Winter Ventilation for your Greenhouse

Control humidity and excessive condensation by maintaining proper ventilation through the year

We all know the importance of adequate ventilation of your greenhouse during the hot summer months to control high temperatures, but winter ventilation is just as important in maintaining a healthy environment for your plants.

Greenhouse ventilation, simply stated, is removing the inside air of the greenhouse and replacing it with fresh outside air. This can be accomplished by natural means with curtains and vents or mechanically with fans and shutters. Ventilation systems, regardless of the type, should be sized to meet the demand of each of the growing seasons.

The primary need for winter ventilation in a greenhouse is to control the high humidity and excessive condensation caused by the difference in the inside temperature and the outside temperature.

 A heating system is generally needed to create a proper environment for plant growth during the winter months. When the warm humid air that’s been created in the greenhouse comes in contact with the cold surface of the greenhouse frame and covering, condensation occurs.  The colder, fresh outside air must be brought in to ventilate the greenhouse and remove the moisture filled air within the greenhouse. This moisture filled air, if not removed, will result in increased plant disease, mildew build-up within the greenhouse and accelerated deterioration of the greenhouse structure. Winter ventilation is also beneficial in maintaining healthy balances of carbon dioxide and oxygen.

It is important to determine a ventilation rate that will maintain safe humidity levels while keeping heating costs to a minimum. If the air exchange is too high, giving you too much ventilation, you will have increased heating costs and possibly a relative humidity that is too low for healthy plant growth. If the air exchange is too low, you will not get enough ventilation resulting in continued condensation and related problems. A general guideline is 2-3 air exchanges per hour for winter ventilation, when the inside temperature is being maintained at 60-70 degrees.

The air exchange refers to the volume of air that is being moved in a given amount of time. In order to determine proper fan size, you must first calculate the total cubic feet of space within the greenhouse. To calculate the cubic feet, multiply the greenhouse width x length x average height.

Once the cubic feet have been calculated, size the fan according to its cfm performance rating to give you the desired air exchange.


A 30×96 gothic greenhouse with an eave height of 10’ and a peak height of 18’.

30 wide x 96 long x 14 average height = 40320 cubic feet.

To get an air exchange of 3 per hour, multiply the 40320 x 3 = 120960

Divide this by 60 minutes per hour to convert to cubic feet per minute 120960/60=2016 cfm

For more information on ventilating your greenhouse, contact us today.