South Texas is very hot country for a greenhouse. To make it useable you will need an evaporative cooling system and shade cloth like 40-50% aluminet. I guess my question is what are you growing that needs a greenhouse? I can see a structure to avoid freezes in winter. But what needs covering in summer? I'm growing all kinds of fruit in a greenhouse in west TX at 4500ft elevation. So we're much cooler and lower humidity. This makes a useable greenhouse but only with evaporative cooling, wet wall opposite exhaust fans, and shade cloth. In your climate the best greenhouse will still be 100F in summer and freezing in winter. Mine 32ftx54ftx16ft and covered with inflated double poly still falls to outside temperature at night unless heated. Mine benefits from covering in summer because our rainy season would rot the stone fruits. It also excludes hail, wind, birds, etc.

I'm in MA. We just recently had a hot spell. My green house having been destroyed several times, taken down and folded up for a hurricane last fall is not as bright as it was when I 1st added the new plastic more than a year ago. You might say it is semi frosted. So the plants are not getting total direct sun. It's kinda small now 16'x22' and the height varies from 10' or so down to 6 or 7 feet. I need to add support to push the plastic back up.

Firstly let me say - those of us with a low body temperature who have always had this - it is nothing to worry about ath0ugh it does have its problems. Anyone who has only recently become 'cold' there may be another reason for it so do get it checked out. My daughter is in her final year of a nursing degree and was talking to one of the docs about this. It is unusual but not unheard of - I think most people who are colder than usual are not aware of it, nurses very rarely put down if you have a low temperature as they fear it would make them appear sloppy by not checking properly. The real difference for most people is our 'immunity' to bugs - bugs are very temperature reliant and they do not like our cold bodies. However when we come across a bug that is less susceptible to temp changes we effectively go down 'like a ton of bricks' and take longer to recover. We don't feel the cold quite as much but we feel the effects - so even if we don't feel cold we can get problems like Reynards where the fingers and toes go white and numb. Unpleasant but as long as you are aware of it and don't get too cold for too long it can be controlled. I also find heat to be a problem - I am not really sure why but I will try to find out more.

Keeping an even temp' while ventilating will get expensive if it's even possible where you live.