Ginger, my GH is 10'x12' and the solar blanket material that covers two sides and the roof measured approximately 22'x12'. In other words it took a 22' long piece to cover that expanse. I would think that a 20'x40' piece would easily cover yours and probably leave enough to cover another side. But don't trust me to be correct, take your own measurements. Karen says her solar blanket material has lasted 10 years and still looks fine.
David, yes the solar blanket material looks like bubble-wrap (see my previous pics) but is way heavier (20x?) and is UV resistant. I have only had my material covering my two GH's since November, so time will tell whether I leave it as-is year-round. If I understand Karen correctly, she leaves hers' on year-round and it has lasted ten years.
