MollyD,

I would highly recommend an exhaust fan, not just circulation fans if you plan to use your greenhouse over the winter. If you don't remove the warm moist air before it cools to the dew point, the condensation will persist. Circulation will help and so will the additional layer but it won't solve the problem. What is your coldest night time temperature? How warm do you plan to keep your greenhouse? How large is the structure?

Mollyd, the advice from gardenallyearlong and cuestaroble is good. I would add that condensation is particularly a problem if it drips on the foliage. Therefore, controlling drips is an important part of managing the condensation. Many greenhouses are designed with internal "gutters" to collect the drips and channel the water off and down, minimizing the amount of drips onto the foliage.

Condensation is caused by the surface being colder than the dewpoint of the air. If you are trying to control condensation on the glazing, add another layer of glazing - crystal clear shrink film is a cheap alternative.

Condensation indicates a large heat loss problem.

Thank you all.

The GH is a hoop house 8' w x 17' long and almost 8 ' high. I've got 6Mil film on the outside and will be adding the solar pool cover on the inside once all the wiring is done so that will fit in with Nathan's advice. I am planning on an exhaust fan. I found a 10" one that was in line with my budget but it doesn't have any shutters. I did find shutters separately. I'm assuming the shutters are installed on the outside and the fan on the inside? Will shutters close well enough to keep the cold out? This is on the west (and windiest side ) or can I install the exhaust fan on the east (more sheltered side) ?
I need to keep night temps above freezing and day temps in the 50 to 60 range as a minimum. I don't want it to go above 75 in the daytime. I have an indoor/outdoor thermometer in there so I can read the temp in the GH from inside my house. This morning it was 33 outside and 37 in the GH (that's with unfinished insulation and no solar cover yet). I have a heater in there set to run at 35 degrees and raise the temp to 45 if need be.
I can bring in outside air but it's very humid here so how much drier will that air be?? I have no benches so far and as I said the floor is pea gravel over sand. Ground water must also be adding to the humidity in there. Should I have covered the floor with plastic before putting in the sand and gravel? If so now is the time to fix this!

Thanks!

MollyD

MollyD,
A key point about humidity is that it is relative to the temperature of the air. Cooler air holds less water than warm air. (which is why you get condensation when the air cools on the glazing). So, in the winter, if you bring in cooler, dryer air into your warmer, humid greenhouse, the relative humidity will go down as the new air warms.
As for the floor covering, if you think that ground moisture is entering the greenhouse, then a plastic layer on the ground under the gravel would help. Be sure you have some slope to the ground so the water can drain away from the greenhouse.

Here is a link that might be useful: relative humidity in greenhouses

Thanks Cuestaroble, now I get it.

MollyD

MollyD,
Regarding the shutters and exhaust fans. They should be located on the leeward side of the greenhouse. Standard shutters usually do not close very completely, but it is probably not economical to use electrically controlled shutters to get a better fit. Just make sure they close as well as possible, and add some door insulation if there are gaps when they are closed.

Molly, I rely on passive ventilation in the winter. I completely seal my motorized intake shutters and cover them with 1" foamboard. I built a box that covers my exhaust fan and put that on in November to prevent infiltration there. I leave a small crack in the screen in the door and leave the roof vent functional. Just some ideas for you.

I have a so-called "Hot House" by Handi-House (Swainsboro, Georgia manufacturer) and condensation dripping is a big problem. The roof and the walls are all made from corrugated (M or W shaped) polycarbonate, while frames are made from either aluminium or galvanized metal (not sure). The manufacturer calls the frame "steel studs" and they seem to absorb both heat and cold, so usually it is colder inside of the greenhouse than outdoors (if left unheated). The floors are wood, raised from the ground. Floor is covered by ground fabric that lets water through in a couple minutes. There is a fan at the end of the greenhouse. The g/h size is 12 x 24 feet. I had to upgrade the receptacles and wiring to 50 Amps as it comes with a standard 110 V / 20 Amp wiring. Receptacles which are a joke for this size of a greenhouse. I have sealed the gaps under the greenhouse with the same polycarbonate right into the soil.

Depending on how you fastened your film you might want to try removing it and putting it back on top of an inner film layer of Tufflite Infrared dripless and light diffusing film. I think you will be happier to not have the hastle of attaching anything from within. Naturally a wire-lock attachment would make this process easier.

I'm using the inner dripless film on 2 larger structures now and although there a still condensation much of it runs along the film to the sides before dripping. Also the light diffusion has been noted by a few researchers to be significant in growth of many crops.

I doubt you will be able to stop condensation but you might be well off to try to control it somewhat.

Condensation is a typical by-product of owning a greenhouse. The commercial suppliers do sell anti-condesate films. Yes, water vapor condenses on the walls, and you can't insulate well enough to keep a side wall warm enough to prevent it in your zone at those temperatures. G'houses are a high-humidity or wet environment. That's why they are built to specific codes for safety. If it's to the point it's causing disease conditions do as custaroble suggests. When I used to run sensitive crops like poinsettias, I'd crank up the heat to increase the load of water the air would hold, then vent out and bring in colder, dryer air a cycle or two before shutting the houses up for the night. No, earthen floors with gravel cover are fine.

I have a simple plastic greenhouse with black barrels holding water, for heat. My greenhouse also has so much condensation it drips. The herbs love it, but my fern has turned light brown. NOT a dry brown, but perhaps over moisturized. What can I do for my fern? I have taken it out, but I'm wondering if it is going to make it. Don't ferns love moisture?