Greenhouse condensation: reducing persistent damp

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Greenhouse condensation occurs when warm, moist air comes into contact with cooler surfaces, such as glass panes or metal frames, causing water vapour to liquefy. Persistent dampness in a greenhouse is typically a result of poor air circulation and an imbalance between temperature and humidity. To reduce this, gardeners must implement a combination of ventilation that suits the weather, adequate plant spacing and careful watering to prevent the air from becoming oversaturated with moisture.

Diagnosing the causes of persistent damp

Identifying why a greenhouse remains damp is the first step toward a solution. Condensation is most prevalent during two distinct periods: winter and the transition into spring. In winter, the temperature difference between the heated interior and the freezing exterior is extreme, leading to heavy droplets on the glass. In spring and summer, dampness is often caused by high transpiration rates from plants combined with insufficient airflow.

Signs that condensation has reached a problematic level include:

• Water dripping from the roof onto foliage, which can encourage fungal growth.
• A “steamy” atmosphere that persists long after the sun has risen.
• The appearance of grey moulds or white, fuzzy patches on the undersides of leaves, which may indicate downy mildew.
• Persistent dampness on the floor or benches that does not dry out between watering cycles.

Optimising ventilation to clear moisture

Air movement is the most effective way to remove excess water vapour. When air is stagnant, humidity rises, and water clings to the structure. Effective ventilation requires a flow of air that enters at a low level and exits at a high level, carrying moisture away.

There are three primary points of ventilation to manage:

1. Roof vents: These are the most critical as they allow hot, moist air to escape from the ridge.
2. Doors: Opening doors provides a large volume of air exchange and can compensate for a lack of roof vents in smaller structures.
3. Side vents: Often louvred, these are less effective than roof vents but help create a cross-breeze.

Use roof vents, side openings and the door together when weather permits, while protecting plants from cold draughts and securing openings against wind. The amount of ventilation needed changes with weather and planting density; a fixed vent-area ratio does not guarantee an air-change rate.

Managing humidity through watering and spacing

The way plants are watered and positioned directly impacts the amount of moisture released into the air. Transpiration—the process where plants lose water through leaf pores—increases humidity. If the air is already saturated, this water condenses on the glass.

To reduce the moisture load:

• Avoid evening watering: Watering late in the day leads to high humidity and leaf wetness that persists throughout the night, creating an ideal environment for diseases like downy mildew. Water early in the morning so surfaces dry quickly.
• Limit overhead watering: Avoid spraying water over the foliage. Instead, water at the base of the plant to keep leaves dry.
• Increase plant spacing: Avoid dense planting. Leaving gaps between pots and controlling weeds ensures that air can circulate freely around the foliage, speeding up evaporation.

Using shading to regulate internal temperature

Shading is useful for excess solar heat, but it is not a general cure for condensation. Cool surfaces can still collect moisture at modest air temperatures. Use shading only when the plants need it, and remove unnecessary winter shade so available light reaches the crop.

Different shading methods offer varying levels of control:

• External blinds: These are the most effective for cooling because they stop sun rays before they hit the glass.
• Internal blinds: These are easier to automate but less effective at cooling since heat still enters the glass.
• Shade netting: A cost-effective polyethylene mesh fixed inside the glasshouse.
• Shading paints: Diluted paints applied to the exterior glass in spring and washed off in autumn. These are not recommended for polycarbonate or acrylic glazing as they may leave permanent traces.

Winter damp and insulation challenges

Condensation can occur in heated and unheated greenhouses when moist air meets a surface below its dew point. Keep watering appropriate to plant growth, remove standing water and decaying foliage, and ventilate in suitable weather. Heating alone does not remove moisture.

To mitigate winter damp:

• Ventilate on mild days: Even in winter, opening doors and windows on sunny, mild days is essential to flush out accumulated dampness.
• Seal draughts: Replace damaged glazing and renew the correct seals where necessary. Keep designed ventilation and drainage gaps open; do not use sealant as a repair for cracked glass.
• Strategic insulation: Adding a layer of bubble wrap can reduce heat loss and slightly raise the temperature of the inner surface of the glass, reducing condensation on some surfaces. It does not lower the air’s dew point by itself. However, this reduces light by approximately 10%, which may affect light-hungry crops like winter lettuce.

Useful products for this job

The links below open Amazon UK product searches. Check the exact item, seller, current price and compatibility before buying.

• maximum-minimum thermometer: Records temperature extremes to support decisions about ventilation and plant protection; temperature alone does not measure humidity. Ensure it is a max-min model rather than a digital instant-read to track overnight temperature drops. Search Amazon for maximum-minimum thermometer.
• automatic vent openers: Helps maintain consistent airflow even when the gardener is absent, reducing the risk of humidity spikes. Check the compatibility with the specific vent frame width and the temperature setting for opening. Search Amazon for automatic vent openers.

Related garden guides

• More greenhouse & growing guides

Sources and further guidance

• Greenhouse: Ventilation and Shading Tips | RHS Advice
• Greenhouses: Heating Efficiently | RHS Advice
• Downy mildews: Symptoms, Causes & Control | RHS Advice

Guidance researched in September 2026. The feature image is an AI-generated illustration. Product guidance is based on published information, not hands-on testing.