Hi Bluebell, I put up a 10′ x 20′ polytunnel in May 2014 and wish I had been able to go to 12′ wide because my paths are very narrow if I have 21/2 to 3 foot wide beds either side and a bed in the middle for tall stuff such as tomatoes, it’s a bit of a squeeze getting along the paths when everything is growing strongly. I would also say go for the largest you can afford and you will still feel you needed to get a bigger one!
I bought my tunnel from First Tunnels and went for the Thermal anti drip cover, (I think called Super Therm now), I find I do get condensation and some drips and this seems to happen where the polythene touches the Twin Support Brace Bars and the drips fall from the bars. I have since put anti hot spot tape along these and that seems to have made it worse as it’s created a barrier for the condensation to hit against, but then, the metal bars were touching the polythene and it’s recommended to fit the tape to stop the polythene deteriorating prematurely due to heat build up.
I went for single sliding doors because this means you don’t have to worry about where a hinged door would swing out to, either inside or out, as I don’t have a lot of room, so the sliding doors just run along the ends not taking up further room.
I have put half inch square solid metal mesh on the top panels of my doors and on the outside I have put runners to hold 4mm twin walled polycarbonate sheets sliding up and down for ventilation, i just screwed on some vertical runners with holes drilled about every 4 inches and I fashioned some 2mm metal rods into stops that I just insert where I need to hold the bottom of the sheets.
My allotment is on a very windy site and is orientated with the ridge east/west (with a southwest prevailing wind). I have been worried about gale force winds meeting a solid object, so I leave a 3 inch gap at the top of both doors all the time to allow any wind to blow through and thus far, it’s been okay.
I have anchor plates and wooden base rails and have buried about 6 inches of the polythene in the ground to tidy it up and also create a bit of a physical barrier for bugs. We are not allowed to use concrete on our plots so the anchor plates are just buried 600mm down. Another plot holder has Aluminium base rails and regrets it, he says the locking centre rails are really difficult to press in and once the polythene is trapped, the locking rail is really difficult to get out.
I didn’t use the supplied nails for all the battens holding the polythene, my thinking was it would be easier to change anything if I used screws all around, this being the first time I had put up a tunnel. This may prove to be useful for the ventilation screen and side vent. I found this to be a bit ‘Heath Robinson’ because the rod holding the 20 foot of polythene is only supported at the winder end and is floating at the other end, just relying on the polythene holding it level all along it’s length and this was quite difficult to get evenly wrapped around the rod. In practise, when I wound it up and down at various times, I trapped my Melon leaves around the polythene, so at the moment I have decomposed leaves wrapped around the polythene either side and I can access the inside to clean them off but not the outside as it has the ventilation screen, (net), fixed via the battens, so it means that I will have to remove the battens to take the net off in order to clean the outside of the polythene screen. If I had used the nails there would have been a danger of splitting the battens with the constant removal of them over time.
I will be looking to try and get some sort of solar powered fan for further ventilation because you certainly need ventilation/cooling in a tunnel most of the time, especially when it warms up and removal of condensation when it gets cold. I figured that I could fit a large sheet of ply on the east side from the continuation of the ventilation panel support rail and the base rail as it shouldn’t affect the polythene cover, a fan could then be fitted in this ply panel. I just need to find a source for a fan a bit stronger than those tiny ones fitted to boats or motorhomes! (Any suggestions would be really helpful).
I think, like Sarah said, you will need to open up every day because the temperatures and humidity change very fast and the air needs changing frequently. I did think about putting up another side vent and screen to get more ventilation and rang up the Company to order it but they said as the tunnel had been up more than 6 months they couldn’t recommend messing about with the polythene cover and I wouldn’t get it drum tight so I haven’t gone down that route. In your case, it may be worth thinking about getting it for both sides.
With regard to the cover tightening, it was only my sister and I who put the tunnel up and we got it almost tight but needed the extra strength that only men can supply in order to get it drum tight. The wooden base and side rails made this tightening easier than just burying the polythene without base rails I feel.
So a lot to think about but it really opens up more possibilities and gives you a month more growing time at each end of the year, and a lovely shelter from the wind and rain! At the moment I have salad leaves, onions, cabbages, pak choi, pea shoots all coming along nicely and today I started on building the frame for my hotbed in there. The possibilities are endless! Good luck. Jeanette.
