Allison Bailes, the creator of the storied Energy Vanguard blog, gives us a compelling summary of Professor John Straube's half-day seminar on moisture physics. Straube is well-known for his practical insights on the transfer of moisture, air and energy through building envelopes, and he reportedly delivered in spades on the moisture part at this summer's Westford Symposium on Building Science.
Here are some of the nuggets of information from Straube that Bailes reports:
Houston in summer is worse [for problems due to capillary condensation] than Vermont in winter. That doesn’t mean capillary condensation in Vermont is OK. Because of the lower temperatures, Vermont just won’t be as likely to support the microbial infestations that rot wood and cause indoor air quality problems. But when capillary condensation occurs on the backside of air conditioned drywall in Houston, the temperature is much more conducive to microbial growth.
... water molecules group themselves into large clusters ... [On] a material like spun-bonded polyolefin, the stuff that Tyvek house wrap is made of, liquid water has trouble going through the pores because of the size of the water clusters.
Professor Straube brought all this fundamental moisture physics home with a discussion of concrete and wood, two materials used occasionally in buildings. (And when I say “occasionally,” I mean in almost every building.) He talked about porosity and permeability and how those properties affect the ability of a material to get wet, store water, change size, deform, and dry out.
To get all the details (including a link to Straube's presentation slides and a list of some of the funny "Straubisms" that make Professor Straube's presentations so entertaining), read more.