A 30 ft. X 100 ft. double poly greenhouse will lose approximately 100,00 btu/hour after the sun goes down. Without added heat or other heat storage system, the inside temperature will be about equal to the outside temperature by very early morning.

I just can't agree. Temps outside hit -18F this winter and my soil did not freeze or have frost. (Gh is 46 x 22 inside 50 x 26 outside WIDE double layer). If inside my GH was near -18F i don't think i would have had stuff survive, never mind harvesting spinnach monthly through the winter.

One of these days i HAVE to buy a thermometer...

Thanks for the comments. i am curious about the greenhouse with wide roof layers. How could those be framed up ? I am learning all the time.
- Jean in Mt

Check out a book called "Four Season Harvest" by Eliot Coleman. He's done a lot of work with hoop houses and is from Maine.

Personally, I find about 5 degrees difference between inside an outside with a 30x96 here in Illinois in the early spring. Mine is double layer, but not very draft free.

Interesting article from Cornell on high tunnel temperatures.

Here is a link that might be useful: high tunnel temperatures

Very interesting research article Cuestaroble. I had read it before somewhere but this time I printed it for unbelieving skeptics- of which I am one. In reality we all know that you can keep plants alive better in these structures than without them. The key in whatever method of season extension that you use is to have at least one back-up plan.

Years ago I looked at the cost of purchasing the extra plastic film and what it might potentially save in heating costs. Compared to other costs the film is relatively cheap. I now have two layers on 2 structures and on a third structure, a smaller benchtop greenhouse, I have 3 layers(the middle is solar pool cover- bubble wrap)and I inflate on both sides. My main problem is keeping this structure from getting too hot at night with a small woodstove in it. A few minutes ago When I checked the stove the outside temp was 37F; the inside temp was 95F.

Bigger structures are much easier to buffer changes in temperature at the plant level. My 30' structure is much harder to heat and cool but the plants always seem more content even if temps fluctuate.

I'll never install anything under 28' wide again and I'll always use 2 layers of plastic.

My greenhouse is 32 by 54ft and covered with inflated double IR poly. On clear winter nights the inside and outside temperatures are about equal, as closely as I can measure, soon after sunset. On cloudy nights it can be as much as 6F warmer inside.

I've not seen the colder night temperatures referenced above or the vast increase that some advocates promote. I have very good thermometers inside, both thermocouple and mercury thermometers. Outside I'm + or - a few degrees on accuracy. But I've watched this endlessly for 7 years and if it's way warmer inside I can't see it.

I also set up 1,000 gallons of water as a passive heat source during a cold spell this winter. I thought it was of minimal value. No where near as much heat as needed to save a frost sensitive crop when it fell to 10F outside.

thanks for the comments.
We have decided to refuse the NRCS funding. It would include a financial outlay on our part of nearly $1000, for that much money we can build another passive solar greenhouse. Here in Montana is was 18 degrees last night. Greenhouse was 38. There is no way we can expect that much protection from a plastic structure, and such temperatures are common here.
It has been great to read the inputs and hope the conversation continues. Thanks.

I have a 15x30 dw-poly - I found a used 75,000 btu furnace. Now understand at 6500 feet in Colorado it extends my season. $12 a head for lettuce in December doesn't fly.

Here is a link that might be useful: Com'on in

We have a little more than 25k square feet of cold frames, and they hold minimal heat at best. In a central CA location with pretty moderate winter temps (night time lows in high 40's) they require pretty substantial heating to maintain a 58 minimum temp. If i recall correctly we have a 150,000 btu heater in each one to maintain acceptable growing temps (20x 100 cold frame). They work pretty well for cold crops such as freesias, callas, etc but for in general a standard greenhouse with raised benches seem to work much better. The good thing about cold frames is they can handle high winds quite well. The ones we have are basically located in a wind tunnel where we regularly have 10-20 mph winds and storm winds in the 40-50 mpg range and with new poly they are pretty bullet proof for a few years.

Hello. I want to do my self duoble layer inflation fan on my greenhouse, have someone detail intruction how can i to do it? In very details? Pls. In google not found any good info about this.