Cob: attic ventilation

[Shannon C. Dealy]

On Mon, 10 Mar 2003, Darel Henman wrote:

[snip]
> I remember reading an article by a U.S. southern state contractor who
> was complaining about the Uniform Building Code, UBC.   He stated that
> the venting requirements for the north were different from the south in
> practical terms, but the building department people, not people with
> actual building experience didn't recognize that.

While I would tend to agree with the general sentiment (venting
requirements vary from place to place), it is definitely not as simple as
North and South.

> So does a buildings attic have to be vented during all seasons or just
> for some seasons?
> During a hot dry or even humid summer it makes sense to cool and keep
> dry this area.  But, I was wonder what about dry winters?   Anybody with
> any experience on knowledge on this?

It really shouldn't matter how dry your Winters are from a venting
perspective (though how warm they are will make a big difference), if you
close/remove the vents during Winter time you will have major problems.
It is important to remember that houses are moisture emitters - cooking,
showers, breathing of the occupants, and other activities release moisture
into the interior air in large quantities.  This moisture will migrate
(along with the interior air) through the walls and ceiling, cooling
(because it's winter and presumably cooler outside) as it approaches the
exterior side of the wall or ceiling.  Because colder air cannot hold as
much moisture as warmer air can, the excess moisture from the interior air
will start condensing at some point (when the dew point temperature is
reached) in this trip through the walls/ceiling.  If there is not
sufficient ventilation available, this moisture will build up and possibly
cause mold, mildew, and rot.  This mechanism is also the reason that
cement stuccos, sealant paints, etc. fail when used on cob walls, because
the venting mechanism with cob is that the material breathes, if you seal
the exterior of the wall with something, the moisture will condense
near the outside of the wall behind the sealant (cement, paint, etc.)
and either:

   1 - Cause the cob to soften and/or separate from the sealant.

   2 - Freeze and expand causing the exterior surface of the cob and
       coating to break apart.

   3 - Both of the above.

Either way it will trash the exterior surface of the building -- they
learned this the hard way years ago in Mexico when attempts were made to
preserve some historical adobe structures by covering them with a
cement stucco, the result was far worse than the damage they were trying
to protect the buildings from.

Shannon C. Dealy      |               DeaTech Research Inc.
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