I recently taught a class called What the Duct!? at the Builders’ Boot Camp in Virginia. Paul Francisco was one of the other instructors (teaching about indoor air quality), and on the last evening at dinner, our conversation turned to building science. (Imagine that!)
I don’t recall exactly how we got there, but somehow the question of condensation came up. Paul talked about how Bill Rose, author of Water in Buildings, often asks the question, “Can you get condensation on a sponge?”
If you’ve read Rose’s book, you probably already know the answer. Well, let me rephrase: If you’ve read and absorbed (hint, hint) the material in Water in Buildings, you know the answer. If you haven’t read the book, you may now be able to guess where I’m going with this. (But you should still get yourself a copy and read it anyway.)
What is condensation?
On page 81 of the book, Rose quotes Leonard Haeger from the 1952 Condensation Conference: “I suppose in the beginning we should have a definition of condensation, and to practical men condensation is what you find on a highball glass at 5:30 in the afternoon.”
It’s also what you get on single-pane windows (see Image #2, below), bathroom mirrors, and grass on a summer morning. It happens when air with a certain dew point temperature finds a surface that’s at or below that temperature. It occurs under precise conditions of temperature and water vapor content of the air. It’s a point on the psychrometric chart.
The difference between sorption and condensation
Sponges, of course, can have a specific temperature and interact with the water vapor in air. Unlike a glass surface, however, some materials — like wood, brick, and perhaps sponges — collect water…