I prefer dual-skin/twin wall panels. They're light, rigid, don't cut out huge amounts of light, easy to work with, cheaper than single skin, etc. Just remember to use something like Aluminium tape over the open cell ends - otherwise all types of bugs will crawl in and die.

A dead air space slows down conductive heat loss and the slope of temp change will be shallower as there is more resistance in the wall. In the heat you will be venting regardless of wall type, and the twin wall will help with light transmission as well. If you need additional buffering in winter then water storage will do that for you.

Dan

Thanks for the replies. Unfortunately neither really help much with the immediate issue of exactly how much difference twin makes vs. single. Moderating temperature swings is fairly self-evident, but what I really want to know is the extent of the difference. Are we talking minutes or hours? When the sun sets, usually a plastic unheated structure gets close to ambient air temperature at some stage (huge number of variables). How do we quantify the differences between the two cladding types?

I guess put the quesiton another way. Imagine two identical structures side by side, unheated. One has twin-skin plastic, the other single. How much difference would you expect in thermal performance between them?

On a related note, has anyone got evidence for or against the use of 'thermal' GH plastic films on heat retention in uheated structures?

I'm just a reader and don't know much about warm climates, since I'm living in a zone 3. I'm more concerned about cold nights in the spring, so I do know about heat loss.

One comment here made me wonder why you are you are so hung up on the exact thermal difference.

If the double is cheaper as eurotrash states then what is the sense of going for the single.

These facts remain--Double will definitely slow the heat loss down because of the dead air space.
Nothing will slow down the heat loss if the structure isn't air tight in the first place.
Making it air tight, as I have my greenhouse, may make it harder to cool in the day. So take your pick.

HI Oilpainter,

The reason is very simple. I am nearly broke, and single plastic film is about 40% of the cost of double plastic. The panels Eurotrash speaks of are many steps of sophistication above what I am talking about. I am talking about an extremely simple structure essentially just a hoop tunnel on legs. The reason I'm pushing this is simply to attempt to find out from more knowledgable people if the additional financial outlay will be worth it. I could retrofit my design with a second skin later if necessary, or use a moveable internal thermal screen. Will see what happens.

Warm climate? Very subjective. I know 'no frost' sounds warm compared to zone 3, but I am cold much more often in the year than I'm hot. Mean annual air temp is 60F. Temperature range more like San Francisco surrounds than Miami.

Thanks everyone for comments, I apologise if I came across ungrateful!

Not ungrateful Banyan just not fully explained.

Before I had my greenhouse I used cold frames. Mine were simple structures; just hoops stuck in the ground and covered with plastic stapled on to poles.
You understand this was back in the days when they had dinosaurs for computers and I had never heard of UV plastic if there even was such a thing.

I used a double layer of heavy guage polycarbonite film like they use for vapour barriers in homes. I just checked Home Hardware on line and it's $12.99 for 10' by 25'. It held up for 3 or 4 summers. Once I used the covering off of a new mattress we bought. It work well for me and my flowers grew well under it.

Why not try it or you could go with the UV film on the outside and this as a second skin. The plastic will last longer if the outer layer cuts the UV rays. It doesn't really matter what the air temperature is. Any way you cut it, under plastic or in a greenhouse, it will be cold at night and too hot in the day if the sun is shining. That is the age old problem of growing under glass or plastic

Hi
Curious as to why you feel you need a sealed structure at all?? The species you name can be grown even down to 32. With some types COOL days is a very good thing lol.
Generally I find a structure with a covering to be downright harmful on a year around basis.
I'm not too familiar with 10 California but I know all the species you name are being grown as "yard " plants there.
gary