From: "John Straube"
Sorry I wont see you, but at least we have email :)
Bruce, being the super editor he is, prodded me to provide
quantitative recommendations in the book. So, I did. These are
pretty general however and the permeance of the interior layer will
depend on numerous factors, the primary ones are:
1. how cold is the climate and how much sun does the wall see
2. the interior temperature and RH during the winter months,
3. the exterior plaster skin permeance, and
4. the rain wetting and drying capacity needed.
1. The recommendation for 5 or 6 perms applies to the North East.
However, in the Green Mountains, 5 perms is likely necessary,
whereas a house south of Boston (much warmer) could work fine with 10 perms.
2. if the house is operated at 65-68 F and 25-30%RH during cold
(less than 30 F) weather, you can pretty much do what ever you want
with the interior permeance. If the conditions are 72-75 and 35-40%
things are very different. Just to let you see how important this
point is... 65 F and 30% RH has a dewpoint temperature of 32
F... 75 F and 40% RH the dewpoint is 49 F (the temp at which
condensation will occur, and a direct measure of the air's moisture
content). The higher temp/humidity air contains almost 30% more
vapor. But, given the physics of diffusion through a wall, this
means that the likliehood of wetting the back of the exterior
plaster/straw interface will be about 4 times higher.
3. The higher the exterior plasters permeance, the less the interior
matters. The Scandinavian values of permeance ratios are another
attempt to simplify advice. The ratio's they use (5:1 or 10:1) are
valid for layers that have low permeance, such as polyethylene on
the interior (0.1 perm) and thus 0.5 to 1 perm on the outside. When
the permeances are higher this ratio does not work as well. For
example, with a painted drywall (say 5 perm) interior then a 10 perm
exterior is usually fine, even though the ratio is just
2:1. However, it is critical NOT to apply Scandinavian rules to
much of the North east. Except for coastal Maine and similar
climates, the rest of the NE actually gets warm humid summer
weather, in which case the vapor flow reverses. This is not
something the Danes or Swedes understand, and applying their rules
in, say, Boston can result in disaster.
4. The reason we want a high permeance of the interior skin is to
allow for inward drying during sunny days and after rain in summer.
If you don't have much rain to dry, or if that rain can dry to the
exterior, the interior permeance can be much lower with success.
This is certainly the experience with cement plasters (which have
much lower permeances). They only seem to fail when gets in somehow.
Given all these variables, and the fact that each can vary by a
factor of 2 or 3 or more, changing the interior permeance from 5 to
9 perms cannot be seen as likely to result in failure. The 5 or 6
perm recommendation was a pretty conservative and pretty general
recommendation only. The reason for up to 10 perms is that with
planned and controlled ventilation I know that the interior RH
levels will be controlled, and the difference in interior conditions
I do not, however, believe the contention that SB homes are
inherently airtight. The few blower door tests I have seen have
shown that some are tight, and some are loose. Depends on the
details, not the stucco wall!!!! Realise that drywall which lines
most wood framed homes allows no more airflow through than
cement-lime plaster that lines SB homes. The reason both leaks are
joints at rim joints, around windows, at baseboard, at roof
insections and penetrations, etc. These joints exist in both types
of homes and so leakage varies in a similar manner. In fact, SB
walls are often built with pretty air leaky windows and doors, and
so DO have more than normal accidental and unplanned ventilation.
So, I would not freak out in the least. I would try hard to get the
outer permeance greater than the interior (since during winter this
is the predominant drying direction) but I would try harder to
control the interior RH (since this is the source of both diffusion
and air leakage concerns).
I would be very concerned (meaning something like a greater than 10%
chance of a serious failure) if the exterior skin was cement stucco
(low perm) the interior was gypsum(high permeance, over 15 perms)
and the interior was occupied by numerous people, in a cold place
(e.g, Burlington Vermont) with no mechanical ventilation. If this
extreme case were already built, I would go back at about this time
of year and measure the interior RH over several days and cut some
cores into the concrete plaster to measure the moisture content of
the plaster and straw. If the RH was low, and the MC reasonable, I
would provide a written direction to the occupants that they need to
continue to control interior RH and then go home and sleep very
well. If I did see high interior RH problems and moderate to high
MC, I would install an exhaust fan and wire it up for continuous
operation. If the RH was not too high, and MC was high, I would
first check for rain, and then add two layers of good latex paint to
the interior. Then sleep less well and come back next winter.
RE: The Detail. The top seal of the wall needs to be an air barrier,
not a vapor barrier. This air barrier can best be formed by straw
reinforced earth, or a weak mixture of thick lime plaster (applied
in several layers to avoid cracking). This not only stops air
movement, it accepts and redistributes any short term condensation
that might occur. A sheet of peel and stick or poly would be a bad
thing. Building paper over a thin plaster skin would be darn fine.
Hope this helps . .. .
Dr John Straube
Dept of Civil Engineering & School of Architecture
University of Waterloo
Waterloo, Ont., Canada
Building Science Consulting
519 741 7920