No, the fan used to inflate the space between two layers of poly is called an inflation fan, and that's its only purpose in your g'house. They are just a squirrel cage blower with a modified cage making it easy to connect to plates installed between the plastic.

Whether you want to use a double layer will be implacted by how expensive your heat source is and whether you will be running the structure all four seasons. If you are using the structure as season extender and your fuel is cheap a single layer might work for you, especially in a milder climate. The nice thing about air inflation is that if the two layers of plastic aren't perfectly installed, the air will poof out the difference so that the air pillow is maintained keeping that insulating pillow along the whole structure. If you stuff stuff in between the layers, it will sag, sooner or later, where it shouldn't, and your pillow is lost. It also tends to accumulate condensate between the layers and you could get a wet situation, encouraging algae growth. Not harmful until it gets so thick it blocks light.

Didn't think about the condensation. So on these double and triple layer polycarbonate panels what stops the condensation and algae from growing inside?

The instructions for the rigid sheeting is to seal the open channel ends with something like duct tape before seating them in the U or J channels, themselves a good barrier to water seeping in from overhead. They are essentially air-tight then, which isn't the case with poly where no matter how tightly it's sealed will have air exchange. The inflation fans, of course, will pick up ambient air from inside the greenhouse, and it's always humid in a greenhouse. Having it move freely tends to keep it from condensing, just from the scrub of the air movement. If you seal the plastic off, the air will change up and down with the relative humdity, and any moisture in there when it's hot from solar gain, will settle and condense between the layers when the greenhouse cools off. If you've ever worked a lot in greenhouses in winter months, even the anti-condensate poly film will have water clinging to the inside and 'rain' on you if you have a whip of wind, or something to bump the structure.

I have had inflation fans crap out in late spring, and with summer coming, and a shade cloth on, not been in any hurry to order another one and install it. I can tell you from experience, there will be a certain amount of cruddy algae growth between the layers even when they're just collapsed upon the other. It's not a major deal, but if you want to keep the plastic pristine (and for me, since I am a winter grower I need to do for light transmission) I should have gotten my touche in gear and replaced the fans sooner, rather than later. With the proper care and shade cloths on in summer and an inflation pillow, I have had four year film last as long as eight years before degrading to the point it needed replaced. The film will fail first anywhere it touches the structure, especially if it's metal. Think about how hot the metal gets in full sun, and how cold it gets at night in mid-winter. That air pillow keeps the outside layer from touching the structure and in double poly houses, it's the outside layer most important and gets the most abuse from the elements.

The inflation fans should be installed with OUTSIDE air being pulled in between the two layers of polyethylene.

"We recommend using outside air for inflating double layer poly because it has less moisture and less chemicals which can come in contact with the film."

Here is a link that might be useful: inflation fan placement

Thanks for your help. I originally wanted a greenhouse to over winter some aloes that my wife made me remove from the house because she found a bug on one of them. lol. I lost about 6 due to the cold outside, so now I have a few in my shed wrapped in cardboard with a 60 watt light bulb. They seem to be pretty happy but I'm not. The original aloe was given to me by my grandfather years ago (he has passed now so they have sentimental value to me). After i got to thinking about what all i could do with it I started looking in to buying a pre built one. They cost to much for the size, so i decided on building a hoop house. Anyway I would not only like to over winter my Aloes and a few other plants but try my hand at some vegetables and getting a jump start on spring for plants for the yard. If i decided to go with a single ply of 6mil then i will have to worry with keeping the house warm at night. I work for a propane company so the propane would be a little cheaper than most would pay, however not free, so i would like to look at the fan option in a little more detail. It sounds like it extends the life of your plastic pretty well. Could you recommend a web site (preferably one you've done business with) so i can get a better idea of cost? Thanks again, Scott

Most of the companies on the attached list are good to work with, including AgraTech, AM Leonard, FarmTek, etc.

Here is a link that might be useful: hightunnel suppliers

Is a solar pool cover an option? I see a lot of talk on this site about them working well for heat loss. This maybe a stupid thought but has anyone ever used the cover instead of the plastic to cover a hoophouse? its twice as thick and has air bubbles for insulation.

Yes, it is recommended the air should be pulled in from outside, but if it's minus 23 out, and I have run Easter lily crops when it got that low at night for three consecutive days, the last thing I want is to be pulling in frigid air between my poly when the heaters are cranking full out just to hold a their temperature rise. I do not use aerosol chemicals, or sprays in the houses, just an occasional systemic granular and as said, I've had tremendous luck with longevity on my poly. The particular fans available from my equipment broker were not designed in any way to mount them inside and yet capture outside air. The uptake is all along the sides of the fan itself and the only manifold is the output.

I think you are really wise for going the hoop house route. Always plan on advancing in what you are growing in a g'house and you outgrow them quickly. The larger houses are also more 'temperate' than smaller ones. Don't heat up or cool down so quickly.

I retired last summer, and dismantled several of my structures, but I kept one g'house for what you are doing. I keep the stats just a tad above freezing and moved all the urns and houseplants I use outside, to the inside and they're wintering over well. I also use it to house my newly grafted trees and young fruit whips. I shall crank the stat up slowly as the weather warms and I do veggie starts for a big garden this summer. The gas bill has been rather modest keeping it at this temp. The higher the heat rise on a furnace, the more it costs per each degree rise. Be aware that single/double poly or poly carb..............if you are not planning on heating it, don't expect it to maintain a safe temperature. If I lost my power on a cold winter's night, I could only sit on it for about four hours before they lost enough heat to kick on the generators. They'll not hold much more than five degrees higher than outside with no solar gain.

Thanks again calliope for the help, sounds like i will need to invest in a fan for the poly as well as a heater. Do you use a solar pool cover for your house as well? or would a solar cover interfere with the fan doing its job?

I would only need one in winter, right? Well that's when I need the best light transmission. I grew floral quality potted ornamentals primarily for my winter crops and needed the best light transmission I can get at my latitude to prevent the plants from stretching. No, I don't see one as interfering with an inflation fan at all. Most of what you will or won't need will center around what you are growing or using the hoophouse to do. For houseplants or wintering over tender stock you may not need that light level.

Here's what I have done. Two ridgid structures (you can see the greenhouse within the greenhouse) though i wish i had used something less expensive for the interior frame.

I have no heat and no power. Causes me some limitations but i am happy so far.

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