wrote:

http://www.cibse.org/pdfs/8cimbabi.pdf has an equation for the dynamic
metric U-value of a breathing wall, as corrected:

Ud = VRhoaCa/(e^(VRhoaCaRs)-1) W/m^2K, where

V is the air velocity in meters per second,
Rhoa is air density, 1.2 kg/m^3,
Ca is the air's specific heat, 1000 J/(kg-K), and
Rs is the wall's static thermal resistance in m^2-K/W.

Using V = 1/3600 (1 meter per HOUR :-), and Rs = 5.7 m^2K/W (a US R32 wall),
Ud = 0.058 W/m^2, like a US R98 wall. A more typical V = 10 meters per hour
makes Ud = 1.7x10^-8 W/m^2K, like a US wall with an R-value of 334 million :-)

Ud becomes infinite as V becomes 0, which seems odd...

No... it approaches 1/Rs, like it orta, as e^x -- 1+x :-) For instance,
Rs = 5 and V = 10^-8 makes Ud = 1.2x10^-5/(e^(6x10^-5)-1) = 0.19994...

Profs Imbabi and friends at http://www.environmental-building.com
are now testing a commercial "dynamic breathing wall" product in
Scotland, Holland, Italy and Dubai. Coming soon to the US
(in February in Houston.)

Nick