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Greenhouse
Energy Conservation
Much of the below is an excerpt from the book
Greenhouse
Gardener's Companion
© Copyright 2000,
2012 - Printing or reproduction
of any kind is prohibited
Want to read more?
See the book
Greenhouse Gardener's Companion
Scroll down for
more resources on this subject
With the
price of energy getting more expensive I am hearing from many greenhouse gardeners wondering how to use less energy or how they can
benefit from more solar heating. The following is a discussion that should
help guide you down the road towards less heating needs:
These
are the basic requirements for an energy conserving or solar heated
greenhouse. Even if you have a conventionally heated greenhouse, you can
still reduce the need for heat and reduce your heating bills by applying
some of these basics for higher energy efficiency.
Above shows some
of the basics of a passive solar heated greenhouse (in this case,
the Cheyenne Botanic Gardens).
Notice the different sun angles between summer and winter.
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1. Insulated walls. If you
have a freestanding greenhouse, can you insulate the north wall? If you
live in a sunny area, you may also consider insulating the east and/or
west walls. Start with the side receiving the winter prevailing winds.
Some people retrofit an existing greenhouse for winter by using a foil
backed "bubble wrap" type of material on the north, east and
west walls and north portion of the roof. This material is commonly sold
as "Reflectix" insulation. It is found in lumber yards, hardware
stores and sold by greenhouse supply catalogs. For even more insulating
ability you can double-up this material.
2. Insulated foundation.
The foundation of your greenhouse should not be a heat sink. If the ground
is freezing cold outside, then it will conduct directly through your
foundation making it cold inside. The solution is to insulate your
foundation’s outside perimeter with Styrofoam board, usually 1 to 2
inches (2.5-5 cm) thick. This insulation should be placed vertically and
extend down to a minimum of 1 foot (.3 m) in most temperate areas. If you
live in gardening zone 4 or less (as determined by the USDA garden zone
map), consider even deeper foundation insulation. Set the foam-board
insulation against the outside of your greenhouse foundation. What if your
greenhouse has no foundation? Place the vertical insulation in a trench
around your greenhouse perimeter.
3. Double or triple glazing.
(Note: "Glazing" is the term for the greenhouse skin which the
light shines through.) This is a must for an energy-efficient greenhouse
and essential for any solar greenhouse. Of course,
glazing should be
tightly sealed, free of dust or dirt and not yellowing (which is common to
many older plastic glazings). If your existing greenhouse already has
single glazing you should pursue some type of retrofit. You could opt for
an expensive solution: tearing out the old glazing and replacing it with a
newer material. Other cheaper options include applying an inner film such
as vinyl or other greenhouse polyethylene type of film.
Some
greenhouse supply companies carry clear bubble insulation. This is the
very same stuff that is used for packing material that kids and some
adults love to pop. First you need to spray a translucent adhesive in
spots on the framework or glass. Then you press the bubble insulation in
place. This stuff can be removed whenever the extra insulation is not
needed. The spray adhesive however, is not recommended for use on acrylic
or polycarbonate glazings. For these glazings you need to figure out some
other way to get the bubble insulation to stay up against the glazing. I
have used different types of tape, hooks and wedging it in with success.
Click
here for R values of glazing
4. Seal every nook and cranny
prior to winter’s arrival. Vents and doors must be weather-stripped,
and cracks must be caulked and checked for wear every year. I usually do
an inspection for any potential air leaks in fall before the cold weather
comes. I always find an area that needs some sealing up. A good way to
search and destroy winter air leaks is to close up the greenhouse and
light up some incense (If you are a Buddhist or were around during the
60’s, you might even have some around the house). Light the incense and
hold it near suspected leaking areas. You can visually see the smoke
respond to incoming air currents as you move the incense around. If the
smoke streams in a new direction, there is your leak. Plug it up with
caulk, foam, weather-stripping, or whatever is appropriate for the spot.
Improvise if you must. One small air leak can make the difference between
above and below freezing!
5. Thermal storage. This is
the use of rock, water or other dense thermal mass material in which to
store the incoming solar heat. Water is by far the most efficient and
easiest material to use (cheap too!). Solar greenhouses are notorious for
their accumulations of black 55-gallon (208-liter) oil drums filled with
water placed along the north wall to store the sun’s heat. They are
cheap and efficient. For smaller greenhouses look for the 30-gallon metal
drums. Many owners of heated greenhouses have also installed containers of
water to increase heating efficiency. It’s easy to use a water drum as a
support for a plant or soil table. Read more about thermal mass in the
next chapter.
click
here for more information on water as a storage medium
 6. Air-lock entry. If you
have a free-standing greenhouse or if you use your attached greenhouse’s
exterior door with any frequency in wintertime, you should consider
installing a second door that creates an air-lock entry. This is a
two-door setup with enough space between them so that when you open the
outside door, you won’t get a blast of freezing air onto your poor
little plants. Even if you heat your greenhouse conventionally, an air
lock entry will save an immense amount of energy. For freestanding solar
greenhouses, an air-lock entry is essential equipment. Attached
greenhouses owners can get away without one as long as you always go
through the house! Never use the outside door when it’s cold.
7. Site orientation. If you
have a solar greenhouse, you had better hope it’s oriented in the
correct direction. If it’s not, I guess that there isn’t much you can
do about it now. A solar greenhouse should face within 20° either side of
true south.
8. Night curtain. This is
an option for people in very cold areas. It is not for everyone. A night
curtain is usually an insulating cover you roll across the inside of the
greenhouse glazing at night like a blanket to prevent excess heat loss. I
often view night curtains with a bit of skepticism. One problem is that
they are costly and often have problems with reliability and are not easy
to find off the shelf. Homemade ones are even less reliable. The farther
north you live, the more they may make sense. An alternative could instead
be the addition of an extra layer of glazing. If you have single glazing,
consider double. If you have double glazing consider making it three
layers thick. This can be achieved by either re-skinning your structure
(that’s not what you want to hear, huh). A cheaper alternative is to
apply a new inner layer of vinyl, or polyethylene film. Read a few
paragraphs prior (Double or triple glazing) about using plastic bubble
packing. By the way, a light evening snow on the glazing is a great
nighttime insulator and is free. Always wait until morning to knock the
snow off unless you have concerns about the weight of the snow on your
greenhouse structure.
More
resources
Want to read
more? See the book
Greenhouse Gardener's Companion
Great site on solar
greenhouses:
http://attra.ncat.org/attra-pub/solar-gh.html
This is a wonderful
compilation of 6 case studies of renewable energy/ energy efficiency
from cold Minnesota!http://www.cleanenergyresourceteams.org/blog/renewably-extending-seasons-growing-greenhouses-powered-clean-energy
Crazy but it is worth
pondering . . . insulate your solar
greenhouse with Bubbles!!!
http://www.solarbubblebuild.com and
http://www.solaroof.org/wiki/SolaRoof/SolaRoofTech
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