[permaculture] Re: Greenhouse Design

Paul A Cross charybda at newmex.com
Thu Jan 1 14:16:32 EST 2004


Kirby wrote:

> number two.  We decided to build a single gable with polly carbonate panels
> ($16 per panel for 38 panels - ouch!) on both sides of the gable.
>
> The south, east and west walls will have 32" by 60" single pane windows ($79
> each) and two clear, 36" wide storm doors ($99 each) as continuously as
> framing will allow.  The north wall will be buried 4' into the hill side and
> be completely covered with hardy panel siding.  We are installing a louvered
> fan and vent at the gable ends that will be on a thermostat switch, but as
> of now we haven't exactly figured out how to passively vent the ridge, and
> are still flipping through catalogs looking for ceiling panels that can be
> opened with a hand crank.

Knowing the dimensions of the actual structure itself would be helpful, too.

If you can design a ridge vent that opens continously along the entire length
of the greenhouse for about 4 feet, along with sides (or in your case one
side?) that open up continuously along length for about 5 feet high, you will
have great ventilation. Something like this will be passive. The commercial
guys use a motor and a drive shaft attached to  arced gear plates that open
the top roof panel all in one section. You could probably buy such a system
from a greenhouse

> Are their some industry standards for greenhouse ridge / ceiling vents?  Do
> you think we even have a chance of growing out the micro-greens in the
> summer?  I wonder what the difference is between a conservatory and a

Yes, with enough venting (active and passive) you should be able to do MGs in
summer.
A good book I've seen that covers the mathematical science behind greenhouses
is Greenhouse Engineering by Robert A. Aldrich & John W. Bartok, Jr. There
must be others, but that is one I am familiar with. Not sure who else carries
it, but I see it's listed in the Hummert International catalog on page 592
(www.hummert.com).

> I imagine that there will be a shade cloth we roll out in the spring, summer
> and fall that is somehow attached just below the collar ties.  However,
> their will probably be an overhead irrigation system that we'll have to stay
> above.  Maybe we should drop the collar ties to wall height and roll out the
> shade cloth above them.  The louvered fan and vent would then be well above
> the shade cloth and I guess we would need at least four or so circulatory
> fans below the shade cloth.

You really ought to consider putting the shade cloth on the outside of the
structure rather than inside. You are trying to reflect light / heat away
before it enters. Once the light is through the glazing, it's already making
heat.

> Are windows on three sides enough for summer ventilation?  Maybe we should
> put in a continuous row of 2' x 1' windows as high up as we can get them on
> the north side for letting the prevailing summer breeze through.

Catching the prevailing breezes is definitely worth doing. A continuous row of
windows 2' high sounds too small to me; better to go 4' high if you can.

> Do you think we should insulate the north side of the green house?

Yes, most definitely insulate the polewards wall if you can afford to do it,
assuming this is the detached structure.

> Should I build some openable panels underneath our south and east facing
> windows to let the prevailing breeze in as low as possible?

Yes. Ideally the side walls would open up continuously for 5 feet tall. I have
a greenhouse where the 100' sides have 5 slider panels that are 2' tall in 10'
sections evenly spaced. This is not enough passive ventilation in the summer,
but is used in emergency situations when the power goes out and there's no
alternative cooling. Power goes out often here.

> What would you recommend for the floor?  At the moment we have one pallat of
> bricks on site and some pieces of flat stone.

Well, we just used base coarse from the local gravel pit, smoothed it out with
a backhoe and a landscape rake as best we could. Crusher fines would make a
smoother surface if you need that. Then we "carpeted" the entire space with
DeWitt Pro 5 weed cloth held down with ground staples pounded in with a rubber
mallet. Cheap, fast, and easy. It comes in rolls 12' wide x 250' long, I
think.

> should our walls be lower than stud height?  Presently, I'm considering
> running the top plate just above the door header (the finished wall topping
> at approx. 86.5").  That puts the bottom of our windows at 26.5", then
> considering the structure is on one row of 8" tall cinder blocks our work
> bench height will be around 32" if we are to stay below the windows.

Side walls should be straight vertical for as tall as you can afford. 86.5" is
fine for microgreens and perennial overwintering, but too short for vine crops
and tree crops. If you're wanting to make your structure more general purpose,
raise the side walls and the ridge higher.

> In order to fit the 24" x 24" louvered fan and vent we will need the ridge
> to be about 4' above wall height giving us a 10/12 pitch.  Is that too high?

That's not too high. Standard commercial tomato houses used to be built 4 or
4.5 meters high. The new ones are often 5 or 5.5 meters high. I am not sure
about whether the 0.5 meter is in there or not; I can't recall the exact
figures. But the point is that although the high ceiling makes heating more
expensive, it makes cooling so much easier. The hot air is high above the
plant canopy, where it is less of a problem. 24" x 24" is probably an
undersized fan for your application, but it's hard to know for sure with the
information at hand.

Lastly, a disclaimer: I haven't seen your plans or the site, so I am going off
of just what you've told me thus far.

Good luck!





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