This past year has been frustrating one for projects on my property, mostly due to a new job and young family taking up most of my time, but it's all good.
I did however deal with the frustrations of a crappy above ground greenhouse this spring, freezing, blowing away in the wind, getting too hot, etc.
I went cheap and it ruined the fun for me.
I am in the planning phase now for a sunken greenhouse similar to a walipini/mike oehler type underground greenhouse and will be buying Oehlers books this christmas. So far I've got: Facing south, at an angle of latitude +23 degrees, with good polycarbonate glazing, and a passive solar mass behind the north wall.
I might be fine with this not being a year round greenhouse and just being a growing season extender, but year round would be cool if I could utilize multiple tricks to keep the structure above freezing temps. I do believe I can have a pretty solid budget for this.
For now I am trying to get some plans on paper and had a few questions for those in the know.

-The pit walls/foundation, I see alot of cement blocks, I would prefer to avoid that due to cost and deficient skills in masonry. I'm not against it though, are there ok alternatives I could try. I currently have as much wood on my property as I'd ever need, including straight white pines that need thinning, and red oaks that are dying. Neither of those woods are super rot resistant, but might work if I were to dig them in and cover them with a plastic moisture barrier right?

-Insulating, inside or outside the foundation wall? both? Should I plan on insulating the earth mass behind the north wall or is that better to be non insulated?

-The actual growing beds, what height in the greenhouse should they be to maximize growth? I've seen alot of terraced looks, I just don't want to excavate deep and have my plants not get enough sun.

-How effective is the cold sink pit? i assume this will be part of the mike oehler book, but it seems like it could be alot more digging.

Thanks to anyone with some thoughts, I'll certainly be posting pictures once I start.

Hi Russell;  I don't have answers for your specific questions, but I have a few things for you to look into. The first is "SOLEXX' as a greenhouse covering, rather than polycarbinate . I've been using their roll solexx for over 6 years now and its as good as new. Not cheep but neither is polycarb panels. To many good features for me to go into , but well worth your time to checkout.   The other thing to seriously look into is a rocket mass heater. Hands down no better way to heat a greenhouse!  My 12x20 used to use 12-15 cords to stay warm all through a montana winter, with my wife staying up late to feed it and me getting up early to do the same. For the last 4 years since I built a RMH we use LESS THAN 5 CORDS OF WOOD ALL WINTER!!!  No fire at all from 9-10 pm thru 6-7 am and at 0 Fahrenheit it will be 40 F in the morning !     Building a RMH may seem daunting but... its not.  
And working with cob is fun compared to masonry work. Lots of good information right here at permies !

I would like to suggest 55 gallon barrels as the walls of your pit.
Filled with growing medium,rocks,soil or water they could both be structural and provide thermal mass.

Another thing to consider is a way to store the high daytime heat in the earth below the greenhouse.
Check out this link:

http://forums.gardenweb.com/discussions/1607983/subterranean-heating-cooling-system

I put such a system into the foundation of my chicken coop/run/greenhouse.
It's not deep enough,nor is the mass insulated,but I hope that  by running this system I will be able to capture the heat from a small rocket stove without making room for mass inside the greenhouse proper.

I have an opaque roof and front glazing that is made up of  crystal clear vinyl shower curtains. First year,so I don't know how they will hold up.

I am looking forward to seeing how your project works out.

Thanks for the ideas,
I have considered the RMH, but am hesistant to go with something like that for this "version" of my greenhouse. I intend someday to build a much more impresive structure for tropical type plants, this is more to overwinter certain things. start trees, and extend the growing season.
Any idea where to get 55 gallon barrels? I like that idea, plus i could reuse them if I upgrade someday.

When we built our studio/greenhouse it was intended to be a greenhouse alone.  After the artist (my wife) spotted a well lit ,warm space (Thank you RMH) it BECAME an artist studio / succulent greenhouse rather than the vegi sort of thing I had in mind...  Oh well the point of my post here is the mass was used the first year as it was intended to be used in a vegi greenhouse ...  all of your starts, sit on the mass keeping their little baby roots nice and warm ! Larger more cool tolerant plants can be moved to the raised beds. Once your mass is heated up for the season your whole room will be warm and comfy , no hot and cold spots just a bone pleasing warmth. So in my opinion, the the mass inside serves a very useful function rather than taking up valuable space.

55 gal barrels with removable lids are sold at the "North Forty store" here in the northwest.  Apx. $30.00 . This is a farm & ranch type store. Where ever you are located there should be a similar place.

Craigslist has used food grade barrels for less than new.
I have finally tracked down a cheap and reliable source of used barrels.
The sausage company near me sells barrels at $2.00 apiece.
Some with screw on lids ,others with bungs, all the same price.


A barrel full of water weighs 463 lbs. They are designed to be stacked,full of liquid,up to 3 high.
Drums are about 35" high, so two stacked  makes almost 6'.
A subterranean "course" of barrels could support an above ground course,as the northern wall of the greenhouse.

My next greenhouse will have something like this,but all above ground,since  my soil is mostly urbanite on the build site.

Russell Olson wrote:-The pit walls/foundation, I see alot of cement blocks, I would prefer to avoid that due to cost and deficient skills in masonry. I'm not against it though, are there ok alternatives I could try. I currently have as much wood on my property as I'd ever need, including straight white pines that need thinning, and red oaks that are dying. Neither of those woods are super rot resistant, but might work if I were to dig them in and cover them with a plastic moisture barrier right?

I just finished reading Mike O's book and it has a good option for you using wood as the walls/foundation.  In my own design I'm torn between the affordability of wood and the solidity of blocks.  Other folk's designs may not have the same issues I'm facing.  But for my structure, block walls would allow for the roof to be self-supporting (rafters without joists).  I have limited masonry skills myself but the option to dry stack them and surface bond them in conjunction with filling some cores with rebar and cement seems interesting.

Russell Olson wrote:-Insulating, inside or outside the foundation wall? both? Should I plan on insulating the earth mass behind the north wall or is that better to be non insulated?

Great question, I hope someone chimes in with a good answer.  I think we should insulate outside the foundation so that foundation acts as thermal mass.  As for the earth outside the wall, my GUESS is that it depends on how warm you want your greenhouse to stay.  IE does the temp of the dirt around you help, hurt or store energy. 

If I want my Wisconsin greenhouse to stay just above freezing in the winter, I'd probably use a skirt of insulation 1' below grade and 3' out from the foundation so that the 40? degree dirt under the insulation helps keep your foundation a "toasty" 40 degrees.  That would use the heat of the Earth to help heat the greenhouse.

In my case I want my greenhouse to stay around 60 degrees in the winter so I'm not sure what to do.  The soil could act as thermal storage as long as the stored heat doesn't disappear somewhere.  So I'm not sure if the insulation should be right against the foundation (no storage) or outside the foundation to contain some of the dirt as thermal mass.  Kind of like the wofati umbrellas the lab ants are building.

As for thermal mass in barrels, one thing I read this evening suggested finding smaller containers (5 gallon) to hold water.  Then you have more surface area for the heat to enter through.  You'd have to do some thermodyamics to find the ideal size and surface area for your application.  IE a daily heat up and cool down of the thermal mass may work well for small jugs, but a 3 day heat storage to help you survive a cloudy spell may require more volume and less surface area (barrel or tank). 

Another interesting idea I had was to use cinder block holes as thermal mass storage tanks.  I had a plan to use a 3 block high wall of cinder blocks as planting beds.  I was then going to put a kitchen garbage bag into each hole and fill it up with water.  The block would support the column of water and the bag would keep it from draining away.  I calculated an impressive amount of thermal mass for the planter I was designing.  I'd love to come up with a similar system for an 8' high block wall but I don't know of a good way to keep water in the holes.  Food for thought.....