No you don't have to deflate two layers of plastic in a heated greenhouse. You shouldn't have to, anyway. I never have, and I run a commerical range. It depends on what you are growing in your g'house. I run winter runs on things like poinsettias and Easter lilies as well as the long-production items like liners for pots and hanging baskets, and the light quality is important to crops grown in winter at your latitude. You can keep them 'alive' or run some foliage in low light, but it can really bring on quality and disease problems in winter if you decrease light transmission too much, like three layers would do, or running with shade. I even have to change plastic, whether it's damaged or not, with double layers because of normal clouding and resulting lower light levels.

You may lose in solar gain, what you save in R value, or you may not. It's very difficult to document savings, because it's very difficult to compare one heating season with another, because of the typical solar gains in sunny winter weather, even if the 'degree' days in charts say they are the same for household heating.

I found you can do this, but let me give you some IMPORTANT tips. I thought you could just tack another layer over the top like you do a shadecloth with another piece of wiggle wire, but actually you have to pull the wire holding the two pieces you already have, and fasten all three pieces in with the same wire.WHY? Because the wire in between the first two layers and the third layer you're trying to add causes too much air to escape--and your new added on second inflation fan won't inflate the third layer. But other than that little problem we had to work out, it's working great.
I just overwinter tropicals and baskets for spring starts, I don't care much about them growing in this house. I have noticed that my LP is lasting longer, even with the cold weather we've been having that's colder than last fall.

Hey,

Take the shade cloth off if you can and let all the light in during the winter. This helps to keep it warm during the day with no gas heat. In your area it would probably make a big difference with a 3rd sheet of poly. I read something about 3 layers of poly once and it gave some percentages of savings. I think it would be worth it. I am thinking of doing the same next time I change poly. Also make sure your run your fans 24/7, this will knock out any dead air pockets and help keep the place warm. Also, remember that a full greenhouse heats less than an empty one. The more space you have without plant material, the more it will cost you.

The shade cloth is off. I take it off every fall.
I don't think it's a problem about dead air pockets or the greenhouse not being full. Thankfully it's not as full as last year, but if anything grows very much, then I'd better be losing some weight...lol...wrong time of year for that....
You always want to run your fans 24/7--especially if your greenhouse is full, I hope everyone knows that.
I just wanted to pass on this tip, that I couldn't find on the net anywhere. If you want to do triple layer plastic, fasten all your layers on together so the air will stay in. Don't think you can put the third layer over the top with a wire in between. It doesn't work! I wish someone had been able to tell me that, because we just now figured out what was wrong when we fixed it a week ago. And it would have been lots easier to do that when it was 60 degrees out instead of 30!

I know these are from 12 and 13yrs ago, however here goes. 3 layers - issue- light transmission and keeping the heat in. IT WORKS - i installed 3 layers (top layer is what i call my sacrafice layer do to our severe weather in Lake Tahoe/ Carson City NV. i kept the thickness of either 6 or 8mil each i can get specifics if you like. i also installed black steel barrels filled with water on the North end as thermal mass and escavated aproximately 8 feet down below frost line. i also have two 250gallon totes filled with water. the sun heats up the barrels and during the day and at night the barrels release heat slowly through the night, along with the 8 feet down im using the natural heat from the earth. i ran spiral HVAC Duct to the green house from the main house. natural gas is way cheaper. the green house gets too hot late morning so fans come on to compensate. first year second design of greenhouse. oh i also have a saw tooth roof with motorized reflectors to maximise sun rays and have led grow light to compensate. im still learning but i sure wish i could find a cutting edge greenhouse book since summer is around the corner and im hoping i dont get a 200degree hot house. im thinking the 8 feet down will cool it a bit but we shale find out quickly. any insight would be greatly appreciated. GM

I would like to know what the difference in heating costs were between 2 layers and the 3 layers of plastic. I currently add this greenhouse bubble insulation to my greenhouse every winter and it works quite well, but going with 3 layers of film would be less maintenance for me.