High Tunnel Gets Colder Than Outside

[clkeiper]

Our dbl walled structures are much sturdier than the single walled ones we have. if our electric goes out the fans are off and there is a lot more movement to those structures than normal.
I also see a big difference between the small structure and the huge one for temperature. the larger the air mass the more stable it is. my 26x96 has yet to freeze. I keep my plants covered because I am expecting it to happen any day now but it hasn't dropped below freezing yet. as a matter of fact I have a cherry tomato from Burpees in there yet that is amazing. still green still fruiting over 6' tall and flopping down to the ground... now it is getting some grey mold on it due to the cold damp but I ma still picking tomatoes off of it.

[berryman]

Exact same thing happened to me years ago Cole. I had a 16X30 ft high tunnel with one layer of plastic and one night in the fall the humidity was high and the temp went down to low thirties. Plants outside were okay but there was heavy frost in the greenhouse...killed everything!

[POAJoan]

I use 16 mm crystal clear solar pool cover over the hoop house over a layer of 8 mm clear plastic.
It's usually 4-5 degrees warmer than outside. Solar cover usually lasts six years.

[Cole_Robbie]

Thank you to all the people posting here. It is valuable to hear your experiences.

28 degrees as a low for me tonight and tomorrow night. I stapled extra layers of plastic inside the ends of my greenhouse, around the doors. I also tacked up some strips of old carpet, about 4' wide, covering the area from the hip board to the base board, on part of the south side and most of the north. That area has always been a problem with air leaks. My roll-up sides never did work correctly. If I re-covered the greenhouse at this point, I would only have plastic over the top. The sides would be plywood or osb, that I could open and close like shutters.

I have about a thousand gallons or so of water in various troughs. I should have filled them up earlier in the day, though, to catch today's sun. I will probably go place a small electric heater out there late tonight. It's only 1500 watts, but might make a degree or two of difference.


edit: forecast changed. 25 the next two nights.

[clkeiper]

Yeah, ours is 22 tomorrow night. I don't think I can keep anything when its that cold. I am bumming. I still had oodles of tomatoes out there ripening up very slowly. I picked about a bushel of green to light red ones today.

[Cole_Robbie]

Heck with it, I'm going to cheat. I just went to town and bought a 50k btu forced air job site heater that runs off of diesel fuel.





I just fired it up. The thing is like a jet engine, huge amount of heat. It costs about a dollar an hour to run.

[Salsacharley]

A dollar an hour can get real nasty if you need it very long. 4 hrs per day for 30 days is $120.

[bower]

I've tried to heat my greenhouse and it couldn't be done. Might as well try to heat up the outdoors. But if you have a good skin on it, why not.

I do think covering is the most important thing when you get those clear still nights when the cold air sinks and gets trapped inside the greenhouse.
In my place the wind is a big factor because it's a leaky structure. I can up the comfort level quite a bit by hanging row cover around the walls to block the drafts if it's windy, which is better for the plants when light levels are low and the cold is pushing in horizontally instead of vertically.

[Cole_Robbie]

I checked on the heater about two hours after starting it. The inside of the greenhouse was 20 degrees warmer than the outside. Unfortunately, I seem to have triggered a safety shutoff switch at some point. Everything was ok, though. I am going to try to mount it in a different place. It stinks of diesel, and I don't like being around it when it is running, but it is certainly powerful. I had the thought today that I could easily heat both my high tunnel and greenhouse with just that one heater. I was planning on using the high tunnel more like a greenhouse, anyway, with tables of potted plants. I may have to mount the heater externally, to give it enough space to not overheat.


And I am hoping to not have to run it much, maybe 4-6 hours on a cold night, but at this point that is only occurring once or twice a week.

[clkeiper]

Cole, do you have some way to vent that? the exhaust can and will kill your plants. unless you have a ventless furnace/heater you need to run it outside or have the structure somewhat open defeating your purpose.

[Cole_Robbie]

It certainly does stink, like a city bus. I had the realization that I was probably burning up all my oxygen, which is what made the heater go out. I will have to experiment with ducting, maybe mounting the heater externally.

[bower]

You be careful with that Cole. People die from carbon monoxide poisoning using appliances not vented outside - we had fatalities a couple years ago on a January power outage. It is so not worth the gamble!

[Cole_Robbie]

The heater stinks badly enough that I don't want to spend any time around it. I think propane would probably be a lot more dangerous, because it burns cleaner and does not drive a person away with the smell.

I didn't know the fumes could kill plants. It does make them smell a little bit like a city bus.

I would not try ducting the heater if anyone lived adjacent to or near the greenhouse, but I have forty acres of space around me. If I had a fire, it wouldn't be much of a loss of property. I'm about done with mums for the year, so I might not run the heater again until spring.

[KarenO]

Carbon monoxide would definitely be a high risk with any fuel burned in an enclosed space. Venting would be required for any fuel burning heater indoors.
KarenO

[Cole_Robbie]

I must have done a good job of sealing up the greenhouse. The plastic is old, and far from perfect. If I had skipped that step, the heater might have been able to draw in enough fresh air from all the leaks.