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Greetings permies,

Yet another question for you all, this time regarding a trench greenhouse.  I like this concept because it eliminates some construction materials.  I am concerned about drainage, and that is my main question for this thread.  Is drainage in a greenhouse critical?

Idea in brief:

Situated on the northern edge of a flat garden, the edge slopes down to the north (I don't know the slope, approximately 1ft.drop per 10ft. horizontal) Several concrete block compost bins about 4 ft. high, totalling about twelve feet wide, for the north wall.  Dig a trench down the center, give or take, about three feet deep.  Southern wall is glass, with cedar posts and lumber framework, approximately two or two and a half feet tall.  This would leave me with approximately two twelve foot long by two or three feet wide "tables" of clay to work on/plant in (north "table" should be apprx. four feet wide to accomodate steel drums and give work space).  Only one door, on the west or east (prevailing wind from the west, thus I should set it on the west?), so the other end will have an additional small sq. footage of "table" space.

My main concern is drainage.  Is drainage critical in an underground greenhouse?  I know it is of huge concern in building a residential structure; what about a building meant for plants?  I don't mean for this greenhouse to last for perpetuity.  Ten years would be nice; we have more cedar trees.  I will line the floor with gravel and am strongly considering digging a trench from the doorway down the north-facing slope as long as it takes to create a proper level for draining the greenhouse floor, if drainage is necessary.  Another idea was to scoop out a hole in the middle of the trench, cover with a board, and from time to time bail out any water in the hole (we are working in heavy clay).

I know this isn't a perfect plan, but I have some hopes for it.  What do you think?

Additional concerns are creating 6.5 ft. of head height in the middle of trench, and how to balance this with creating an appropriate slope of the roof for our latitude (37 degrees N).  As well as how to create a solid roof where snow will melt off instead of create rot issues.

In the image below, on the right are the compost bins. Above, the red is some old roofing which will be used to provide partial shade.  Resting against the north wall will be 55gallon steel drums painted black (whoops, they weren't painted in the drawing!).  There will be additional wood framework not shown in the drawing for clarity's sake.  Gravel lining the trench floor.

If anyone could tell me whether or not they think drainage is necessary for a greenhouse, I will appreciate that!  Any other ideas, thoughts, and questions are very welcome.

Thank you!
Greenhouse rough sketch.png
[Thumbnail for Greenhouse rough sketch.png]
 
Kate Nudd
Posts: 115
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Hi
I stumbled upon a design, and actual build, of a self watering greenhouse. ( I love it and want to try something similar once I have more land.
Would any of his concepts help in your case?
www.envisioneer.net
He's got lots of other super stuff,too.
I like your idea of incorporating the thermal mass of the compost bins into the greenhouse.
 
Abe Connally
Posts: 1502
Location: Chihuahua Desert
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yes, drainage is always an issue.

You could make a drain that goes down slope, just dig a trench and drop a pipe in it....
 
                        
Posts: 107
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Right, Vela, that's what I will resort to if it seems necessary.  Why is drainage an "issue" for you?  How much water could I reasonably expect to build up inside a roofed structure normally occupied by plants soaking up water?  I can see the possibility of an over-zealous waterer creating puddles at their feet.  I am trying to think of as many possibilities as I can before we begin construction in the fall.

Hi Katee, thank you for the link!  I'm not sure how many more photos are in this gallery, I'm enjoying the photos and info so far.  Plastics are a material I am hoping to avoid, thus some major aspects to that project are a little out of the realm of possibilities for me at the moment.  I do want to capture the water and store it in a 55 gallon steel drum, with the capability to run this water into the greenhouse for watering.  I was doing some slight math earlier this evening and realized I'd have to partially bury the drum to keep its top below the bottom of the roofline (apprx. 2.5 ft. in my current design, while a steel drum is maybe 3ft. tall).  I think rainwater harvesting is a very important idea.

What a neat website, very detailed.
 
            
Posts: 177
Location: California
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There's a similar design in M.G. Kains' book, "Five Acres and Independence". In his diagram the house is built on a (more or less) level plot. The wall material is buried 3' deep on all sides in his model, leaving just enough exposed at the top to fit the roof onto. The trench is dug 3' 5" deep and 2' wide with beds on either side. His design incorporates drain tile for the walking path, but the book was written in 1940, so.. updated version - If it were me, I'd run a perforated drain pipe under the gravel for the walking path, bare minimum. It's the lowest spot in the design and you don't want to be slogging through mud during the winter months. I'd dig the trench wide enough to run a wheelbarrow down as well, and reinforce the walls of the beds that form the sides of the trenches with lumber. If you wanted better longevity out of the exterior walls, you could trench around them as well and lay in drains and/or gravel if/where you have problem water areas.
 
Abe Connally
Posts: 1502
Location: Chihuahua Desert
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to me, drainage would be an issue if water could do 2 things:

1. threaten the stability of the structure (in this case, the walls would start crumbling away).

2. it drowned any of the inhabitants (water could pool in this design, sit there for a long time, and drown plants).

How much water could you expect?  Enough to fill that hole.  Water will flow through the soil and fill any cavity.  I've seen it down many, many times.

It should be easy to test things out.  Dig a hole, put a tarp or something over it resembling a roof, and leave it.  Check it after every rain. 

Just prepare for the worst and put in a drain.  As well, use berms and earthwork to direct all surface runoff away from the building.
 
                        
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Great suggestions you two, thank you.  Now you've made me realize the error of my laziness, Vela.  I just finished putting in a woven wire fence, and two post-holes within 100 ft. had to be re-dug because they were constantly filling with water during the rainy season... couldn't find a window to sink a post in and re-fill, even though I'd bailed them several times (I'd even covered them with little tin roofs out of frustration!).  I see now that a drain will be necessary.

I like the idea of the wheel-barrow accessibility, especially with the compost heaps so nearby.  If I can make the entrance wheel-barrow accessible, then I should be able to make the trench wide enough as well.

Thank you!
 
                            
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If you can, check out mike oehler's "The Earth Sheltered Solar Greenhouse" book.  He applied the same principles he used in his $50 Underground house to a greenhouse.
 
Abe Connally
Posts: 1502
Location: Chihuahua Desert
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I second that, Budro.  That is a good book to look at.
 
                        
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Thank you, Budro.  I read mike oehler's $50 & Up underground house book.  I differ slightly in my preference towards glass.  I am sure a book of his dedicated to greenhouses would have something to teach me.  I tried looking for a video posted on here by paul wheaton that is a tour of a greenhouse built by Mike Oehler.  Unfortuantely, I did not find it.  It did leave me content remembering how Mr. Oehler mentioned having low pockets of air for the cool air to collect.

Another thought, if you will bear with me:

If a drainage trench is necessary, I ought to build it with the slope of the land in mind.  In this case, that is to the north.  I've looked over the potential site a few times in the past couple of days and the slope looks to be pretty decent, very do-able.  I wonder, given the design image above, if I could somehow split the back wall of concrete block compost bins in two, and put the greenhouse's doorway in the north wall, trench included.

Are doors in the north wall of greenhouses undesirable for any particular reason?  I know it would interrupt the look of things, but now that I think of it I could build a small little tool storage area right over top of the drainage, using the door and two opposing walls of concrete block as the tool shed's boundaries.  It could be like a boot porch for the greenhouse!  Getting out of hand perhaps.  I am concerned a bit about the difference in heat loss/thermal mass between a north wall composed of solid concrete block backed by compost piles and one that is concrete block backed by compost piles with a gap of a wooden door and open air behind it.  One advantage to the latter is that I would extend the west-east length of the building, making it bigger as well as splitting it symmetrically north/south rather than west/east.

I think I need ventilation in the west and east-facing walls of the greenhouse, especially considering that is the direction of the prevailing wind, so that is why I initially planned for the doorway in one of the two.  Now I am wondering if a northern doorway would be more efficient.
 
Kay Bee
Posts: 471
Location: Jackson County, OR (Zone 7)
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I put in some trenches for partially underground/bermed greenhouses this past November.  they are situated on a south-facing slope, approx 2-10% over the area.

They range from 4-5' deep and ~5-6' wide, ~25' long. The excavated dirt was placed mostly on the uphill side as a berm to block runoff and to support the roof angle.

I had planned on getting them covered before the rainy season really hit here.  Didn't end up happening and within a month, I was amazed that they were all completely filled with water.  Still are with the wet spring we have had.  Now they are building up a population of water beetles, skippers and a good number of tadpoles and other fun critters. 

While we get ~24-30" of rain a year in our location, most of the water looks to have come in through lateral downhill flow through the clay.  We've seen this in our hugelbed areas, as well.  I was very surprised to see how much flow occurs through the first couple feet of clay.

So, I will be constructing my bermed greehouses uphill of the trenched areas (ponds) and on or above the existing grade.
 
Fred Winsol
Posts: 155
Location: Sierras
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There is a wonderful concept called PAHS (Passive Annual Heat storage) that started in the 70's and have some great science behind it (Dr. John Hait of RMi) check it out at:

http://www.norishouse.com/PAHS/UmbrellaHouse.html

It uses the heat of the summer for winter heat, and winter cool for summer cool.  It defies most of our currrent architectural practices of thermal envelopes, Passiv(euro style)  homes.

A lot of people tried building these in the 70's, ended up 5+years later with water intrusion.  In Dr. Hait's book on the subject he recommends a permaculture approach to resolving this:  observe and nudge nature for natural drainage AWAY from the structure.

Lo-cost suggestion: one can use visqueen for thermal barriers...

Fred
 
                                    
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I tried something like this on a larger scale, I was actually trying to make a walipini, same idea as a trench greenhouse.  I had water infiltrating from below, and the surrounding soil immediately, and now the hole is full to the top...
If I were to do this again, I would either
1. build on a slope.  Channel the water over, and under with vapor barriers in the soil.  Some water would get in, and maybe a trench could be dug on the backside to catch that, and a pipe to channel it out.
2.  Go no more than 2 feet down, and create a gravel bed with piping running through it.  The warm air would be forced by a fan from the warm top, through the gravel, depositing it's heat for nighttime use.

Since I already have a flooded hole, #2 option looks better.  BTW, this is already being done, google "subterranean heating and cooling" for more info.
 
Fred Winsol
Posts: 155
Location: Sierras
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Has anyone ever done a trench greenhouse with the PAHS method?  Seems pretty easy as you're not moving as much soil as with a house.  just 2 feet or soil around the trench greenhouse, put down an impermeable barrier (visqueeen) and refill the soil.... and then you gotta do the same under the trench.  Should keep it warm all winter.. and cool in the summer.

... could also work on a hotbox?

 
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