> 2) If the cladding or brick veneer are wet then heated
> with sun, the vapor pressure in the cavity will go high, so the vapor drive
> is strong toward the inside of the building.
>
>On #2, if you get all this water/moisture/vapor from the heated wet masonry,
>isn't it (theoretically, at least) going to condense on the cold surface (the
>housewrap), and dribble down the housewrap until it dribbles out the weeps
>(assuming you had weeps to begin with, that they didn't get clogged, and that
>the 10,000 holes you put in the Tyvek  to nail in brick ties didn't cause the
>Tyvek to leak like a sieve anyway), rather than passing through the Tyvek?
>
>Have the Fine Homebuilding guys (and guyettes) observed these conditions?
>
>
Ralph,

This is a problem mostly in the southern US. The wrap is not a cold
surface--it's at the same temperature practically as the back of the brick.
The cold surface is either polyethylene sheet on the inside of the wall or
vinyl wall covering. The condensing goes on at the poly or the vinyl during
air conditioning seasons. The American Hotel and Motel Association noted
around the Gulf Coast about ten years ago that they were losing $600M per
year in lost revenues due to actinomycete and mold growth behind vinyl
siding, mostly Streptoverticillium rubrireticuli, which produced a luscious
red/orange pigment. Same flora and pigment, apparently, that affects poorly
drained exterior marble.

Regarding water shedding of wraps, I've seen thousands of weeps and, to
best of my recollection, I've only seen three weep dribbles where the wet
spot around the weep is noticeable. We reckon in single-wythe brick
construction, that, for design purposes 1% to 3% of the water that is
applied to a vertical brick surface gets to the back surface. Then most of
that water spreads on the back of the brick and the mortar mess and, over
time, evaporates into the cavity. Liquid water gets to the wrap either from
bad flashings or across mortar bridges, with flashing the real culprit. A
nail and nailhole will leak water, but a tie nailed to sheathing through a
wrap will be closed up, at least as long as the nail stays driven tight.
When a ten-yr old wall is disassembled, there are usually non-serious black
spots at the fasteners. Asphalt felt seals better to nails than any of the
wraps, and has a little moisture storage to boot.

Fine Homebuilding hasn't really tackled this but the Journal of Light
Construction has. February 2000 issue reports on a very non-standard set of
tests by Paul Fisette at UMass, where asphalt felt wins and the losers are
bad flashing, poor design, and any of the perforated housewraps.

Regarding guyettes in construction--I read somewhere that the circular saw
was invented by Sister Tabitha at one of the Shaker communities. Any
verification out there?