I've built a cattle panel greenhouse with 6mil plastic. Doors on east and west end. Sides capable of rolling up. I'm in north TX (zone 7) and temps lately have been above 75 quite often (today 85 and WINDY!). Temps in the green house are beyond 110 (today was 126). With open doors they come down to 103. I haven't put any water containers in yet to retain warmth at night -- afraid to! Afraid daytime temps will be even more unbearable. Goal is to put black 55 gal water drum in there or 10 -12 five gal. buckets with black weed fabric draped over. Without either night time temps are same as outside. I have yet to put plants inside for any length of time -- afraid they'll cook!
This is to be a temp. greenhouse -- stripped down to cattle panel in summer for vines to grow on, then up again in late October hoping to keeps greens going all winter -- granted we have mild winters with a couple nasty ice/snow storms blown in to keep us from thinking we live in south TX.
Any suggtestions?? Oh, it would be impossible to put a roof vent in, but my doors are as tall as GH minus 4 inches.
Do you have electricity available? You could easily put an exhaust fan in one end, with a thermostat. Open the door on the opposite end, and that should create a cooler air flow. Put the fan as high as you can...possibly mount in one of the doors?
This is a real tough issue in this climate. I have already moved many things out due to sun scald and then tonight, I had to drag them back in or cover them 'cause it is dipping to freezing. Grrrr!
The liquid shade Dave suggest is interesting but I wonder how it will work on your 6 mil plastic.
I didn't see what size your GH was but will suggest you look for shadecloth at Wmart or Lowes or HD, etc. You may be able to find a roll or two that will adequately cover it. When I had the cattle panel plant shelter, I hung shadecloth on the outside of the greenhouse. The temp reduction was minimal but noticeable.
My greenhouse is 16 x 8. I have two black 55 gal drums (half full of water) inside, thinking that would keep it warm enough this spring. It did dip down to 31 inside last night -- if I can trust the thermometer. But, I did notice several of my 50 tomatoes (1-3" tall) that were on the outside edge of the wall had a bit of damage. . . so wondering if my water barrels did any good. I'm sure glad I thought to throw some pillow cases on the tomato plants.
So, maybe I should fill the water barrels full, cause I know there will be a few more freezing nights before this spring is official.
I'll check out the shade cloth, but I do not think the paint shade will work on the plastic. So many challenges!
What is the idea daytime temperature to aim at?
Where should the thermometer be placed? I finally realized -- not in the sun, which is where it was when registering 126. I'm trying to place it in shade. Is that right?
Were your tomatoes up on a table or sitting on the ground level?
Could you feel warmth in the water in the barrels?
Could you put a thermometer in the water and monitor how cold it is?
The reason I suspect it won't be that warm is while sitting on the soil it will stay colder.
Full barrels might do better.
I had bought a smaller piece of shade cloth and thought it was fairly priced. Probably 8' x 20' and if I recall I paid $20 or $25 for it.
Yes, place your thermometer in the shade. I have two, one in the cooler end and one in the warmer end. I did that so I could place my plants accordingly for winter temps. You will also find the temp will be different from hi to low. If you used a heater, the heat would rise.
I don't keep the seedlings in the GH so not certain what a good daytime temperature would be.
I used the shade cloth for years. It will keep the leaves from getting burned but it still made the inside of the greenhouse to warm to work in because the black cloth still absorbed the heat. The Kool Ray is bright white and reflects the heat. My inside temps now are hardly any warmer than the outside temps in the warmest months of the year, and the Kool Ray works perfectly on 6 mil plastic.. I wouldn't think of ever using shade cloth again after using the Kool Ray. If you have any questions, just go to one of your local garden centers or a nursery that uses it and see what it does first hand.
Last night the plants were on the ground. Today I placed most of them on the barrels and filled the barrels at least 4/5 full. I also put a flannel sheet on top for the night. Temp right now (10pm) in greenhouse is 52.3. Outside temp is 53. (These two thermometers never register the same even when side by side.) Tonights low should be mid to low 40's, so I should be okay. Oh, if I use my imagination, I can feel a bit of warmth from the barrels. Tomorrow I'll look for a thermometer to place in it, wondering if my compost thermometer would work. . .H-m-m-m-m???
Uncle, thanks for the info on the Kool-Ray. I'll research that. Sounds good.