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RE: GSBN:re: Moisture Problem

Rob Tom's analysis seems spot on.  Diffusion can be ruled out except in
cases with essentially no plaster on the interior and thick and/or
impermeable plaster on the exterior.
Air leaks are very very likely

I would not run 40%RH in outdoor temperatures much below 30 F, and most
windows (not good ones) will have condensation below this temperature at 40%

Dr John Straube
Associate Professor
Dept of Civil Engineering & School of Architecture
University of Waterloo
Waterloo, Ont. Canada
<a  target="_blank" href="http://www.civil.uwaterloo.ca/beg";>http://www.civil.uwaterloo.ca/beg</a>

-----Original Message-----
From: GSBN [<a  target="_blank" href="mailto:GSBN@...";>mailto:GSBN@...] On Behalf Of Rob Tom
Sent: Tuesday, November 15, 2005 2:04 PM
Subject: GSBN:re: Moisture Problem

On Fri, 11 Nov 2005
Paul Lacinski paul@... wrote:

> pattern of wetness at the outside top of the north wall, which
> appeared as if water had been poured along the top edge of the wall,
> and allowed to run down.

> found ice on the underside of the sheathing,

> outer 2" of the bale, and into the plaster.  The outer 12-24 inches of
> cellulose was also quite wet

> dense-packed cellulose in cathedral ceilings.

> The walls had approximately 1" of lime stabilized clay plaster, with a
> lime finish coat, >and limewash.

> The ceiling is 1/2" drywall with latex paint, poly, and dense-packed
> cellulose.

> In late February, Jim took a set of moisture readings which showed
> alarmingly high moisture levels in the outer 3-6" of the walls, often
> in the range of 25-40%.

> in mid-June. many readings still in the high teens and 20's, there
> were none in the 30's, and all high readings were in the very outer
> inch or so of the straw, just behind the exterior plaster.  The inner
> third of the bale was consistently at 8-12%, and the middle third at
> 8-15%.  Even the outer third of the wall, except for that last inch,
> ranged from 10-20%.

> highest exterior numbers were on the east wall, which is most exposed
> to rain, and >receives some spray from an adjoining roof.

  >But direct air leakage could not have been possible for depositing
> in all of the problem places

> For some time now, I have been musing on the possibility that the
> plaster system that we use- equally permeable on the two sides of the
> wall- may not be suitable for this climate.

> generally high moisture levels in the field of the wall also indicate
> that it was working through the plaster by diffusion,

Paul and GSBN Amiguettes and Amigos;

I haven't had a close look at the long,long long (and very nice, thank you
very much) report yet but my first response is  that I doubt very much if
vapour diffusion through the wall plaster is responsible for the wet walls
or that it's even an issue.

It never has been with conventional construction systems in Cold climates
and I think that it is even less likely with the thick wall plasters that
are typically used with SBC.
Besides, the greatest vapour pressure will be at the ceiling and its vapour
permeance (1/2" drywall) is higher than that of the wall plaster.

I suspect that the biggest culprits are rain wetting and air leakage.

The ice formation on the underside of the roof and the wet ceiling
insulation and top of the walls are strikingly similar to a classic example
of poor air sealing in the ceiling and poor attic ventilation.
Cathedral ceilings are particularly prone to the above symptoms because they
simply do not provide the attic volume that triangular trussed roof
systems provide.   The additional volume somewhat helps to alleviate the
poor ventilation condition somewhat by dispersing the trapped moisture but
of course, exhausting it altogether is preferable.

That the attic moisture then ran down to the bottom of the underside of the
roof and then rained onto the walls is not at all surprising.

While it is mentioned that there was a poly sheet used in ceiling beneath
the drywall and that there was tarpaper placed beneath the wall-ceiling
junctions, I am more than a little suspicious that
    (1) the poly was punched full of holes when installing the drywall
    (2) there are pentrations for electrical boxes and wiring that weren't
    (3) the joints between sheets of poly weren't made air-tight ... or in
short, the air barrier is not.

As for the 40% interior relative humidity in winter, that's not excessively
high IMO.
I like to maintain at least that level in order to avoid the health problems
that are associated with the too dry winter conditions that are typical of
air-leaky buildings.

=== * ===
Rob Tom
Kanata, Ontario, Canada
<ArchiLogic at chaffyahoo dot ca>
(winnow the chaff  from my edress in your reply)

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