Thermal sheet

I've always overwintered a few tender plants in a cold frame, but bearing in mind that the glass alone is no real frost protection, I usually make a roll down cover out of a plastic foam material, to keep the frost away.
Last year this was ripped on a couple of occasions by cats, presumably fighting on the frame, and by spring there wasn't much left of it.
So does anyone have any ideas as to a strong flexible material that has some thermal qualities and will not tear or rot, that I can use.

 

If it doesn't have to be translucent you could try getting hold of a roll of the insulation that can be slipped behind domestic radiators Pete. It's foam backed shiney foil and quite tough. Same stuff is used as sun shade/frost shades in cars.
Otherwise bubble wrap is fairly cheap if bought in bulk and you could simply renew it if the blasted moggies stike again :rolleyes:

 

Hi Pete. I don't really have much experience of the practical side, But as a physicist I am interested in what is going on. I may be wrong but my feeling is that the temperature inside a cold frame is not much higher than that outside. In fact my summerhouse (with a glass front) sometimes records a lower minimum temperature inside than out - I am not entirely sure why. I always used to think that the minimum temperature should be higher. But of course there is no heat source inside the shed, or cold frame.

The reason your plants get protection is because they are out of the wind (ie wind chill factor). Also they get protected from the wet, which could be just as important.

During the day a greenhouse warms up because it receives radiation from the sun. On a clear night that process goes into reverse and low level heat is radiated back into space. As space is very cold, effectively no heat is radiated back from space into the greenhouse. However on a cloudy night your green house will radiate out the same amount of heat as on a clear night, but there are clouds in the way and they radiate back almost as much heat because they are much warmer than outer space. Consequently cloudy nights are warmer than clear nights. In physics terms, every object radiates heat in proportion to the fourth power of its absolute temperature.

Sorry if that it a bit complicated, but what I am suggesting is that if you cover your coldframe with a cloud (in the form of a white sheet or any other material) you may well conserve the heat as much as any insulating material, as a major source of heat loss at night is radiation. This is not totally intuitive - hence the physics lesson.

 

Dave I will look into the radiator material and see, bubble wrap is an option also.

Peter, I must admit I'm not really sure as to what you are saying, is it that any thin covering would have the same effect, regardless as to wether it was an insulator or not.
I'm not really up on the physics side of it, but as I understand it, unless there is a heat source, a glass covering is not a real protector from frost overnight. I do know from experience that if its covered with a blanket or similar it holds the daytime warmth longer, in most cases long enough to get it through the night without freezing.
I agree that protection from wet is also a help, but not sure that plants suffer from wind chill in the same way we do, after all the temperature of a plant is the same as the air temp. therefore the air movement cannot affect the temp of the plant.
To me wind chill is a human response.

 

Hmm! Interesting point but in that case, how do plants suffer from 'wind burn'?

 

Pete - I do think there is a wind chill factor for plants. For humans there are two forces at work. 1) Wind will blow the warm air out of your sweater - this doesn't apply to plants. 2) If your face is wet, a cold wind will make it feel much colder. When water evaporates it extracts heat from the body that it is in contact with. This is how refrigerators work. The refrigerant gas is made to evaporate inside the fridge and extract heat. Plants are full of water. In summer water evaporates from the leaves, which is why you have to water them. If a wind is blowing if will evaporate much faster, and in the process cool the plant. A cold wind in winter will also evaporate water and cool the plant. One way that a plant can die in winter is if its roots become frozen and it can't draw up any water. But if there is a strong wind it will still be losing water by evaporation from its leaves, and it dies of dehydration.

Fleece is full of holes and the wind can go through, so it isn't really warmer inside the fleece - but fleece does offer protection by making the air still. The same with a greenhouse. After several days in winter without any sun, the temperature inside a greenhouse might be exactly the same as the temperature outside. But the still air works the same way as a fleece.

Sorry about my previous post. On reflection it was rather clumsy. I was trying to explain how radiant heat works, but the physics got in the way. :D Yes I was trying to say 1) that still air is your friend, and 2) that a major loss of heat at night is caused by radiation. Anything covering the glass - even a sheet of paper, would cut this down a lot. In fact something opaque may well be better than bubble wrap, which being transparent will allow heat radiation to pass through outwards at night.

 

Two links Pete -

Wind chill affects runner beans in the same way as human beans. The main difference is that they can't shiver in order to generate more heat and feel warmer (for a while).
Moving air strips away temperature faster than static air does. A similar process happens with humidity when a breeze dries washing faster than static air.
What I'm trying to say is that cool air doesn't really care what warms it - animal, vegetable or mineral. It will eat the heat regardless. :D

http://www.bbc.co.uk/devon/weather/wind_chill.shtml

http://www.ncdc.noaa.gov/oa/climate/conversion/windchillchart.html

 

Hi Terrier - I was composing the above as you posted.

I think wind burn is what I am talking about above. In winter a wind will extract water, and if it can't be replaced fast enough you get the burn effect.

 

Dave in the case of plants the wind removes moisture quicker than the plants can take it up from the roots a process called desiccation this is what causes the most damage in winter, the fact that root growth and photosynthesis is all slowed down due to lower temperatures adds to the problem, it is important that plants still have moisture in winter to prevent this happening.It doesn't have to be frosty for wind burn to take place.Peter I think the last 2 posts explain it well.

 

I can see where your coming from, and definitly not being a scientist or what ever, I can only argue as I find.
Firstly I cant agree that wind burn/dessication is an effect of cold, as you have answered that one yourselves, it happens when the roots cant keep up with the water loss through the leaves, be it hot or cold.

If a wind temp, ie the air temp is above 0C, say 1C, how can the fact that the air is moving lower it below 0C.
It surely only appears lower to us, because we have internal heat that, as Dave says, gets stripped away faster.

I see it similar to the fish thing, how do they survive in cold water, simple their body temperature is the same as the water temperature.

Anyway getting back to the cold frame thing, I do know that a thick blanket placed over a plant has more frost protection than say one layer of fleece, but it must cover the plant completely to the ground, there by using the ground as a storage heater.

 

Pete. A good quality bubble wrap attached to the inside of the glass is the best bet other than a heavy fleece. The plastic foam type fleeces are likely to tear too easily as they are very weak in one directionor the other due to the methods of manufacture.

 

If a wind temp, ie the air temp is above 0C, say 1C, how can the fact that the air is moving lower it below 0C.
Pete the wind chill factor does happen temperatures are reduced cosiderably the stonger the breeze, see the table on this linkhttp://www.r-p-r.co.uk/wind_chill_table.htm

I am an engineer not a scientist but I have had practicle experience of this one of the jobs we used to do as apprentices was to drain water out of air receivers, it used to accumulate in the bottom of the vessels because the air was warm when compressed then codensation formed and considerable amounts accumulated in the bottom,it was under considerable pressure when released through a drain valve so the air and water was forced through a narrow orifice at great speed plugs of ice used to shoot out formed by the wind chill effect, it can happen in liquids as well when they are moved at high speed but thats another story.

 

Hi Walnut - didn't know you were an engineer - very similar field to physics.

Pete. Cast your mind back to school - ugh!. 'Latent heat of evaporation'. I tried to Google this and there were hundreds of entries but none useful. The best I could find is this http://www.thepigsite.com/articles/5/housing-and-environment/601/why-evaporative-cooling-works Its how to cool your pigs down. :D

The basics is that if you have water at say 10C and you blow air (also at 10C) across it. The air will evaporate the water and in doing so will take heat out of the water. The net result is that the water will cool below 10C. Many years ago after I left school, I hitchhiked around the USA for several months in the days before cars had air-conditioning. In the summer the USA can get extremely hot, and I was surprised to see cars with bags of water attached to their roofracks. The bags were slightly porous so the the outside was always slightly wet. I experienced the purpose in one car that picked me up. We were driving through near desert conditions in temperatures of 90F or 100F, with one of these porous bags. When we stopped we had drinks from the bag and the water was really cold. By driving at 70 mph, the water was evaporating furiously from the outside of the bag and had lowered the temperature of the water from 90F to about 40F.

 

Walnut, is this some thing to do with latent heat, seem to remember something like that in science classes at school.
Thats still a wind chill table referring to people the ACTUAL AIR temp is along the top.
If you place a thermometer in that wind, what would it register?, if it registers lower as the wind speeds up, then surely the ACTUAL AIR temp is lowering with wind speed, which would make the whole table pointless.
I have an idea that when you compress air it can form ice, but in these cases we are talking about very high preesures compared to wind speed.

Chobart, I agree these foams do tear very easily in one direction, trouble with bubble on the inside is getting rid of the condensation.

 

I was wrighting this as you did Peter just took me longer to post it.
Amazing we were thinking the same latent heat etc.