This is part of my series "The Basics"
Principles of a Monotub
This guide is not focused on bulk substrates per se. It's really just the basics of how the monotub works. Having the basic concept of how it works is a good foundation to build on.
A monotub is a all-in-one container to mix your spawn with bulk substrate for colonizing and then also serves as the fruiting chamber.
There is more than one approach to making and maintaining a monotub. I will attempt to explain the most commonly used monotub here. This monotub is considered a "set and forget" mono. I will refer to it as a mono from here on out.
How it's madeTypically, a monotub is made of a clear/transparent tote/storage tub. It has 6 holes total; that's two holes on each long side just above top of substrate and a hole at each end as close to the top as possible.
While colonizing, all holes are covered with tape. This locks in moisture and allows CO2 to build up. The tape should not be breathable. The imperfect lid will allow for all the GE you need.
Once 100% colonized and ready for fruiting conditions, the tape is removed. The bottom holes then get polyfil stuffed tightly in them and the top holes have loose or no polyfil in them.
Sometimes, there is also a fan in the room near, but pointing away from the monotub. You have to experiment with this to see if you need one. Sometimes, a fan seems to dry out my substrates.
Principle of operationThe dilemma:We know humid air rises because it's less dense than drier air. So, the humid air inside the mono should rise up and out the top holes, right? Because it's less dense than the drier air in your grow room, right?
You can see evidence of this if you put a substrate in the mono and leave the holes completely open. You will see the substrate drying faster near and around the bottom holes. This is because the humid air flowing out the top creates a vacuum that pulls fresh dry air in through the bottom and causing drying.
We don't want this to happen because mycelium weakens and the substrate becomes more prone to contamination as it dries.
Stuffing polyfil in the bottom holes lowers the volume of fresh air coming in. The tighter you stuff it, the less fresh air.
Looser poly will allow more fresh air, but too loose can cause the drying mentioned earlier.
If you stuff the bottom holes tight, you will protect your substrate from drying, but you restrict the FAE your substrate needs.
What Can We Do To Fix This?I know of two basic answers:
A) Adjust your polyfil just right so you have a balance of FAE without having dry spots around the holes. Then, make up the difference with fanning and/or misting.
B) Stuff the bottom holes tight with polyfil and maybe put a fan in the room.
I'm here to explain option B) Set and forget.
A fan in the room will create air movement. Because the top holes have loose, or no poly in them, the air can move in.
Think of your car window. When going down the road, a fully opened window will cause air to blow in as it goes by. Albeit exaggerated, this is basically what happens with a mono through the top holes with a fan in the room.
The fresh air comes in through the top, mixes with the humid air inside, and then flows out the bottom holes. This allows for a more even fresh air and prevents dry spots because it becomes humid air before it reaches the substrate.
The force of the moving air created by the fan is greater than the force of the rising humid air and is also greater than the force of the circulating air inside the tub.
Essentially, by stuffing the the bottom holes tight and putting a fan in the room, we have changed the natural flow of air inside the mono. Humidity at the substrate surface remains high because the fresher drier air coming in is humidified by the moisture released from the substrate. This is how you can get good FAE without sacrificing humidity.
Having holes down low ensures that when the air comes in at the top, it will circulate and mix with the humid air inside because it will want to flow in up top and out the bottom due to the force of the fan in the room.
This is not a perfectly closed loop system. Some of the air will flow in and out the top and bottom, but most of it comes in the top holes.
If all the hole were high, then the air may mostly circulate in and out the top and have little effect on the substrate. If all the hole were at the bottom, then you would have to keep the poly tight or risk dry spots.
Tub SizeI use a clear 64 quart Sterilite 1928. If your tub is too big, you may not get enough air circulation inside. If it's too small, it may get too dry inside. 50-80 quarts seems to be a good range for this tek.
It needs to be tall enough. It needs to be tall enough to accommodate at least 2-4 inches of substrate and 4-8 inches of mushroom anything taller than 16-18 inches is likely unnecessary.
Some Things to ConsiderI place the bottom holes so that they split the side in to thirds. The distance between the holes is the same as the distance from the corner.
To help prevent side pinning, line the inside of the tub with plastic. As the substrate shrinks, it plastic shrinks with it and thus prevents a nice climate on the sides for the mushrooms.
Leave at least 4-5 inches of space between the tub and walls in your grow room. This will allow for the needed air circulation created by the fan.
No need to tape the bottom of the tub. That was done when we used to think light was the main cause of side pinning. Just don't bother.
Frank's Bulk Spawning Tek (in a monotub)His has pictures

Now it's time to
Make A Monotub