greenhouse heating

hi im looking to build a 15x 60-foot aquaponics greenhouse in southern Minnesota. The plan is to sink in about 4 feet in the ground with a 5-foot double plastic wrapped greenhouse tunnel above. there will be large water pools (1500 gallons) and many many concrete blocks for constructing grow beds that will make wonderful heatsinks. id like to heat this year round with a rocket mass heater but I've never seen one heat something this big in a place that it gets so cold in something that's not very well insulated. Is this possible? how big do you think it would need to be? how much wood do you think id go through? sorry if this is alot to ask of you guys but i really wanna learn what I can before i commit. Thanks!

Hi Gage; Welcome to Permies!

I heat a 12 x20  greenhouse/studio  all winter in northern Montana.  I have an 8" J tube and use less than 5 cord of wood. We do have a fire from morning until 9-10 pm.
15 x 60 is very big. You certainly will want to build an 8" or larger batch box, with mass.   Possibly 2 , one at either end.
Your masses can double as heated grow beds.  We use ours for starts.
How much wood ?  I'm guessing 8-10 cords,  in a poorly insulated greenhouse ?  Your water tanks may help.  Thing is I have been in Minnesota in the winter... makes western Mt seem practically tropical.  

Hi Gage, welcome to Permies!  

Can you orient the greenhouse so the long axis is E/W?  And if so, could you insulate the north half of the hoop?  Maybe rockwool between the layers of plastic?  That would really help.  

Is the land flat or sloping?  The reason I ask is that the south 4' earthen wall (plus 3' of snow) will block a lot of winter light.  If you can build it into a south facing hill it could help as well.  Solar gain is a good thing.

Mike jay,
thank you very much for the welcome, i plan on orienting it east-west yes the land is very flat. Ive considered insulating the north side but im worried even though its on the north that ill love alot of useful sunlight. It is dug into the ground to add high and act as good insulation.Thank you though for the advice and i welcome any more you may have
-Gage

Careful what you ask for, there's lots of good advice here  There are also a bunch of threads about different greenhouse builds.  Yours sounds a bit like a walipini so searching on that may return some stuff.  

Two of the biggest challenges in our area for sunken greenhouses (I believe/suspect) are:
1.  Low winter sun angle leads to long shadows on south side
2.  Earth is cold.  Better than -20 air, but still could be colder than you need.

I'm guessing you're not going to insulate the perimeter?  If the earthen walls around your sunken greenhouse are frozen (I'm guessing you have a 4' frost design depth?) you have some very cold dirt that would be constantly sapping heat from the greenhouse interior.  Assuming you have some snow around the greenhouse, I think the frost wouldn't actually get 4' deep.  But it's still going to be really cold.

We also get a modest amount of snow.  This year if I hadn't shoveled snow from in front of my greenhouse I'd've had 8' of snow leaning up against it.  Picture in your mind how much light will get into your greenhouse in January.  Unless you clear the snow all the time which is a different story.

We're also cloudy a bunch in Nov/Dec.  Sunny days can really heat up a greenhouse but 10 cloudy days in a row leave you chilly.  So it's good that you are thinking ahead to heat.

Do you have a place to put the excavated dirt?  What if you use it to make a hill on the north side?  And then set your hoop on the ground on the south and on the hill on the north?  Might need a retaining wall to hold back the hill.

Regarding insulating the north side, it is a balance.  In the winter, insulation up there probably won't block any light from hitting the plants.  It might actually reflect some down that would otherwise come in the south side and escape out the north.  But in summer, the sun is obviously much higher in the sky.  Some people put thermal mass along the north wall for the sun to hit in winter and then it's in the shade of the insulated north roof in summer.  But then you give up growing space.  Doing a side view drawing of the building with the summer, winter and equinox sun angles on it may spark some ideas.

This should be fun!  Good luck!

Mike Jay wrote:
Two of the biggest challenges in our area for sunken greenhouses (I believe/suspect) are:
1.  Low winter sun angle leads to long shadows on south side

Yes, I agree, I am not a fan of sunken greenhouses. We made only one, and the shade thrown in midwinter was ridiculous, obviated the point of having a greenhouse! And we're a lot further south than you. Your sun is a lot lower than ours is.

Similarly, in such a northerly latitude, you won't get a lot of useful sunlight from the north side, especially in winter when you would want it.

Don't forget a good way to increase the warmth of growing beds is to set up some kind of additional cover over them inside the greenhouse, that you can close on the coldest nights of the year. Since there's no wind inside the greenhouse those can be very simple, not fixed or tied down.

i am planning to build something similar and some things i have considered are:
the sun angle is quite low in dec and an insulated N side is a good thing
but by the time it is warm enough to bring plants outside the sun is much higher in the sky
even starting the day ne and ending nw

so maybe have insulation on a roll being rolled up and down nightly for the south facing part
and a temperature controller roll up motor controlling insulation on the top/north facing side/s
or just as the season progresses remove insulation from the top opening up more area above
the roll would have to be inside to operate properly
i might even go triple poly
solar panel and batteries inside structure to be warmer and free of snow
i was also looking to build with fence top rail and hoop bender at first and i still plan to make this style of pit tunnel hoop house
but for the deep winter greenhouse i think wood on a steep slope would be better since it does not transmit the cold as much and water will not condense on it like with the steel pipe

also looking into jean pain compost heating
possibly the pile being in proximity/part of the north berm to further heat the thermal mass

ideally the s facing area will have room below the poly for lots of snow to be able to pile up without the light being blocked

also a solar water heater inside the structure to circulate heated water into youre reservoirs whenever it is warm enough to do so would help

the RMH would be nice too

my plan is to throw everything i possibly can at it and hope it works