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Greenhouse Doesn't Heat Like Expected

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Greenhouse Doesn't Heat Like Expected

Postby packrat79 » May 14, 2008 1:21 pm

I recently started moving plants out to the greenhouse I've been building, it's airtight now and I figured it was safe to start using it. I knew a greenhouse isn't the easiest thing to keep warm at night, but I never figured it would be this hard. Even with a 1500 watt space heater running full blast all night, the temperature still fell to a hazardous 43 degrees (about 6oC). It seems that the plastic I used (a special clear tarp) simply doesn't insulate like it's supposed to. The cold seems to seep down through the roof, making the top shelf much colder than the lower one. (see photo)
Most plants don't appear to have been damaged, with a couple of exceptions; some Squash plants are getting dead edges on their leaves, and some Cardinal Climber totally wilted as soon as the temperature got below 50 (I promptly took them back in the house, where they recovered).
My concern is that even if the remaining plants don't freeze, they just aren't going to grow very well, if at all. What should I do?
It's too late to make any major changes to the greenhouse, and I can't afford a more powerful heating system. I'm going to try adding a second layer of plastic (on the inside instead, forming a (sort of) vapor barrier) and see if that helps, but that's about all I can do for now.
Thanks for any advice.
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Postby dlb » May 14, 2008 3:28 pm

I know the guy I get my heritage tomatoes from has a layer of bubble wrap on the inside. He also uses water jugs as a heat conservator, and has a stone floor.
Dora
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Postby Lizcordysmum » May 14, 2008 4:09 pm

Dora took the words out of my mouth...staple bubble wrap on the inside of the framing, and put the big milk jugs full of hot water in when the sun goes down...and maybe be prepared to add more hot water to them at midnight before you go to bed, and early morning when you get up with the birdies.

Maybe an ordinary electric light bulb would help too? Up high to warm the top?

Good good luck.
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Postby packrat79 » May 14, 2008 6:55 pm

Will the plants still grow normally under these conditions? That's the main concern to me.
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Postby green thumb guy » May 14, 2008 7:28 pm

Adding a second layer of plastic to the inside will help reduce the heat loss of your greenhouse; as long as you seal up the seams of the plastic to prevent drafts from getting in between the layers.

If you are using Super 6 construction plastic, keep in mind that it is not UV protected. I'vre used it before and had it last a few years before it started to disintegrate. Of course it wasn't at a convienent time

A really good material to use is a twin walled polycarbonate. It is already UV stable and has a natural insulating ability. A company called Warehoused Plastics sell it

From my experiences, you are not really going to be able to keep your greenhouse much warmer than the outside temps without an external heat source.

My greenhouse fluctuates also. I'm only concerned when the temp goes below 5C. Most of my plants can take some cool.

I've also trid the 1500W microfurnace. One spring, I had to run it so much that I melted the extension cord and nearly started a fire. I had to upgrade my extension cord with a heavy duty one. This was fine to keep the frost and freeze away only.

One year, I was going to try a camping heater that could work off of a BBQ tank. Produced lots of heat but made me really nervous about the burning part and my wooden benches.

You could look into a radiant heater or 2 for the size of greenhouse you have. I've seen them at Lee Valley and Canadian Tire also carries them. I can't say that it works but it is like at a hockey arena heater and it is really cheap to operate.

Good luck
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Postby Lizcordysmum » May 14, 2008 7:30 pm

The answer is pretty well YES. All you are doing is providing warmth, and of course they will get the light they need.

edited to say...if it was me, I would add an old garage sale hibachi to the mix, and keep it going...just make sure you open the greenhouse up well, before you spend any time in there. Depending on the size, maybe two hibachis. do not stay in there with them going.
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Postby Durgan » May 14, 2008 7:45 pm

Reality check. 1500 watts is like hearing a normal house with a coal-oil lantern.

A backyard greenhouse allow the plants to get good light and offers protection from FROST if the temperature is not to low.

Second. The part of a make-shift home garden greenhouse close to the outer walls is not the same temperature closer to the center or source of heat.

Often the greenhouse can only be used during the day, since the temperature due to sun exposure rises dramatically, aad often ventilation is required to lower the heat level.

Your expectations are too high if you expect to have normal growth in our cold climate with a bit of plastic between your plants and the elements.
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Postby Lizcordysmum » May 14, 2008 8:03 pm

I do NOT want you to be discouraged. Yes, your plantings will be slower growing, but again, if the temp inside doesn't freeze, you will have success.

May I suggest you take a look at Alaskan greenhousing, from folks like yourself, home-builders...on the net?

Yes, your effort will be more, but you seem the type to want to put it in.

Good good luck.
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Postby green thumb guy » May 14, 2008 8:06 pm

Good Point Durgan. Simple logic.

You must have been leaving Brampton just as I was building my greenhouse.

I was lucky, I was able to use the glass from window replacement in our house and another house. My mistake was not using pressure treated lumber. Now 12 years later, I have to start replacing the wood.

I crapped out due to money. I used super 6 on the roof. First 2 layers. Then a couple years later added 4 more layers.

Then replaced it all with twin walled polycarbonate.
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Postby savona » May 15, 2008 12:44 am

Last year we bought a heater attachment for a propane bottle from Princess Auto when the forcasted temps were to go down to -22 in March, 2 weeks after I took many flats of plants to the greenhouse. I had a small woodheater in there but knew the back corners would get very cold. With the help from the propane heater I didnt lose anything. Using a fan to help move air helps too..savona
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