On Nov 2, 2:08*pm, nestork wrote:
I'd like to address this issue of whether or not painting stucco will
prevent it from "breathing", and thereby eliminating moisture from the
walls of the house.

Moisture gets into the exterior walls of a house through leaks in the
vapour barrier, such as at electrical outlets in exterior walls. *That
moisture accumulates over the course of the winter, forming condensation
and frost during the winter, and that frost melts in the spring time.
It's true that mold needs moisture to survive, but an occasional wetting
once per year isn't enough to support mold growth.

People should be aware that all latex paints (both interior and
exterior), "breathe", whereas neither interior nor exterior oil based
paints "breathe". *The term "breathe" means that humidity can pass
through the dry paint film, but not liquid water.

Oil based paints don't breathe because they crosslink so densely that
there isn't sufficient space between the oil molecules or the parts of
the alkyd resins to allow water molecules to pass through the paint
film.

Both interior and exterior latex paints "breathe" because the gaps
within and between acrylic resins are larger than the diameter of a
single H2O molecule, but smaller than the distance between H2O molecules
in liquid water.

Consequently, individual H2O molecules can pass through a latex paint
film relatively easily, but liquid water cannot pass through it. *It's
that ability to allow humidity to pass, but not liquid water that's
really what's meant by that term "breathe".

An acrylic resin can best be thought of as a long copper wire scrunched
into a small ball. *No matter how strong the person doing the
scrunching, tiny gaps will remain between the segments of wire that make
up each ball. *Those gaps will be wide enough to allow a sufficiently
fine powder, like sand say, to flow through the wire ball. *Water
molecules attract each other, and so liquid water cannot pass through
that scrunched up wire ball unless the gaps between the wire segments
are wider than the distance between H2O molecules in liquid water, and
in latex paints, they're not.

So, I don't see why painting stucco with a latex primer and exterior
latex paint is going to prevent moisture inside the wall from
evaporating through the paint film.

Masonary paints are latex piants that have particularily wide gaps
between the wire segments, but not so wide that they allow liquid water
to pass through the paint film. *That allows for the greatest
"breathability" of the paint film to minimize freeze/thaw damage in
masonary walls from moisture getting inside the wall and freezing.

I really don't know how well elastomeric coatings breathe.

--
nestork

Thanks for the detailed explanation.

It is my understanding that here in Arizona the moisture barrier is
reversed compared to homes in areas that are cold/freezing. Think
about it we run AC here, making the interior colder than exterior at
least 9 months a year. I don't run it at all if I can help it. but
you see the concept of the inside being cooler than the outside.

However, when working on the exterior wall to reposition the stove top
vent hood; the barriers looked like they were still set up for a
'freezing' climate. consisting of
interior paint
joint compound skim coat
gypsum drywall
nylong netting[ chicken wire gaps
fluffy fibre glass insulation blown in between the studs
1/2 inch sheathing
two layers of tar paper
stucco
exterior paint

Upon running the stove top fan am convinced the house is, nor could
ANY house could be, tight enough to need super breathing requirements.