[infowarrior]

  I'm nearly ready for the cover to go on my polytunnel, just need to finish the raised bed system inside first. I'm still undecided as to how much ventilation to include though.  Tunnel is about 40ft long by about 3 metres wide.  My plan is to just have the doors (one at each end) covered with netting to create a constant draft going through the tunnel and my cover along the long sides just attached to the top of my scaffold board raised beds but been told by couple of people that I need the netting all the way round the tunnel at the base as it will get too hot otherwise.  I disagree with that. I think that the netting on the doors will be enough to keep things from scorching in summer. What are peoples thoughts on this??   My main concern is that i want the opportunity to really extend the growing season in the spring and autumn if I need to (depending on what I'm growing at the time) by having it easy to seal by just tacking some polythene or something over the door netting if it's cold and having a heater in tunnel but if I had a foot or two of netting all the way round the tunnel at the base this would be alot more difficult and would let heat out from heater and I've seen quite a few tunnels online with no ventilation at all with the cover just going straight into the soil or attached to a base rail at the sides!  I understand the importance of fresh air to plants but don't think they need the netting all round.  Do people reckon I'd do fine with the doors only covered in netting??

                           Infowarrior.   

[japagow]

Amateur and non technical reply.

1)In all the books I've read on p tunnels they state that ventilation is paramount and illustrate this with areas above both doors at both ends for that purpose. 

2) For this reason the books also state that the p.tunnel should be placed running North South not East West because in the summer it gets v hot.

I believe if you have the right position and ventilation above doors therefore you could avoid netting.

3) Wait for a technical member to assist you further.

[japagow]

Just re read your original enquiry.

Do you mean the whole poly sheet- longsides- is tacked onto the top sides of the boards used to make your raised beds and therefore isn't secure to the ground at all?

My tactic is going to be to dig a trench along the long sides of the tunnel, bury a portion of the sheet in the trench weighted down by soil etc and then pull/stretch the sheet over the hoops to bury it again on the other side - with some fiddling about and swearing no doubt.

The raised beds will then sit inside the sheeting and not form part of it if i read you right.

[mumofstig]

As you say....if you buy a polytunnel from one of the big companies,they don't have netting all round, do they?

It is dificult to manage ventilation in the winter, I found. If you keep them closed up for long in the winter they tend to get quite damp and mouldy inside. So on all but the coldest days the door netting needs to be open for air flow.

What are you thinking of growing as you are talking about a heater?
The main advantage of the polytunnel is the protection from cold wind, rather than being able to raise the temperature a great deal IMO

[N.WalesIdealist]

The ones at the garden centre down the road don't have any ventilation along the long sides, but the walls at either end are completely net made during the summer then replaced with polythene when the weather is colder.  That should be enough ventilation I should think!

[arugula]

Mesh doors at either end are good and could be enough, depending on the postition relative to the prevailing wind, but the addition of solar powered fans to help circulate the air may also help, as air movement is paramount.

[infowarrior]

Yes, I'd thought of solar fans but the ones I've come accross were pretty poor ones.  Prob not powerful enough.

[arugula]

Would a few wired up in series do the job?

[sonnycbr]

This is what I've recently done when building my 20ft X 12ft tunnel. I used mesh up to 30inches and then put the cover on. The temperature inside is already several degrees higher than outside and I appear to have all the ventilation I'll need. It could be an option for you to use mesh and roll the polythene sheet down in the winter to save heat.