High Tunnel Gets Colder Than Outside

[Cole_Robbie]

That graph tells the story. The full article about it is here:
http://www.hort.cornell.edu/hightunn...isons_reid.pdf

http://www.hort.cornell.edu/hightunn...stic_study.pdf

I'm still struggling to grasp the physics behind it, but on clear nights atmospheric radiation can make a high tunnel drop below the outside temperature.

I thought this idea was worthy of a thread, for everyone growing under plastic. Whether you have a high tunnel, or just a few shelves with a drop cloth over them, it's quite possible for your plants to freeze even when the outside temperature never gets to freezing.

[RobinB]

Yes! I noticed this with my greenhouse. It's weird, but true. With a heater on low, I can more than make up for it though.

[BigVanVader]

Interesting, thanks for posting. I'm leaning toward a double layer inflated on mine since the fans are cheap and I already have the extra plastic. The Hort instructor where I work said it is the best way to regulate temps since you have a buffer to slow temp gain/loss. Just need to get power ran to it first.

[Cole_Robbie]

I started with a double layer, but the diluted latex paint I used as shade paint wouldn't come off, so I had to take the top layer off after the second season. I'm on year five with the layer that is left. It's barely holding together. I had to sew up several splitting seams with fishing line.

6 mil plastic has an r-value of .6 - I think the inflated double layer is like R1.3

If you use an inflation blower, use outside air to inflate, not inside air. Hook a hose to the blower's intake and connect it to a hole to the outside. Inside air has a higher humidity, and is more likely to cause condensation between your two layers.

[clkeiper]

Originally Posted by Cole_Robbie

I started with a double layer, but the diluted latex paint I used as shade paint wouldn't come off, so I had to take the top layer off after the second season. I'm on year five with the layer that is left. It's barely holding together. I had to sew up several splitting seams with fishing line.

6 mil plastic has an r-value of .6 - I think the inflated double layer is like R1.3

If you use an inflation blower, use outside air to inflate, not inside air. Hook a hose to the blower's intake and connect it to a hole to the outside. Inside air has a higher humidity, and is more likely to cause condensation between your two layers.

I don't see how we can inflate using outside air, even though I know that we are supposed to. None of our inflators have a "connector/connection" to go outside. there is a "restricter of airflow" ( I have no idea what this piece is really called. It moves to allow more or restrict air into the layer) cover attached to the intake side. there is no where to put a hose on the fan to draw outside air in to blow it into the layer. None of the ones I have ever bought has been made any different. It would literally have to be mounted outside in order for it to draw outside air in.

[KarenO]

interesting. My twin wall polycarbonate greenhouse is easy to maintain an even temp of 12 + degrees inside to -10 outside temp (celcius) with just a small 1500 watt ceramic heater. The twin wall is the difference I believe although I have no science to back that up.
How about a "twin wall" poly tunnel? would be interesting to see what double layer of poly would do
Karen O

[Cole_Robbie]

I was wrong, the inflated layer is R1.7:
http://greenhousegab.com/consider-the-r-value/

Your twin wall poly carb, according to that site, is either R1.5 or R1.7

Nothing clear has an r-value that is high. The most expensive glass is r2.0 - everything else is less.

[Cole_Robbie]

This is what I have:
http://www.farmtek.com/farm/supplies...roductId=36527

Vacuum cleaner hose attached to the intake of the blower would accomplish the same thing, poke the other end outside.

I've never used inside air for inflation, so I don't know for sure if it causes any problems. I'm just doing what Farmtek and Clearspan recommend.

[henry]

Soap bubble greenhouse.
http://www.tdc.ca/bubblegreenhouse.htm

[Cole_Robbie]

Neat.

Regarding carbon footprint, if you count running grow lights inside, I will use about $60US of coal-fired electricity over the course of the spring. I also have to buy my supplies, and someone had to ship those to me. I will produce about 5,000 plants. About 1,500 of those will be grown on our farm, and I know local agriculture reduces greenhouse gas emissions. The other 3,500 that I will try to sell I think are also Earth-friendly in that my competition is the Bonnie plants from big box stores that are trucked here from Alabama. A lot of those plants die on store shelves, so they have to ship a lot more than they sell.

[AKmark]

Thoughts. In the spring the ground in the greenhouse is still cold, at night with no sun and being trapped it drops your temps. Up here, it is an issue for me for several weeks, my ground temp just hit 50 in the greenhouse. However, it just cost me a lot of gas at night for another week or so. The good part, some plants are waist high already, so I like the early start ups.

[RobinB]

My greenhouse has the inflated walls so it does not fix this weird temperature inversion. I think before next winter I'm going to line everything I can with bubble wrap or something similar.

[PureHarvest]

I'm going to run a radiant tube heater this winter.
Heats like the Sun and is supposed to reduce fuel use by 30-50%
They have been slowly retrofitting all the chicken houses around here with them to get rid of the forced hot air heaters.
Radiant tubes heat every surface including the ground, and then the air is heated from those surfaces by redirecting or convecting.
There are other benefits. Here is a long but interesting paper on RTH:
http://www.rg-cloud.com/RG/manuals/I...ual-lo-res.pdf

[whoose]

Try adding some thermal mass, water in containers, concrete or stone. This will help shift your curves to the right. I have 200 gallons of water in mine.

[Cole_Robbie]

Thanks. I have a 220 gallon horse trough of water, and another trough of about 80 gallons. My floor is about 8-10" of gravel.

If I had the heavy equipment to make it possible, I would have dug a big pond and made a floating greenhouse.