Re: SB: Re: Santa Fe style, breathability

[Rick & Katy Blanchard <urbaneagle@cybermesa.com>]

Robert le Tom wrote:

>I think that we can all agree that the Santa Fe style, with its 
>parapetted walls
>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 
plaster surfaces.


>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 
>significant amounts
>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 
>the exterior
>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 
schemes?

>			=== * ===
>re: Breathability

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
>walls themselves.
>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.

Cheers,
Rick
-- 
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
  La Otra Banda   36 N 11.25  -  106 W 33.75
http://www.urbaneagle.com/otrabanda/otra.html
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

-------------------------------------------------------------------------
To unsubscribe from the list, send a message to:
   <strawbale-unsubscribe@crest.org>

or for the digest to:
   <strawbale-digest-unsubscribe@crest.org>

Please send any list administration questions to
strawbale-owner@crest.org