I started with an inflated double layer when I first got the greenhouse, which will be seven years ago in the spring. I had to take a layer off, because I used diluted latex paint as shade paint, but then couldn't get it off. I do still have a spare piece of plastic from my high tunnel, so I could throw it on the greenhouse if I wanted to.
I never saw any difference in strength of the structure with the double layer, as it is an expensive Clearspan brand building, and quite sturdy. I believe the Amish have a design for an inflated double layer that uses the wind, and a swiveling wind vane cone to catch it. That design does add strength, because the inflation pressure is greatest during wind gusts, which is when that strength is most needed.
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