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Produce => Under Glass => Topic started by: Garden Manager on November 21, 2005, 16:46:45
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Can anyone give me some idea of the effect temperature wise that bubble wrap insulation has?
I ask this because at present it seems to be making little difference at all! I was getting very small temperature differences at night before fitting the insulation (sometiems no difference at all) and now with insulation fited I am STILL recording only minor differences between inside and outside temperatures. I did wonder if my electronic max/min, in/out thermometer was on the blink but it seems not. Something is wrong with the greenhouse it seems.
Please help
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I may be wrong here, but I think that using bubblewrap will only reduce the rate at which temperature drops, not the end temperature.
So, if you had 2 glasshouses, one with bubblewrap and one without, and measured the temperature in each every couple of hours, you'd find that the one with bubble wrap cooled down more slowly, but probably by the end of the time period, all the heat would have been lost from both anyway. The thickness of the bubblewrap would affect this too, as would the specific heat capacity of any materials within the building.
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Double or triple layer the bubble wrap :)
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I placed a min/max thermometer in the bubblewraped GH and one outside, the difference in the min temp at the end of 24 hours was the the GH stayed 3 degrees warmer i.e. outside temp -1 deg C, inside temp +2 deg C.
I was using small bubblewrap, large bubble wrap will give better insulation.
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I am not sur what is not working - the insulation or the thermometer. It only shows 0.5c diference between inside and outside, so if the temperature falls to even -1c outside it will still fall below zero inside. Doesnt sem to have done any harm to the plants inside, though with the cannas i am not too sure as I have never overwintered them before.
I am using small bubble wrap and as yet have not used my heater in conjunction with the insulation (i was hoping not to have to use the heater so much with the insulation!).
Quite frankly i am at a bit of a loss as to what make of it all. ???
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I think you will find Richard, Jenny has explained it perfectly. The greenhouse will still freeze, even with bubble wrap, when the temperatures are low enough for long enough. Even with my bubble wrapping and fleece and polystyrene chunks, I use a thermostically controlled electric heater on a frost free setting. Nothing is wrong with your greenhouse, just look at them, gaps galore, thin glass, metal frames. Is it any wonder they get so cold! All I can suggest is either getting a heater or start using fleece as I think with the very cold winter that is forecast, we are all going to be desperately trying to keep our greenhouses warm!
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I've got my tubular heating thermostat set to 30 by day and 15 by night and I 'm lucky if it gets to 10 day or night in this frosty weather, I've had to resort to a fan heater (alright today though). I'm going to add yet another tube and another layer of bubbles and see if that works.
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Emma and jennym have explained it perfectly. You will also find you have various 'leaks', where draughts can get in, and these will rapidly pull out any retained heat.
Derekthefox :D
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I understand now. Insulation only slows the heat loss, it doesnt prevent it. With the temperatures i have recorded since installing it i dread to think what the internal tmperature would have got down to !
Thanks for the help.
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No wish to worry you Richard ;) but the thermometer registered -5C the other night, in my unheated greenhouse ! I move my Cannas into the heated conservatory, as have lost them in previous years, even wrapped well with fleece!
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What really annoys me is that if i could have an electricaly powered, thermostaticaly controled heater in the GH then i wouldnt have to worry. I did plan to have one in the GH, and have got one (I now use it in the conservatory) but in the end couldnt get electricity to the GH so am having to rely on insulation and a little parafin heater.
Not ideal but there we go
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I tried using a paraffin heater one winter, but found it so frustrating and tiresome to keep an eye on, but that's just me in winter hibernation mode!! ;)
Do hope it works for you this winter Richard.
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I tried the parafin heater first time with the insulation last night. It was a cold night and made about 7 degrees difference, which i was pleased about. thatsaid i will only put it on when there is a cold night forecast, otherwise i will be running out to get more parrafin every five minutes!
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Can anyone give me some idea of the effect temperature wise that bubble wrap insulation has?
I ask this because at present it seems to be making little difference at all! I was getting very small temperature differences at night before fitting the insulation (sometiems no difference at all) and now with insulation fited I am STILL recording only minor differences between inside and outside temperatures. I did wonder if my electronic max/min, in/out thermometer was on the blink but it seems not. Something is wrong with the greenhouse it seems.
An issue you may be confronting is the phenomenon that was unexpectedly found by early users of poly tunnels (showing my age here as I was around to be told of it!). On any night soil will radiate it's accumulated heat from the day in the form of long wave radiation (radiational cooling). When there is cloud cover overnight, or simply moisture held in the atomsphere, this radiation will be reflected back to the ground as shortwave radiation. On a night where temperatures are approaching freezing this reflected radiation can make the difference between a frost or none. As these temperatures approach 0C however the presence or absence of wind can in turn make all the difference too. What can happen in a plastic greenhouse/tunnel with few places for wind to enter is that the greenhouse can end up colder than the outside air. This is due to plastic being relatively impervious to short wave radiation (but it does allow long wave radiation to escape) and there not being any exchange of air with the outside resulting in excess cooling of the atomsphere inside. Glass will allow the short wave radiation to pass through more or less unimpeded so, all other conditions being equal, glasshouses will remain warmer overnight than plastic ones.
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That is genuinely fascinating ...
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Monitored from 2pm yesterday until 2pm today
(http://i10.photobucket.com/albums/a149/mike_brennan/Bubblewrapped.jpg)
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There is no arguing with data like that ... they are the results I would have expected ...
Derekthefox :D
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There is some fascinating info coming out here. Thanks to all who have contributed.
Its strange i didnt seem to have any problems keeping the greenhouse warm last winter, and that was without insulation (but WITH a heater i might add). It does not bode well for this winter when one cold snap in November has caused worries.
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Just out of curiousity Richard, what are you over wintering in your greenhouse this year??
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Just out of curiousity Richard, what are you over wintering in your greenhouse this year??
I usualy overwinter things like stock plants of pelargonium and half hardy fuschias, plus a nice but tender salvia i have. This year i also have cannas which i grew for the first time this year. I grew them in pots otherwise i would have tried leaving them in the ground with a protective mulch to save space in the greenhouse.
I think the cannas are the only thing in there which wont stand it geting too cold in the greenhouse, since the other things have survived lower temperatures in there before.
I am considering setting up one of my old mini greenhouses as an 'inner chamber' to keep at least one part of the greenhouse a little warmer than the rest, and put some of the more sensitive things (like the cannas) in there.
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Good idea with the mini greenhouse Richard. I think some cannas are tougher than others as I have had some outside forever, and they are fine, then others pop of on a nippy autumn day!!
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There is no arguing with data like that ... they are the results I would have expected ...
It occured to me afterwards that I probably hadn't made it clear that the problems I had described only occured in a tiny percentage of plastic greenhouses (although large enough for part of a college lecture on greenhouse materials to be given over to the subject) sited where a confluence of factors come together. Remembering back to the discussion Richard and I had last winter (?), and his later photos, concerning the positioning of his greenhouse it seemed possible to me that he may be encountering such issues. These conditions are very rare- I have never encountered them or heard of them directly.
One solution that Richard may be able to adopt, and springs from Jenny's comment regarding Specific Heat Capacities, is the use of containers filled with water to absorb heat during the day and release it at night. A grower friend of mine had a lean too plastic greenhouse (the 'lean-too' wall was cement/breeze blocks which would also have absorbed heat) in which his benches were stood on 220L drums filled with water. Even though there was no other heating involved he was able to produce a merchantable crop of basil right into December even though night-time temperatures are commonly as low as -10C. Being so far south of the U.K. helps of course but I would imagine that water filled drums in the U.K., even with Richards sheltered location, could absorb enough latent heat to keep tender plants from freezing during the night.
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Hello John. Yet another interesting posting there. Yes I do remember the discussion about siting the greenhouse last winter (after a bit of memory jogging). Should i be 'honoured' to have such rare conditions in my greenhouse? I mean where the inside temperature can actualy be lower than outside? You think the positioning is the cause of this?
Reading back over your earlier post it seems to suggest to me that i may actualy have made things worse by insulating the thing, whereby the bubble wrap may be blocking out the heat energy. I have noticed the 'house is staying cooler in th morning when the temperatures would normaly soar on a sunny day. However with the addition of heat on a cold night the insulation works well and creates a significant temperature difference.
I wonder whether under such unusual conditions the bubble wrap insulation might be as good if not better than shading come next summer? if it keeps the greenhouse cooler during the day and warmer at night then why take it down in the spring?
Water containers as a heat storage method? Sounds a good idea. Its only a small greenhouse so would a couple of watering cans or buckets full of water help?
Just one more thing, can you (or someone else) explain why positioning my greenhouse the way i have has caused this effect? I dont really understand.
Many thanks for the advice.
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I agree on the use of water containers as storehouses for thermal energy. The large capacity is necessary because of the slight temperature difference between the water and the ambient temperature (inside or outside, it matters not). Small containers would not hold enough heat to make a significant difference.
Derekthefox :D
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The subject of using water containers was covered in October under "early tomatoes" Tim supplied the address to a web site that covered the subject.
http://www.yale.edu/ynhti/curriculum/units/1983/1/83.01.13.x.html#h
Basically you need 1 gallon of water for every (1) square foot of glazing
MikeB
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That equates to over 200 gallons for even a small greenhouse !!! However, I presume that is without the bubblewrap on? That would be adjusted for the change in heat transfer coefficient ... fascinating article by the way.
Derekthefox :D
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Hello John. Yet another interesting posting there. Yes I do remember the discussion about siting the greenhouse last winter (after a bit of memory jogging). Should i be 'honoured' to have such rare conditions in my greenhouse? I mean where the inside temperature can actualy be lower than outside? You think the positioning is the cause of this?
The positioning is the only explanation I could come up with as your experiences are so atypical. I would imagine that when it is windy you probably have a wind tunnel but when it is calm the air is really still and is contributing to excessive heat loss. The bubble wrap is probably also blocking the energy radiated from the surrounding structures at night from also helping to raise the temperatue inside your greenhouse.
I wonder whether under such unusual conditions the bubble wrap insulation might be as good if not better than shading come next summer? if it keeps the greenhouse cooler during the day and warmer at night then why take it down in the spring?
It would certainly provide some shading. The problem I envisage is that the bubble wrap is probably not treated with UV inhibitors and will break down quite quickly. You will end up with an unshaded greenhouse and a mass of brittle plastic flakes
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that seems like a brilliant idea ;D
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When/if I ever get my greenhouse up I think I might try to insulate just the north side, and then during the day the sun should warm the soil etc. and that might work. Providing we get any sun. ::)
Rosemary