A lit room without controlled climate is a room that cooks. Every watt of lighting ends up as heat, every plant transpires, and these two flows add up in an enclosed volume. This step deals with air: renewing it, moving it around, holding your temperature and humidity targets, and managing odours.
Why air must be renewed continuously
Air renewal fulfils three functions simultaneously, and forgetting it makes all three fail at once. It removes the heat produced by the lighting, it removes the humidity produced by plant transpiration, and it resupplies the room with carbon dioxide, which photosynthesis consumes continuously.
In an enclosed, well-lit volume, the carbon dioxide concentration drops within a few tens of minutes to a level where photosynthesis slows markedly. The plant may receive all the light in the world, but it no longer has the raw material to use it.
