Cob: Breatheable cob walls

[Shannon C. Dealy]

On Thu, 6 Dec 2001, Darel Henman wrote:

[snip]
> > I do have a question though. My understanding is that cob needs to breathe and
> > that insulation and a vapor barrier would inhibit this and may lead to possible
> > moisture problems?
> > Would appreciate someone more qualified sharing their views on this. 
> 
> Cob walls can breath and its good.  It's not necessary I dare say.  But,
> the point in using earth is that it does breath and helps regulate
> humidity inside the house. 
> 
> If you used an outside vapor barrier the wall could still breath from
> the inside.
[snip]

Actually, it is a really good idea if cob (and for that matter, most
building materials) are able to breathe, if they cannot breathe, than some
provision MUST be made for the structure to breathe in some other way for
the following reasons:

   1 - It can take a year or more for a cob wall to dry to the point
       where it's moisture level is stable relative to the average
       humidity of the surrounding air.  If the wall doesn't breathe due
       to impermeable exterior insulation, that moisture is trapped inside
       the building unless you make other provisions for getting it
       out of the building.  This may cause problems with mold or mildew,
       not to mention greatly slowing the drying of the wall.

   2 - Occupancy of a building releases large amounts of moisture in
       various forms that are trapped inside the building unless some
       provision is made for it to breathe, regardless of the weather
       outside:

          - Exhaled by occupants
          - Sweat
          - Steam from showers and cooking
          - Standing water in toilets and bathtubs

       all of this can add up to a great deal of moisture over time, which
       for various reasons, can and will tend to concentrate in certain
       areas of the building, particularly cooler areas such as around
       cold water pipes, at impermeable moisture barriers inside of
       exterior walls (which will provide a "cold" surface for the
       moisture to condense on during cool weather), and possibly other
       places as well.

   3 - For the health of the occupants, it is necessary that air in the
       house be continually exchanged with the outside air to prevent
       build-up of toxins from sources inside the house such as plastics,
       formaldehyde based building materials, etc., not to mention
       providing fresh oxygen (you did want to breathe didn't you :-)

While the above does not preclude having non-breathable cob walls, if you
don't make some provisions for exchanging air and eliminating moisture,
your building could develop serious problems including: mold, mildew, rot,
separation of plaster from walls, bubbling paint and possibly other
problems.

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
                      |    Embedded Systems, Real-time, Device Drivers
Phone: (800) 467-5820 | Networking, Scientific & Engineering Applications
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