Greenhouse V Polytunnel

[Rich45s]

Hello,

Long term planning here. Are there significant temporeture differences (or other differences but I’d have assumed temp was main one, possibly light levels) between the two?

Polytunnels remind me of the old corrugated Nissen Huts which I think I prefer over greenhouse shapes

Thanks

[peter831shaw]

There are lots of differences between these 2 styles of greenhouses.

Heat loss is a function surface area of the covering, temperature difference between inside and out, and the rate of transfer of the heat energy through the covering. The roof is the worst as heat rises. This is why the "Dutch" style greenhouses have a low profile and the roof angles are very low compared to a typical A framed house. These low profile greenhouses reduce the surface area of the roof.

If you choose a poly tunnel you need a second layer filled with air to create a layer of dead space. This second layer makes for a pretty well insulated covering compared to other double layers rigid polycarbonate or acrylic panels and way better than glass.

Venting (cooling) is also problematic with a bow shaped roof, so its not often included which is a huge mistake for cooling as hot air rises requiring fans and vents on the end walls.


a few links for more reading
energy blankets https://ag.umass.edu/greenhouse-floricu ... reenhouses

google this for examples in the UK

greenhouse thermal screen uk

[Chez2]

We upgraded from an 8 x 6 greenhouse to a 30 x 18ft polytunnel. We used to throw shade netting over the greenhouse but put it inside the polytunnel.

We have double doors at each end of the tunnel and its windy where we are. As its windy we didn't want to put vented sides in our tunnel. You would probably need them if you are not in an open, windy, rural location.

This is our first winter with it. We have gas heating to the main tunnel, around 5 degrees minimum. We have put a smaller tunnel up inside the big tunnel with a fan heater for plants that need to be kept warmer. The gas heaters failed on the coldest night put plants in the inner tunnel were fine.

The tunnel isn't pretty but nobody really has to look at it. Our land is sloping and terraced so we can just see the top of the roof from the area of garden we use.

We bought a professional grade tunnel with high quality polythene. Its very sturdy and the polythene is guaranteed for quite a few years.

In winter we sealed up one set of doors and have a fan going to circulate the air inside. It is ventilated by opening the doors on most days.

[Rich45s]

Fantastic chaps,

This is really helpful info. It’s going to lead me onto things I didn’t even know I needed to know! Definitely going for a read up now.

Cheers,

R

[agavemad]

I have both polytunnel and greenhouses.
The greenhouses are 10mm polycarb.

I would not like to try and heat it as it has no insulation values and I heat the greenhouse to 7 degrees

In summer the temperatures are scorching. The polytunnel will easily reach 60 degrees and I have seen 65 degrees with both double doors open. When we put another one up we will use vents down both sides.
In comparison the greenhouse temp has never been above 45 degrees

Condensation can be another problem in the polytunnel - however I use it for alpines etc and it does get more water than the greenhouse.

Hope that helps

[Phil_SK]

I want the one of these: https://www.youtube.com/watch?v=7kaDXbCOL54 (Sorry, that's not helping your quest, is it?)

[Paul in Essex]

They look brilliant!

[Chris L]

I want the one of these: https://www.youtube.com/watch?v=7kaDXbCOL54 (Sorry, that's not helping your quest, is it?)

I've often wished that Greenhouses could open at the top. It would make watering so much easier. You'd just open the two halves of the roof on a rainy day and hey presto plants are watered.

[Aiko]

Same with heat. An easy way to cool the greenhouse from 50C to 30C...

[Chez2]

I have both polytunnel and greenhouses.
The greenhouses are 10mm polycarb.

I would not like to try and heat it as it has no insulation values and I heat the greenhouse to 7 degrees

In summer the temperatures are scorching. The polytunnel will easily reach 60 degrees and I have seen 65 degrees with both double doors open. When we put another one up we will use vents down both sides.
In comparison the greenhouse temp has never been above 45 degrees

Condensation can be another problem in the polytunnel - however I use it for alpines etc and it does get more water than the greenhouse.

Hope that helps

Wow, how did it get that hot? Are we talking centigrade? Ours was up last year in the really hot summer. It is in full sun all day and ours got nowhere near that hot.

We did have a problem with condensation so made sure we opened the doors on days it wasn't too cold. We have a big tree fern in there throwing out moisture and I wouldn't say it was that bad. It was worse initially before we put in the fan. The fan keeps the moisture in the air so it gets blown out when the doors are opponent. If it didn't we had considered moisture absorbing traps.

Professional polytunnel covering is a little better at uv light transmission and a bit better at insulation, obviously I'm not trying to say its like a bubble wrapped greenhouse.