Increasing humidity and increasing temperature are contrary operations - when you increase humidity you will cool the greenhouse down. For this reason it is good to be sure that you really do need higher humidity.

If you do, a water feature would be a good option.

well i actually just want to increase the temp a little while maintaing the humdiity. well i already am using 2 big water containers filled with hot water so that it will help raise the temp while maintaining the humidity.it seems to be working pretty well..but in the summer its going to be really hot and dry so i might need a humidifer or something then

You can't do that. If you increase the temperature, the humidity drops. Using hot water seems wasteful, as you are just going to lose that energy out the window. How do you define 'working well'?

Best operating practice is something like: heat in winter and leave dry, evaporatively cool in summer.

Hottielover, take a look at the humidity link below. Air at 20*C (68F) holds 17.33g/m3 water, which is 100% RH and VPD of 0 kPa. Increase that air temp just 5*C to 25*C (77F) and the RH is 75% with a VPD of 0.79 kPa. To get 100% RH and a VPD of zero at 25*C, you need to add another 6g/m3 water vapor to the air. If you were able to do that (and it's not that easy) all of that water vapor would condense at night when the GH cooled down, and you'd be back to 17.3g/m3 at 20*C by morning.

My point, and I think it is Nathan's point, too, is that it is not easy to maintain a certain humidity level while the temp in the GH increases. The capacity of the air to hold water increases to such a degree that it is hard to keep up. The plants that you list may prefer a higher temp than your 68-70*F, but they will do fine at anything higher than 55-60*F as long as you don't overwater.

Here is a link that might be useful: Humidity link

well i define working well because when i add the hot water to my GH it stays a little warmer and the humdiity stays in a good range. also at night its stays a little warmer.

stressbaby, ok but lets just say i wanted to, how would i go about doing that? ok but they also need higher humididty and when summer comes around it gets pretty warm here and drier so i would have to add a humdiifier or something to keep the humidity up right?

also i forgot to say that i have a plastic humditity cover over my GH too.

stress has yet again demonstrated remarkable ability at speaking scientist and layman at the same time! The only thing I'll add is that to add those 6g/m^3 corresponds to adding 13kJ/m^3, and that a higher total humidity than outside will result in a continuous heat loss as you ventilate.

To make up some numbers, a 30m^3 10x12HFGH greenhouse with air replacement every 10 minutes means you're using an extra 660W (2200btu/hour) heat. Every air change every minute and a half and you're looking at 15kbtu/hour - a serious amount of extra power!

"A higher total humidity than outside will result in a continuous heat loss as you ventilate"

Hottielover, I think Nathan rightfully questions whether you want to do this in the winter. In the summer, however, this is precisely the goal...you add water to the air, it evaporates and cools the air.

As I've said before, in reality, humidity is the hardest of the variables to control in the GH. You can control light and temperature more easily. In the summer in my GH, with the addition of a fogger AND misters, the RH barely moves, presumably because the air turns over so frequently. The plants don't seem to mind because the cooling effect keeps the temps in the proper range, even though the afternoon RH might be 40% on a hot, sunny summer day. And I'm growing the same types of things you are, including Dwarf Cavs.

May I ask you how you plan to keep a DC banana in a 6x3 GH? Every leaf on my DC banana is longer than your GH is wide!

I also grow bananas and I have a vanilla orchid, and a key lime or two and they don't seem to need that much humidity. At a constant 80% humidity you will definitely have mold and mildew problems at some point. You might want to rethink that humidity. Besides it sounds like a lot of work/expense too if your plants don't have to have it.

If you use an unvented gas heater in the greenhouse the exhaust gas (mostly CO2, H2O, N2) will raise the temperature and the humidity. Units operated improperly vent toxic CO into the greenhouse. Some also vent C2H4 (ethylene) which affects many plants adversely. Please be careful.

Here is a link that might be useful: Gas Heaters

I want to follow up on Sandy's post. It is, practically speaking, impossible to keep a constant humidity level in the GH. The temperature swings during the day will dramatically alter the RH and the VPD. What we are in reality tryiing to do is avoid the extremes of humidity. So it is more helpful to consider the safe upper limits of humidity.

Plant diseases don't really get going until you get a VPD of 0.20 kPa or lower. This corresponds to a RH of >90% at 68*F and a RH of >95% at 86*F. Even if hottielover decided to let her GH get cooler (and based on his/her name, it seems unlikely), at 50*F, a VPD of 0.20 kPa is still 83%.

This source states "Therefore it is safe to work with RH of 85% as the allowed maximum humidity level." I have seen this statement in other sources.

One other thing to think about is how much sun you get. Radiation is what drives plant transpiration. If you have plenty of sun, then there is plenty to drive transpiration. If you have relatively little sun, you need a lower RH to help drive transpiration and you may want to aim for a little higher VPD (and lower RH).

Here is a link that might be useful: GH humidity and disease

I have radiant heat in other part of my home. When I designed the system, I planed to have enough capacity to add a futre greenhouse to the system. (That will happen this year.) I thought it was a good idea because this system doesn't dry the air like forces hot air. Has anyone done this?

My system uses pvd (plastic) tubbing with a water heater, not the type that uses a boiler. In other words, the tubes don't have extremely hot temperatures. The tubbing can be added to the knee wall as well as the floor.

Here is a link that might be useful: My radiant system

freedee: It will dry the air exactly as much as forced air. More interesting is whether you can use the greenhouse house to heat you main house.

ok well based on what Nathan says and some other people, maybe I shouldn't be caring that much about higher humidity during the winter and save it for the summer. but then Stress baby comes along and says that only humdiity at about 90% will cause diseases and mold. is that a right interputation? ok well im using 4x40W FL lighting fixtures over each tier? would that be enough to have a higher RH?

well hdcochran, thanks for that article and i will consider it but it could also pose a health risk if used improply which could be a big con to possibly getting one. stressbaby, i dont plan on keeping it in there forever, just for now and its taking a while to grow too. and my GH is actually about 6x4.5'.

Sandy, i have read that they all like high humidity though and i want my plants to thrive not just grow. also im getting a Cocoa tree and possibly a Macadamia Nut tree.

So what would you guys say would be a good RH range for my plants in my GH during winter at about an average of 74-75 degrees?

Macadamias certainly don't need high humidity. I would just forget about humidity - your plants will correct the humidity fast enough through transpiration. By all means watch it, but in winter it is fine to just worry about temperatures. A macadamia will certainly take short term 0C (35F) without trauma - I've got a macadamia out in the open.

In summer you will be cooling the air by evaporating water - then you need to be careful to prevent mould growth.

I agree with Nathan. Worry about the temperature. Monitor the humidity if you like, and avoid the extremes. You are fine if RH 50-85% for most things at most temp ranges. It is a far greater return on your time investment to learn and attend to good watering habits than it is to worry about small adjustments in humidity.

So you still didn't answer my question...how are you going to keep a DC banana in a 3x6 GH? :-)

Nathan, How much approx. humiditiy do Macadamias need? Plants will really correct the lower humidity by themselves. But what about my Cocoa and Black Pepper which ive heard need high humidity? ok my high is about 74 but i want about 78-80, if i get a ceramic heater to run off and on for that extra boost, wont the humidity go down quite a bit during the day?

stressbaby, ok i will focus more on temp for the winter and then focus more on humdiity during the summer, but my humidity is about 45% during the day in my GH, is that even ok? ok i already know how to water though. lol well mine grows slow and its in a pot so it wont grow that big but if it does then i will take it out. my GH is 6x4.5' also :)

The macadamia will probably handle a RH of about 5%. They are really tough. In their natural habitat it doesn't rain for months at a time, with temperatures of 30C every day. I grow kava(close relative of pepper) in an unheated greenhouse, where the only humidity source is a fish tank (admittedly a danged big fish tank!).

If you are really set on humidity, put a pond, fish tank or pond in your greenhouse. You also need to forget about relative humidity. It is not useful for plants. Read about vapour pressure deficit and dew point - it has been discussed here before, and I wrote some stuff up on wikipedia here:
http://en.wikipedia.org/wiki/Vapour_Pressure_Deficit

The best regulator of humidity is in fact soil!

Nathan, I like your Wikipedia submission. I noted that you referenced the Ohio State University extension site. I have that site bookmarked as well, but for some time I have been unable to access it. I noted that your Wikipedia link doesn't work anymore either. Do you have a new or revised link?

ok thanks stressbaby and nathan for all the info youve given me..ok i wont focus primarly on humdiity as much as temp during the winter. but im going to get a ceramic heater to help boost the temp and im thinking of putting a small container of water in there and a water heater like one for a aquarium to raise the water temp and help speed up evaporation. so that its not to dry in there..

stress: no. I'd like to find a more up to date site too.

hottie: A bigger tank will help even out the temperature fluctuations from day to night - in my case it means that in our current weather, cycling from 40C in the day to 20C at night the greenhouse is an even 26C all day. But I have a lot of water! I've got 4.5kL (about 1200 gallons I think). I keep fish and bog plants (such as sarracenias) in the tank.

There are several discussions about this on this site.

ok nathan. ive been looking at some ceramic heaters, any suggesions for my small GH?

I don't have a heater, as it is not needed in my climate. I'm sure that others will have plenty of advice. Whereabouts are you? Zone or whatever?

i live in Vancouver WA, which i think is zone 8.

hottielover14,

Why ceramic... see my post on the blue flame/radient thing.

I have, and would recommend an oil filled heater and a small table type oscillating fan.

Why? I don't trust an exposed element heater as I have seen flaming objects come out of them.. (bugs) and a ceramic heater is very directional.

I admit I have used, and will use an exposed element heater with a fan but it has a very fine filter on the air intake.

Here is a link that might be useful: Fan vs ceramic (electric) or Blue flame vs Radient (gas)

well chris thanks for the info. but my GH is pretty small, being only 6x4.5' so i think a blue flame/radiant heater would make it to hot or catch something on fire or release toxic gasses or something like CO. i need something small and not that powerful.

hottielover14,

I was trying to say ditch the ceramic heater and go for a fan heater.

Even better get one or more of those cheap plastic vicks vapour humidifier things that look like spaceships.

They provide humidity and heat.

whats the difference between a fan heater and ceramic heater? but then i have people telling me to go for more heat in the winter and focus more on humidity in the summer.

A fan heater blows warm air. It warms the air.

A ceramic heater is like a real fire, it warms what is in front of it.

If your GH is 6ft by 4ft a ceramic/radient heater will burn a hole in the wall.

oh ok. i looked for a fan heater on some sites like home depot. but couldn't find any. all i could find were ceramic heaters

how about either of these fan heaters for my GH?

http://www.emeraldelectronic.com/store/proddetail.cfm/ItemID/288462/CategoryID/19000/SubCatID/8761/file.htm

http://www.shoppingwarehouse.net/prod-72141.html?src=froogle&src1=sw&src2=gbase

what does everybody think about those for my GH?