Greenhouses. Basic Rules For Solar Heated Greenhouses, permaculture

Greenhouses. Basic Rules For Solar ...

Greenhouses. Basic Rules For Solar Heated Greenhouses, permaculture

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The Solar Greenhouse That's Right for You
http://www.survivalplus.com/foods/page0009.htm
Welcome To Our "Survival Foods
& Their Preparation" Section
The Solar Greenhouse That's Right for You
(Text & illustrations for this web page came from the
August, 1978 issue of
Organic Farming & Gardening
)
This Site's Pages
Updated Every
Friday Evening
Between 7 & 9
PM CST
Site Table
of Contents
Here is a new gardening tool that produces
fresh food when the snow flies.
BULLETINS
CATALOG
NEW STUFF
PHILOSOPHY...
SURVIVAL FOODS
FORWARD!
THE OUTRIDER
SELF-DEFENSE
JACK RUTTLE
ALMOST ANY STRUCTURE that is built to look like a solar
greenhouse will work. That is to say, the solar greenhouse concept is so
right that you can ignore (or not know) the fine points of solar design
and still build a house with much less need for supplemental heat than a
traditional greenhouse. But once you understand a few basic
solar-greenhouse design ideas, you can easily put together a greenhouse
that truly lives up to the label
solar,
and provides remarkable
efficiency.
A TEST
Dave MacKinnon, Ph.D., ORGANIC GARDENING greenhouse
designer, has put it all together after three years of experimenting and
has created a design formula that gardeners in any climate can follow.
His newest solar greenhouse, which he has built and tested in Flagstaff,
Arizona, epitomizes a good solar shape. It has produced food through
two winters without requiring any outside heat source. Almost all the
floor space is usable for growing beds because the energy storage is on
the walls. And it uses a minimum of materials because the design,
insulation and heat storage are in balance and arranged to complement
each other.
The best measure
of a
solar green house is the plant-growing
environment it creates. When the building is skillfully made, you will
get midspring soil and air temperatures in the depths of winter on sun
power alone.
Our experiences suggest that solar greenhouses can maintain that kind
of environment in most parts of the country. ORGANIC GARDENING
researchers have built two different greenhouses that have worked well
despite unusual winter weather. The Flagstaff greenhouse performed
well with much less sun than is considered normal, and the one at our
Maxa-tawny, Pennsylvania, research center worked through the coldest
winters in recorded meteorological history.
In December and January we harvested enough salad greens every day
for three or four people. Cold-hardy plants, all very rich in vitamins A
and C, produce best. Escarole, lettuce, parsley, corn salad, chervil,
chives and other salad herbs are dependable. So are kale, chard and
chicory, which grow so thin and tender in the weak winter sun that they
are best in salads too. In spring and fall the harvests are bigger.
Succession plantings make heat-loving plants like tomatoes and
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The Solar Greenhouse That's Right for You
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cucumbers possible far beyond their normal seasons.
Dave MacKinnon's solar greenhouse
greatly expands
his crops of homegrown food. He picks
salads every
day through winter. Frost nips his outside
garden early,
so greenhouse protection has meant the
first
heavy-producing tomato plants he's ever
grown.
SOLAR
GREENHOUSE
BASICS
At the least, a solar greenhouse
should have three features. One
of the long walls should face due
south rather than east or west.
The south wall should have two
layers of glazing. All the surfaces
that don't face south are
insulated. But there's a little more
to it than that if the greenhouse is to live up to its solar potential. The
new Flagstaff greenhouse is a perfect model.
Dave MacKinnon says the greenhouse should be about twice as long
(east to west) as it is wide. Accordingly, his Flagstaff greenhouse is 20
feet by 12 feet. The two-to-one relationship offsets the effect of the
shade that the opaque east and west end walls create. The building thus
captures more solar energy for each square foot of growing space. If the
building is made much deeper than two to one — that is, closer to a
square floor plan — the heat-storage material in back is shaded too
much. These proportions are recommended for greenhouses
everywhere.
MacKinnon has learned another rule of thumb for sizing the
energy-collecting south face properly, and has built it into the Flagstaff
greenhouse. The peak should be made about as high as the building is
wide (north to south). Heat-storing materials in the back of the
greenhouse will then get the direct exposure to the sun they must have
if the storage is to work efficiently.
The slope of the north roof is an important feature of MacKinnon's
greenhouse, though the precise angle of slope is not critical. Sunlight
which enters the greenhouse and strikes the aluminum-foil-covered roof
(white paint works well too) is aimed back down to the growing beds.
From the outside, the interior of the greenhouse looks almost black
because very little light is bouncing back out to the viewer. If designed
well, solar greenhouses with reflective walls can actually deliver up to a
third more light to the plants in winter. In the traditional all-glass
design, much of the light passes right on out the clear north roof and
wall. Angles between 60 and 75 degrees for the north roof will work
well in the United States and southern Canada.
The slope of the sun-collecting south face might appear to be trickier to
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The Solar Greenhouse That's Right for You
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decide upon. The angle does affect how well the translucent face
collects sunlight, but for greenhouses it's not as critical as when setting
up a compact solar-heating unit. Actually, a wide range of angles will
work equally well at any given latitude. The simplest thing to do is to
add 20 degrees to your latitude. A south face with that angle will give
optimum performance in January. But if that particular angle proves
hard to work with, go to a slightly shallower one, and you will be
favoring solar collection in spring and fall. If you use 50 degrees rather
than 60, which, for example, you may figure is your ideal, you still
have sacrificed very little midwinter light.
Given this leeway, other factors like convenient construction can help
determine the south slope. The south face of the Flagstaff greenhouse
was made steep all the way to the ground to shed snow quickly. That
feature lets sunlight in sooner after storms. The Maxatawny greenhouse
has a vertical glazed knee wall from which a shallow, clear roof slopes
up to the peak. There is much less snow to worry about there, and this
shape makes working in the front of the growing beds easier.
When it comes to putting in insulation and heat storage, however, solar
greenhouses can get needlessly expensive. The key is to have sensible
amounts of both. Great thicknesses of insulation can't do away with the
need for heat storage, and are wasteful. And obviously, adequate
heat-storage material without a certain amount of insulation in the walls
is
equally wasteful. Even if you build a greenhouse that is not
completely solar-reliant, using a balance of these component parts
guarantees an economical building that will work well. (The
information on the map indicates the proper proportion of materials, as
well as recommending minimum amounts for a fully solar structure.)
HOW HEAT STORAGE AND INSULATION
TEAM UP
The connection between heat-storing materials and insulation works
like this. Without heat storage, solar greenhouses are something like a
thermos bottle — all the energy is in the sun-warmed air. Drafts will
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quickly drain off the heat, because even the most tightly made building
will have a fair amount of tiny cracks. Energy held within storing
materials is not lost along with escaping air. The energy is released
slowly as the greenhouse cools, and the building stays warm much
longer.
A no-less-important effect is that heat-storage materials keep the
greenhouse from overheating during the day. We quickly learned that
without storage the inside temperature can soar into the 80's or 90's on
cold, bright days. That is quite hard on a winter greenhouse
crop.
With storage absorbing some
of
the incoming energy, the result is a
milder daytime environment.
The amount of heat storage for full solar heating seems enormous at
first, but is manageable in practice. Both of MacKinnon's greenhouses
use about 1,000 gallons of water stacked vertically on the rear walls.
That amounts to four gallons of water for each square foot of floor
space. The best method we've found is to use rectangular five-gallon
honey cans with a rust inhibitor added to the water.
"Five-gallon honey cans make
efficient
heat-storage containers" says
MacKinnon.
They pack the maximum amount
of
water into a given space.
Why do we rely so
heavily on water? It is
admittedly hard to work
with because it tends to
corrode containers and to
leak. But water is about
the best heat-storing
material known and is
cheap. The best alternative is rock (in any form from sand through
concrete), but water holds about five times more heat. So water
reservoirs on walls make compact heat storage that gets a good share of
direct incoming sunlight.
MacKinnon favors smaller containers over 55-gallon drums for two
reasons. Drums leave empty about a third of the space they occupy,
because they are big and round. They also permit warmed water to
gather into a few large areas, which causes both greater heat losses and
poorer collection in those areas. Smaller containers keep the energy
more evenly distributed. On the other hand, the large barrels are
certainly worth using if they can be had cheaply. We've also used
translucent plastic cider jugs filled with water dyed black, and have
heard reports of success with beverage cans sealed with tape and
stacked right-side-up.
The amount of insulation that MacKinnon judges to be practical in
various regions is roughly the same as local, energy-efficient
recommendations for homes. If that seems lavish for a greenhouse,
remember that homes get a lot of extra heat; the greenhouse is designed
to get along with none. To me, the need for plenty of insulation is a
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reminder that people aren't much different from plants
in
their
requirement for warmth, among other things. To use less insulation,
however, is to need more heat storage, which demands more space and
money.
Two other simple things are crucial to the success of our greenhouses.
The earth below them is insulated to a little below frostline with plastic
foam. And at night an insulating curtain is drawn over the clear south
wall to reduce the high heat losses there.
Regional recommendations for
minimum
amounts of
insulation in walls and roof, below ground
and of water for heat storage.
Zone 1
wall and roof, R-40
below ground, R-15 to 3 feet
deep heat storage,
4 gallons per
square foot of floor
Zone 2
wall and roof, R-22
below ground, R-15 to 3 feet
deep heat storage,
3 gallons per
square foot
Zone 3
walls and roof, R-12
below ground, R-10 to 2 feet
deep heat storage,
3 gallons per
square foot
Zone 4
walls and roof. R-8.5
below ground, R-10
to
2 feet deep heat storage,
2 gallons per
square foot
Zone 5
walls and roof, R-6.5
below ground, R-5 to 1 foot deep heat storage,
2 gallons per
square foot
Zone 6
walls and roof, R-6
below ground, R-5 to 1 foot deep heat storage,
1 gallon per
square foot
Zone 7, 8, 9 These regions need insulation and night curtains,
but to much lower insulating values. Greenhouses
in
these
regions do not require double glazing, but it will help. No
heat storage or below-ground insulation is needed for
minimum performance. About half the
north slope of
the roof should be glazed.
It pays to insulate the earth below the greenhouse because earth is a
relatively poor insulator, contrary to a lot of lore.
A few inches of most common insulators match the R-value of ten to 15
feet of earth. But earth is a good heat-storing material, lying somewhere
between rock and water. So insulating around the perimeter builds heat
storage into the structure while stopping steady heat losses to the
ground outside. We checked the advantage of doing this at the
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