Humidity

Hello,

It’s been a while since I posted but glad to be back. I am having trouble understanding humidity in a greenhouse. I have a small 12x 16’ greenhouse. I get how to reduce it but what about in Winter. I’ve read articles that say for example if temperature is 70 ideal RH is 91%. This makes no sense to me. Can anyone tell me what is a good humidity level in a greenhouse in the winter. Our temps. can get in the single digits.
Thank you.

Hi Nance -

Humidity is somewhat misunderstood, and for this I blame the penchant for using relative humidity expressed as a percentage. This is because as the temperature of air increases, so does its ability to hold water vapour. So your example air mass with 91% RH at 70 F would be only 65% at 80 degrees and 47% at 90.

I like to use dewpoint instead, because that stays constant. In our example, the dew point is 67 degrees and this does not change with temperature. If you cool the air to the dewpoint, the water condenses out. This is why you get water on the inside surfaces when it's colder outside the greenhouse. The air cannot be cooled below the dewpoint without getting rid of some of its water, so humidity helps to moderate the temperature somewhat.

If you've got growing plants in the greenhouse and it's closed up, you are going to have high relative humidities. This is a given. Over winter, if you're letting things go dormant and not watering so much, the dewpoint will go down but the RH will still be high. The three ways you can control moisture are to heat the air, dry it using a either a dehumidifier or a dessicant, or ventilate with drier air from somewhere. The trick is figuring out how to do one or more of these things without great expense, and if you're introducing outside air then heating it is probably a given.

Thanks so much for your response.I have a thermometer in there that tells me the temp.and humidity level. I’m assuming I’m reading the RH,but not sure.  I do heat the greenhouse,in the winter as I do grow microgreens in there. In the wi tee though if I heat the greenhouse to 75 the humidity does not go,down much,this is where I get frustrated and confused. Is there something I’m doing wrong. I have now put a dehumidifier in there hoping that will help my problem. It’s also difficult to keep the humidity down when it’s raining outside.

What's happening when you heat the space is that you're increasing the air's ability to hold moisture, and since there's actively growing greenery in there it's happily pumping water into that air. So everything is finding an equilibrium: warmth means the water in the soil and the plants can move into the air and that keeps the RH high. As long as you've got growing plants (with the exception of cacti and succulents) the only way to change that will be to actively dry or change the air as well as heat it.

Do you have problems caused by high humidity affecting your plants? Or are you just concerned about the numbers you're seeing?

Plants need nutrients.
And the water in the soil has the dissolved minerals that they need.
So the higher the concentration of minerals in the water the less water the plants needs to take up.

To take up more water the plants leaves need to get rid of the excess water that it already has, or else it will rupture/exploded.
If the humidity is too high the plants cannot transpirate the excess water and it will take up less dissolved mineral+water mix, and pretty much kind of starve.

Pest love to attack these weaken plants, but even without pest, the plants are already stressed.
Without the necessary mineral to build compounds to  fight back and heal, they are just sitting ducks for the pest.

Greenhouse (just like for humans a 40%-70% humidity is best, but like us they will work with whatever you give them)
Night = 50F and 70% Humidity (5.5g water/1000g of air )
Daytime = 68F and 40% Humidity (5.5g water / 1000g of air)
https://www.lenntech.com/calculators/humidity/relative-humidity.htm

Yes I’m concerned about the plants,I’m getting a lot of damping off. For example this morning the greenhouse temp read temp 64 humidity 69.When I went in the greenhouse the exhaust fan was going. The dehumifier was full. Shouldn’t  the humidity level have been lower than this. It seems like it will be fairly expensive in the winter to heat. How did you figure out the dew point? It seems I need to focus on that,however what is considered okay for the plants. Is there an article you can recommend I read to familiarize myself with this?Thank you so much for your help!

To answer your question of what is a good humidity range for your plants/greenhouse the answer is 40%-70%

In the tropics/florida the humidity is super high as in 85% and up during the day and every morning you will find dew on the grass/leaves or just see the fog rolling in. So don't worry about it too much.

You mentioned that you had vent open in your greenhouse, which suggest that you are just bringing in cool super humid air from outside during the night/day break.

So if the air outside is 64F and the fog rolled in at 11pm last night and dew is on grass aka 100% humidity.
And you bring in the outside air in the greenhouse will have the exact same air temp and humidity.
It's like opening all the doors, windows and vents and then turning on the AC in the summer and asking why is the house temperature and humidity so high and hot.
You have to cut off all incoming air.

So you might be wondering isn't it kind of bad if I prevent fresh/new air from coming in because:
1) C02 levels will be depleted so you will have to get CO2 enrichers (humans breathing, fire burning, animals/fungi)
2) Wind to strenghten plants, just get some fans
3) Wind to increase evaporation rate, get some fans
4) Wind to bring in some less humid air, just get a humidifier
5) Wind to help polinate, get some fans

Nance, if you had 64 F inside temp with 69% RH first thing in the morning, then that is fine. If that air warms up to 75 the relative humidity goes down to 47%. The dew point stays at 53 degrees. See how it works?

Here is a simple online calculator: http://andrew.rsmas.miami.edu/bmcnoldy/Humidity.html

And here is a detailed discussion with some charts: http://extensionpublications.unl.edu/assets/pdf/g1849.pdf

If your local weather station uses dew point numbers, follow them to see how the outside air is changing. Print out a chart and keep it next to your thermometer so you can do the conversion when you're checking temps.

If you bring in outside air through a vent, its influence on the inside air will depend on what the respective water content of each air mass was before mixing. It's far easier to think about this if you use dew points instead of RH, because RH changes as soon as the temperature goes up or down.

If you had a vent open overnight and the outside air had a dewpoint of 40 F, then mixing equal volumes of outside and inside air would have brought your indoor dew point down to the mid 40s. It does not matter in our example if the temperature of the outside air was also 40 F (in which case you might have had fog or at least a heavy dew) or higher.

I would NOT advise keeping a greenhouse sealed up. Find a way to manage some air moving in and out of the structure, especially during a sunny day. That is when you get the best bang for your buck in terms of "free" drying.

Okay I think I’m beginning understand but how do you know what is a normal dew point. In other words how did you know 53 dew point is okay

To decide if it's ok, you just use the table to to see whether that dew point is in the relative humidity range you're aiming for. A DP of 53 means you're in the 40-70% "sweet spot" across temperatures from 60 to 75 F. That's pretty good.

I've made a couple of tables (imperial and metric) that show the 40-70% range in green to make it easier to spot:

Hi I just saw your questions regarding  your greenhouse and it brought back some memories when I had my greenhouse years ago.  We are setting up a new one for a friend's of ours and will be setting up one again for us very soon too. One of the most important things to have in your greenhouse is "air flow."  It you don't have enough air flow things start happening that you really don't what to happen in a greenhouse, mold. Three things that causes mold to grow is:
1. Low Light,
2. Low or no Airflow,
3. to much moisture/humidity.

The easiest thing to control is air flow. Moving air causes the moisture to evaporate off of the plants and will help the life cycle work better. Hope this will help somehow

Unless it rains a lot where you are, the outdoor humidity should be fairly low during the winter.  Assuming the outdoor dewpoint is lower than the greenhouse dew point, then simply ventilating the greenhouse during the daytime (when it's warmest) should reduce the humidity in the greenhouse.

If it's very cold outside, you might use an air-to-air heat exchanger.

I had the exact same issue with my 10x12 greenhouse and I upped the air flow and did a deeper gravel bed (5-6"). The deeper gravel bed was a MUST and I'm sorry I ever tried to "cheat" and stretch on that score.

As for air flow, I simply cut a square in the bottom of the back panel and stuck a box fan in there. I took the plastic panel I cut and stuck it back in in such a way that there's now a little "roof" over it. I also have four fans in each corner of the greenhouse that blow air in a circle pattern, and then an exhaust window fan up top. It doesn't seem to really matter where I position the fans or anything, as long as there's a "flow" it can be extremely soaking wet in there, and it doesn't feel "damp" at all and my plants, seedlings, cuttings, orchids, etc, are all thriving and happy.