Venting is the default and usually the right answer: a short vent in the middle of a cold day exchanges humid inside air for dry outside air at a modest heat cost. A dehumidifier makes sense in a small closed propagation space in cold weather, where the heat you would lose by venting costs more than the electricity to condense the water out. In a full-size greenhouse a small consumer dehumidifier cannot keep up with the transpiration.
Venting cost
Heat, but outside air is dry
Dehumidifier cost
Electricity, heat retained
Best time to vent
Midday, when outside air is warmest
Air movement
Needed either way
Researched from published specifications and verified owner reviews · Updated 2026-08-16
A greenhouse in winter generates moisture continuously. Plants transpire, wet media evaporates, and the air cools at night until it cannot hold what it is carrying, at which point condensation forms on every cold surface including the leaves. That is the condition botrytis and downy mildew require, and it is the main reason winter crops fail under cover.
Both approaches here remove water. They differ in what else they remove.
The comparison
Figure 1. Dehumidifier against venting·7 rows
Figure 1. Dehumidifier against venting
Measure
Dehumidifier
Ventilation
Removes moisture
Yes, condenses it out
Yes, exchanges the air
Heat lost
None, some is returned
Real, but modest at midday
Running cost
Electricity
Heating to recover the loss
Capacity against a full greenhouse
Inadequate for consumer units
Adequate
Works on a still cold day
Yes
Yes, briefly
Water to empty
Yes, a tank
None
Best use
A small closed propagation space
The whole house
Researched general information rather than professional advice.
The midday vent
Outside air in winter is cold and, in absolute terms, dry. Exchanging inside air for it removes a great deal of water even though the relative humidity outside may read high. The best moment is the middle of the day when outside air is at its warmest, so the heat cost is lowest and the exchange is most effective. Ten minutes with a vent open and a fan running does more than an hour at dusk.
Automating it is worth doing. A humidistat such as an Inkbird IHC-200 Humidity Controller ($42.99) running an extract fan vents on moisture rather than on a timer, so the house is not throwing heat away on a dry day.
Where a dehumidifier makes sense
In a small enclosed propagation area, a cold frame with a lid down, or a closed cabinet in a garage, venting means losing the heat you have paid to build up, and the volume is small enough that a modest unit can keep up. A BeOkay 1,657 ml Small Dehumidifier ($35.15) in that context is a reasonable purchase.
In a 10 by 12 greenhouse with a crop of tomatoes transpiring all day, a small consumer dehumidifier is a rounding error against the volume of water in play. Buying one for that job is money spent on an appliance that will run permanently and change very little.
Note
Cultural measures beat both
Water in the morning rather than the evening so surfaces dry before nightfall. Water at the root rather than over the foliage. Space plants so air can move between them. Clear dead leaves and spent crops, which are where botrytis establishes first. None of these cost anything, and together they do more for winter disease than any appliance.
Air movement is required either way
Neither venting nor dehumidifying reaches the thin humid layer sitting directly against a leaf. Only moving air does that, which is why a circulation fan running continuously in winter belongs in any humidity plan. Set the vapour pressure deficit target you are aiming at against the humidity and VPD chart rather than working from relative humidity alone.
The decision, summarised
A full-size greenhouse: venting, ideally on a humidistat, plus a circulation fan.
A small sealed propagation cabinet or frame: a dehumidifier is genuinely appropriate.
Condensation dripping onto the crop: vent at midday and get air moving before buying anything.
Botrytis already established: remove affected material first, then fix the air.
Any winter house: water in the morning, at the root, and clear dead foliage weekly.
Managing winter moisture
The controller that automates venting, the appliance for small spaces, and the fan that both need.
Broadly, somewhere in the region of 50 to 70 per cent relative humidity suits most crops, with propagation wanting more and ripening fruit wanting less. Relative humidity alone is a poor guide because it changes with temperature, which is why vapour pressure deficit is the more useful measure. Check the crop against the VPD chart rather than aiming at a single number all year.
+- Why does my greenhouse drip with condensation every morning?
Because the air cooled overnight past the point where it could hold the moisture it was carrying, and the coldest surfaces, the glazing and the leaves, collected it. That is normal to a degree, and a problem when it drips onto the crop. Reduce it by watering in the morning rather than the evening, venting briefly at midday, keeping air moving, and reducing the amount of standing wet surface in the house.
+- Does venting in winter waste all the heat?
Far less than it feels like it should, if the timing is right. A short vent in the middle of the day, when outside air is warmest, exchanges a great deal of moisture for a modest amount of heat, and the drier air is then cheaper to heat than humid air was. Venting at dusk or overnight, by contrast, costs a lot and gains little.
+- Will a dehumidifier stop botrytis?
It can reduce the conditions botrytis needs, but on its own in a full-size house it is rarely enough, and it does nothing about the still humid layer against the leaf surface. The measures that reliably help are air movement, sensible watering timing, spacing, and removing dead and infected plant material promptly. Once botrytis is established, sanitation comes before any appliance.
Working out the figures for your own house and season? The Greenhouse Build & Growing Planner is the
paid version of these pages: 8 printable worksheets you fill in with your own
numbers, plus the full PDF, $29.
