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Economically Heating (& Insulating) the Greenhouse

Updated: 12/13/98

Heating the greenhouse is a 'h2ot' topic this winter (probably most winters).  I've copied some of the e-mails that we've been passing around:

What is the best (and most economical) way to heat my greenhouse?
A low power, inexpensive, heater solution?
What material to use to insulate the base of the greenhouse that won't rot? R-board?

Question: Cool H2otbox? I've been reading some of your how-to articles and at http://www.h2othouse.com/html/how-to_articles_0.html you mention the use of something called a Cool H2otbox .  I am planning on constructing a green house on my balcony which is 25 x 5 feet.  I'm trying to figure out the best (and most economical) way to heat it as I live in the Pacific NW and it gets chilly here at night. Any suggestions??
Thanks! Maia

Answer: Believe it or not, Pacific Northwest winters are similar (same minimum temperature) to southern Mississippi, but longer.  We moved from Whidbey Island about 5 years ago and prefer short winters and the heat here.  The Cool H2otbox will not heat much (unless you add a boiler and radiant heated floor).  It is mainly designed to add humidity in winter, and cool in summer.
If using an electric heater, make sure it has a temperature control so it automatically turns on and off.  Make sure your greenhouse is 'tight' -- no cracks around doors, vents, parameter around the balcony, etc.  A small leak will let a lot of heat escape. During winter, you can line the inside of your greenhouse with 4-mil polyethylene film.  Double layer is better and can reduce heat loss up to 40%.  Charley's greenhouse has bubble insulation that doesn't need to be inflated or installed with a gap to separate the layers.
Other things to save heating costs:
      Keep greenhouse cool at night - don't heat above 50 degrees.      Grow plants with low temperature requirements or short growing season.      For germinating seed (must maintain 70 degrees) build a small propagating bench with cover to keep warm.      Solar heat storage -- add water filled dark drums or other material to capture and store heat during the day and release it at night.
If you have any suggestions to improve my web site or have a story or learning to share, please let me know and I'll pass the information on to others.
Thank you for contacting me and great growing,
Joe Zeeben
h2othouse.com


Question:  We have a 9' x 12' greenhouse that has been heated for 9 years with two milk house heaters, both now inoperable; one won't turn on, the other won't turn off! They are 1600 watts which is expensive to operate. Do you have a low power heater solution other than a new milk house heater?
  Thank you, Dee

Answer:
Gas is much more economical than electric.  We've set up an old ceramic heater($0) that we salvaged from one of the bedrooms in the house and are in the process of adding a recirculation system (~$200).  I'm planning on building a humidifier/evaporative cooler to control humidity in winter and hot temperature in summer: http://www.h2othouse.com/html/how-to_articles_0.html . You'll want to pull the heat down from the ceiling and distribute it close to the floor. The more heat sinks (things like oil drums full of water that  collect heat during the day and distribute at night) the better.  We light our heater in the evening and usually turn it off in the morning and keep the greenhouse just warm enough to prevent the plants from freezing.  However, if you want to automate it, check at your local hardware store or http://www.charleysgreenhouse.com/ .  If you want to start seeds, you'll probably want to build a covered propagation bench with a heater coil under the soil.  See Frit's Farm, www.guineafowl.com/fritsfarm/greenhouse/, for photos of their propagation bench (slow to load) and for supplies check Charley's Greenhouse or one of the other suppliers on my supplies page.
You might also check my link on greenhouse temperatures.
I took a look in my Charley's Greenhouse catalog and it gives a formula for calculating the approximate heating requirements:
A x D x 1.1 = Btu's where 'A' is total wall and roof surface area, 'D' is the difference between coldest outdoor winter temperature and the night temperature desired in your greenhouse.
Btu's is the heat requirement.  Subtract 30% if the greenhouse is insulated using double glazing or polyethylene liner.  Subtract another 30% if it is a lean-to greenhouse on heated wall.
Charley's has a few gas and electric heaters available with a range of prices.  The least expensive is an Electric "Rival" Heater with fan and thermostat for $64.  It delivers up to 5,120 Btu's.   I'd be interested to know what you choose.  Keep me posted.
If you find other sources of information,  please let me know and I'll add them to my web site.
Thank you for visiting http://www.h2othouse.com

Up to the top!
Question:  Hi, my name is Chad

I have a greenhouse that doesn't work.

First, the problem was the temperature fluctuated wildly.
I put insulation in the walls and foamed in the cracks
to fix that.

Second, I needed something to cover the insulation in the walls.
(fiberglass on a paper backing). The insulation needed to be
covered because of the water and humidity in the GH (GreenHouse).
I used clear plastic and lathe boards to fasten the plastic over
the insulation to protect it from moisture.

Third, the plastic is splitting, brittle, and coming down.
The paper backing of the insulation is brittle and tears easily
exposing the fiberglass. The sunlight has degraded the paper
backing and the plastic.

I need to cover the insulated walls with something.
Considered:
  * plyboard- not resistant to moisture and sun;
  * masonite- cheaper but not resistant to moisture and sun;
  * bath board- resistant to moisture but not sun and more expensive
             than plyboard
I need something inexpensive, resistant to sun and moisture,
and sturdy. I've thought about using corrugated fiberglass-plastic
but the corrugation is a problem if used inside, on the walls.

The GH has solid walls to the front, back, right, and left.
The light enters through the roof.

To this day, the GH was a bad investment because of the problems
it has had. I hope that I can find something that will work for
the walls. I've included a simple drawing to show you the basic
shape of the GH.

What should I use to cover the walls inside my greenhouse?

Sincerely, Chad

Answer:
I've used rigid 1" thick rigid foam board insulation with a water resistant / paintable surface (also called R-board) which can be purchased in a variety of sources.  It comes in 4'x8' sheets and can be cut with a utility knife.  Attach it to the frame with long screws.  I've also used treated plywood that comes in 1/4, 1/2, and 3/4" thickness.  It does warp when it dries and must be securely fastened.  Some other options to investigate world be exterior siding or exterior plywood -- these will rot if sitting on the ground or in water but the treated plywood will not.
If using film, make sure it resistant to ultraviolet rays.  I'll see if I can find a source.  I'll look at your drawing later this week and see if I have any other ideas.
Hope that helps,
Sincerely, Joe
 
Reply:
Joe, thank you for that information and your quick reply.
Treated plywood, I compared prices and discovered 1/2" is
the thinnest size available and the lowest price I found
was $24.22 per 8'x4' sheet. I need 6-8 sheets. I'm not
ready to spend that much money, yet- not frustrated enough.
Approx. a month ago, I found a piece of plastic. It is
keeping my hope alive that I can find a material less
expensive than treated plywood.
Description of plastic: black; stiff and doesn't stretch;
one side is glossy; the other side is dull; the glossy side
has words printed in white THIS SIDE UP, BEMIS, UC2-98;
the BEMIS is black letters inside a white oval- like a name;
size of piece is approx. 3'4" x 4'2"
I think there's something called 'BEMIS BAG COMPANY' in
Idaho Falls, Idaho- 30 minutes from my house.
I sincerely appreciated your reply to my plea for help.
I sent e-mails to other greenhouse related addresses.
You gave me the best suggestions. Thank You.
 
More on R-board:
I checked on the insulation board this weekend at the local lumber
store.  It's called "R-board" and is $6.95 for a 1/2"x4'x8' sheet.  You
can get it in other thickness' and make sure it has the fiber coating
on both sides.  I painted it with porch enamel and use it to seal up
under my house in winter.  I'm not sure if that's in your price range
but it would probably last longer, looks better, and offer more
insulation R-value than the plastic!
How much is the Bemis going to cost??
Good luck & Great Growing!
 
A Resolution? BEMIS vs. R-Board:
BEMIS Company Inc
see also
IDAHO Package Co Inc
2140 W Heyrend Way
Idaho Falls, ID 83402
phone: (208)529-0891
I looked in my phone book map for that road. Couldn't find it.
I found a map to that address using Yahoo. Later, looking at the
Yahoo and phone book maps, I realized the road was off the edge
of the Idaho Falls phone book map.
Thanks for checking that price and name. The 'R-board' has a good
price, I think- less than ordinary plywood. I hope its available,
locally. 'R-board' sounds easier than plastic to install. The old
paper-backed insulation, I will leave it in the walls. If I remove
the lathe boards and bad plastic, then I can fasten the 'R-board'
directly on to the 2x4's.
I don't know how much BEMIS plastic would cost. I don't know if they would sell it to me. I tried their phone number this morning, but
an answering machine told me they're open Mon-Fri 8am-5pm.
Do you think 'R-board' needs to be painted, to improve its
durability, when it is used inside a greenhouse? What color is the
'R-board'? During the summer, light colors absorb less heat than
dark colors. That would help keep the temperature down. However, that wouldn't help keep the temperature up, during winter.
Thank You, Joe. I've had this greenhouse for about 4 years, but only
used it for the first year. Couldn't use it in the winter- too cold.
Tried using it in the summer- slightly too hot. After I quit using
my greenhouse, it has been a reminder of something expensive that
didn't work- and isn't returnable. You've been more helpful than
anybody I've asked about my greenhouse problems.
 
More Q&A:
Q: Do you think 'R-board' needs to be painted, to improve its durability, when it is used inside a greenhouse?
A:  No, painting is just for aesthetics.
Q: What color is the 'R-board'? During the summer, light colors absorb less heat than dark colors. That would help keep the temperature down. However, that wouldn't help keep the temperature up, during winter.
A:  The R-board is usually off white with black lettering of instructions and advertisements (mine says "Lowe's").  I'd probably paint it a light color to reflect the light.  For winter, you may want to add some passive solar heat sinks in front of the insulation to absorb heat during the day and release it at night: steel drums filled with water, concrete filled cinder blocks, brick, stone, adobe, rock, etc. all make good heat absorbers in winter.  I'm working on a how-to article: 'Economically Heating the Greenhouse' that should be ready next week.  I'll notify you by e-mail when ready and list it with my how to articles: http://www.h2othouse.com/html/articles_news.html
Thanks for the help and sharing your learning's; an unused greenhouse is a sad thing!
Joe
 
The end??

 

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