Greenhouse humidity control

Understand what the reading is telling you

Relative humidity changes as air warms and cools, even if no water is added. A high reading at a cold dawn and a lower reading in a warm afternoon are not directly comparable. Put a hygrometer among the plants, away from dripping glass and direct sun, and note the reading at roughly the same times over several days. Pair it with a temperature reading and a visual check of wet leaves or glazing.

Condensation itself is not a diagnosis. Drops appearing overnight on cold glass are normal; water that continues to drip onto leaves and benches through the day suggests excess moisture or inadequate air exchange. The RHS ventilation advice also emphasises maintaining airflow around greenhouse plants. Different crops and life stages prefer different conditions, so a rigid universal humidity target is less useful than observing the pattern.

Trace the sources of moisture

Watering is the largest controllable input in many small greenhouses. Check compost before watering, apply water near the root zone, and avoid routinely soaking foliage, benches and paths. Water early enough for splashes to dry before a cool night. Empty standing water from trays unless they form part of a deliberate, monitored capillary system. Find a more consistent plan in the drip-irrigation guide.

Dense leaves release moisture and block the air path. Give plants enough separation for their mature size, remove diseased material promptly, and keep the space under staging clean. If you are overwintering plants, group them for efficient protection without packing them so tightly that leaf surfaces stay wet. The greenhouse hygiene guide covers cleaning and plant-health routines.

Exchange damp air without chilling everything

On mild days, open a roof vent and lower inlet to replace humid internal air. Adjust for wind, rain and the needs of tender plants. An automatic roof or compatible louvre opener can help, but its temperature response is not a direct humidity controller. Check the hardware and actual airflow rather than assuming an opener cures condensation by itself.

A clip fan is useful where foliage is crowded or a corner remains stagnant. Aim for gentle movement across the canopy, not a constant blast at one pot. Circulation is not extraction: if every opening is shut, the fan recycles the same moisture-laden air. Any mains equipment in a wet growing space needs a suitable protected supply, installation and placement in accordance with the maker and electrical-safety guidance. The fan guide distinguishes the two jobs.

A practical week-long check

Record temperature, humidity, vent position and whether foliage is still wet mid-morning. Change one factor at a time: for example, water earlier and at the base for several days, then improve airflow if dampness persists. Clean or replace a defective hygrometer if its readings seem implausible. If the greenhouse is cold and closed for winter, balance ventilation against frost protection; the winter-temperature guide explains why an unheated structure should not be assumed frost-free.