Preventing Humidity Problems

[Richard Mong]

Hello all,

My system is going up at this very moment (slowly, of course), so during the next few weeks I'll be asking for a lot of help. I'm trying to think of everything that can go wrong and humidity issues is at the top of my list. I've been warned by people in the know that the fish tank shouldn't even be in the same greenhouse with the plant and human population (I'll be living in it every other week for the next year) because of humidity issues. I have come up with two ways of dealing with it, and I'd appreciate any feedback I can get.

Ventilation: During the day I will have to keep greenhouse temperatures down. To facilitate this I went with what somebody told me is called a "poly-clerestory," so I'll go with that name. Basically, there is a 19m by 50cm vent at the top. Wind across the top will pull air in from the sides. I have a 30m by 3m roll of expensive insect netting on the way. This top venting out bit will use about 10m2 of it and the remaining 80m2 will be strategically placed along the lower areas of the greenhouse walls. During the day, the plastic gets rolled up, exposing the netting and allowing the air to come in. At night, the plastic gets rolled down, keeping as much warmth in the greenhouse as possible. The problem here is that I can imagine a lot of humidity during the night. I think the ventilation during the day will be OK.

Tank specific ventilation: My idea for meeting this challenge is pretty simple. If you look at the second illustration, you'll see that I'm thinking of draping greenhouse plastic over the tank and venting out (completely out of the greenhouse) using a simple bathroom fan or something similar. It seems to me that any condensate on the inside of the plastic over the tank will just dribble down the sides of the plastic back into the tank (conserving water). An alternative would be to do a chimney kind of thing instead of a fan which wouldn't require electricity.

Any thoughts? Here are the illustrations:

And here is the "solution":

[Jim Troyer]

Hi Richard - good luck with your project.

check this out for controlling the atmosphere in the the greenhouse

[Chris Carr]

I was curious where you got your info regarding your greenhouse design. I am interested in more info on this type of design as I haven't been able to find much. I have seen this more commonly referred as "tropical" design. I recall reading that 1/3rd vent area to greenhouse floor plan is optimal for tropical climates but since you mentioned you are in a cooler micro climate your sketch seems appropriate. I guess even though a unique micro climate, you still get insane humidity like the rest of the area.

I ask because as of the new year, I plan to make a pretty drastic life change and move to the Mekong delta. Hot as a mofo and 100% humidity.  I am trying to gather as much info as I can before I start my own life experiment :)

[Richard Mong]

Hello Cris,

I sort of made it up after reading everything I could about "tropical greenhouse design" and variations of the same, so you are correct. You also have to take into consideration how the insect screens worsen ventilation. I think it is something like a 30% lessening of ventilation compared to no screens at all. You should check that, though. I would have gone with more insect screen area but I don't want to buy two rolls. If this design works I'll do the same next in Vientiane then in Thailand. Same design but much, much more ventilation. Mostly left open at night, I would think. The fish will love the warmer temps so it might be geared more to fish production than veggies. I'm really trying to use locally available, cheap materials. The hoops will just be PVC water pipes with rebar inserted into them for some more rigidity and springiness. Both materials are cheap here. The water pipes are smooth so gentle on the greenhouse plastic. I'm getting high quality greenhouse plastic,as well as good PVC pond lining material for the hydroponic troughs (about 30cm deep). I'm not worried about uv so much as they will have the rafts over them at all times. The divisions between the three troughs will be made with earthbags. Just plain rice bags full of dirt, tamped down, and covered with replaceable burlap bags to protect from sunlight. These can be walked on and are as wide as the usual paths between rice paddy fields, so while I might stumble and fall in my workers should be right at home. The upside down tomatoes might prove a bit of an obstacle, however. I'll keep people posted as to how it works but there will be more details in my normally self-depricating style on my website at www.wrongwayhome.com.



Chris said:

I was curious where you got your info regarding your greenhouse design. I am interested in more info on this type of design as I haven't been able to find much. I have seen this more commonly referred as "tropical" design. I recall reading that 1/3rd vent area to greenhouse floor plan is optimal for tropical climates but since you mentioned you are in a cooler micro climate your sketch seems appropriate. I guess even though a unique micro climate, you still get insane humidity like the rest of the area.

I ask because as of the new year, I plan to make a pretty drastic life change and move to the Mekong delta. Hot as a mofo and 100% humidity.  I am trying to gather as much info as I can before I start my own life experiment

[RupertofOZ]

Most greenhouse ventilation controls are a combination of wind up side curtains (shade cloth screened), stirring fans, and extraction fans (an/or roof vents)...

 

For that size greenhouse... IMO... you need all three...

[Richard Mong]

Oh dear, I hope not. Of course, the sides will roll up but only as far as I have insect screening. Running a few fans for air circulation will be done if necessary. I've read, though, that breezes passing over such a design as mine have a tremendous "sucking" ability, not to mentione the convenient fact that hot air rises. Most desigs that I've seen are of the tunnel type, so mechanical removal of the hot air is a must. So I will have two out of the three necessary items you mention. Fingers crossed.