[Strawbale] cement stucco problems?

[Rene Dalmeijer]

Peter,

The whole discussion going on now I think is giving you the wrong 
impression. A lot has been discussed regarding permeance of plasters 
and I think therefore many people reading this list are starting to 
confuse the issue. I will attempt to put things back into perspective.

Study has shown that moisture damage in house walls, whatever building 
method, is mostly due to (in order of importance, #1 plain old leaks, 
#2 air leaks and then a long way back #3 condensation due to moisture 
traveling through the surfaces. In practice it means nearly all 
moisture damage I expect at least 99% is due to leaks.

There are exceptions though where correct management of moisture 
permeance is of importance in preventing moisture damage. This is when 
the structure in question is almost steady state cold on one side and 
warm on the other and even more so when the relative humidity content 
is higher on the warm side. This situation exists for instance in cold 
storage wharehouses situated in for instance Florida. In such cases the 
best way of preventing moisture damage is to ensure that the moisture 
permeance of the surface after the insulation is higher then before 
including the insulation. This is the reason why closed cell insulation 
is so popular for this  type of structure. Correctly sealing joints 
between the insulation panels is of paramount importance otherwise you 
are back to moisture cause #2 and all your efforts regarding moisture 
management are null and void.

In most building envelopes the above steady state allways heating and 
always cold on the other side do not exist. This means that the 
envelope experiences drying and wetting cycles either every day or 
longer periodes up to seasonal. The rational  for permeance of the 
surface finishes is thus; if moisture somehow works its way into the 
structure it can work its way out during a drying spell, the faster the 
better.

Regarding your own project. I only have a reservation regarding the 
external earthplaster thickness 30mm is very thin I would suggest at 
least 35. I cannot say if you have sufficient protection from wind 
driven rain causing possible erosion of the earthplaster this depends 
on how exposed the site is and what type of weather the site 
experiences. I would suggest to go ahead with thicker external 
earthplaster and possibly afterwards protect the wall with weather 
boarding. Besides this make sure you have no air leaks specifically on 
the inside plaster surfaces. This means carefull detailing of all 
plaster intersections with other materials, a simple butt is not 
sufficient. At least attach a strip ( 300mm wide) of hemp mesh to the 
other material before the finish coat and lap it into the finish coat. 
All interior plaster edges should be finished in this manner. Besides 
this measure it is good to create a labyrinth at plaster intersections 
whereby a simple butt is transformed into a notched groove.


Rene
On Sep 28, 2006, at 02:26, strawbale-request at listserv.repp.org wrote:

> Next year I hope to start building our SB house in foothills (550m) of
> French Pyrenees. Quite wet (1000mm/yr, about 80mm/month in winter 
> months),
> not too cold (mean Tmin/Tmax 0?/10?C in January), with quite a lot of
> clouded humid days in the winter. It's an exposed location (mainly 
> from W to
> NW), so W/NW side will be protect by garage with wood cladding; N, E 
> and S
> sides will all have 100cm roof overhang.
> I'm planning to put earth plaster (3cm) on all exterior and partially 
> earth
> plaster (3cm), partially wood panelling on the interior of these walls.
> I cannot guarantee average internal humidity to stay below 50%, but 
> will
> install small hygrostat-controlled extractor fans in bathroom, toilet 
> and
> possibly the kitchen area. Some internal walls (e.g. in bathroom) will 
> be in
> adobe so could store/even out humidity peaks, I suppose.
> Does your study give any indications that I could be in trouble with 
> the
> above settings?
> And would adding weatherproofing (almost certainly decreasing the 
> vapour
> permeance) to the finish coat (e.g. to better protect the N wall)
> significantly increase the risk?
>
> Peter van Balen
>