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.

Turns out I have not much choice - I've got it heated to 50F because that seems to be the lowest my heater can be set without going down to pilot only which is too cold. All of my babies were okay this morning - I guess I'd better get back to work here so I can pay the big gas bill that's coming... many thanks Lori

basie9 wrote: >I haven't used a greenhouse before, but my dad found a great deal on these little walk-in greenhouses. Yes, they are pretty cheap, but… I had one… >how cold is too cold, and how hot is too hot? It depends on the plants and the exterior temperature you have there. I see you mostly have flowers. If you have some tropical flowers, as Cole_Robbie said, they will NOT like low temperature. Some might not even like very high temperature either. Check the optimal temperature for the plants you have there. As Cole_Robbie said, during the day, roll up the plastic door. And if it's hot during the night keep it also open. This kind of GH get VERY hot VERY fast. If outside is 60’ F, inside can easily get to 80-90’ F. if outside is 70, even if the door is open, you will get 100’ F or more. If the door is closed, you will get 120'F +. You can do little to control the heat. For example you could use a shade fabric. These shade fabrics come in different degrees of shade. Last summer there were almost 100’F for few days and it was VERY hot inside, probably 120' F - OBVIOUSLY the door roll up - the plastic cover of the GH - the roof - started to melt. Of course, I couldn’t use it anymore. So I took it down, and I built my own GH, technically a cold frame. >I see posts about ideal ranges, but what will start to hurt my plants/seedlings? Like I said before, it all depends of the plants you have there. If vegetable seedlings, 70’ F would be the maximum. They will not die at 80’ F, but they don’t like it. And at that temperature you’ll have to water them every day. >I installed a little electric temperature instrument that I can read in the house, but I didn't know what I should be watching for? You should watch for temperature and humidity in the GH. Over 85% humidity is dangerous, because mold will develop. In my GH, I installed a fan which worked very well. The fan also help lowering the temperature in the GH. You can also put 2-3 Silica Gel packs. It helps, but make sure you refresh them often. Regarding low temperatures, inside will be at least 5 degrees higher than outside. Even more if it’s windy outside. For vegetable seedlings you will need about 55’ F inside your GH. For flowers most likely higher. So if outside temperature is lower than 50’ F you might have a problem. Yes, you can cover your GH, with a “large fleece blanket” - as Slimy_Okra suggested - but be careful not to be very heavy, because GH’ structure is pretty fragile. Do NOT use a blanket if rain is on the way. Use a poly tarp instead. VERY IMPORTANT: As Cole_Robbie said, you’ll have to anchor your greenhouse to the ground. I made a structure, a frame of 2 x 4’’ covered by a ¾’’ plywood. I fixed the GH to that structure with screws, and it passed a mini-tornado. You could also use these Hook Style Tent Stakes. Please see here my cold frame. You could get some inspiration. This post was edited by Daniel_NY on Tue, Apr 30, 13 at 23:23

While your body does generate heat, it doesn't produce nearly enough to heat up your greenhouse by 20 degrees in 15 minutes. I did a rough engineering calculation based on the approximate volume of your greenhouse, and it would take about 4 times the amount of heat that your body produces to heat up the air by 20 degrees in 15 minutes. That does not even take into account that you would also need to heat up the structure (which has a much greater mass than the air inside it) as well. Have you actually looked at the thermometer before you open the door and let cool air in? My guess would be that the greenhouse is at a certain temperature from solar gain, it gets cooled down temporarily when you open the greenhouse, and the solar gain plus the heat retained by the structure (plus a very minor contribution from your body heat) raises it back up while you are in there. One experiment you could do is to go into the greenhouse for 15 minutes at night when there is no solar gain, and see what difference your body heat makes then.