What is the reasoning behind having the vents at the bottom rather than at the top? I have similar auto vents in my ("normal") greenhouse but they open windows in the roof. They can be open for many hours at a time so I imagine the temperature in your greenhouse could climb considerably after your low-level vents open.

The goal is to have natural convection getting air flowing everywhere without fans. For this to work well you need to have everything sealed up tight. Have vents at the lowest possible place and the top vents be at the highest place.
Since I am in a hot climate I have a lot of venting.

The water in the drums will also help cool the greenhouse in the summer.

Cord of Smart greenhouses in CO. does not even use a fan in the summer and has great air flow for the plants.
He says on a hot day he will go into his greenhouse to cool down.

Here is where I will be adding vents next the whole way across the top of the back.

http://ecorenovator.org/forum/attach...1&d=1451916123

There will be about 10-12" between them to allow side venting. The two H's up top will be solid.

It was cold yesterday evening and last night South Carolina cold anyway.
I still have plastic for the back vent and only one row of 55 gallon drums. I still am very interested how the greenhouse is performing at this stage.
Yesterday at sunset it 35.6 f and windy it got down to 23 degrees here are the charts. The greenhouse did not go below 50 degrees.

Keep in mind it was very sunny yesterday

http://ecorenovator.org/forum/attach...1&d=1452002918

http://ecorenovator.org/forum/attach...1&d=1452002918


Later down the road I will be putting in probes so I can show graphs on how it is performing. I will monitor greenhouse temp and fish water temp.

10:30 am windy and it is up to 32 degrees but sunny. This is the greenhouse temp.
http://ecorenovator.org/forum/attach...1&d=1452008048

Woo hoo! Is that magic or what? Just don't open the door for too long.

What is keeping inside the greenhouse so warm overnight?

Jeff it does seem like magic. You are correct any air leaks and that warm air will go away and not come back.

I have eight 55 gallon barrels on water and about 1,000 gallons of fish water. The black barrels heat up during the day and release heat at night.
The back walls and the roof are insulated with 4.5” Dow blue board foam insulation. That makes 22.5 R all OSB is covered with Grace Ice and water to prevent any air getting into the building.

This is better insulation and air sealed than my house.

A little cold and windy to work outside on the greenhouse. I have started painting the inside it all needs to be white to reflect as much light as possible.
I want to paint the back wall before the pallet racking goes in it will be so much simpler I should have done it before the first row of barrels.

http://ecorenovator.org/forum/attach...1&d=1452086983

http://ecorenovator.org/forum/attach...1&d=1452086983

Today I went to the specialty steel shop. I picked up the rest of the steel for the back vents.
I got my welding gas filled and my new torch gas cylinders filled.
All ready for the weekend.

It is a little cold and windy to work out side. I painted some more this morning. I will be painting everything white to reflect the sun rays. The barrels will stay black to soak up the sun.

I painted everything that I over the grow bed before I built it. But wood that was not over a grow bed I left unpainted. Now it is time to make it white before the pallet racking goes in.

http://ecorenovator.org/forum/attach...1&d=1452178784

Reflector backing?
 

Those barrels are going to emit IR in all directions.
When you might want to direct the heat forward, and not into the wall.?.

Perhaps a highly reflective surface on the wall behind the barrels will prevent
heating up the wall, and bounce the heat back into the barrels on that side.
Keeping them warm longer and allowing more BTU emission forward.?.

When I installed this under-counter LED, I was able to increase the light on the counter-top work area using foil tape.
I looked for the well lighted areas underneath and slapped on some tape.. It worked great..
http://ecorenovator.org/forum/attach...1&d=1452181557