GH vs Poly?

[Bill HH]

I have a polycarbonate green house and a small polytunnel, In the GH I had a max min thermometer and recently bought one for the tunnel. What has surprised me is that the one in the tunnel shows a whole degree warmer on the minimun temperature. I suspected a variation in the thermometers but last night I swapped them over and the polytunnel was still a degree higher. I know its only a degree but when the weather is hovering on the frost level that could make all the difference.

Is it generaly known that a polytunnel is warmer? or is it something to do with my particular situation, although they are side by side.

[Kristen]

Is the greenhouse more draughty? Gaps between panes, ill fitting vents, and so on?

I imagine that the only "join" on the Poly tunnel is the door frame, so absolutely no where for draughts to enter.

[bearded bloke]

I wonder if glass dissipates heat faster than polythene,does the greenhouse heat up faster than the poly-tunnel Bill ?

[Kristen]

R-values of some cladding materials for greenhouses. "2" would transmit twice as much heat as "1" would do.

4 mil polyethylene = 1.20
6 mil polyethylene = 1.15
3 mm (1/8") glass (single layer) = 1.13
Double layer insulated glass = .45
4 mm (5/32") twinwall polycarbonate = .70
6 mm (1/4") twinwall polycarbonate = .62
8 mm (5/16") twinwall polycarbonate = .58
10mm (3/8") twinwall polycarbonate = .53

So on the face of it I think that 3 mm glass looks to be marginally better than 6 mil polyethylene (assuming that is what the Polytunnel is covered with ...) and a little bit better than 4 mil. Either way: much of a muchness by the looks of it.

Twinwall polycarbonate would be a lot better but panels made from that have a habit of blowing out

[Bill HH]

I wonder if glass dissipates heat faster than polythene,does the greenhouse heat up faster than the poly-tunnel Bill ?

I currently have no data on that one BB.

[bearded bloke]

It just seems to me that glass is hotter "to the touch" than plastic,so maybe it transfers heat faster,but how it could be practically tested is beyond me Bill

[Bigmallly]

I'm confused with all this talk about glass when Bill said it was a polycarb GH................

[Bill HH]

I'm confused with all this talk about glass when Bill said it was a polycarb GH................

Yes it is poly carb, but if glass is better its good to know.