VPD — vapor pressure deficit. Temperature and humidity in isolation are misleading; the same 75°F / 55% RH is great for veg and dangerous for late flower. VPD combines both into one number that predicts whether plants can transpire, drink, and eat properly. Dial VPD in and most deficiency, burn, and stretch problems disappear. Ignore it and no amount of perfect feeding compensates. Start with the VPD Calculator; the rest of the environmental tools are secondary.

Yes, with different targets. Cannabis wants moderate VPD (0.8–1.6 kPa across stages) and moderate humidity. Mushrooms want low VPD (0.5–1.0 kPa for fruiting), high humidity (85–95%), and different fresh air exchange requirements. Every environmental tool in this hub accepts a crop selector — pick cannabis or mushroom and the stage targets adjust automatically. Same math, different target ranges.

No. These tools work whether you're running a sophisticated climate controller (AC Infinity, TrolMaster, etc.) or just a basic thermometer and hygrometer. The tools tell you what targets to aim for; how you hit those targets depends on your equipment. Manual adjustment works; automated control works better. Start with the measurement tools, upgrade equipment as the data tells you what's limiting your results.

Likely VPD, even with individually "perfect" readings. A common trap: 78°F at 65% RH feels reasonable but gives VPD of ~0.9 kPa, which is fine for mid-flower but too low for late flower (where you want 1.2–1.5 kPa to encourage transpiration and prevent bud rot). Run your numbers through the VPD Calculator with your stage selected. Often the fix is lower humidity (not higher), which surprises first-time growers.

Sensible heat is air temperature. Latent heat is moisture content (humidity). An AC removes both, but the ratio depends on the unit. Grow rooms produce a LOT of latent heat from plant transpiration — a unit sized only for sensible load will keep the room cool but leave humidity sky-high. When the BTU/CFM Calculator ships, it'll account for both; for now, size AC ~25% larger than pure sensible BTU math suggests to compensate for latent load.

Only if your light intensity is above ~800 PPFD at the canopy. Below that threshold, CO₂ doesn't help because light is the limiting factor; adding CO₂ just wastes money. Above it, CO₂ can increase yields 15–30% but requires a sealed room (no exhaust during lights-on) and tight climate control (higher temps, higher VPD to compensate). The CO₂ Supplementation Calculator will walk through the break-even math when it ships; until then, the short answer is "probably not yet" for most home growers.