browfish wrote in message ...
Hi, new here so go easy

i very nearly ordered a polytunnel the other day but read that they are
not suitable for overwintering plants such as palms, bannanas etc that i
currently put in a small greenhouse. I understand they are slightly less
efficient at heat retaining but by how much.

I wanted a large 20' x 10' (around £300) and the equivalent greenhouse
would cost me a fortune. Do you think a ploy tunnel would be ok? I plan
on adding some heat and can always line with bubble wrap etc but then i
do that in the small greenhouse. Im in southampton by the way

cheers

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.