Celebrating the Spring and Autumn Equihume

Condensation on a window. (Ozgu Ozden, https://bit.ly/3kytUIV)

By Nate Gusakov

The practice of marking and celebrating the spring and fall equinoxes is a human custom that is older than Stonehenge. The two points of the year when day is equal to night (equi-nox, equal-night) are still noted by billions of people, all around the globe (this year, March 20th and September 22nd). Somewhat less common is the practice of marking the equi-hume (yes, I just made that word up) – the point in the year at which the average daily relative humidity inside your house is equal to the average daily relative humidity outside. This is a moment that should be celebrated, as it represents a time of building science peace, during which there is very little risk of condensation anywhere in the building assembly of your house.

Here in the northeast U.S., there is a huge seasonal variation in average relative humidity. During the winter months, the air is cold, often well below freezing and even below 0°F. By the laws of physics, this air is much drier than the warm air inside our homes. Because the warmer inside air can hold more moisture than the cooler outside air, there exists a vapor pressure differential that causes interior moisture to drive against outside surfaces. If these surfaces are cold enough, we get that vapor condensing into liquid water on them. By contrast in the summer, the outside air generally carries more humidity than cooler inside air (either cooled by shade or by air conditioning, in which case it is much drier than the outside air). In this situation (which is much more common in the air-conditioned southern states), a vapor pressure differential is formed with vapor driving inward. If this vapor meets a cool enough interior surface, voila – condensation!

The picture shows a familiar sight. Condensation on the inside of a window pane during a cold winter day. Change the scene to a 95°F muggy day in the southeastern U.S. and take the picture from the inside of a house that’s air-conditioned to 55° and dry, and the condensation would be on the outside of the window pane. On equihume days, well, no need to worry in either direction.

As you can see, the progression of seasons in our climate subjects a house to near constant vapor pressure drives, potentially from both directions, leading to condensation risks that have to be addressed with proper design and detailing of the thermal envelope. The proper protection against condensation is a sufficient and consistent thermal envelope with adequate insulation on the correct side of a consistent and continuous air barrier. Each spring and fall however, regardless of the state of the building envelope, every house gets to enjoy the equihume and a respite from the risks of condensation in the building structure.

Nate Gusakov is a building envelope consultant and AeroBarrier installer for Zone 6 Energy.