From the beginning of this project, I knew that humidity in the bioshelter during the winter months was going to be the biggest challenge. Most other issues like pests, disease, structural and material challenges could be diminished or solved. But, humidity maintenance is a whole other can of worms.

Why is humidity such a threat?

Have you wondered why summers can be unbearable when it’s humid, yet we never complain of winter being humid? That’s because hot air expands and can hold more moisture, and a lot of it. A cold air mass and warm air mass come together high up. The point where the masses meet is where water “falls out” of the warm air, and condenses forming clouds. Enough moisture accumulates and it rains. Thank goodness for thunderstorms.

Ever take a cold bottle out the fridge and place it on the table in your kitchen on a warm day. What happens? The bottle sweats and creates a small puddle of water on your table. It’s because the air surrounding the bottle is just cold enough for the warm air to turn cool. The cool air can’t hold the water anymore so it “rains” against the bottle. This change from dry to wet because of moisture condensing is call the dew point. Yes, exactly, the same process happens on grass in the morning too!

Back to the question why high humidity is such a threat?

One of our subtropical plants, an air potato. Lots of mold on this leaf.

The answer is, Mold

Like all living creatures, mold needs water to survive, in fact under high humidity mold thrives. Greenhouses have potential mold problems during sunny days and cool nights. This is because the sun comes in, warming the space and all the surfaces, including plant leaves. When the sun goes down, and it is cold outside, the temperature drops pretty fast. Remember, cold air holds less water, what’s going to happen? Your right, the leaves become the cold bottle you took out of the fridge.

Oops, now the leaf surfaces have condensation on them. In fact all the surfaces that have cooled off faster then the air temperature have started to collect water (moist air accumulates in the greenhouse from soil water evaporation and plant transpiration). This will happen to the point where it’ll rain inside the greenhouse! If the leaves have condensation for long enough they will grow mold. Mold will smother the plants and eat them as food.

Strategies for mold prevention due to high humidity and condensation include:

– Water only the root zone, not the leaves, don’t overwater. Water in the late morning.
– Open plant spacing improves air circulation around leaf zone transpiration.
– Use anti-drip plastic or a 6:12 roof pitch to prevent water dropping onto plants.
– Move interior air with fans.
– Control heating and venting with humidistats.

What is a desirable humidity level? It depends on temperature. We use the chart below:

Chart from: http://extension.umass.edu/floriculture/fact-sheets/reducing-humidity-greenhouse

The most effective and consistent control of mold from the strategies above is through the use of controlled heating and venting. Our bioshelter is powered by a 100 watt solar panel and battery. It was not designed to run fossil fuel heaters, fans, vents and controllers. In the spring and fall it is simple enough to open the window and door to control humidity. But, without the fans and heat strategy we are getting mold in our bioshelter in the winter.

So far we’ve found that during a sunny winter day, or cool low humidity cloudy day we can open the door and window for a few hours and drop the humidity to acceptable levels. Unfortunately there have been long stretches of cold cloudy, or rainy days, when we really can’t open it up to lower the humidity.

If we can vent the bioshelter often enough we might be able to hold off the mold from taking over everything. We’ll see how things progress. The experiment continues…