Renfro
Well-Known Member
I thought this would be helpful for some.
Three things are at play, major factors anyways, aside from ambient humidity and whatnot.
1) Biomass - As the plants grow the amount of water respired is directly proportional to biomass. When the plants hit a certain size (I like to call it critical mass lol) the dehumidification is at some point unable to keep up with the plants respiration. When this happens, the room will suddenly start having humidity spikes at lights out since the dehu can't keep up and the humidity just builds up. It only takes a few minutes when the lights go out and my AC stops running for a large biomass to push the RH% up from 40% to 50%. Defoliation and lollipopping help to mitigate some of the biomass and allows for more airflow inside the plant.
2) Temperature - Cooler air holds less water. So with the same absolute humidity, cooling the air say at lights out means the relative humidity rises. This is a bigger factor than you might imagine. For example, lets take some air that's at 80 F and 50% RH. Lets cool that same air to 70 F without pulling a single drop of moisture out of the air. We now end up at about 74% RH! The chart below shows the absolute humidity for air at a given temperature, you can reference this number on the same table in a different temperature row and see how the relative humidity number changes.
So in the chart we look at 50% column and 80.6 row and we get 11.34 grams of water per kilogram of air. Looking at numbers that approximate that value in other temperature rows, we can see that at 69.8F we are up closer to 75% with 11.73. This is also why using cold air can be a great dehumidifier. So in the winter you can use cold air intake (filtered) to counter your heat load from lighting but also combat humidity levels. In the summer things change unless you are lucky to live in a dry climate that has cool nights. Air that's at 35F and 75% is very dry and blowing it into a 80 degree room will drop the humidity fast as the absolute humidity is around 2 grams per kilogram, when at 75 degrees that's less than 15% RH.
One may notice how powdery mildew seems to always start on the bottom and inner parts of a plant? Well it's cooler there! Those few degrees make a big difference. Lets say we are at 80F and 50% canopy, but the bottoms are at 75F, we are now at 60%! So keeping some balance of the canopy drier at say 40% or the bottoms warmer at 80F would be ideal.
3) Dehumidification - This is actually temp related and I touched on it a little bit already but dehumidifiers pull a lot more water from the air at 80F than they do at 70F. Those few degrees make a big difference in the RH% in the room AND the ability of the dehumidifier to pull water.
Managing humidity in your grow can become a very delicate balancing act when the biomass is large. This can be a constant problem for those in climates with high humidity. Understanding the factors involved will give you the tools you need to prevent powdery mildew and bud rot from ruining your crop.
When all else fails, grow a strain that is mold and mildew resistant such as Durban Poison.
Three things are at play, major factors anyways, aside from ambient humidity and whatnot.
1) Biomass - As the plants grow the amount of water respired is directly proportional to biomass. When the plants hit a certain size (I like to call it critical mass lol) the dehumidification is at some point unable to keep up with the plants respiration. When this happens, the room will suddenly start having humidity spikes at lights out since the dehu can't keep up and the humidity just builds up. It only takes a few minutes when the lights go out and my AC stops running for a large biomass to push the RH% up from 40% to 50%. Defoliation and lollipopping help to mitigate some of the biomass and allows for more airflow inside the plant.
2) Temperature - Cooler air holds less water. So with the same absolute humidity, cooling the air say at lights out means the relative humidity rises. This is a bigger factor than you might imagine. For example, lets take some air that's at 80 F and 50% RH. Lets cool that same air to 70 F without pulling a single drop of moisture out of the air. We now end up at about 74% RH! The chart below shows the absolute humidity for air at a given temperature, you can reference this number on the same table in a different temperature row and see how the relative humidity number changes.
So in the chart we look at 50% column and 80.6 row and we get 11.34 grams of water per kilogram of air. Looking at numbers that approximate that value in other temperature rows, we can see that at 69.8F we are up closer to 75% with 11.73. This is also why using cold air can be a great dehumidifier. So in the winter you can use cold air intake (filtered) to counter your heat load from lighting but also combat humidity levels. In the summer things change unless you are lucky to live in a dry climate that has cool nights. Air that's at 35F and 75% is very dry and blowing it into a 80 degree room will drop the humidity fast as the absolute humidity is around 2 grams per kilogram, when at 75 degrees that's less than 15% RH.
One may notice how powdery mildew seems to always start on the bottom and inner parts of a plant? Well it's cooler there! Those few degrees make a big difference. Lets say we are at 80F and 50% canopy, but the bottoms are at 75F, we are now at 60%! So keeping some balance of the canopy drier at say 40% or the bottoms warmer at 80F would be ideal.
3) Dehumidification - This is actually temp related and I touched on it a little bit already but dehumidifiers pull a lot more water from the air at 80F than they do at 70F. Those few degrees make a big difference in the RH% in the room AND the ability of the dehumidifier to pull water.
Managing humidity in your grow can become a very delicate balancing act when the biomass is large. This can be a constant problem for those in climates with high humidity. Understanding the factors involved will give you the tools you need to prevent powdery mildew and bud rot from ruining your crop.
When all else fails, grow a strain that is mold and mildew resistant such as Durban Poison.
