1612 K Street, NW
Washington, DC 20006
|Strawbale Archive for October 2001|
|236 messages, last added Tue Nov 26 17:42:19 2002|
[Date Index][Thread Index]
Re: SB: Re: Santa Fe style, breathability
Robert le Tom wrote:
>I think that we can all agree that the Santa Fe style, with its
>and lack of overhangs offers no rain protection whatsoever to the exterior
>wall cladding if the direction of the rainfall deviates the least
>from straight down.
While true, there are a significant number of walls that have a
portico (porch) of some kind protecting lower sections of wall, and
this too is part of the 'style'. The Pueblo styles (mothers of the
Santa Fe style) also have the same problems only worse, as the
traditional materials are earth plasters and not many overhangs for
protection. The reason this system works at all is that the climate
provides short rainshowers with drying breaks in between storms, low
overall rainfall (9-10"), and the acceptance of yearly renewal of the
>Presumably the straw to which the exterior plaster has been applied will be
>drier than rain-wetted plaster, so naturally , moisture will tend to move from
>wetter to drier areas... into the straw.
With more permeable surfaces and plasters, the tendency is for more
of the moisture to stay within the plaster itself and not pass as
much through. If a stucco plaster and more permeable plaster both do
this, the stucco would keep the moisture longer and water would stay
in contact with the bales longer, allowing more penetration.
Additionally, given the slightly humid levels in the straw vs the
dry, low humidity air on the outside, the tendency would be to dump
the moisture into the air. We have had rainstorms with the air
humidity level staying around 17%!
>When the rain has stopped, there may be some drying of the plaster towards
>the outdoors due to wind and sun, but the sun can also drive
>of moisture inwards, into the straw.
If the plaster acts as a wick it can suck that moisture back into
itself when conditions change, and again it becomes the primary
holder of the moisture. It takes more energy to break the capillary
action within the plaster than to move into the bonds available
within the straw but more widely dispersed, drying out the straw
before the plaster.
>In addition to the above, significant amounts of moisture can find its
>way into the straw via wind-driven rain through cracks in the plaster,
>past ineffective or non-existent flashing details and via just-plain-leaks.
This is not exclusive to the Santa Fe style, but may have a higher
potential of occurring on unprotected walls.
>Given the above, it should be apparent that a design which permits
>skin of plastered SB walls to get wet (ie parapet designs) run a
>very high risk
>of acquiring elevated levels of moisture within the straw and over time
>(measured in decades not years) if the drying capacity of the wall does not
>exceed amount of wetting, it is almost guaranteed that the accumulated
>moisture will result in deleterious microbial activity.
True, but the low rainfall and high solar presence make this type of
wall capable of working fine within this climate, passing sufficient
moisture back out of the walls to exceed the wetted amount and stay
dry within. I have also seen many plastered houses in the area that
are not parapet designs but only have a 6" overhang. These walls are
as susceptible to driven rain as the parapet design.
>In a loadbearing design, decaying straw could result in structural failure
>(ie no straw-plaster bond = no sandwich action = no lateral support
>for the skin
> = possible failure of the skin by buckling)
Loadbearing designs get a two sided plaster skin the same as
non-loadbearing, and it is bonded to the straw in the same manner,
unless there is a bunch of wire mesh in the way. Is there a different
dispersal of the forces in the wall skins with the different bearing
> === * ===
I am not sure if the comparisons are equal here. Are you saying the
straw plus two breathable skins are going to be less permeable than a
sheet of gypsum board? What about the stud wall, insulation batts,
vapor barrier/house wrap and outer skin of stucco or whatever? You
are assuming two skins of cement based plasters for the strawbales. A
permeable earth/lime/sealant on the outer wall, a permeable straw
infill and a permeable earth/lime plaster on the interior wall sure
seem to me to equate to something better than a 12mm cement board.
Even if the comparison is equal in terms of permeance, the ability of
the interior plasters to absorb odors will give a 'fresher' smell to
the interior, which may translate into "fresh air entering the room"
- even if it is not moving in at a greater rate. It is likely though
that your point that air movement through a wall system in a sealed
type of house is not enough to satisfy the required air exchange
levels sounds true.
>>From the above, one may conclude that if a house with
>plastered straw bale walls has a higher air change rate than
>another house, then the air exchange, if due to the walls,
> will be due to air leakage through gaps and cracks due to sloppy
>or faulty construction practises rather than through the plastered
>That being said, I think that we can assume that the higher air
>change rate in Karin's houses is not due to sloppy or faulty
>wall construction practises, but rather, air is finding its way
>into the building through areas other than the walls.
Well, I don't believe I can say that or reach your other conclusions
without more empirical evidence.
>My *guess* is that the "fresher" smell is likely attributable
>more to the wall plaster's hygroscopic properties than it is
>to increased air change rates.
Does this mean you are relying on lower air humidity levels (that
carry less smells) to create a 'fresher' smell? Or are the odors
absorbed and trapped in the higher moisture content of the plasters?
There was a compatriot of yours from Mooseland at a workshop that
Beel and Athena held here recently who was working with professor
types on finding out more about the movement of moisture in and
through walls. Hopefully there will be more data coming out of his
studies in a year or two.
La Otra Banda 36 N 11.25 - 106 W 33.75
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