Well, for others reading, I will share some info I found a while ago... Kirk, that looks pretty good, other than they compared Tyvek Commercial (28 perms) to their product rather than Tyvek (residential) 58 perms, though that wouldn't look so good... unless a commercial product only.
Tyvek (58 perms) is very open, Typar (11.7 perms) is open.
Here is a test that matches an earlier lab test where no difference in WRB's exists. In FL, not somewhere with a lot of rain, and injected water, not under a leaking window flashing; pp. 15 and 18; http://www.buildingscience.com/docu...nce-hot-humid-climates-real-weather-real-time
Snippet from a report sited on pp. 24;
"A laboratory and modeling research program on wall drying in hot and humid climates identified
air leakage and point source water intrusion as prime contributors to moisture accumulation in
wall systems. The research cites two studies, one that reports air leakage accounting for 6 to
100 times as much moisture transfer as vapor diffusion, and another that concludes that point
source water intrusion accounting for 20 times the moisture associated with air infiltration, while
air infiltration accounts for 25 times the moisture movement associated with diffused moisture
(Boone, 2004). This study also compared drying rates of OSB samples inside WRBs with
various perm ratings in a controlled laboratory environment. In addition, it included
hygrothermal modeling of wall assemblies with vinyl siding and different WRB options. The
study concluded that at laboratory conditions of 75ºF and 55% RH, the breakpoint for promoting
drying a wet OSB specimen was between 18 and 24 perms." Found here; http://www.energy.ca.gov/2007public...achment_14_Impact_of_Title_24_Residential.pdf
IMO, if I have a water leak at siding, I'd rather have it open enough than closed too much in the permeability ratings... at 90%RH to dry in 8 days vs 27 days (pp.13 Fig.14), or 55-75%RH (pp.15, Fig. 15); http://repository.tamu.edu/bitstream/handle/1969.1/4625/ESL-HH-04-05-25.pdf?sequence=4
Of course, OSB vs. plywood makes a big difference, as does a water reservoir siding on a HW. Or just use Advantech sheathing.....if it gets wet... anyone have any idea how vapor open (perm rating) that is?
I used to think it was 16 perms, listed by them for the barrier coating, but...
Lol, we're going to see a lot of problems/complaints/questions on the forums in a few years, from the heating climates...