Humidity and FAE balance in a monotub — the real numbers
47 replies · Cultivation
I keep reading conflicting advice about humidity vs. fresh air exchange in a monotub. Some people say polyfill holes are enough, others say you need to fan. My first tub had great colonization but then the pins aborted — small 4mm pins just stopped growing. What's the actual mechanism here?
Aborting pins are almost always a CO2 problem, not a humidity problem. Psilocybe cubensis needs CO2 below roughly 1000-2000 ppm to pin reliably. In a sealed tub, CO2 from the mycelium builds up and stalls pinning. Your polyfill holes may not be enough — especially in a large tub or cold room with stagnant air. Solution: fan briefly twice daily (30 seconds each) while misting. This is the standard monotub FAE protocol.
Relative humidity should be 90-95% during fruiting. Below 85% and pins desiccate. Above 97% and you risk mold and wet rot on pins. Mist the walls and lid — not directly on the substrate surface. Check by pressing a moisture probe into the top layer of substrate; you want the casing moist but not waterlogged.
My setup that works: 6-quart tub with 6 x 0.5-inch polyfill holes (3 per long side), tub elevated on small blocks for airflow underneath. I fan 30 seconds morning and night without misting (my room humidity is 55-60% and the polyfill alone isn't enough). After switching to this from just polyfill, my pin set density tripled.
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