Planning a Greenhouse

My wife and I are building a home in Northern Minnesota.  I am from California, and can't quite wrap my mind about what Winter really means here~  But I am assuming a greenhouse will really help if we want to grow much of our own food!  I would like to share some of our thoughts about our dream-greenhouse and would love any thoughts and feedback you might be able to share!  

We are planning an underground greenhouse, probably about 8' to 10' deep.  I would consider going deeper, except I'm concerned we may hit the water table, since the site is about 14' above a surrounding wet-land area.  I am thinking I would like to have the greenhouse connected to the main house so that the two units can share warmth together.  This essentially means building a retaining wall under the foundation of the house -- I am definitely factoring in the labor involved!  I am thinking of making this retaining wall/foundation out of stone masonry so that it will have thermal mass on the North wall of the greenhouse.  I am thinking of keeping our compost pile in the greenhouse for easy access in the winter.  We plan to compost humanure in this pile, and I am wondering about odor issues in such close proximity to the house; though, I am not too worried since I plan to use adequate cover material.   I will probably run the tail-end of our indoor rocket-stove flue through the greenhouse.  I am considering building tubes under the greenhouse with exhaust fans to have more control over internal temperature, but I'm not sure if it's worth the effort?  I am thinking about having tall windows on the greenhouse to meet the living/quarters/home above so that we will be looking out through double-windows in our kitchen and dinning area -- this will create more insulation on the South face of the house where we want to have more glass.  This double story greenhouse will hopefully allow us to grow some well pruned fruit and nut trees that otherwise would not thrive in this climate.  I am concerned about enough light reaching the greenhouse in Winter as there are some evergreens to the South of the site. . .  we are up on a hill, which helps, but I am considering careful and selective pruning high up on these spruce trees to allow at least dappled Winter light into the Greenhouse if the light proves not to reach the area.  I am considering building the greenhouse out of recycled windows as there seems to be a nearly endless supply of them at our local recycling center.  I am thinking of making the greenhouse extend about 15' from the house and to wrap about 20' around our circular home on the South side, as this is all that will fit into the space available.  I am wondering if this size greenhouse will be worth all the effort that will go into creating it?  

Tons of thoughts and ideas here, and I would love to hear any experience, knowledge, thoughts and ideas you might have!  

Welcome to permies, and welcome to Minnesota!

My greatest concern is the amount of heat you would need. Exposed area of your greenhouse would be 1000 sq. ft. Presuming U-factor of those recycled windows to be 0.4, at 80 degrees F temperature difference, it gives 32k BTU per hour, or about 10kW. You would be burning a cord of wood a month just for the greenhouse.

I would start small and see how it goes.

What kind of trees are you going to grow there?

I'm a big fan of the attached solar greenhouse. I've been living with seasonal attached greenhouses as our only source of heat for the past 20-some years in Ladakh, at high altitude with people ice skating outside.

But! But I'm at only 34N latitude, and 10,500 feet altitude (thin atmosphere above us), and in a desert with usually sunny dry winters, so our greenhouses get nice and warm even in January. But Minnesota is a lot further north, a lot lower, and maybe less sunny in winter. So your results will certainly vary.

I've seen that even dappled shade makes a big difference in the solar heat gain in winter. Around here, snow evaporates (well, sublimates) by 9am wherever sun hits it, but in the dappled shade of leafless deciduous trees, it can sit on the ground for days.

I also was underwhelmed by the one underground greenhouse that we made. The southern side of the greenhouse shaded pretty far into the greenhouse, especially in mid-winter when the sun angle is low. We've had better results with having only slightly recessed beds in a ground level greenhouse, so we can cover seedlings with an extra layer on especially cold nights. However, I've seen convincing discussions about how a recessed greenhouse can be better in a place that doesn't get much solar gain in midwinter. Then again, if you will have heat tubes running under some of the beds, maybe that could replace the underground function.

I don't try to have a tropical climate in my greenhouse, I just extend my climate a couple of zones warmer. It does go below freezing for many nights between late Dec and early Feb, but certain greens and herbs don't mind, and others just kind of hunker down for the winter and resume growth in Feb when things warm up. It lets me keep perennials that might not survive outdoors, like certain herbs and flowers.

I love having the winter greenhouse attached to the house. I just love staggering out into it with my morning coffee when there hasn't been any greenery outdoors for months, and doting on the latest seedlings and admiring my fresh greens and herbs. The flowers that bloom in the winter greenhouse get much more attention than any summer flower. Smells in the greenhouse can permeate the house, so you may find that you don't want to keep your compost inside the greenhouse after all -- but this sounds like something you can simply change if you don't like it.

15 x 20 feet sounds big enough to be worth it. Mine is 7 x 30 so it's a little smaller than yours; I'm not producing huge amounts from it, but the benefit to my mental state in winter is enormous -- fresh greens, basking in the sun, doodling with greenery, enjoying flowers, all round cheerful start to the morning. The cats love it too. I don't even use all the space in mine, since I have to haul the water by hand; I'd like to have more of the floor planted, but then it can cause moisture and condensation.

To learn more about growing food in the winter greenhouse, I keep going back to the Eliot Coleman books, The Winter Harvest Handbook and The Four Season Harvest.

Thank you for your wonderful and detailed experience!  And for the insights~

I have attached a (VERY) rough sketch of the situation, excuse my drawing skills!  I believe we should see the Sun at about 20 degrees above the horizon at Winter Solstice.  So we are planning to angle the Southern wall/floor of the greenhouse to allow as much light as possible and create few shadows -- this will also make it so only the deepest portion of the greenhouse will actually be below 10'. . .  it makes me curious to see if/when/where the frost hits this theoretical greenhouse!  The frost line in our area typically reaches about 5' deep.  

Thank you for your experience with dappled shade!  We could certainly choose to prune the trees more aggressively, to create a clear window of light. . . or may simply top the 2 or 3 trees in the areas needed.  But this is something we can experiment with as time goes on.  And we can certainly relocate the compost pile if the odor becomes obtrusive.  We will have to plan for a back-up location!  If we do find it's reasonable to keep the bottom of the greenhouse from freezing, it would be nice to continue composting through the winter.  

To clarify about the use of a rocket mass heater.  This heater will be located inside the house and will heat a large cob bench and a section of the earthen floor before leaving the house and having the tail end of it's flue run through the greenhouse.  We will only be running this rocket heater when necessary for heat in the house, and any run-off we get into the greenhouse will be considered a bonus.  Our goal with the greenhouse is indeed only to boost our climate zone a point or two.  

Truth be told, I don't have much of a green thumb yet~ but I am trying to think ahead as I build, knowing I want to grow my own food in the future -- it will be a life-long mission!  I don't know exactly what trees might work in a situation like this, but I will have to experiment.  In my dreams, I would love to find some reasonable way to grow walnut, almond, pecan, avocado, fig, and lemons. . .  I doubt very much that we would be able to grow oranges and figs!  It seems to me it would be a miracle to have even a few green things growing in the middle of this deep and powerful Winter!

As for the material covering the greenhouse, the quality of glass used will surely vary since we will be salvaging it.  We may over time be able to install double layers of glass to increase the insulation value and heat-retention.  I have indeed considered plastic options, like the thick double-layered poly-whatevers. . .  but my wife and I are both quite averse to plastics, and would rather not surround our lives with it if at all possible.