[Maud]

We are renovating a farm building with stone walls by the addition of a polycarbonate roof to make a greenhouse. I can use standrard 10mm clear twinwall polycarbonate or a recycled version, 'haze' which lets through only 40% light but has the same thermal properties. As this is a large building, about 11 m x 7m,x 3.5m high, and I have never had a greenhouse before, I am not sure if clear polycarbonate might get too hot and suffer from too much glare when working inside? Should I use alternate sheets haze/clear? Or do I need as much light as possible for successful growing?
Because of the exposed site, ventilation may be difficult to control- all or nothing I suspect. Any tips most welcome please.

[mumofstig]

Everything needs as much light as possible specially through winter and spring, although if the summer is good most of us have to rig up some shading.

40% light would grow weak spindly plants

[JayG]

Glass has a typical light transmission of 94%, clear polycarbonate around 85-90% depending on type. 40% would be disastrous in my opinion given that your building has solid walls.

You probably also need to give some thought to providing some sort of opening vents to assist with ventilation (as you say it's quite a large building.)

[Maud]

thanks for both these comments.  How about alternate sheets in the roof? Also how can I work out the amount of ventilation I would need?
I'm sure the info is out there on the internet somewhere but I must have missed it in my research so far so I appreciate your comments.

[JayG]

Maud; I'm a very long way from being an expert on greenhouse design but if you alternate clear and translucent panels you will presumably finish up with an average of about 70% light transmission.

Given that greenhouses are normally all-glass constructions (or all-polycarbonate, polythene, or whatever) and yours is something like two-thirds solid stone I really don't think you should compromise on the light issue.

[Paul Plots]

Go for glass if you can.... you can always add shading but you can't improve light transmission once you have polycarbonate.

A glass roof could quite easily allow opening vents. "Lots of" openings would be good. You can always close them but you can't put holes where there are none when it gets too hot and you wish you had more ventilation. 

Hope you follow my drift.   

[TerryB]

All glass would be great but very costly and heavy. 
Go for tripple clear plastic if you can afford it and as much ventilation as you can.
If you have electricity on site put a 9in fan at each end, 1 input 1 extract, these could be controlled by a stat and would reduce the number of openings required.   

[Paul Plots]

All glass would be great but very costly and heavy. 
Go for tripple clear plastic if you can afford it and as much ventilation as you can.
If you have electricity on site put a 9in fan at each end, 1 input 1 extract, these could be controlled by a stat and would reduce the number of openings required.   

Not sure how many years the fans would be running for but by the time the electricity bill has been accounted for glass might be cheaper.... it would also outlast poly.