Pit greenhouse ideas

[gregkdc]

I have been looking into passive solar greenhouses for quite a while but I have never actually built one.  From what I can tell it is very hard to almost impossible to build a greenhouse that will actually store and keep the heat in at night.  I think the focus should be on how to insulate the greenhouse at night.

As you probably already know a person needs to insulate the north, east and west walls, as well as the perimeter or foundation of the greenhouse. It needs lots of thermal mass/storage like water barrels on the inside. While the south side should be angled to catch the most sun possible, but what about at night? This is were things get vague on the internet and were other members of Growing Fruit who actually have greenhouse say the biggest problem is. Some web sights recommend throwing a blanket over the greenhouse but I doubt this is enough. I would think you need something like an insulated automatic garage door to cover the south side, it just seems like a lot. Also did you see the greenhouse that actually makes foam for insulation at night?
As an alternative a person could build an underground greenhouse to avoid the need to insulate the walls but as was already mentioned the plants are not going to get enough sun due to the angle of the light at that time of year. Mabey large Mylar reflectors could catch and direct sunlight into the greenhouse? They sale Mylar film for relatively cheap on Amazon and it wouldn’t be hard to glue it to sheets of plywood. Just a few thoughts
P.S. If I lived in a place with a lot of wind I would build a turbine that is wired directly to a heater in the greenhouse.

[PatapscoMike]

I grow lemons, limes and grapefruit and a variety of succulents and cactus and hot peppers year round. I overwinter a few window boxes most years. I’ll do the occasional flat of lettuce in the winter and I clone coleus and a few other annuals all winter. Cloned a dozen king coleus just today. I start a few hundred veggies each spring, which is really what my greenhouse does best.

[warmwxrules]

In this climate i would (if my wife would let me) use our attached garage///insulate the heck out of it (wouldn’t be hard to do) and put up a bunch of leds and heat it. Would also be nice to put a drain in the floor. Then just move everything outside for summer. Far easier then dragging things into the house/basement.

[swincher]

@swincher

On a fully sunny day around the winter solstice here (sun angle of 19° at solar noon) it probably would struggle to top +20°F. Sun angle matters a lot!

We had 2 straight hours of full sun right around solar noon today (20° max solar angle) and I was slightly off, the greenhouse managed +25°F before patchy clouds arrived, and then sunset (~4pm):

You can see how that compares to a fully overcast day yesterday (in blue).

[GeorgiaGent]

Have you seen this guys post?

http://www.bananas.org/f2/my-semi-pit-banana-greenhouse-18518.html

[snowflake]

Funny. They are definitely stressed in some of the shots at 7/20. If he raises the roof of the greenhouse to make room for them, he might drop his temps too much in the winter.

[warmwxrules]

Came across a geothermal greenhouse they are working on in the Netherlands.

So drilling a well 6000ft deep…pumping out 175F water at about 1500 gallons a minute. Extracting heat and dumping cold water into nearby well. Not sure the size of a greenhouse you can heat in that climate (pretty mild compared to here). This stuff is amazing//using the heat of the Earth’s mantle to grow your crops. Would be interesting knowing how much energy this type of system consumes (pumps/etc).

[Sue-MiUPz3]

I’ve had a simple not heated modified pit attached greenhouse (and underground house) for about 40 years. Experience is pretty much as Rob @warmwxrules has said for the upper Midwest. It’s great for winter greens; or a short inside extended season for more tender plants. One could heat it though we don’t (except the sun and being attached to the house. Here’s a link to a brief description when we rebuilt it in 1999:
https://www.manytracks.com/House/Rebuilding.htm
Sporadic info and reports on using the greenhouse can be found here:
https://www.manytracks.com/Garden/greenhouse.htm
Might give you some ideas.

Here’s a great old book, very hands on and definitely leading edge folks. Might be able to find a used copy.
Winter Flowers in Greenhouse and Sun-heated Pit, by Kathryn Taylor and Edith Gregg. Chls Scribner’s Sons. Even if they don’t talk much about vegetables, I gleaned more good how-to greenhouse garden from these ladies than I did from oodles of more academic tomes. Even if you don’t have a greenhouse and never want to grow a flower, their style and attitude is worth it to read this one.

[bopcrane]

I was stuck with a hard choice recently, re-do my aging pool fence and in-ground pool (with a failing liner, heater, and pump for a crazy amount of money I don’t have!), or figure out what else to do with that “sunk” cost that is literally now a giant fenced-in pit on my property.

So, a pit greenhouse is my next big project over a few years that I’ll be digging into. I don’t mind going somewhere else to swim, really, I swam in creeks when I was little (don’t think I’d swim in any of them around here now though lol). So I made the choice, maybe to my wife’s disappointment, that we’d have way more utility in another greenhouse to grow food, than another place to swim during our sometimes-short swimming seasons. Maybe the pool maintenance requirement weighed on that decision a little, too. Has anyone seen the price of chlorine lately? - I should have bought stock in Clorox.

I was also kind of compelled to do this project in part to remove an egregious tripping/falling/drowning hazard from my premises, if I wasn’t going to fix the pool and cover it over - I have 3 kids and although I padlock the fence down tight it is a worry that itself won’t be fixed until the pool area is.

I still have a few major details on the implementation to plan, but building the previous greenhouse that’s right next to it and growing citrus etc in the ground inside of that for a few years now has been the sort of proof of concept that I wanted first in order to be confident enough to try and take this pool area on and make it into a greenhouse. The dilapidated pool area is already piped for natural gas heating, and I’m already stuck with the giant concreted hole in the ground…

I’ve got an engineering friend I’m talking to to see if he thinks some type of bracing may be warranted around the rim or walls due to not having water pressure up against the side walls; the deep end is 12ft deep, so I was considering doing some kind of heat exchange and possibly digging it deeper there for that and/or put a catch basin and grate for sump access after I remove the liner. But I’ve not settled on whether I will do the heat exchange thing or just heat with natural gas thermostatically. Due to cost, I’m leaning towards natural gas.

Water ingress, sump pump and access to it, heating/cooling/temps, humidity, air flow will all be of potential concern, but it will be extremely well insulated, good sun exposure where it’s located, and with a solid and reflective north wall I feel that most potential problems could be designed around, at least in part. I’m trying to do some math on heat loss, thermal mass, solar gain, etc. to try and get the right amount of sidewall height in order to grow well, but not to lose too much heat.

I don’t think I’m really going to be aiming for a passive set up, I’d be happy with just an improved version of my current greenhouse setup (the one in the picture to the right of the pool area); it is poly skinned w/ air inflated between the two IR/drip resistant poly skins for extra R value, north wall heavily insulated and reflective. The electric oil radiant heater placed in the middle of the GH is set on a thermostat and so far it has done a pretty admirable job of keeping the temps in check until the sun comes back out. It is not a pit greenhouse.

[disc4tw]

As I think I suggested earlier in the thread, heat pump technology has drastically improved in the last few years and a 20 SEER (or better) unit that you can install yourself that has pre-charged lines is affordable and will pay for itself in a couple of years versus paying for natural gas. Plus it can run in fan only mode or to cool in the summer. Wins all around.

[bopcrane]

Nice, I will look into those. I want to be as efficient as possible, within reason. I’m currently seeing how sound the gas line infrastructure is in the pool area and if it involves much work I may very well go that route instead