I just put up a greenhouse in SW Washington state, and used the single-wall polycarb. It's readily available and easy to put up. The double-wall is very hard to find, and is a major science project to put up, because of all the aluminum extrusions. But if you are planning to heat the greenhouse in winter, you should probably use the double-wall, since its thermal resistance is so much higher.

I am giving serious thought of using a double wall pool cover. Birdwidow mentioned she had one, but so far I have been unable to see it on the net.

1eyedJack and the Dawg

John,
The major difference between polycarb sheets and twin wall panels , besides the cost, is the heat loss factor. Single layer- 1.20, twin wall- 0.65. That means about a 40% savings on winter heating with twin. The light transmission is: single- 87%, twin wall- 82%. The framing for the twin wall can be as simple as a slotted wood frame. My greenhouse uses this technique, works very well and no metal is needed. Mold should not be a problem with the twin wall, if the ends are sealed according to the recommendations.

Why not check out your nearest pool supply Place? They may have just what you need.

How often will you need to heat? How many nights do you have below freezing? Do you plan to actively grow plants in winter or just warehouse them until the next season? I am just warehousing my plants. In Alabama zone 7b/8a, I have about 30 nights per winter when I need heat. I used single ply corrugated plycarb. My greenhouse is 9'W x 16'L x 8'H with a 9'Wx6'Lx10'H addition. On a 26-28 degree night a 10K BTU heater keeps it above 50 degrees.

tsmith,
I think our winter climates are similar - only maybe 10-15 nights of freezing temps. Mostly I want a GH to extend the seasons, warehousing, and getting seedlings started in spring - and if I find that the cost of heating is not prohibitive, then I'll try some winter lettuce/toms, etc. What would you estimate your winter GH heating bills to be?

Thanks all for your replies.
john

Hi, John. I use K-1 kerosene heaters for heat. I have a primary and a backup heater. If we have a really cold nights I will use both heaters. I keep a log of my heating costs. I've used about 40 gallons of fuel this winter which costs $3.50 per gallon or about $140. There were maybe 10 nights when I used both heaters. I usually have about 30-40 nights when temps dip below 35 degrees. When I first started using kerosene it was 99 cents a gallon. Heating bill for the fiorst season was about $45. How times have changed. A greenhouse is definitely not a poor man's hobby,