a “walk-in” coldframe, sunken into the ground. At that point, I realized I was really thinking of a pit greenhouse, traditionally a walipini in South America.
I am in Central Maryland. Zone 7. Average winter highs lows are generally 40s/20s. We do see single digits most winters, but not all, and an occasional mildly subzero low (I’ve logged a couple -2/-3 readings in the Polar Vortex winters of ‘14 and ‘15, and again in 2018 and 2019).
I won’t be using it year-round, most likely. Mainly to overwinter citrus and keep a few winter cool-season veggies going (which can be done with row covers as well but it is a bit more labor-intensive in my mind). I’d also use it for late winter/early spring seed starting for veggies.
The idea here is that the insulation/heat exchange of the ground should minimize the need for additional heat (although I assume I might need at least SOME), and probably temper ventilation needs as well (but I’ll still add ventilation).
Has anyone attempted to do this? A few questions:
How likely is it that I would need supplemental heat? I can’t find any reliable calculator for this.
All the articles mention using solar and ground heat, but mathematically, due to the low sun angle in winter (28 degrees on 12/21 at local noon for me), the vast majority of the structure will be in full shade until sometime closer to the equinox. I did some math – if my greenhouse is 4’ deep into the ground on the south side, the first 8’ of ground area inside will be in shade all day at that time of year. I could elevate plants into the sun, but otherwise, the only sunny spot in a 10’ deep greenhouse would be right against the north wall!
Flooding. My water table in the area I’d build this is at least 20’ deep. Would I have any issues with flooding inside as long as I grade the outside properly?
Would you consider this “not worth it” for what I want to do?
I have no personal experience with one, but I actually just watched a YouTube video the other day about one. They give a lot of construction details and lessons learned since the video is covering a walipini built some years ago.
So I have a big plan for next summer:
To build, in a hill, basically a normal modern basement, on the deep side being 10 ft walls with 8 ft of dirt depth, but with a couple 4ft square window wells(one per side N,E&W sides), and then a roof over it with a couple skylights in the roof, and this be my home… It will be an underground home with only the roof and top 2 feet of wall above ground, plus a couple nice window wells…
And but make its South side have big full glass windows to see out in the greenhouse and a door to walk right out into the attached second similar underground area but with dirt floor a foot lower than my basement home cement floor, and a solid clear poly roof, this will be the attached underground greenhouse with ground stable temps and solid clear roof.
For cirtus to be inground, etc
The sun will warm it so much that I will make parts of the roof raise to vent it…
Also I will bury lots of 300 ft long runs of 3" rib pipe 8 ft deep to circulate air with a simple fan when temps get above 85 or for the short-lived times that a cloudy January blizzard drops temps below 45/50F, as this guy in Nebraska did it is above 45 in coldest times in 5a NE and very low energy use:
I might add a simple heat source in the greenhouse as I will in my home portion of a Blaze King long burn time King size wood heater and a propane wall heater or two in the house.
I have a greenhouse in 7b Maryland and I can’t imagine a pit greenhouse would do much to keep the above-ground part of the greenhouse warm at night without some kind of underground piping system like the Nebraska example above. That works but unless you own heavy machinery and have room to use it the process would be very expensive. I really don’t think you are going to be able to do it passively. The winter nights are just too long. It takes 3 small heaters to keep my 10x12 above 45 all winter long. All three of them run most nights from late November through March.
It’s a crapshoot digging a 4’ trench in central MD. Where I live it’s quite rocky and would probably be extremely difficult digging. More like digging and jackhammering.
As to water, that depends on how well your soil percs. If you are up in Carroll County red clay you might have drainage issues. If you are further east then it’s probably not going to be a problem.
Be nice if you could just dig right into a hill about 6-8 feet deep…Then cover the top. The key would be to get a stable temp (should be in the 50Fs all winter). I know my basement floor stays above 50F even with very little heat and in the coldest temps of winter. I would imagine your zone soil temps are a few degrees higher. I think i sit about 7 or 8 feet below grade. I can say last winter i overwintered a good sized plumeria plant and it did great. Leafed back out in spring//have it back down there for another winter. Problem i see is carrying large plants up and down steps. Its not fun//a walk out basement would be great–better yet would be a small garage door opening.
Paul, that sounds awesome, although I have to ask, if you have the technology to install 300’ of pipe at 8’ deep, is there a reason you aren’t exploring geothermal with a liquid antifreeze loop instead of just air circulation? It is the most efficient form of heating and cooling and if you have room for 300’ of tubing, a horizontal structure is a lot cheaper than a vertical one because you don’t have to hire someone to drill. As it pertains to this thread, it would make temperature regulation for a greenhouse very affordable if you are using geothermal too.
Yes, our unheated basement stays 50F or above and is not even that good, 100 year old and no insulation and uninsulated windows and only 5.5 ft below grade.
So yeah my plan for home/greenhouse 8 feet below grade will I expect it to not need any heat for winter for citrus and easily stay above 40F in the coldest nights. Above 50F i expect. Especially if I put some black plastic water tanks to absorb heat in sunny days here in sunny KS 6b. The air circulation plan is just a luxury and more for summer cooling which will be my biggest issue with the clear poly but I will make roof areas hinge open for venting too.
Well, we have a old excavator so i can dig a trench and run lots of pipes say 8" apart at the 7 to 9 feet deep. A 300 ft loop like the NE guy, say 120 ft out, 60 ft over, then 120 ft back. Something like that. It will just need a box fan at the return box to blow the air.
And I have heard those loop systems are good and more ideal, but isnt the system that the loop hooks to expensive? Also more expensive to maintain over the decades than just replacing the box fan on the wooden return box i make from a sheet of treated plywood that all the pipes connect to for the air circulation system?
All I will need is to dig the trench, put in the cheap black flex ribbed 3" pipe, make it connect to the wooden box on one side that has the box fan, and make the pipes reenter at different spots in the greenhouse to distribute the air.
In the video I linked to they had 55 gallon drums along the back wall that would be filled with rainwater from the roof and used for watering. The design allowed the sun during the winter to hit the barrels and warm them, but keep the sun off them in the warmer months. This was their main internal thermal mass, plus the deep trench in front of the grow beds where people walked would allow colder air to sink away from the grow beds while being moderated by the lower soil level temps. I think the goal was just to avoid freezing temps.
That Walipini is huge. Scale is super important with greenhouses. The bigger they are the better they work, and a lot of those commercial houses can’t just be made at 1/20th the size and still function. People told me all sorts of stuff about solar energy storage, thermal mass and how to get “free heat” when I first got my greenhouse. None of it worked for me.
A sunny climate also helps a ton. Around here Oct-Dec are usually extremely cloudy so trying to maintain a warm greenhouse is all but impossible without lots of heating and add to that growth will be limited with shorter days and no light so additional lighting is probably needed. I view my basement as more a place to overwinter plants dormant in which it has worked well…but to keep things actively growing i need to use heat pads and LED lights.
I agree. The guy by Alliance, NE has many sunny days during the winter. Also, he has a very good mature cedar windbreak on the north side of his setup to help protect from awful winter winds (which they get a lot of in that area). By wind, I mean the awful stuff, not the light breezes that most of the country sees.
Haha yeah it blowzzz like a pipeline from ND to TX and then back lol!!! Me and you both are in its path but a treeblock really helps!! As does our many intensely sunny days in winter! Behind a glass it can become comfortable fast even on a 20F day when that prairie sun is shining bright!
It would be so nice to be able to build this. There’s a Guy with a pick yer own fig farm in central PA who has succeeded, But I suspect the size and scale of the project it would let him succeed, and a much smaller, backyard sized greenhouse with probably fail
Are you planning on installing supplemental heat or cooling in your new home? As in, would you be putting in an air conditioner, natural gas furnace, or electric heat? If so the geothermal should completely remove those as one unit as it can both heat and cool. You could even get a system set up that has different air ductwork for your greenhouse and home, possibly have some sort of automated system that allows you to heat/cool one or the other separately, or have separate systems running on the same heat sink loop.
Regardless, if you choose to install the in ground loop for air, I HIGHLY recommend spending the time (or the money to hire a consultant) researching how to design it so you could potentially retrofit to a heat pump loop in the future. That includes making sure that the way you install your pipes will allow them to be filled with coolant, proper sizing and placement of the pipes, etc. You wouldn’t want a low spot in the line causing an air lock or something like that.
I spent less than $3,500 (including labor) after credits from taxes /the power company to replace our furnace and air conditioning with a high efficiency air source heat pump. I imagine you would expect to pay a bit more for the geothermal hardware but I haven’t researched it recently. This tech keeps getting better and less expensive as it is more widely adopted. I’d start looking at www.hvacdirect.com where I bought my Mrcool Universal. They had good customer service to help with figuring out which model worked best for me.
Feel free to message me directly or make a lounge topic if you have any questions, I don’t want to derail the greenhouse side of this thread.
I’ve been doing some actual analysis and I’m coming to some very different conclusions.
Where I live I have only 800 to 1200 hours per year below 40 degrees. Many of those are short bursts and I think if the passive heat retention gave me 5 or 10 degrees (taking the edge off) then I’m down to probably only 300 or 400 hours a year that the greenhouse needs help. Looking at a window unit heat pump (cost about 400 to 500 dollars) my electric bill would be under 100 dollars a year to run it for just those 300 or 400 hours per year. Seems like a no brainer to me.
Everyone has these goals of being totally without any energy input. I say if it is just 300 hours and it is under 100 dollars per year, why overcomplicate things trying to reach some ideal? Just run the math…
I think this is an assumption that won’t bear out. It might give you a few degrees for an hour or two, but winter nights are long and there is zero room for error. One freeze and everything is toast (depending on what you grow of course). But why not try it and just throw a cheap heater on a Thermacube just in case?
I’d love to read about someone who makes it work. I’ve been looking for 5 years and haven’t found a single example yet but I’m going to keep looking.