Last summer my wife and I added an attached greenhouse to the side of our house. When it is cold outside it sweats an awful lot. What can we do to eliminate (or at least minimize) this sweating? We do have a dehumidifier there that we use, but it doesn’t solve the problem.
Plants constantly pump moisture into the air through their leaves—a process called transpiration. More moisture evaporates from the soil. For those of us whose homes become dry in winter, the extra humidity given off by a roomful of indoor plants is welcome. But when humid air comes into contact with a cold surface, like window glass, you get condensation.
Greenhouses are humid by nature. You don’t mention what types of plants are growing in your greenhouse, but because most plants prefer a humid environment, the moist air is probably doing more good than harm.
If drippy glass is an annoyance, you could try adding a second layer of glass, plastic film, bubble wrap, or polycarbonate sheeting inside your greenhouse. This added layer of insulation means you’ll get less condensation because the surface won’t be so cold. Add a fan, if you don’t already have one, to keep the air constantly moving. Raising the temperature in your greenhouse slightly will also help reduce condensation (but will increase your energy costs). When you’re watering, take care to not splash water on the benches, foliage, and floor.
These tactics will help somewhat, but in the end, you have to choose between dry glass and an environment that encourages plant growth. —Doug Hall