Dear Full, (kidding)

While I admit to having only a layman's knowledge, I thought I
understood the principle of a vapor barrier had to do with the fact that
warm air can hold more moisture than cold air hence the "warm in winter"
adage on what face of any insulation the vapor barrier should be
installed on.  In your scenario, since we are going outside to inside,
to me it doesn't seem like there would be much change in the insulating
value of the block or the brick and if that is the case there should be
very little opportunity for condensation in this wall area.  Ideally the
wall is properly insulated farther inside and the vapor barrier is on
the inside face that insulation.  If that is the case, there should be
very little water vapor to condense between the two masonry units.

As far as the Tyvek rather than membrane discussion goes, I thought that
Tyvek was like Goretex with properties that are indented to resist wind
and water penetration while allowing moisture vapor to pass through.  If
true, I think the joint question is moot (FYI Dupont recommends the
joints be taped but I think that is for maximum wind intrusion
prevention, something that your brick/block wall should do pretty well
on its own) and that Tyvek would be a better choice than an impermeable
membrane inside a wall if any moisture happens to be passing through.

One thing I have always been puzzled by, based on my layman's
understanding of the vapor barrier principle, is the typical practice of
aluminum and vinyl siding contractors of installing foam insulation over
any existing siding before installing their new siding.  I know that it
creates a nice smooth working surface but it seems to me that it would
cause a condensation zone on the inside face of the foam.  Since the
synthetic siding is usually going over a wood product (to cut out the
maintenance requirements of wood) and since it is less likely that the
typically older building getting the facelift has a proper vapor barrier
and insulation, it seems to me there should be condensation problems
with this type of installation.  I know there is a Preservation Brief on
the subject but I have wondered how often problems resulting from this
practice are encountered in the field.  Any eyewitnesses to the problem
or lack of one?

Hoping my vapor barrier theory isn't all wet,
T. Gale






[log in to unmask] wrote:

> Alright, boys and girls, I have another pain-in-the-ass technical
> question, this time regarding vapor barriers and flexible membrane
> flashing inside masonry walls.
>
> I am working on a couple of c. 1950-60 NYC residential buildings, into
> which I want to stop water penetration.  What I have recommended in the
> past is removal of face masonry (in both these cases, brick) and
> installation of flexible membrane flashing on the outer face of the
> backup wall (concrete block), followed by rebricking and installation of
> weeps, drainage mat, etc.  The theory is that whatever water migrates
> inwards through the face brick will be stopped and drain down the face
> of the Bituthene, and out the weeps.
>
> One of my least-favorite contractors said yesterday that there have been
> concerns that sealing up an entire building in this way may prevent
> moist INDOOR air from passing through the wall and outwards, and that
> mold may grow as a result.  What this alleged individual says is that
> they have installed one "course" of Bituthene at shelf angle level to
> drain water out and flash the shelf angles, and above that they install
> Tyvek so that the moist indoor air can pass through the Tyvek instead of
> being blocked by the Bituthene and being trapped forever inside the
> wall.  This guy claimed that the Tyvek was sealed (including horizontal
> lap joints) and nailed or glued to the backup wall, so that condensation
> forms on the INSIDE face of the Tyvek.
>
>  Seems to me that sealing the Tyvek, esp. at horiz joints, will trap
> moisture, too, and trap it at the INTERIOR face of the Tyvek, where you
> really don't want it.  For the Tyvek to do you any good, it seems to me
> that the horiz. Tyvek joints have to be left open so that moisture that
> condenses will run down the interior face of the Tyvek and eventually be
> conducted to the Bituthene and drained out.
>
> OR am I full of shit about how condensation inside walls works, and
> specifically on which plane within the wall it condenses; I have a
> feeling that inside moisture condenses at a different plane within the
> wall than cold oudoor air condenses.
>
> Please advise, and thanks.
>
> Ralph

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