Re: [GBlist] Re: Wall permeability

["Mike Rogers" <rogers.mike@verizon.net>]

Caveats:

> I second Lewis' opinion.  I have had good results with the approach he
> suggests.  I also rely on an interior air/vapor barrier...

NB:  The interior vapor barrier is for cold climates only.  It is a bad idea
in hot humid climates.

> However, the details I looked at
> (and checked again today) by Building Science Corp. (for buildings in cold
> and semi-cold, warm climates) show an outside layer of foam insulation
that
> is sealed at the joints.  So, it would not really allow the gases from
> inside to get outside, as the foam would act as a vapor and gas barrier
and
> trap both within the building envelope.   The approach of placing the
vapor
> barrier on the outside of a building in a cold climate is in opposition to
> what I have been taught and practice.

There are several considerations with the foam on the outside including:  Is
there an *air* barrier on the inside?  Is there a *vapor* barrier (or
retarder) on the inside?  (Note:  the air barrier and vapor barrier CAN be
the same, but they do not have to be!)  For the given climate how often is
does the temperature of the inside face of the foam drop below the dew point
given the relative humidity inside the house.  What is the vapor
permeability of the exterior foam?  How much attention was paid to
detailing?  Lots of questions that need to be answered.  Nonetheless, if
done properly, the exterior foam insulation can work well in a cold climate.

As several have stated, controlling interior sources of moisture is
important.  I would add the obvious (but often improperly addressed) point
that exterior moisture needs to be controlled as well.  Flashing, flashing,
flashing.

Mike


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