Moke: good ideas, but I really don't know about additional fans, vents and tubes. There are a lot of panels in a 12 X 16 ft. GH. But air circulation in general won't be an issue, with 4 roof vents on automatic openers, 2 powered intakes set at the base at one end and an exhaust fan set high at the other, coupled with my "ultimate continual air movement weapon": 2- 52" ceiling fans, turning in opposite directions 24-7, will privide a lot of circular air movement. If the surface of the GH wall panels are perfectly dry, and the inside ones are sealed on all sides; if they were then sealed against the GH panels, would they STAY dry? Or- leave a few inches at the tops and bottoms open, to allow moisture a place to run off? Or- and this is one my husband and I discussed: cut the inside panels to fit just between the channels on the polybars, as not to block them, and seal the interior panels against the metal, creating dead air space between them and the exterior panels? I could also use that shrink plastic used on windows, but I'm really looking for a more permanent solution. Frankly, if I had thought more about this issue, I'd had given up my desire for the sloped roof on my lovely Cross Country, and just gone with triple wall polycarb and now, I'm sorry I didn't. But... the R value still isn't all that different between double and tripe, until you get into the ultra heavy, commercial weight polycarb panels and for that cost, you might just as well blow it all, and go with thermopane glass. Oh well, the months ahead should give me some time to develop more nutty notions. The GH gets some decent dappled shade, and between the trees, misters and all those ventillation gadgets, I'm hopeful of keeping it reasonaly cool, but do want to come up with something better than draping the whole thing with that heavy solar pool cover again. All notions nutty sounding or not, are welcome. BTW: my "electrician" (and carpenter, plumber, etc,) is my husband, and one of the reasons why I'm anxious to find a better solution than climbing ladders to haul that cover onto the GH. It worked, but oh, what a chore, and the wise old wife simply does NOT tell her 73 year old farmer he's too old to climb ladders; she just tries to avoid the need for him to get anywhere near one.

A good reference book about building greenhouses in our climate is Gardening Under Cover: A Northwest Guide to Solar Greenhouses, Cold Frames, and Cloches by William Head. It discusses siting, styles, and heat sinks. Also shading during summer if you want to use it year round. You need clear for winter use as we have such low lighting to keep things growing. The KCLS has it if you're in King County. I scanned the greenhouse structure page, so email me if you'd like me to share it. It talks about pitch of roof, type of roof & functions. Best wishes & post a photo for us to see. Corrine

Thanks for your help. I am using a heavy duty plastic which is supposed to last a few months - long enough for what I need at the moment. Hopefully by next winter I will have a bigger and better one done with uv plastic but that has to wait. I plan on putting 30% shadecloth on the roof over the plastic to cut the sun down when it gets hotter. As it is only a temporary one I don't want to go to any great expense so am trying to do it with what we have lying around (there's always plenty of junk around here!)or something cheap. I've thought about the plastic ties and fencing clips (maspro clips) but I wanted to avoid putting holes in the plastic. The only thing I can think of is using small pieces of polypipe cut into semi-circles and used like a washer with screws. The gal pipe is about 2.5cm (1 inch) and the poly we have is 3.5cm - I had to actually measure it to the disgust of my welder husband - as he uses steel everyday (makes horse floats and trailers) he only needs to glance at it to tell the size. I prefer to work with wood, I hate working with metal that's why I don't really want to screw into the gal pipe - I always have trouble with it so have to wait for the other half to do it (all you girls out there would know what it's like waiting for "the man" to do something!). Any suggestions are greatly appreciated.

The only greenhouse film I ever used was 6 mil which is plenty thick. It comes with a 4 year warranty, but will often last 6-10 years. The point that nobody mentioned is that the films sold in box stores are almost always not UV protected and thus will rarely last one season without self destructing. What you need is the greenhouse grade film. Those films are available from certain landscape supply centers and from farm/greenhouse supply centers. These are heavy enough that you probably would be better to pickup rather than have shipped. My latest 42x 100ft. roll that I picked up at Central Penn Irrigation Supply a few weeks ago must weigh at least 100 lbs. since it took two of us to lift it onto my truck. I have seen Lowes carry solar pool covers that are UV protected and might be suitable for cold frame covers if they are the clear version.