Topic: Polytunnel

[John85]

Does anyone have one with a double layer?
If so what system do you use? blower or spacers?

[Vinlander]

Interesting engineering - I can see how air pressure would be cheaper than spacers - but only for massive commercial tunnels.

For "small" tunnels like my 5m x 3m jobby I'm pretty sure twinwall or triple polycarbonate would be a lot simpler and probably cheaper (the triple would probably also be better) - certainly longer lasting* - I was very tempted to go this way when my last cover gave in to UV, I may do it next time especially if the Spanish double-inhibited stuff doesn't make the expected difference - but it's looking pretty good in its 3rd year.

Cheers.

*PS. when PC goes yellow after 10 years or so, you are left with a very valuable durable resource that's good for all kinds of projects - even raised beds. When PE sheet fails you're left with a pile of useless flakes that blow all over the place and are a bugger to clear up and out of your soil.

[pumkinlover]

I thought twin wall was in sheets rather than flexible

[John85]

Unfortunately to use a blower you need electricity and that is not always available in a field or allotment.
A solar panel and a big battery + inverter is a very expensive solution.
Spacers are for sale in USA for commercial tunnels but I found nothing else on the web.
Polycarbonate sheets need  junction bars and those are not flexible enough to fix on the frame of the tunnel.
Any suggestions?

[Obelixx]

Mine is a single layer but we also bought the green shading net that fits the whole thing and which I shall leave on over winter, partly because it would be a faff to shift and partly for protection.   There wasn't an option for a double skin but would bubble wrap not work, attached to the inside of the frame?   

[Vinlander]

I thought twin wall was in sheets rather than flexible

It's often more flexible than you want it to be - even the triple stuff can be warped out of its fixings by snow - on several occasions enough to leak, and occasionally enough to create a visible gap when all the movement happens at one side. (A good reason for minimising the expansion gap - a little too much expansion is much less troublesome). Yes, I know I should pin them centrally on each sheet - I do at the top, but it looks horrible at halfway, and risks leaks when done at the bottom.

Anyway, it can easily follow the curve of a 3m diameter tunnel if you lay it lengthways across the curve, and not so easily but do-able if the length of the sheet follows the curve (but only in warm weather - it's very, very tough but I wouldn't risk bending it "against the grain" below 15C).

Clamping it to pipes is probably more trouble than its worth - so it's best to design it from scratch (same applies to any other method in this thread). I would go for a frame-and-stringer approach like an upturned boat - it's easy to cut curved or elliptical frames from marine ply or even OSB.

Presumably both methods turn a polytunnel into a greenhouse, and building in roof vents becomes a breeze (sorry).

NB. You will probably need a few glazing bars when laying the sheets across the curve, but it's possible to cut into the cells both sides of a lengthways join so the cells interlock like hooked hands - losing a few cm of sheet in the join is a hell of a lot cheaper than an extra glazing bar, though the unclamped sheets will need extra stuff to hold them in place onto the frames (ropes or straps over the outside on every frame is the obvious choice, though metal strip or UPVC laths would look better).

The same kind of clamping could work for sheets along the curve too - but you would need dozens, scores of those pin-through fixings on the stringers - I'd probably want to put some stringers on the outside instead - U shaped aluminium would be good as it could create an array of gutters for harvesting rain - I wouldn't pay DIY prices for it, but I've already got a fair amount from skip-diving .

Cheers.