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By PlantsWithTeeth_01
Posts:  61
Joined:  Mon Jul 15, 2019 2:49 am
I am building a small greenhouse, roughly 9ft square, in Adelaide. Our humidity is fairly low: Avg summer humidity is around 30%, but can get lower on some days like 12%, avg. winter humidity is around 60%, but can get higher and lower than that. My nepenthes actually do just fine outdoors in the shadehouse, once they have become tough, but I need a greenhouse for cuttings / seedlings / acclimatizing weaker plants I just bought.

My particular concern is ventilation, which I can't find much online about. The greenhouse will be unheated, no humidifier or any tech; weak plants have done just fine in my cheap $50 greenhouses, but the problem is they are breaking, hence the upgrade. I grow all of my plants in water trays, and use live sphagnum as a top dressing, so that's where most of the humidity comes from, which works great in the small greenhouses.

So how important is ventilation? Any advice? I assume the low humidity outside might have an impact.

One suggestion from a friend was having a vent of sorts down low on one side, and then a second vent up high on the opposite wall, to let cool air flow in down low, then the warm air out the top, but I'm not so sure about that. Possible to use flaps to open and close so that the vents don't have to be open all of the time - or should there be air flow all of the time?

Also what are the problems to look out for without good ventilation? Can nepenthes get problems like mildew? How much does ventilation matter? Need no-tech solutions that just involve openings in the walls / flaps, etc. I'm a little bit iffy about flaps or solutions that require too much manual handling, in case I forget or weather makes it difficult to get out there and open stuff, preferably it would be either open all the time or closed all of the time if that were possible. Dunno if that's a bit of a reach for keeping the humidity up.

Another possibility is: I'm planning to use corrugated perspex, UV rated (sun tough I think it was), for the roof, so maybe just leaving both ends open is enough ventilation? Would that give enough humidity?

Also, I plan to build the frame out of perma-pine, and use greenhouse plastic for the outside. Someone suggested using 2nd hand timber to save money for the shelving supports, since they are off the ground and away from white ants, but I worried about wood rotting. Problem or not?


Edit** I did a bit of further googling, and came across something called "Automatic Bayliss Openers", here: ... entilation

and here: ... 0694099183

They seem to not be electric, though I do not know if they would work with my setup of a no-tech, unheated greenhouse? Or whether I should have other ventilation as well? Maybe that could be a good solution though, or part of the solution.
By iacp
Posts:  34
Joined:  Thu Jan 24, 2013 4:34 am
That type of opener uses a piston with a substance inside that expands as it heats up and contracts as it cools down. So as your greenhouse heats up they gradually open to release excess heat. You would still want a vent down low I would think.
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By sanguinearocks101
Posts:  1648
Joined:  Mon Jan 06, 2020 1:56 am
iacp wrote:That type of opener uses a piston with a substance inside that expands as it heats up and contracts as it cools down.
I talked to a person near me with a greenhouse a while ago and I think she said that it is filled with beeswax. Not 100% sure though.
By steve booth
Posts:  913
Joined:  Mon Jul 18, 2011 11:15 am
I don't grow neps, so whilst knowing that ventilation is required to prevent mould and heat build up, what amount is a 'proper' amount I can't comment. However, in general terms, ventilation I do know about, the most efficient is a low-level inlet (the larger it is the more air you will shift, depending on the temperature difference between the low-level inlet and the temperature at the top of the greenhouse, termed the stack effect) and a high-level outlet on the opposite side of the greenhouse, to get cross-flow ventilation and encompass as much of the greenhouse as possible.
The wax cylinder openers are great for adjusting this airflow, I have had one in my greenhouse for 10 years and it is still going well.

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