heating system need help choosing

Hi Scrungee Thanks for the info. Would you think a double layer of poly would make much of a difference. I could relatively easily put a second layer on the inside of the wooden structure there would be a 2 inch air pocket

 

I don't have any experience of it, but I have seen video of a smaller 'tunnel' contructed within a large one, and that had some heating used in it.

 

Hi Tommy - in my polytunnel, I have found that the temperature drop at night has become much less since I put more area of brick pavers down for standing pots on. Basically they seem to act like a low grade storage heater from when it heats up in any sunny days (even in winter). So if you expect to stand pots there (and table and chairs for enjoying yourself ) then that might help. Though I think it would get VERY expensive to keep the temperature high enough for tropicals as you suggested using electric heating with only single wall polythene cover. Think more in terms of mimosa, datura, brunfelsia... (and wine bottles). Twin wall polycarbonate is supposed to be much better (i've even seen a fancy 9 wall UV stabilised version)

 

You should be able to calculate roughly how much heat you`ll need based on your dome dimensions and in/out temperature difference. It looks like you have roughly 600 ft2 of surface area, 1930 ft3 of volume and a 61ft perimeter. I personally wouldnt encase the timber frame between the inner and outer skins of poly due to heat and condensation.