[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

GBlist: Dense pack and diffusion



<snip>
I still have a retrofit question:
How does one protect a blown in insulation from water vapor migration?
Is vapor barrior paint the only path?
<snip>

Dear Mike,

Forgive me for talking down to you. I answer a lot of letters on
weatherization and generally pays to start out with basics. Still
friends?

Invasive examination of a zillion houses that have had cellulose dense
packed (DP) into their old walls, has shown no moisture troubles in even
the most severe climates (wet and/or cold). It's just a super process.

Here's why: consensation in walls requires 4 things. A large resevoir of
water vapor, a path, a motivation, and a cold surface. Generally,
weatherization contractors clean up the water problems before insulating
because they know the problems in tight, wet houses. DP effectively cuts
off the path for water vapor between the warm living areas and the cold
exterior wall surface. We also balance and seal duct systems so that
mechanical pressurization is eliminated as a motivating force.

So while we can't eliminate the cold surface, we can to a good job at
eliminating all the other factors that contibute to condensation
building failures associtated with retofit insulation.

But there's something else that I generally don't mention because it is
too complicated for most novices. I won't make that mistake with you
again. It's the *huge* moisture storage potential that DP imparts to a
wall assembly. Building scientists agree that they cannot accurately
model the moisture content of a wall system without accounting for
storage potential and, of course, more is better. Well, DP provides the
most, and this has been demonstrated to be a big factor in protecting
the walls and building durability into the finish systems. Houses
insulated in the '20s with cellulose in Saskatuan (12,000 DD) had
perfectly preserved wall assembles when studied in the late '70s during
demolition. It is the only insulation material that has a this kind of
track record, vapor diffusion barriers (VDB) or not.

And speaking of VDBs, they are generally considered useless at best and
destructive at worse. Special paint is silly and completely undesirable
(along with gloss paint and vinyl wall paper) in the South. Diffusion
has been known to be a second order moisture problem since 1950. Now it
is recognized as a non-issue. VDBs in the form of paint and poly sheet
do not contibute to the pressure boundary needed to exclude convecting
water vapor from wall cavities. So we don't use them in new construction
and don't loose any sleep from their absense in retrofits.

Congratulations on your new old home and renovation adventures, Fred
__________________________________________________________________
This greenbuilding dialogue is sponsored by Oikos (www.oikos.com)
and Environmental Building News (www.ebuild.com). For instructions
send e-mail to greenbuilding-request@crest.org.
__________________________________________________________________