Green House Inside Green House

I don't see this as being necessary....If you want to conserve heat use some large stones or concrete to regulate temp, or maybe a heating pad with thermostat set to lowest setting on a timer...with two layers of plastic you will restrict sunlight reaching plants...

There are companies that make a double-walled polyethylene material for greenhouse walls. Probably less expensive and better than what you are considering. Here's an example of the material:

http://www.greenhousecatalog.com/category/solexx-greenhouse-covering-panels

To extend the length of your growing season, it's not uncommon to build a much smaller 'greenhouse' inside the greenhouse, which would be just big enough to accomodate seedlings for an early start.

The mini greenhouse can be opened on a sunny day to keep the temperature moderate and let more light in. It can be closed at night, and if there's a risk of frost, kept above freezing by filling a pop bottle or two with hot water and tucking that inside the "mini-greenhouse".

The design that you suggested sounds like a waste of space for roughly double the material cost : one foot all around; and will reduce your light transmission all year round. Since the area is large, there's no "quick fix" to do when the temperature gets colder than expected, as can be done with the 'mini' setup. You mention 45 and 30 F, but don't say whether the wind ever blows. Wind causes heat loss by convection off your 4 mil plastic surface, much faster than a still cold.

You might best talk to growers in your area, who will be familiar with the conditions and performance of greenhouses in that circumstance. I suspect that solid foam insulation on your north side and/or prevailing wind side would give better insulative value than a foot of air between two layers of 4 mil.

Good comments so far. I was just wondering about that.Then I had another though.Heat the ground inside the green house. With the black plactic pipe 1/2 comes in coils for got the name of it,but it's cheap. Take that and put in about 100 or 200 feet about 3 inches under the dirt, Then connect these to a coil outside with about the same amount 100 or 200 feet, put in a mix with ant:and H2,0. With a small pump running of a temp. sensor.The sun would heat it up. LOL.

Well... the sun wouldn't heat my greenhouse that way in the winter. Solar for winter is a difficult proposition. Too much cooling.. and not enough heat gain. Days are short, and snow gets in the way!

The idea of using heat storage mass is always a good one, but you must realize, in order for this to work, your greenhouse must be insulated from the outdoor cold or your losses will counterbalance your gains when it gets too cold.

There are some interesting designs that use earth sheltering and insulated foundations to contain heat storage mass. A concrete or stone foundation is insulated on the outside so the foundation also serves as storage mass. A massive chimney fits well with this sort of design, and is a better way to heat up the mass in the winter than solar/water would be - at least, anywhere there's seriously winter.