I would go with outside, simply because it would be easier and not be sagging inside. Plus you would probably have less air leaks. Stretch it tight and/or put something in between to give you an air gap.

But heres where I get confused. Wouldn't the dead air space be warmer if the plastic on the inside. I def agree there would be less leaks if put on the outside, but the dead air space will be colder. and would I really get double protection if I put it on inside and outside or would I just be wasting my time? Thank You for your replies.

It would probably not make any difference in heat loss if you put the extra layer of plastic under or over your glass roof. The heat loss factor for single glass and polyethylene is the same, 7 watts/m2. As far as plastic over and under the glass, there might be some heat loss reduction, but you would also be losing an additional 13% of the available light compared to just one extra layer of plastic. Depending on what you are growing, this might reduce yields.

I would put it on the inside. Practically speaking, it is easier to keep the air gap if it is hanging from the inside as compared to propping it up or using a blower with it on the outside. Also, consider winds and whether or not it is possible to staple in on outside such that it is secure...outside it will likely take more of a beating. I started with my winter insulation outside and after the first year moved it inside where I don't have to worry about winds, air gaps, etc.

gardener1

The temperature of the air space doesn't matter. It's the insulating properties of the air space that helps, a little, to keep the greenhouse warmer.

I don't know how much wind you get but anything stapled on the outside here would last only until the next strong wind. Then shreds. On the inside there will be water build up between the two layers but that's better than shredded poly.

I also have a heating chamber inside that helps to hold in the heat and provides bottom heat for all my seed starting and potted plants. It works really well and serves several purposes so I really get my monies worth out of the little heater. Thanks to all for your replies. I think its pretty cool that people take the time to help others. I really appreciate it.

I agree with fruitnut on this. It is dead airspace that gives the insulation value. If the poly is installed correctly on the outside it should hold as well as any hoop house. The window frames should provide the airspace needed and keep the air still enough to add the insulation factor needed. If it is installed inside then you would almost have to have anther green house frame inside just to keep it tight enough so as top not have condensation puddle.

Thanks to all. I will try both outside on the south wall inside on the North wall. And outside on the ends. There will be about an inch and a half dead air space if I do both sides it would be about 3 inches.