Humidity in the Greenhouse

[Joel143]

With the change in weather here in southern Louisiana at nights been dipping in the lower 40's upper 30's for a few hours then 50's, 60's and even 70's for the day time highs. I have majority of my cactus in my greenhouse. This will be the first winter that they have been in a greenhouse. Prior winters they were on my front porch covered during the frosty nights. But now beng larger plants, as wll as collection, the greenhouse was the way to go for me. My question is can I keep a fan runnng, even with the heater for the colder nights, to keep down the humidity in the greenhouse? We do get quite a bit of rain during winter time and am alittle concerned about the humidity doing harm. During the summer it was no problem with the fan and the open ventalation windows keeping humidity in check. But now being closed...well....I dont want the humidity to linger and/or go up. So am hoping a fan running wouldn't be a problem, even when the heater is running.

Any input would be greatly appreciated.

[iann]

Running a fan won't reduce the humidity although it will help to reduce condensation and stop problems like botrytis. Moisture tends to build up in stagnant spots near any surface and particularly in enclosed spaces like between spines. The plants themselves are continually giving off a small amount of moisture that makes things worse. A fan moves the air about so moisture won't build up in those spots.

To reduce humidity overall you have to ventilate, ideally only when the humidity outside is lower than inside. This is difficult when it is cold and damp outside with no heating from the sun, but very easy if the greenhouse gets heated by the sun. Cold air holds very little moisture, so bringing cold air into a warm greenhouse will drop the humidity dramatically., but it would be expensive to do this with your own heater.

At night it will inevitably get more humid, but this is fine and something that cacti are adapted to. Just try to remove condensation and drop the humidity during the day.

[Rod Smith]

Humidity is a potential problem to cacti in an enclosed space such as a greenhouse, as Ian has commented. I have a small greenhouse of 8 foot x 6 foot and have two fans. One is on permanently blowing unheated air. The other is set to operate at around 5-6 degrees C (about 42 deg fahrenheit). I live in a particularly damp part of the British Isles and last winter I installed a combination thermometer & hygrometer. During the winter it consistently showed maximum readings in the high 90% and regularly above 70%. Of course, when the temperature drops close to freezing and water vapour begins to condense, the reading will approach 100% but I obtained high readings in various temperature ranges.

Despite the high humidity, during the whole of last winter I lost only one plant, a Turbinicarpus. An Echinocactus grusonii was damaged by a combination of extreme cold and damp but every other cactus & succulent came through unscathed. I gave no water from late September, apart from a dribble to some of the succulents. My experience may not be typical but I was very relieved that what I perceived to be high humidity had not damaged the plants. I'm sure the cool fan was a major contributor to this.

[Joel143]

Thanks guys.....I have two small fans that stays on continuously plus a larger fan I turn on on occasions as well. I also have a heater w/fan set to turn on when it gets in the mid to low 40's. During the day its no problem to control but its at night that worries me once winter really arrives since in my area down south we have wet winters.

[Tiggy]

Glad that you brought this problem to light Joel, I too was a bit worried about the humidity in the greenhouse. Like you it isn't too much of a problem during the day but at night. I am at present using a small anti-frost parafin heater, which doesn't throw out much heat at all but just keeps the temps. a few degrees above those outside and delaying the time when I will have to use the larger heater. I did a trial run on the larger one a while back and was amazed at the condensation. I had the window slightly open as it wasn't really freezing cold. I went in with a ktchen roll every day that I used it and wiped all the inside of the greenhouse to dry it off and then if the sun was out left the door open a little. This dried it out well. When it gets really cold though I think I will have to leave more air circulation in there. Luckily my greenhouse is so small the large heater gives more heat than I need so fingers crossed it should be alright. I am constantly in and out checking the situation in case in starts to get out of hand (one perk of being retired). If I manage to get a larger greenhouse (hopefully next year) I think I may look at using a thermostaically controlled gas heat and fans.
My larger heater is also parafine and I had a sharp lesson when I gave it a trial run. SOOT everywhere!!! It ook me days to clean everything. I had cleaned the wick and kept it filled as directed, but had not thought about the funnel on it! I just hope it doesn't do that every time or it will be in the bin. I'm a bit worried about using it again but will have to try it sometime. Anyone else ever had experience with one of these?

Susi

[Rod Smith]

Soot can be a problem with paraffin heaters but the major downside is the amount of moisture they produce. This will significantly increase the amount of moisture in the atmosphere, unless ventilation is very good: but in the winter, ventilation will simply let in more cold air which will reduce the effect of the heating.

[Tiggy]

" Catch 22" - I am just going to have to struggle through this winter because, 1) I can't really afford to buy anything better at the moment, (too many things to sort out in and around the house) 2) My greenhouse is very small and there's not enough space to have a larger heater of the type that I would like. If I can get a bigger gh (hopefully within the next year) I tam thinking of a thermostatically controlled gas one. Electric is out of the question as it is far too expensive, unless I win the lottery and can then go solar power. Until then I will be in there with the kitchen towel everyday that I have to, at the moment I am not getting the condensation as it isn't too cold at night, and it's usually warm enough in the day to have the door open til the sun starts going down. Hey ho! All in a day's fun!

Susi

[Joel143]

Would like to ask when placing a fan or fans in a greenhouse for air circulation, at what level should it be placed? Ground level? Up near the roof? Heat rises, so now with fall and winter on coming, I would think having a fan at ground level would just keep things cold leaving the heat near the roof. In my greenhouse I keep a fan running at all times to keep down the humidity and to keep air flowing but have noticed a temperature difference between the ground, where the fan is, and the roof area. Any input would be greatly appreciated.

[daiv]

A fan for air circulation is to prevent the air from "stagnating" around the plants. My greenhouse is quite drafty and so I have not had a need for a fan. However, a well-sealed greenhouse with all doors and vents closed can cause problems fast on a sunny day especially. If you have no air circulation and the air temp in the middle of the greenhouse is - say 90F, then the temperature on the surface of a given plant in sunlight would be well over this - hot enough to roast them alive. So a fan that moves the air around the plants will push that 90F air past the surface of your plant and cool it down. Hence, I would probably place the fan at a level that is just above the top of the plants.

Other considerations with ventilation are preventing condensation and that sort of thing, but I always move my plants out of the greenhouse before this becomes a problem and so others will want to chime in. I know this just came up in another post.

[promethean_spark]

I've noticed that my GH tends to condense water on the walls most of the time - it's the coldest part of the GH (especially at night). I think this actually helps dry out the plants because the plants don't condense water - effectively it acts like a solar distiller. Of course if a plant happens to be in a spot where condensed water drips it'll get watered all winter, but drips are usually in the walkway where the hoop-house roof is relatively flat.
If water is condensing and running down the walls of your GH, you may be able to collect it in a gutter to run OUT of the GH, or let it fall to some gravel on the ground where it'll run down and away, that way the moisture will work its way out of the GH even if the vents are closed most of the time.

[Joel143]

I posted another new post concerning the positioning of the fans in greenhouses especially during winter wanting to know the best position especially since heat rises. The other post is titled Proper Air Ciriculation. After Daiv's reply figured it could of gone all on the same post. So if any can suggest or state where their fans are positioned in their greenhouses...would be greatly appreciated. Me I had it at ground level but figured this is were the cold is and that its just circulating cold air while the warmer air is near the top of the greenhouse. I repositioned my fan near the roof figuring this would circulate the warmer air better. I would hope this would still take care of any stagnant air around plants as well as help wih humidity.

[Sharpy]

Other considerations with ventilation are preventing condensation and that sort of thing, but I always move my plants out of the greenhouse before this becomes a problem and so others will want to chime in. I know this just came up in another post.

I think that was me. I have since spoken to several greenhouse builders in Florida here and they say the exact same thing, (summer time speaking) blow the air accross the plants. My fans also will be just above the container level and blowing accross/through them. Unfortunately I can not really chime in on moving the hot air down (in winter), since it never really gets to be "winter" here. This whole week has been near 80F day and upper 50s at night.

I would suggest looking through some of the "winter heating" threads. Sure there are lots of suggestions or different ideas.

[promethean_spark]

I don't have a fan, so the air is stagnant at night. There's about 20' clearance between the top of the top vent and bottom of the bottom vent so I get a chimney effect when it's hot and the vents are open.