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solardan1959

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Reply with quote  #11 
Mattie,
  
Not really,  With all the glazing I think it's unrealistic to try an keep the heat in, even with twin wall.  It generally runs 10 to 20 degrees above outside temp by the end of the night.  I'm just thinking of some heat to keep the plants from freezing at night when we are still getting the occasional 0 degrees outside.  Greg Wilson mentioned heating pads or an electric blanket under the seed beds a while back in some post.  That may be the easiest thing to do or a little electric "milk house" heater.
http://www.amazon.com/Patton-PUH680-U-Milk-House-Utility-Heater/dp/B0000BYC61

I have one of those and an oil filled heater that may do the trick but was just fishing for other solar based alternatives.

Dan

mattie

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Reply with quote  #12 
A fair point, keep it simple is allways a good motto,the heat blanket under the beds sounds like a good solution straight to the problem area.
Regards Mattie
mattie

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Reply with quote  #13 
Hello all,
Something else that crossed my mind is instead of having a vertical wall and the need for structural support regarding soil collapse,would a much simpler solution be to slant the walls back so for safety, the roof area will increase so more light enters the walipini and if a fall occurs it is less dangerous.
Regards Mattie
kcl1s

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Reply with quote  #14 
Mattie,
Your idea has pros and cons.

The sloped back wall would be more stable (good) but your longer roof span will need the bows or rafters to be beefed up especially if you are in an area where it snows which will run up cost (bad).

Letting more light in is only good if it falls on plant foliage. Having heat escape at night out the extra glazing would be bad.

Maybe a terraced or stair step design would be good

Keith

teraced.jpg

mattie

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Reply with quote  #15 
Hello Keith
I can appreciate that ,pros and cons being the nature of pretty much everything .Your idea of a stepped inside will actually increase the growing surface area so not a bad result.My main point was highlighting safety concerns.The angle of the walls wasnt going to be massively low, 60 degrees say or over just enough to let gravity do its work in holding them in place.
The snowfall problem might be solved by using center supports instead of making the roof beefier.This could be done using anything really(whatevers available) with the required strenght at a few points or a lot depending on snowfall rates.(pros and cons thing again lol)
walipini.png
There will be more heat loss to due to the area of the roof increasing.This u value could be reduced by using a suitable material.Polycarbonate glazing layers or twin wall polythene with air pump.
They key to it all is what you want to achieve and what you are willing to invest(Within reason ofc)
Regards Mattie

Garage_Hermit

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Reply with quote  #16 
FWIW, digging an unshuttered pit generally follows a "rule-of-three" as regards the wall slope; that is, the earth is classified in one of three categoires:
  1. highly stable;
  2. moderately stable;
  3. highly unstable.
The depth of the wall is a known.

For soil type 1, the wall should be sloped back by one-third of the depth
For soil type 2, it is sloped back by 2/3 of the depth.
For soil type 3, it is sloped by the same dimension as the depth (= 45 degrees, therefore).

G_H

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kcl1s

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Reply with quote  #17 
Mattie,
Yes I was thinking larger angles. Your design should alleviate the cave in problem without adding to the cost much if the soil is fairly stable.


Keith
mattie

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Reply with quote  #18 
Some more info for anyone interested in reading about soil temperatures
http://www.builditsolar.com/Projects/Cooling/EarthTemperatures.htm
petset77

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Reply with quote  #19 
I also looked into building a walipini for our greenhouse, but went another route for a few reasons. I liked the earth mass as insulation and buffer, but I questioned light availability up here in the Colorado mountains during the winter. Our land also slopes toward the north, so we'd have to dig a much wider footprint to make it work. We would have also had to damage more trees and land to get equipment to the location we chose behind our solar (PV) house. Finally, there's a short youtube video with heat gain/loss evaluation for an entirely passive walipini, and most design variants have a negative BTU balance on a winter day. However I built it, I'd need supplemental heating. All that explanation brings me to what we chose to do. The building is 36'x16'. We situated the greenhouse as if was a walipini, with long walls east to west, so the south side of the roof faces south. That entire side of the roof is going to be triple wall poly. I started building it a few weeks ago (late June), so watching shadows helped me realize that if I want summer sunlight in more than half of it, I'd need a few skylights on the north side of the roof. Most of the north side roof is R-30 insulated with metal roofing and gutters to catch rainwater and snow melt. To help keep heat in the structure in winter, on the north side, I'm adding simple wooden slide channels for 2" foam panels, to slide into place over the skylights in winter and out of the way without needing storage space in summer. The south wall is a combination of clear glass (4'0"x5'0" insulated glass casement windows I got free for helping Habitat for Humanity load a pile of them after a restoration project at a major ranch up here), and twin wall poly panels I saved from the dumpster after a movie I worked on a while back. The glass will let direct lighting in, and the clear poly diffuses and spreads light, negating hard shadows. The east and west walls each have one of the windows and one panel of poly, situated closest to the south side. Another design element to retain heat, I'm making insulated roll down shades out of inexpensive packing blankets, rolled on dowels like matchstick blinds. Those will go down at night when it's cold. With our extreme cold in winter (negative 20s are common), it's going to have two heat sources: a rocket mass heater, with horizontal flue/heat mass bench under the south wall glazing, and 2 IBCs (water tanks), 300 gallons each, painted black to absorb solar radiation during the day placed on the R-19 insulated north wall. If they don't get warm enough, I'll build a couple of absorber panels to feed them hot water. This winter will be the test to see if we can stay above freezing, but I'm confident we'll do fine. The building is framed, with metal and poly roofing ordered. I start building the rocket heater this week, and the IBCs should be here within two weeks (I'm getting them used from a potato farmer for $45 each, so they'll get here when they arrive). Overall, and in most cases, I would have opted for the walipini, but it didn't seem right for our situation. Mattie, I have watched that video of the long narrow greenhouse. He did a great job with it. I went a little wider to accommodate the mass heater and tanks. Keith, I did just what you said about insulating the ground on the perimeter, but I only insulated 16" down (I'm getting older, and hand digging a trench totaling almost 100' long that much deeper in the hard ground up here was balanced with the possibility of having to place closed cell foam just under grade at a later date). If I find that cold permeates from the sides to an objectionable degree, I'll also go horizontally, like I did with the house. There's a great website with data on shallow foundations and insulation for Scandinavia, but I lost the address when my last computer crashed.
petset77

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Reply with quote  #20 
...I just proof read my post (after adding it as a reply, of course), and saw a funny typo. "...so the south side of the roof faces south." I guess that would be opposed to the south side facing any other direction. It's kind of funny to read, but sorry. I guess it should read "...so the long south side faces the winter sun."
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