A year and a half later, and I am still pondering the initial topic of this thread, especially how to avoid the inside of the greenhouse getting colder than the outside. My best guess right now is humidity. I have troughs and bins of water through my greenhouse. You can see in the pic below, I sprayed down the gravel a little as well when I was filling the bins. Outside temperature was in the 40's, and it was almost 90 inside, and more importantly very humid. Condensation forms on the inside of my plastic. The layer of water droplets has a nice diffusing effect on the daytime sunlight, but I also think it adds an insulating value, and possibly even a shield against atmospheric cooling radiation. The humid air may help, too. The cold air outside is very dry.

Lows have been around 30 the past few nights. All of the outside tender plants froze. I have a few ghost pepper plants with my greenhouse mums. They can be my canary in the mine shaft as far as plants freezing. I'm not using any heat, and my single layer of old plastic is far from perfect, but so far, so good in the low temperatures.

cole, I also cover inside of my high tunnel with floating row cover.

I've wondered about why th high tunnel would be colder?

Yes, the low tunnel covers are great over benches. Unfortunately, I have too much to cover up at the moment. In the spring when I have to do that, my seedlings are small, and I can stack them on shelves to take up less space under the cover. My plants really hate it when the leaves touch the plastic, though. Maybe it's the high humidity, but they will rot overnight. Everything is fine as long as plastic doesn't touch leaves.

And yes, it is extremely counter-intuitive that a plastic structure can be colder inside. I am still going to accept it, even though it's hard for me to understand. What I am most concerned with is trying to create a different environment in my greenhouse than the high tunnels at Cornell. I have a thick gravel floor, which helps. I also have a lot of water, humidity, and various mass filling the structure. Those things make me a little different than the typical high tunnel, so I hope those differences can help avoid the colder-than-outside phenomenon. It's fun to experiment, at any rate. I think the subject is important for high tunnel growers.

Indeed it is an important subject. We're having a very warm fall, but we did have one night that dropped to -2 C (per weather station). It killed every tomato plant in the farm greenhouse, brown stick dead! Many of the outdoor crops are still standing tall and doing fine - even the lettuce (granted it is in the lower field so may be microclimate differences too). So it did make me think of this thread. We naturally expect a few degrees extra warmth in the tunnel but it can be the opposite on those frosty nights!

Forgot to mention, it was quite dry and minimal watering the past month in the tunnel. So opposite of your water strategy, Cole.

frost can form at temps well above freezing.
Have you tried covering the plants inside the hoop house? (with fabric as though they were outdoors).
KarenO

My local garden center has an indoor greenhouse within their sales floor greenhouse and covers malbar spinach seedlings (which are on a heat mat) with fabric row cover as above to gain that extra degree or two for germination in spring.
Probably a good technique for hot pepper too.

- Lisa

Cole, thinking outside the box, but in similarity might help?

The coldest places inside a home (Especially an older home) in the winter is under lower cabinets, under the sofa, under beds, etc. Some have simple fixes like under a sofa/couch - you can roll up bed sheets to stuff under the edges to stop the cold draft. The kitchen and bathroom cabinets you open the doors so they can warm up to keep the waterline from freezing. On VERY cold nights, I don't just open the cabinet doors under the sinks, but I also put a fan near them blowing warm air in. Circulating air does a lot. It can cool things off when it's too hot as long as windows or doors are open - or an AC is on, but in cold temperatures, fans will circulate the heat above to the lower areas.

I can't see this being different in a green/hot house. Of course there would have to be heaters too.

Yes. I think having plants on benches helps avoid the coldest air, simply by being up off the ground. Covers are great, when I can fit everything under them. They are especially great with a space heater. I can still be 40 degrees above the outside temp at night, in a small space with a heater.

And bower, that is a story that confirms the topic of this thread, everything in the greenhouse freezes, but the outside plants are fine. I'm sure most people who hear that story find it hard to comprehend. It is indeed odd.

I'm also wondering if that effect happens in structures covered by an inflated double layer of plastic, instead of just one layer. The cushion of air between the plastic layers might be enough to stop it.

I was going to say a double layer would be your best option. Not only for freezing temps but it would strengthen the structure tremendously. The system is cheap as well.

I've seen the same benefit of raising off the ground in my greenhouse. Cold air sinks, so it is coldest next to the ground. I've also confirmed that by moving my thermometer around. :yes:
You may be right about the double layer. :?!?:
My greenhouse is attached to my home and double glazed with glass - which is different I guess. But it is regularly 10 degrees C warmer than outdoors in the morning. I have seen it get down to the same as outdoors in extended periods of cold and cloudy weather, partly because it doesn't have a proper skin on the west wall IMO, but it is never colder than outside. :no:
Then again, glass does behave differently in so many ways.

I started with an inflated double layer when I first got the greenhouse, which will be seven years ago in the spring. I had to take a layer off, because I used diluted latex paint as shade paint, but then couldn't get it off. I do still have a spare piece of plastic from my high tunnel, so I could throw it on the greenhouse if I wanted to.

I never saw any difference in strength of the structure with the double layer, as it is an expensive Clearspan brand building, and quite sturdy. I believe the Amish have a design for an inflated double layer that uses the wind, and a swiveling wind vane cone to catch it. That design does add strength, because the inflation pressure is greatest during wind gusts, which is when that strength is most needed.

A properly inflated double layer is as hard as concrete. They can carry huge snowloads and up to 100mph winds. The college's got tested recently by a EF 2 tornado and were unscathed. Albeit they weren't directly in the path but wind speeds near 100 were reached in their location.

my greenhouse in zone 3 had twin wall construction. Another person knew had single wall polycarbonate. My greenhouse was easy to regulate the temp in, His was freezing. As you state, I think 3 things would be important : plants up off the ground, the ground was easily degrees colder than the temp in the middle of the greenhouse. If you have a thermometer in there, put it at the level where your plants are, second layer of insulation either cover the plants or if feasible, a second layer of poly over the whole hoop house but I think that would be hard to secure. might be better to line the interior so the wind doesn;t play havoc.
Even a small heater easily kept my greenhouse well above zero with temps of -10 outdoors. The friend with single wall could not heat his effectively at all.
how about rolls of wide clear bubble wrap to line the inside of the hoop house. That would add some significant R value. but the light would still be good.
KarenO

I believe ya'll about it being colder, just don't understand the "why" of it. Puzzles me.