[Strawbale] Dealing with high humidity in bathrooms (was Re: moisture)

Robert W. Tom Archilogic at yahoo...
Thu Apr 7 01:20:03 CEST 2011

On Wed, 06 Apr 2011 04:56:02 -0400, Nikolay Marinov <nikvesmar at gmail...>

> I would like to ask you for advice and share your experience about
> preventing moisture, penetrating into SB walls from bathroom, and other
> spaces with high humidity level.

for full text of message, see:


Moisture (vapour-phase) transport into the wall (or ceiling for that  
matter) insulation materials via vapour diffusion through the broad  
surfaces of the wall's "skins" has never been an issue of concern for  
low-rise residential-scale buildings.

The primary means for moisture intrusion is via air leakage, usually  
piggy-backed on heat loss, through discontinuities in the air barrier.

Those discontinuities are typically at the junctions between dissimilar  
materials (ie plaster-wood,metal-plaster etc), different planes  
(wall-ceiling,wall-floor) or different building components and  
penetrations for services (electrical boxes, plumbing pipes etc.

That is to say, the focus should be on implementing a competent and  
effective air barrier strategy rather than choices for interior finish  
materials, although the choice of materials does matter some.

Somewhere in the (Christian) Holy Bible there is a phrase:

"If thine eye offend thee,
      Pluck it out "

A variation of the above would apply to high humidity levels-- there
should be a ventilation strategy that will provide a means to exhaust
excess moisture,odours and pollution in a timely fashion (ie a matter of  
seconds or minutes after the moisture/odour/pollution event as opposed to  
hours or days).

The means of exhaust may be a simple as cracking open a window or turning  
on a bathroom exhaust fan during/following moisture/odour/pollution events  
but obviously,neither of these will provide any means for heat recovery  
 from the exhaust air, which would likely be an issue in Cold Climate  
regions during the heating season and perhaps less so for milder climates.

While trying to take advantage of the hygroscopic properties of a  
high-clay-content earthen plaster to control humidity is a nice idea, in  
reality, such a strategy would in most cases, be inadequate for
avoidance of potential moisture-related problems (ie moulds, mildew, fungi  
(yes, fungi ... see below) on surfaces of interior finishes and  
furnishings and if the air barrier strategy is ineffective, inside the  
building cavities, in all but the most arid regions.

In conventional construction in high humidity and wet areas like  
bathrooms, good building practise typically includes installing something  
like an asphalt-impregnated felt (AIF) between glass mesh-faced cement  
mortar board (aka "tile backer board, preferable to paper-faced gypsum  
board)  and wood framing to avoid the potential rot that would occur where  
there is wood-to-plaster contact, just as one would install a damp-proof  
course between any wood framing and concrete or masonry.

In a house with SB walls, would you really want to have a high-capacity  
moisture storage medium with high humidity levels and wet service  
conditions on one side, directly in contact with (hopefully dry)  straw on  
the other side  ?

ie If you were moisture sitting in the plaster, are you more likely to
choose to move to the already-crowded (ie humid) air of the room or to the  
less densely-populated,big mass of dry straw inside of the wall ?  
Actually, you would have no choice. The Laws of Physics have already made  
the choice for you.

==========This is "Below"=========

Years ago one summer when I was a student on a co-op work term sharing a  
house with 6 others, one of the housemates was grossed-out when she found  
mushrooms growing in the corner of an upstairs bathroom. (Poor choice of  
floor finish material by a landlord more interested in appearance than  
function (very thick/deep plush broadloom), poor ventilation (east-facing  
shaded window) hot/humid August conditions in Southwestern Ontario  

=== * ===
Rob Tom
Kanata, Ontario, Canada
< A r c h i L o g i c  at  Y a h o o  dot  C A >
(manually winnow the chaff from my edress if you hit REPLY)

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