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moderating temps in portable greenhouse


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#1 carolm

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Posted 23 April 2012 - 08:20 AM

I have a ~5' x 5' x 6' tall portable greenhouse that's situated on the east facing side of my elevated deck. While it warms up nicely during the day--about 20-30 deg. above ambient, it doesn't hold the heat at night. By the morning its about 3 deg. below outside air temp.

So, two questions:
Is there anything I can do to hold the heat in at night? (Ground cover, bubble insulation, I'm thinking passive heat retention).
And, given the temperature fluctuations, is it of any use to keep plants in there, even if its to start the hardening off process?

#2 Daniel Davis

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Posted 23 April 2012 - 09:20 AM

Plastic is a terrible insulator. Some sort of cover or wrap would be needed to hold the heat in.
Bubblewrap, try an old blanket over the top. Anything that can create those extra pockets of air will help reduce some of the heat lost. I have seen one setup where the green house had a couple of wires run across the inside of the greenhouse (see diagram I attempted to draw) and then put a blanket (it was silver reflective thermal sheet made by taping together several mylar thermal wraps handed out at the end of running races. Although you can get them for a few dollars each on ebay.) over the wires. Then during the day they pulled the sheet to the side and at night pulled it across.
They also pulled the sheet across as the days got warmer and they did not want the greenhouse to get too warm.
greehouse.jpg

Another suggestion would be to add passive solar heater. A container(s) that is/are black or painted black and filled with water. With a 5'x5' you might try some 5 gallon pails with lids, possibly stacking a few. During the day in the solar energy is absorbed by the black and transferred into the water and stored and in the evening that energy is released into the cooling greenhouse.

#3 carolm

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Posted 24 April 2012 - 12:23 PM

I like the interior cover idea. Wonder how it would work with bubble wrap. That way I wouldn't have to pull it back during the day.

#4 Janet Macunovich

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Posted 29 April 2012 - 10:16 AM

Some of the row cover fabrics are decent insulators, too. An available in big sheets so you can throw it right over the whole house, maybe. The higher the fabric's insulation value, the more light it blocks, but even the heaviest lets some of the light through. Here's a link to a source that gives the particulars -- we haven't bought from this source, so can't make a recommendation in that regard, but simply point it out now as one that has a range of row covers available and the particulars of insulative value and light transmission.
http://www.veoliaes....s-landfill.html

Notice the row covers are rated by weight, and if you "see details" of each it tells how much frost protection it provides as well as how much light gets through. Lightest weight, 4 degrees of frost protection and 85% light transmission; heaviest 8 degrees of protection and 30% light transmission.


Other words that will help you find information about these materials are thermal blanket and frost blanket. Lots of growers use a combination of thermal blanket and poly to cover pots over winter. Info at http://www.ces.ncsu....hil/ag454b.html

#5 carolm

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Posted 29 April 2012 - 07:50 PM

Found some greenhouse film remnants on sale at Growers Supply co. Ordered a couple of pieces and will try that. Its supposed to be infrared reflective. I'll report back.

#6 Nell Jean

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Posted 10 January 2013 - 11:13 AM

I am a latecomer to these forums. Have you found a way to retain heat, carolm? Thermal mass is key to mediating heat but it is hard to put enough thermal mass into a structure to eliminate auxillary heating. The best thermal mass is water. A few milk jugs won't do the trick. I have 200 gallons of water in barrels in a 10 x 12 greenhouse and it helps, just helps. I add all the brick, stone, ceramic and concrete pieces that I can load in which also helps but I still need two themostat-controlled electric heaters if temps fall below 35 degrees outside.






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