If your space heats up during the day, yes, jugs of water will help some. I used this in the spring last year until I could figure out something more reliable. In the fall I was only looking for a lift of up to 4F so I used two kerosene lanterns (Dietz) - they burned all night and actually did keep the temperature from dropping as much as the outside air.

I now use a gas heater at the moment because the outside temperatures have been near the 0F for quite some time now because I need more than just moderation.

The space heater will be much more expensive as they are much higher wattage than bulbs. Check the wattage on the heater. If it says, for example, 1250 watts, it means it will use 10 times as much energy in an hour as a 125W bulb.

The kerosene lanterns helped me through until hard frosts. As I say, they provided approximately 4F - and my little greenhouse is 6 x 8 single layer poly on my deck - so I don't even have the ground to help me.
I used approximately 1 cup of kerosene per lantern per night which is quite a few fills if you buy it by the gallon. Cost of the Klean (which barely has any smell) from Home Depot is $10 a gallon. So pretty cheap to run when you need them. Don't be fooled by their multi gallon jug - it actually works out to be more expensive!
I spaced the lanterns a few feet apart to try to get a little convection pattern established and that seemed to work.
I got the lanterns through Lehmans and even though I now have a gas heater, I do not regret their purchase.

thanks all for the advice. we are bracing for a 21 F freeze in three nights, and i am starting to panic. it has been a few years since it's gotten that cold here. (average dec-feb low is about 40 F). i am concerned for my large container citruses and tropical plants. i don't know if a space heater or a kerosene lantern can provide enough heat to keep this uninsulated structure from freezing through such temperatures.

i have only two days to batten down the hatches with a workable solution.

this morning a friend of mine lent me an electric heater that runs at 1500W. do you think that MAYBE if i fill some grey sterilite tubs with water and leave them in the greenhouse, run the electric heater, cover the plants inside the greenhouse with blankets, and throw a few large blankets OVER THE TOP of the greenhouse (it's only a 15' x 7' x 7' greenhouse), that might be enough to stave off a freeze inside???

it sound risky! might it work??? if this method proves effective then i might employ it for every forecasted freeze.

i am also still determined to start my seeds in this greenhouse (well, after this upcoming freeze danger). i have my unsown trays sitting in there right now and the thermometer indicates a 75 F soil temperature--perfect for my needs. perhaps running the electric heater on a thermostat at night and snaking some rope lights underneath the trays will keep that soil temperature adequate on cold nights.

THANKS FRIENDS! i would look into the kerosene lantern but i don't have one and have just been lent an electric heater...differential analysis seems to indicate the electric heater is a more cost-effective choice at this point...

The link below will tell you if your heat will work. I think it will be fine. I can't make your link open, so I don't know your dimensions. The r-value of your plastic is about .6

The heater will probably work fine on the low setting, which typical draws about 750 watts. The low setting will also not make your cord get nearly as hot. You want at least a medium duty extension cord for a heater on low, and a heavy-duty cord if you run the heater on high.

Also, pay attention to what you plug in to the same circuit. Most homes in the US are wired with 15 amp breakers. One space heater on high will take up almost all the power that one 15A circuit can provide without flipping the breaker.

Here is a link that might be useful: Heat Loss Calculator

nifty tool!

the heater is equipped with a thermostat (no degree markings though, so i have to experiment with it a bit), and so i will make sure to set it such that it is not constantly running. if the heater is not running all that much, this greenhouse MAY just turn out to be more energy-efficient than running a bunch of grow light cfl's indoors all day.

i also just got finished filling a few sterilite tubs full of water and situating them in sunny spots in the greenhouse.

even better however, i just scored a solar pool cover from my neighbor (he had a spare)! i am curious to see how much it helps with heat retention and reduces the need for supplemental heating. on an average winter night (~38-45 degree lows) the pool cover and thermal mass should render the electric heater unnecessary. I HOPE.

i will try these methods out tonight or tomorrow and report back with comparison temps.

HOW EXCITING!!!

also can anybody explain the thermodynamics behind the curious cooling effect of the uninsulated greenhouse at night? i just came in from outside and while the air temperature outside is 56 degrees, the temperature inside the greenhouse is 54!!!

no, it's not a thermometer problem. i've tried several. you can even feel the difference. it's noticably colder.

i'm still a bit fuzzy on the science, and there aren't any professional articles explaining this phenomenon.

this thing retains zero heat. i assumed the heat absorbed by the ground and plants during the day would surely be enough to help up the temperature a couple of degrees, but i guess not.

while shopping around i read tons of gushing customer reviews on several polyethylene-covered hothouses. nowhere was it ever mentioned that they act like a refrigerator at night. why do people even get these things?

You're measuring the temperature of the air inside and outside. During the day, when there is solar gain, the air gets hotter inside than outside. At night, there is no significant mass to retain the heat and it starts to cool down and will get as cold as the lowest temperature the outside...but no lower. The plastic prevents the inside from cooling effects of winds, but the plastic has next to no insulating value so heat is lost through it fairly quickly. In the morning, it will begin to heat up again, but there will be a bit of a lag because of the plastic; at some point it will match the temperature of the outside air and then exceed it.

I can assure you as an engineer who is quite familiar with thermodynamics that there is no "refrigerator effect"...it's an impossibility. If your thermometers are accurate, you're likely just seeing a mismatch due to the lagging effect. Bottomline: in a 24-hour period, the temperature inside the greenhouse will never be colder than the coldest it gets outside, but it may not recover from that low as quickly as the outside air heats up. If you don't believe me, you can see for yourself by having a recording thermometer inside the greenhouse and one outside the greenhouse, and then compare the readings over a day or two.

I've given you the simple version of what is going on thermodynamically, and there are some complications, like air infiltration, but overall, that is what is happening. People have double and triple pane greenhouses so that they can slow down that heat loss and so it will take less energy -- solar and/or supplied heat -- to moderate the temperature swings. So, even if you buy a several thousand dollar greenhouse, you still have to deal with these issues...they're just not quite as challenging.

My 16ft tall greenhouse covered with double inflated poly will not maintain much if any higher temperature after midnight than outside on clear nights. If there is a heavy overcast it will retain about 6F warmer than outside. I can't explain the difference but have observed it many times.

The greenhouse covering is designated as IR meaning, I believe, that it diffuses some of the infrared back into the greenhouse. But that still doesn't slow the heat loss by much.

So no normal greenhouse will retain much heat at night. It needs extra insulation, ie blanket etc at night, or a heat source to stay warmer at night than outside.

What I've learned is that you cannot plug anything else into the outlet where you plug in the extension cord to the heater if you have it on the high setting. You run the risk of something turning on during the night and blowing the breaker and shutting down power to the heater. A small fan to circulate air inside the greenhouse does some sort of magic when it comes to distributing the warm air. It makes a huge difference. The pool cover will change everything, just drape it and clamp it down so that the wind doesn't blow it off. The jugs of water work great - for about an hour. They don't give off enough heat for an all night cold snap. What will work is to add another layer of plastic inside the greenhouse. I make a tent with plastic storage shelves and drape it with extra thin plastic sheeting. You'd be amazed at how warm it keeps things. I have this greenhouse within a greenhouse set up kind of in the middle of my large greenhouse and place the electric heater inside it with the thermostat set at 40 degrees. On cold nights that tent of warm air keeps the whole structure from getting super cold. I use an old wood burning stove as my main heat because firewood is free for me (but you lose a lot of sleep keeping the fire going on really cold nights) and I also use a portable kerosene heater set on low just in case the fire goes out and the power fails (happens a lot here).

If you can't keep the whole space warm enough consider sectioning off part of it and focusing your efforts there.

Don't panic. I've had super tropical plants survive 26 degrees more than once.

cuestaroble-
Thanks for the info. I'd never heard of that phenomenon. I read the link you provided and it looks like the research was designed properly, so I won't dispute the results. However, I don't fully buy into Albright's statement about what is actually going on thermodynamically. For example, why would plastic's transparency to IR be an accelerating factor compared to the air outside which has no barrier at all? I'm also curious whether this is a factor only in the high tunnels the research was done on, or whether it would apply to any size/shape of plastic covered greenhouse, to the small unit the OP has, or even to a cold frame. For example, what I know of cold frames is that they are like a low tunnel and the covering is ofter plastic, but they don't seem to have the problem described.

Anyway, quite interesting. I am going to try to chase this a little more, and I'll report back if I find anything. If you have any other links about this, I'd like to read them. Thanks.

Click on the link and read the results of the same study that used that quote. Both their high and low tunnels maintained a higher minimum temp than the outside air. The research seems to have proven that quote to be false, rather than true.

Here is a link that might be useful: USE OF LOW TUNNELS TO IMPROVE PLANT GROWTH IN HIGH TUNNEL

Here is a link to another paper comparing clear polyethylene and UV poly on a high tunnel. It agrees with the Albright et.al. 1985 research.

Here is a link that might be useful: high tunnel night temperatures

Temps here are minus 11F - my heater is running and running, I figure these really cold days are going to add up to at least a $50 additional to our natural gas bill if not even more. Our weather has provided - last month 40% more cold days, and this month possibly 75% more - in comparison to last year.
Even our house furnace is cycling much more often.
Felt like giving my hubby the side of a rolling pin last night because he was complaining about the gas bill before we even have it and I had already agreed to pay the difference between last year's and this year's bottom line.
This means that I am subsidizing the house heat due to this colder winter and the two price increases that were levied this year.
Funny - he never complains when my share of the bills - car insurances, house insurance, maintenance, groceries etc. inflate....
Anyway, he can grumble all he wants - it's not going to cost him a penny and I think I will teach the plants to grin at him.
So back to why I'm writing this - my first estimates for the costs of heating are wildly out of whack- so please take them as total drivel. When heating season finishes, I'll post a separate thread showing average temps and actual cost of heating along with the rate we pay for gas so that the numbers are meaningful.

Thanks., Lori. That will be interesting

I can see the research about the mystery cooling effect:
http://hortsci.ashspublications.org/content/44/2/235.full
Previous work indicates that, depending on the characteristics of the polyethylene covering, the minimum air temperature may drop to lower values than outside the high tunnel. This is the result of the transparency of the film to IR radiation and has been documented by several investigators (Baytown et al., 1994; Maia et al., 1990; Montero et al., 1985) and modeled by Albright et al. (1989) and Montero et al. (2005).

My problem is that it still doesn't make sense to me. Look at their data in that Cornell link. They are showing a ten degree difference in temperature. That really seems ridiculous to me. If that were the case, then on every clear night that was below 41 degrees, my greenhouse plants would freeze and die. No one would have high tunnels and greenhouses if that actually happened.

There is much more research to demonstrate a higher than outside minimum night temperature than a lower one:
http://organic.kysu.edu/Temperatures.shtml
http://hightunnels.cfans.umn.edu/structures/temperature/

If high tunnels didn't protect crops, they would not be exploding in popularity like they are right now. So whatever the variables were that created the Cornell data, they don't seem to be typical. I can't argue with the research, but I am uncertain as to the extent that it applies.

Cole_Robbie-
I'm in about the same place as you are on this. I've done a little more reading and have yet to see any in-depth thermodynamic explanation that is convincing...just the assertion that it is due to the "transparency of the film to IR radiation." While this is consistent with quick fluctuations in temperature, it does not explain to me how a temperature inside could go below the coldest outside temperature. I wondered if some kind of supercooling phenomenon was causing this, but, in looking at the temperature graphs in the last link provided by cuestaroble, none of the plotted outside air temperatures came close to freezing, so I don't think this can account for what is happening inside the tunnel. The data look persuasive, but I can't come up with a plausible theory that would explain such a situation.

This post was edited by kudzu9 on Wed, Jan 8, 14 at 13:25

i posted an update here yesterday, but just realized that perhaps it didn't submit properly--it's nowhere to be found!

anyway, i have good news about the state of the greenhouse last night. i ended up nixing the solar pool cover because i underestimated how gigantic and cumbersome it is to toss over the greenhouse. i can't be bothered to do that every night. it's also made of blue plastic and so cannot be kept over the greenhouse in the daytime.

rather than drop $$$ on another cover i happened upon one of my late grandfather's old car covers. while it didn't appear too insulating, it fits over the top 15'x7'x7' greenhouse and reaches about halfway to the ground (i figured this is okay since the heat within tends to rise and escape through the top, not at ground level).

(i also thought about sewing a custom-made cover out of thick quilted mover's blankets, but then i realized i don't know how to sew.)

last night i set the thermostat of my electric heater to 50 degrees. despite getting down to 23, the greenhouse stayed at a toasty 50. i was also pleased to find that the heater was NOT constantly running. i credit the cover for retaining some circulating heat.

all in all it looks like this cheap greenhouse will be well-suited for seed starting after all. thankfully it almost never gets down to 23 degrees here (40 is the norm).

i am elated at the level of interest y'all have taken in investigating this curious cooling phenomenon. at least i am starting to feel less crazy now.

THE ONLY THEORY I CAN PROPOSE as to the cause is that at night, the outside winds quickly strip the greenhouse interior of warmth across the flimsy polyethylene. also unlike the exterior, there is no wind flow in the interior which renders it more susceptible to radiational cooling. this may be two theories. i'll leave the deciphering of my pseudo-scientific drivel up to the experts here :]

i also think that even if it falls to freezing temperatures both inside and outside a greenhouse, it is preferential for frost-sensitive plants to be within the greenhouse because the relatively high humidity better protects foliage from the drying effect implicated in frost damage.

makes sense to me, but correct me if i'm wrong!!!

ydfz-
I agree that frost-sensitive plants will do better inside the GH. It is true that wind flow past the exterior will conduct heat more quickly away than still air; however, lack of wind flow in the interior should have no effect on making it "more susceptible to radiational cooling." Having said that, I still don't know why or if the temperature inside would ever drop below the lowest outside temp. However, as long as you have some solutions that look like they are working, maybe we don't need to nail down all the thermodynamics!

duplicate....

This post was edited by kudzu9 on Tue, Jan 7, 14 at 18:32

After contacting the senior author of one of the Cornell reports, his reply re: inside night temps cooler than outside-

âÂÂon clear cold nights, the loss of heat from the enclosed tunnel through the transparent film is more rapid than can be replaced by the warmer air outsideâÂÂ

A reply to the same question from a University of Calif greenhouse specialist:

âÂÂSay you have temperatures as we have now (unseasonably warm 60s and dry). Pull that through the greenhouse all afternoon long. Go into the night and shut the high tunnel down. The stomata on the plants will still be partially open and since it is dry there will be evapotranspiration and the interior will actually cool about 2 or 3 degrees. Once the rh is over 85% that stops. But it will then be cooler inside than outside.âÂÂ

The second explanation makes sense, if the plants are acting as evaporative coolers. But one would have to be out west in order for the humidity to be low enough for it to happen.

The Cornell explanation still seems to defy my lowly layman's understanding of physics. I don't understand where the cooling energy is coming from.

http://www.physics.louisville.edu/cldavis/phys298/notes/heat_thermeq.html

As we have seen in the zeroth law of thermodynamics, when two objects are placed in contact heat (energy) is transferred from one to the other until they reach the same temperature (are in thermal equilibrium). When the objects are at the same temperature there is no heat transfer.

I would also be interested to know what volume of plants relative to the enclosed volume one would have to have in order for this mechanism to make a difference. In a hobby greenhouse, particularly one that is not well-sealed, and with relatively few plants, there might be little, if any, effect.

cuestaroble- thanks for digging into this further...it's been educational.

This post was edited by kudzu9 on Wed, Jan 8, 14 at 13:24

I'm certain of one thing, the temperature inside my greenhouse never drops more than a degree or two colder than outside and probably not that. The common occurrence in winter is the inside drops to a few degrees above outside by soon after sunset. By morning the inside would be the same as outside unless it is heavily overcast. Then inside can be +6F warmer than outside.

I too can't figure out how inside could fall 12F below outside within an hour or two after sunset as seen in the Cornell data. I'd call that an outlier. Not something I'd expect or worry about.

Years of experience with high tunnels has taught me this general rule:

When it's clear with light winds, it's colder inside the tunnel
When it's cloudy, it's usually warmer inside the tunnel
When it's clear and dead calm outside, it's a little warmer inside the tunnel.
When it's clear and windy, it's about the same inside and out.

The reason is simple - radiative heat loss.

When it's dead calm, both the outside and the GH radiate heat like crazy but the poly makes a slight difference and so the GH ends up a little warmer.

When winds are light, there is sufficient mixing outside the GH with upper level air to keep outside temperatures relatively high, but the GH itself is dead calm inside and temperatures plummet. Hence, GH is colder.

When it's cloudy, radiative heat loss is greatly minimized and the heat being radiated from the ground is trapped in the GH. Hence, GH is warmer.

When it's clear and windy, conductive (and also convective if there are leaks in the GH) heat transfer ensures that temperatures stay about the same both inside and out.

This post was edited by Slimy_Okra on Fri, Jan 17, 14 at 8:51

I like your analysis. Makes sense to me. Wind does affect the degree of inversion and that would obviously be way more outside than inside. Also cloud cover does make a difference.

Interesting thread but it seems to me location would have some effect regardless of winds, cloud cover or sunny. For instance, a hoop house in Florida, on a sunny day, due to the intensity of the sun will get considerably hotter than one in somewhere like Kentucky or even South Carolina. During the day, as the temps climb due to the sun, the soil in the pots warms up, as does the ground. When the sun sets, and temps drop during the night, if the tunnel is fairly airtight, it seems it would take much longer for it to cool off due to all the heat trapped inside.

ydfz did the solar pool cover make any difference?

Powderpuff:

Is an 15-60 minutes a long time? Because that's the max time it takes on a clear night for inside to fall as cold as outside. So if it drops to 32F outside by 3 am, it will take until 3:15 to 4 am to fall to 32F inside. Most equipment can't measure accurately enough to tell that there's any difference.

A couple months after my last post on this thread, I lost most of my greenhouse plants to a late freeze. I had a heater malfunction. The night was clear and windy. I think I experienced that weird physics thing where the inside of the greenhouse was colder than the outside.

Robbie:

Sorry to hear of your losses. I do think it makes sense. A strong wind can mix the inversion and keep the temperature warmer than on a still night. We've all experienced this as the air in low places cools very rapidly on a still evening. Inside the radiation losses continue without the wind to mix warmer air from aloft.

I've been playing around with the heaters, fan, etc. during the last few nights which have been high 30's low 40's. One thing is crystal clear. The more pots and plants that are in the hoophouse, the faster it heats up an it takes longer to cool down. Perhaps because I am in FL and during the days it reaches into the 9o's, the soil in the pots and the ground get warm during the day. It definitely begins to drop rapidly when the sun goes down but the heaters keep it warmer and don't cut on as much when it is full vs semi full or empty. A significant difference. The fan directed up toward the ceiling also makes a significant difference. The hot air doesn't get trapped at the top but instead is circulated all around the inside.

If you are going to have a greenhouse that you need to use in colder temps, please think about installing a simple geothermal system (dig down into the ground, insert some plastic tubing that will vent into the greenhouse) and let the temperature of the earth maintain stasis inside. You can also put any large water containers around the outside or in the center of the inside, painted black to collect more heat during the day. Extending and improving the function of your greenhouses will become a necessity in the coming years. Google "geothermal heat for greenhouses" - you'll get a zillion hits. While you're at it, Google "permaculture". You'll need that knowledge for the future as well. When this country reaches 9 billion population (we are at 7.5 now), we will top out our ability to feed everyone. You should know how to grow your own and hopefully have some left over for others.

I have a greenhouse very similar to this. I only use it in the spring for starting seeds and giving my tropicals I winter over indoors an early start. I usually put it up around late March or early April. Living in Wisconsin, we can still experience some pretty cold nights at this time of year. On sunny days the greenhouse heats up quite nicely... Sometimes to the point that I need to unzip the doors a bit to moderate temps. At nite, if the temps are going to dip much below freezing, I use an electric heater to keep things toasty. I find it very useful for my growing purposes. However, it is not a structure I could ever use all winter long. Our winters are much too extreme for that. Here's hoping for a real glass greenhouse somewhere down the road!

I have a portable "flowerhouse" and use an oil-filled electric radiator heater on a timer that's set right now to turn on at dusk and off at sunrise. it's working remarkably well but here in Oklahoma we are having a very mild winter. I think I would have to get more aggressive with the heat if we were having a very cold winter. I also find that biomass bulk in the form of plants and soil keeps the temp more stable. I actually love this green house and think it does just what I want - overwinter large plants and start seedlings in the spring.