i very nearly ordered a polytunnel the other day but read that they ar not suitable for overwintering plants such as palms, bannanas etc that currently put in a small greenhouse. I understand they are slightly les efficient at heat retaining but by how much.
I wanted a large 20' x 10' (around £300) and the equivalent greenhous would cost me a fortune. Do you think a ploy tunnel would be ok? I pla on adding some heat and can always line with bubble wrap etc but then do that in the small greenhouse. Im in southampton by the way
A polytunnel (as the word is used in New England) is just a light duty greenhouse. They are not really designed for winter use in places that get significant snow loads, but they would work in a mild climate. However, you will have to put double poly with inflation on them if you want to keep your heating costs reasonable. Otherwise you will be spending the difference between the greenhouse and the tunnel on heat. The extra poly cost is worth it in the long run.
Note that whichever you choose, ventilation is essential to keep temperatures reasonable inside the structure. You can easily cook your plants on a sunny day even in fairly cold weather.
You can make your own with PVC pipe if you are really limited financially. However, the commercially built structures are more likely to hold up to snow, ice and wind. Be sure to use the UV stabilized plastic covering. Rated lifetimes are in the 3-4 year range, depending on the amount of sun you get.
besides what dwight says (you have to have two vents, one on each side), there are two things you can do to make a better poly tunnel, relatively on the cheap.
1) add an inner layer of poly. The rule of thumb is 8C for every layer, so two layers will give you 16C. That should be good enough in the UK. The inner layer can be regular poly, which will cost you $10 or so (clamping it may cost you much more). Make sure that the structure you are buying allows a relatively easy addition of the extra layer. You probably won't gain much with a third layer because there is a lot of heat loss at the vents. to do it right you will have to make sure the two layers never touch, specially when the wind is blowing. no way to tell you how without seeing the design.
2) add oildrums filled with water on the north side, not touching the tunnel. I bought mine for $30 each, but they were food grade (they contained vegetable oil). If you find drums that had motor oil, you may get them for free. Unfortunately, it takes 50 calories to freeze one gram of water, but only one to lower its temperature by one degree C, so the drums become much more efficient as thermal ballast at 0C. That is fine for what I do, but it might be a bit too cold for you. Just the same, four drums should strongly limit the night fluctuations in your tunnel. It takes 5 million calories to change their temp by five degrees after all. you will need a small fan to circulate air and make sure that their heat output is spread around the tunnel.
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