We are getting ready to build a greenhouse on our farm near Atlanta, Georgia. I would LOVE to be able to grow veggies year-round. 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
4 Comments
[Farmtek.com](http://Farmtek.com) They have all kinds of greenhouse stuff. I think the insulated plastic panels shown in your photo would be really good, but I’ll bet you could contact them for professional advice. I’ve dealt with them on a much simpler basis for years and found them reliable.
Glass windows do not insulate unless they are double pane which will require a very strong frame to carry the weight. Twin wall poly insulates somewhat. Internal temperature will still reach ambient temperature at night without a heater.
Not directly answering your question but a few thoughts from me
– having a way to pull shade netting over in the summer would be useful, as I expect you may have excessive temperatures if relying on vents alone on a still day
– good airflow at each end really helps with circulation, so maybe look at sets of doors on both end that you can potentially swap out or remove (the ‘summer’ doors could have mesh rather than a solid panel for example)
– I don’t know how well the water will perform when it comes to thermal mass, it’s still quite limited in the heat it will pick up in the daytime. You may be able to use something based on a ground source heat pump to keep the water at a reasonable temperature
– Fleece works wonders in winter if you’re having issues with head loss in the space, I’m based in the U.K. where it has been around freezing to 15C in the last few weeks and a layer of fleece has raised surface temps by 6C in the polytunnel I have
For summers I agree with u/OverallResolve that passive vents might not be enough. Shade cloth could be an option but I’d also run electricity out there and design with fans on either end in mind. Fans also ensure air movement which will help with fungal issues and other things that thrive in still, humid air.
for winter growing… what do yiou want to grow? Most things that need lots of warmth aren’t going to be viable unless you really heat the space but greens, overwintering roots etc will work fine.