We are getting ready to build a greenhouse on our farm near Atlanta, Georgia. It will primarily be used to overwinter tropicals that can't live outside in our zone 8b winters, and to allow us to get a jumpstart on spring growing. We're basing the design off of this image, with a large pond inside for thermal mass and for growing my aquarium plants that I sell. Size wise, we're looking at about 14' x 30'.
Our summers are HOT, so we're going to have plenty of venting on the side and roof. Our winters regularly get down below freezing at night, sometimes a few days where highs don't get above freezing. Lowest lows are generally 15-20F. On a rare occasion, some snow, but not enough that snowload is a concern.
My question is what is the best covering for the greenhouse? Obviously not plastic. But should we go with glass or polycarbonate panels if winter insulation (hopefully without having to add additional heat) is our main concern? If polycarbonate, single or twin wall? Keeping in mind that we're going to have very hot summers…. This will be our "forever greenhouse", so we have to get it right the first time!!
by Clean_Following5895
1 Comment
I am in 8b west GA (Columbus) and used twinwall polycarbonate for both roof and walls in my converted 14ftx24ft space. Single wall is not enough. You will still need heaters – I have to start firing them up under about 50f for what I grow. Begin doing calculations now for heat requirements to make sure your supplemental heat is adequate for the cubic feet of the space and what you are growing. I also have an evap cooler and exhaust fans for warm months, and would consider them mandatory for any days above ~85F. Important to keep in mind that venting will only replace the air inside with ambient air outside, and also lower humidity, which at least for what I grow is a big detriment. Adding a [window-unit-size evap cooler](https://www.greenhousemegastore.com/collections/evaporative-cooling/products/greenhouse-evaporative-cooler) adds back in humidity when the vents are open as well as cooler air.
The central pond feature may help with maintaining temps but just how much requires some math. I would not rely on it as a radiant heat source during colder periods. Running a gas line for a gas heater would likely be cheaper than using purely electric heat, also – if I were redoing my grow space, I’d include that expense.
A glass roof would be prohibitively (imo) expensive and also provide no light diffusion, which isn’t great depending on what you are growing – most shrubby tropicals are understory plants that don’t love direct sunlight blasting at all times. Part of the decision here is your tolerance for energy bills vs initial investment – triple-wall polycarbonate is also an option, which will lower your energy bill even more with a higher upfront cost, and take years to pay for itself.