High Tunnel Gets Colder Than Outside - Page 4

BlackBear · May 23, 2016

Quote:

Originally Posted by Cole_Robbie

The best idea I had for wood heat for my greenhouse, which I didn't have the money for, was to run pex tubing through the gravel floor and use a wood stove to heat the water that ran through it. There's a product I looked at that is a hollow metal element to install in a wood stove so that it will heat the water inside. It is possible to circulate water without an electric pump if the stove is slightly below the system, like an old-fashioned boiler.

The best solar idea I saw is called active solar. Instead of water-filled tubing through the gravel floor, air ducts pull air down into the rock to heat it during the day. At night, the fans reverse, and pull air back up out of the warm rock.

geese what a great idea....Pex ! I have bundles of pex (Blue, Red, White) but I actually use it for "non plumbing " purposes ...LOL

I see the problem is justifying the next "upgrade" step to the full system green house from ..the unheated solar only etc.....

Most can get stuck at the point of only using temp "auxiliary heat sources" to tide them over the sticky times of the extended season.

I have a solar fountain pump in my place I am trialing as development ...it is a cheap one that only is useful at about 1 ft or less but it does move water at the hottest/brightest sun part of the day ......one could make a step system with 4 pumps to get 4 ft of lift etc.(reread your Bernoulli ..LOL) I believe all these are readily convertable to 12vdc

with 12vdc one could have marine battery back up power for circulation as well

I guess a Big green house would take a lot of these cheap pumps ...but it could be done ........

Wouldn't it be nice to have a solar circulation pump with a woodstove optional thermal upgrader in the line ....LOL

clkeiper · October 30, 2017

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

imp · October 30, 2017

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

Cole_Robbie · October 30, 2017

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.

bower · October 30, 2017

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.

KarenO · October 30, 2017

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

greenthumbomaha · October 30, 2017

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

AlittleSalt · October 31, 2017

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.

Cole_Robbie · October 31, 2017

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.

BigVanVader · October 31, 2017

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.

bower · October 31, 2017

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.

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.

Then again, glass does behave differently in so many ways.

Cole_Robbie · October 31, 2017

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.

BigVanVader · October 31, 2017

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.

KarenO · October 31, 2017

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

imp · October 31, 2017

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

October 30, 2017	#2
clkeiper Tomatovillian™ Join Date: Nov 2011 Location: ohio Posts: 4,350	cole, I also cover inside of my high tunnel with floating row cover. __________________ carolyn k

October 30, 2017	#5
bower Tomatovillian™ Join Date: Feb 2012 Location: Newfoundland, Canada Posts: 6,794	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. Last edited by bower; October 30, 2017 at 06:05 PM. Reason: add

October 31, 2017	#14
KarenO Tomatovillian™ Join Date: Oct 2012 Location: Vancouver Island Posts: 5,929	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 Last edited by KarenO; October 31, 2017 at 01:06 PM.

October 30, 2017	#3
imp Tomatovillian™ Join Date: Apr 2009 Location: Wichita Falls, Texas Posts: 4,832	I've wondered about why th high tunnel would be colder?

October 30, 2017	#4
Cole_Robbie Tomatovillian™ Join Date: Jun 2012 Location: Illinois, zone 6 Posts: 8,407	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.

October 30, 2017	#7
greenthumbomaha Tomatovillian™ Join Date: Nov 2011 Location: Omaha Zone 5 Posts: 2,514	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

October 31, 2017	#8
AlittleSalt BANNED FOR LIFE Join Date: May 2014 Posts: 13,333	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.

October 31, 2017	#10
BigVanVader Tomatovillian™ Join Date: Mar 2014 Location: Greenville, South Carolina Posts: 3,099	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.