Lecture notes
Department of Architecture, Cornell University

ARCH 2614/5614 Building Technology I: Materials and Methods

Jonathan Ochshorn

Permeance Example Disclaimer

Predicting condensation within a wall is complicated; the example linked below is quite simplistic, as is doesn't account for all sorts of phenomena, including the ability of wet areas to dry out, or the movement of air within the wall itself, which can transport vapor in unexpected ways, or the complex hygrothermal behavior of specific materials. Nevertheless, it may provide some insight into why condensation — when it does occur — does occur.

Typical water vapor permeance values (perms*, unless otherwise noted)

concrete	3.2 perm-in.**
brick, 4" thick	0.8
concrete block, 8" thick	2.4
glazed tile masonry, 4" thick	0.12
plaster on metal lath, 3/4" thick	15
plaster on wood lath, 3/4" thick	11
gypsum board, unpainted	50
gypsum board, latex paint	2-3
plywood with exterior glue	0.7
polyethylene, 6 mil	0.06
air, still	120 perm-in.**
RCPS insulation (EPS)	2.0-5.8 perm-in.**
RCPS insulation (XPS)	1.2 perm-in.**
aluminum foil	0.001

* Note that perm values for materials listed with a given thickness may be adjusted to account for a different thickness by dividing the given perm value by the ratio of new to given thickness. So, for example, the perm value of an 8" thick brick wall would be 0.8 / (8 / 4) = 0.8 / 2 = 0.4, where 0.8 is the perm value listed for a 4"-thick wall.

** To find the perm value where units are "perm-in.," divide the tabulated value by the material's actual thickness in inches.

For materials not listed above, search online for perm values.

Example of temperature/permeance gradient diagram

An example showing the construction and calculation of temperature and permeance gradients can be found in my exterior wall psychrometric analysis calculator.