[Strawbale] more info. against vapor barriers

[SArjuna at aol.com]

      This article is at http://oikos.com/library/newstrawbale/coat.html if 
you want to look at the photos. 
     Shivani in WI



A Coat That Breathes

The classic, time-proven, straw bale wall consists of bales with a plaster “
skin” applied to both faces. A thick coat of plaster serves multiple functions. 
Protecting the exterior of straw bale walls from wind, rain, and abrasion, it 
also seals bales from birds, rodents and insects that view them as an 
attractive home. Inside, plaster provides a smooth finish to bale walls while adding 
thermal mass. Common choices for exterior plasters include cement/ lime, lime, 
and clay, while interior plasters are often lime, clay, or a commercial 
gypsum or Structolite product. Consider your climate when deciding which type of 
plaster to use, as an appropriate design and finish will require minimal 
maintenance.

These excerpts were reproduced with permission from The New Strawbale Home by 
Catherine Wanek:
     1.     Design Essentials
     2.     Good Shoes & A Good Hat
     3.     A Coat That Breathes
     4.     The Healthy Home


You can buy the whole book from the Oikos Bookstore.

It's important to plaster both sides of a straw bale wall to seal out oxygen 
for fire resistance. For example, if an interior wall were “furred out “ with 
studs to attach drywall without plastering it first, the resulting space 
would act as a chimney in case of a fire. A well-sealed bale wall is also critical 
for energy efficiency, as even minor gaps will allow air infiltration through 
the bale, reducing its effective R-value.

Micaceous clay plaster makes a luscious interior finish.

A cement/ lime stucco retains much of the strength of cement and vapor 
permeability of lime.

Breathability—more accurately known as “vapor permeability”—of plasters is 
also a consideration. While structurally stronger, cement stucco is relatively 
less permeable than lime or earthen plasters. It is known to absorb moisture 
from rain, as well as the ground, and wick it into adjacent wood or straw. As 
long as bales or wood can dry out in time through the stucco, there is little 
cause for concern. But elastomeric coatings—which are often recommended by 
contractors because they tend to eliminate small cracks in the stucco—can 
actually trap moisture in the wall and should be avoided. (In climates with driving 
rain, a siloxane coating on stucco has been tested to repel moisture without 
affecting permeability.)

Clay plasters are the most permeable, with the added advantage of being 
hydrophilic, which means “water loving". Clay readily absorbs moisture and holds on 
to it, actually wicking moisture away from straw or wood. This accounts for 
the preservative effect clay has on these materials, making them much less 
vulnerable to moisture issues. Welsh cob houses and German timber-frame/ 
straw-clay walls provide historical examples of this effect; many such buildings are 
still occupied after centuries of use. Earthen plasters on straw bale walls may 
also have this kind of long-term beneficial effect.

Conventional builders often believe they should wrap a straw bale house in an 
air or sheet barrier (like Tyvek), as this is a standard part of stick-frame 
construction. This is because stud-framed wall systems covered with gypsum 
board (also known as drywall or Sheetrock) on the inside and oriented strand 
board on the outside tend to have many minor cracks where they are joined, which 
allows air filtration that compromises comfort and energy efficiency. “House 
wrap” seals these air gaps and, in theory, repels rain while allowing the 
smaller water molecule to escape. However, testing results from Canada and elsewhere 
indicate this product may not live up to all manufacturer's claims.

In a straw bale home, house wrap is not only unneccesary—as a well-detailed 
plaster eliminates air infiltration—but can be a critical mistake. Such a 
barrier will prevent the plaster skin from “keying in” or bonding with the straw 
bales, reducing the wall's structural strength. Worse yet, as warm, moist air 
migrates from living areas to the colder exterior through the straw bale wall, 
it will tend to condense on the inside of the exterior housewrap. This liquid 
moisture will now dry out slowly, and when the temperature warms, it can 
produce conditions ripe for fungal growth.

Conventional wood frame buildings also typically employ a sheet-type vapor 
barrier on the interior to prevent this warm, moist air from infiltrating 
through surface finishes, into the walls and attic. This practice, however, may lead 
to a false sense of security, according to some studies. The modern life of a 
family of four can easily generate 18 gallons of moisture vapor per week into 
the household air through cooking, bathing, and washing. An interior sheet 
barrier can actually serve as a vapor funnel through any rip or penetration, 
concentrating moisture at this gap. These small air leaks can create big problems 
by concentrating moisture vapor in only a few places; when it condenses, the 
resulting moisture can be significant. In cold climates, the problem can be 
severe, often resulting in the deterioration of the roof or wall materials of 
conventional homes

In a straw bale home, the same principles apply. Plaster finishes, whether 
earth or cement-based, will offer an adequate air barrier to vapor moving into a 
wall or ceiling from the interior, as long as potential cracks and crevices 
are sealed to prevent gross air leakage. Gaps in interior finishes around 
electrical outlets, overhead fixtures, window and door frames, plumbing etc., 
should be well detailed to reduce or eliminate vapor infiltration. The best 
strategy for managing indoor moisture is to install mechanical ventilation at the 
source— in bathrooms, the kitchen and laundry room. Unheated attics and roof 
systems should also be vented. Evidence suggests that clay plasters, which can 
absorb and store large quantities of moisture, can beneficially moderate excess 
humidity in the home.

Ongoing home maintenance begins with observation. If you have concerns about 
specific areas, it's wise to install moisture meters or to periodically use a 
moisture probe in critical spots. Peace of mind is worth the effort.




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