[permaculture] Re: Greenhouse Design
Paul A Cross
charybda at newmex.com
Wed Dec 31 15:54:01 EST 2003
Greenhouse design is one of my favorite topics!
Arlene, it would be really helpful to know what you plan on growing and
whether it is to be marketed or is just for your home use, etc. I am wondering
why you need greenhouse or if a shade house would be more appropriate.
Greenhouse structures often can be customized to the type of growing you are
doing. I need more information on your crops to talk relevantly.
Evaporative cooling will work wonders where you are, but uses huge amounts of
water. Misting systems might accomplish the same for less water uses, but are
more high-tech. Reflective shade cloths are essential and can be added and
removed with the seasons. You could put 50% shade over your structure with no
loss of plant growth. Here in Taos New Mexico USA we see a 14 degree F drop in
temp when we install 50% aluminized shade cloth. The cloth is removed in
winter and reinstalled late spring.
Filtering IR works in two ways. When the sun is shining on the structure, the
glazing can prevent the IR light from entering the structure, keeping the
inside cooler. At night, the same glazing prevents the IR from leaving the
structure, keeping the inside warmer.
Perimeter insulation is also key to keep the ground temperatures moderated
inside the structure. You want to break the thermal continuity between the
inside and outside ground.
As for 55-gallon drums, they are great for home situations. In cold climates
you need about 3 US gallons of water per square foot of structure to prevent
freezing. But every little bit helps, and in warmer climates less water could
be used. Metal barrels have better conductivity than plastic, but metal
barrels needs some propylene glycol in the water to prevent them from rusting
out on you. For commercial scale, water as thermal mass tends to get in the
way of the crops. Imagine a 3,000 sq ft greenhouse needing 9,000 gallons of
water. Where would you put all that water? For most greenhouse situations, the
issue is more about how to cool rather than how to heat.
For overwintering inside, white glazing is typically used. You can get white
rolls of polyethylene films; these last only one season and are thrown into
the landfill. That's the industry standard, nothing sustainable about it.
There are white corrugated plastic rolls that cost about 50 US cents a square
foot and offer greater durability and are R-3. I don't know how many years
you'd get out of it; it really depends on the actual location. You're going to
want roll-up or fully openable sides if at all possible; see below. Removable
shade cloths are key; see above. Yes, the nursery structure will start
overheating in April; that's when you start adding shade cloths and venting.
For winter production, you really do want maximum light intensity on your
currently-growing crops such as microgreens. This is because you are making up
for the short hours of sunlight by have the maximum light intensity. You get
good growth with either moderate light over a high number of hours, or high
light over a moderate number of hours. For overwintering perennials, much less
light is needed than for the microgreens.
For both these structures in Arizona and Texas, you might consider a frame
structure where you can switch glazings between inflated double poly for the
winter and nothing but shade cloth in the summer. You could run a perimeter of
U-channel around the 4 edges of the structure where the glazing attaches. You
place the glazing in the channel and secure it with a W-shaped wire. This
makes installation and removal very easy. In other regions you might leave on
the ploy glazing and just add the shade cloth on with a second wire in the
As for why overhead glazing, I can think of a number of reasons. First, even
plant growth around the plant occurs when the light comes from overhead.
Plants getting light from one side will lean toward the light. You are in
effect reducing the growing area if your light comes from the side. The
economics of agriculture means that you have to design for maximum efficiency
if you are going to survive in business. I can't speak knowledgeably about the
U of Penn system, as I haven't seen it, but (here I go anyway!) if you are
going to concentrate the light to get it through the roof, then you are going
to be faced with having to evenly disperse the light inside the structure
again for optimal plant growth. I'd love to know how they accomplish that.
I've been in passive solar greenhouses with slanted sunwards glazing and
insulated roofs and polewards walls. These are fine for home production, but
not for commercial production, due to the economics and the plant growth
patterns discussed above.
The best way to prevent overheating of growing structures is to have roll-up
sides. These can be manual or motorized and thermostatically controlled. You
are trying to get to 1 air change a minute when the sun is shining. If you can
achieve this level of ventiliation, your inside temp can be close to your
outside temp in hot weather. Of course when overheating in cold weather you
can just roll open the sides partially to regulate the temperature.
Also, you have to take into consideration having air change at night. You want
1 air change per hour all night long to avoid excessive humidty and
condensation on the plants / fruit. Excessive humidity leads to disease
problems such as mildews and fungi. Condensation on the fruit lowers the fruit
temperatures which slows maturation and can effect flavor / nutrient of crops
like tomatoes, meaning lower yeilds and less profit. Again, in home situations
it is less of an issue.
Well, that's enough for now. Happy new year to you all.
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