Calvin, according to google the conditions in Toronto at the moment are -3C and snow with relative humidity of 93%.
If you let that air into your house and warm it up to a toasty 18C., wonder what the humidity reading will be then? We need a boffin to tell us.
If you read the other thread, you will see that i was in big glasshouse the other day where settings were set to simulate montane (cloud forest) conditions. There were lots of orchids there that i would never even think about trying to grow.
Don't know about Pleuros, some of them might be OK.

As mentioned in another post, I'm using humidifiers right now because I have some new miltoniopsis (I expect them to be temporary guests through blooming.) I notice that when I start my humidifiers in the morning and the orchid room is still cool from nighttime and the lights haven't heated it up that it takes no time for the humidity to sky-rocket to over 60% (and that's with one of the humidifiers off). Then as the day goes on and the room warms up, it settles to about 50-55% with both humidifiers running full blast. It's a closed room, so the space remains constant.

Arthur, what is a boffin?

John :>)

It's the heat that dries it up, Calvin. Especially central heating, the forced air kind. I think the old fashioned steam radiators and the closed room warmer heaters don't dry things up as much.

The humidity in my basement light room gets to a low of around 35% as long as the room is warmed only by the plant lights and a space heater that's on only at night. As soon as it's cold enough to have to use the central heating down there (that would be below 20F), the humidity there goes down into the teens for a low. And that's running it only for four or five hours, not all day.

Calvin,
Relative humidity means that the humidity is relative to the temperature at the time of measurement.

If you could "seal" a room at 0­° with 50% R.H. so that no additional water vapor could enter and then warm it up to 30°, the relative humidity would be a much lower value at the higher temperature.

Warmer air has the capacity to hold larger volumes of water vapor.

Warmer air which is relatively saturated with water vapor will condense the water as it is chilled.

Make sense?

--Stitz--

The link below explains a lot of this stuff very well, I ran across it last fall when I started preparing for winter.

Here is a link that might be useful: Humidty and the Indoor Environment

Very nice article, until I got to the part about how to reduce excess humidity in the home - get rid of some plants!

Harrumph!

Good article. John, this explains what I was trying to regarding fresh air exchange. People using sealed rooms and basements do not open doors or windows to let fresh air in and old air out. The use of fans does not prevent mold or fungus if the room is not vented. Fans will cool the leaves and help prevent burning from lights, they will move heat around, but without air exchange they will not prevent mold.

Sealed, damp growing areas are unhealthy, no matter how many fans are blowing. You need fresh air.

Jane

That is a great article and makes a lot of sense!! Since warmer air holds more water, 50% of the moisture capable of being held on a cold day prob. translates to 2% in a warm room. Thanks for the article.

Jane - I think you're right..I just came back from the local orchid society meeting, where a lady was describing how her 8' by 8' basement grow room worked, and she has 2 fans circulating air inside, and a large fan that sucks out the air into the rest of the basement...and then even another one that sucks the humid air out of the basement outside (they found that this eliminated their condensation problems).

Actually I think the same idea applies for people who build terrariums - they always have fans that exchange outside air with inside air in addition to interior fans that keep things breezy.

Thanks Jane. I do take your point. I always think in terms of balance, so it's obvious to me to ventilate. And I figured most people here are growing in open rooms. And that if they took the trouble to make an elaborate setup in their basement they'd know enough about plants, let alone orchids, to understand what healthy humidity levels are, and the need for adequate ventilation.

So, given that growers have these basic concepts under their belts, it's also important to be aware that, in general, air movement is healthy for plants, and that the degree air is moving should be balanced with the level of humidity.

John :>)

Exactly John. But you would be surprised at what I've seen... Sauna's.

Jane

Interesting thread! I have some issues about ventilating because of the way my grow room is set up. I grow in a spare bedroom, 9 x 11 feet.

Once I added the serious grow lights, the room got quite warm. So I bought a dual window fan and ran it to exhaust to the outside 24/7. Obvious problem - we get heavy rains when I'm not home, and the fan and the room get very wet. Bad idea. Plus I can't leave the window open when it's really cold out.

So then I started opening the grow room door to ventilate into the hallway. Problem #2 - the thermostat for the entire upstairs is right across the hall from the growroom. So with all that heat pouring out, it was throwing off the thermostat, and the bedrooms were freezing for most of the night. Family complained too much, so I went back to closing the door.

I do go in there almost every day to check on the plants, so it gets a little ventilation then, and when I water it gets more.

I used to get spider mites periodically on my tropical plants, and I find when I run my room humidifier, they aren't much of an issue. So I've been using the humidifier. I did notice last year that I was getting some mold on the windowsill, so I had to gear it down.

Anyone know the safe upper limit of humidity to avoid mold problems? I want to run it as high as I safely can to keep the plants happy.

Anyone

Do you have double-hung windows, I find that if I open mine a few inches at the top and a few inches at the bottom, ventilation is much improved and even downpours don't find their way inside.

In winter I usually don't have to ventilate during the day and I want to keep the humidity in to counteract the drying effects of heating. The temps stay manageable (under 85 even on warmer days; 80 on colder days). I keep fans going and watch that humidity doesn't peak much over 60%

Then at night, everything goes off. At that point you can open up the room, which should stabilize with the rest of the house within a few minutes and your thermostat will not be affected for the rest of the evening. I actually close my room back up after the heat escapes and open windows to take advantage of cold nights and cool down the room further to about 65 degrees.

Do you have a digital minimum/maximum thermometer/hygromenter?

John

I think that the goal of running humidity "as high as you can to keep plants happy" is perhaps not the best approach to providing a growing environment for your plants. The 60% I mentioned above is *not* a target, it's the maximum allowed. If it stayed that high all the time, there would be problems with mold and peeling paint. At the moment, with some sensitive plants as temporary houseguests, I feel much better when it's around 50-55% for several hours a day, with the aforementioned dry-off period every night.

I tried to mimic, if not duplicate, greenhouse conditions in my grow room for the first few years. I was convinced that with my HID light that my plants would just shrivel up if they weren't getting constant high humidity. But you know what? Finally I got so sick of cleaning my fogger that I tossed it out and thought, if the orchids don't like it, let 'em die. Guess what? They went right on, and I'm having more flowers now than before because I'm buying plants that are appropriate for the conditions that I can apply without breaking a sweat, and all that attention to humidity has been directed to other considerations.

At the moment, I have three sensitive plants that I want to bring into bloom, enjoy, and move on so I'm paying more attention to humidity. After they're gone, I MAY run one humidifier in there to keep things above desert conditions while the heat is on - I'd be happy with above 40%. In the spring, I'm just going to ventilate the room and forget about humidity.

J