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Moisture Properties of Plaster and Stucco for Strawbale Buildings
Moisture Properties of Plaster and Stucco for
Strawbale Buildings
John Straube
1 Introduction
Straw, as a fiber, has been used as part of building materials for several
thousand years. With the invention of the mechanical baler in the early 1900’s
it became possible for compressed straw to be used as the primary building
block of exterior building walls. Although strawbale (SB) houses were popular
for a short while in a local area of Nebraska, they lost favor for nearly half a
century. There has recently been a rebirth in SB house construction and
interest. In many cases the interest stems from the highly insulating, simple,
and sustainable nature of SB walls.
Although there is a large and growing body of empirical evidence that strawbale
buildings can be used very successfully, the scientific justification and
explanation is lacking, and hence accepted engineering approaches to design,
testing, and inspection have not been well developed.
To support the growing volume of rice straw agricultural waste the State of
California supported a research program to improve the level of scientific
knowledge of strawbale wall behavior and performance.
This report is a draft summary of the results of the moisture property testing of
a range of plaster types that might be installed over strawbale walls. It reviews
the literature for previous data, describes the test protocols, and summarizes
the results.
1.1 Plaster -- Strawbale Skins
The most common and time-proven strawbale wall assembly consists of
strawbales with 1 to 3” (25 to 75 mm) thick mineral-based stucco skins applied
to both faces. In modern times, the stucco skin is often made of steel mesh
reinforced cement stucco skins applied directly to the strawbales. For reasons
of performance, cost, sustainability, health, and ease of construction the use of
Moisture Properties of Plaster and Stucco for Strawbale Buildings
non-cement mineral binders such as lime, earth and gypsum has grown.
Regardless of what it is made, the plaster skins provide a finish, a weather
barrier, an air barrier, fire protection, rodent and insect control. Since they are
usually the stiffest part of a wall assembly, the skins also often act as structural
elements, whether intentionally or not.
Straw, like wood, degrades when exposed to a sufficient amount of moisture
for a sufficient amount of time at above-freezing temperatures. Therefore, one
of the major performance-related concerns of strawbale enclosure walls is
moisture control. Moisture control is a complex subject that requires an
understanding of climate, micro-climate, building details, enclosure assembly,
and interior conditions. The material properties of straw and plaster are critical
to the understanding of the enclosure assembly part of moisture control.
Building enclosure strawbale walls with appropriate moisture tolerance is best
achieved by selecting materials and assemblies that ensure a balance of wetting
and drying potentials, with an appropriate amount of safe storage capacity,
given the conditions the walls are expected to separate. Understanding and
predicting wetting and drying is therefore of fundamental importance to
predicting and improving performance, and particularly durability, of strawbale
enclosure walls.
1.2 Project Scope and Objectives
To allow for the prediction of heat and moisture performance, material
properties are needed. In strawbales walls, the strawbale core and plaster skins
are usually the only materials whose properties are of interest. However, there
is a wide variety of plaster types, additives, and coatings.
The most important properties to measure are the vapor and liquid diffusivity
and moisture storage function of the skins, and the thermal resistance, vapor
diffusivity, and moisture storage function of the straw.
Measured vapor permeance values of stucco and some analysis of the level of
vapor permeance required for good performance are needed to assess if
sufficient vapor resistance is provided by interior plaster finishes to resist
diffusive vapour flow into walls in cold weather (and to meet the intent of
some building codes). Drying of walls is predominately a vapour diffusion
driven phenomenon. To predict the drying rate of water stored in straw bale
Moisture Properties of Plaster and Stucco for Strawbale Buildings
walls, the vapor permeance of both the interior and exterior skins must be
Air leakage and rain penetration are usually the two largest sources of moisture
in enclosure walls. Strawbales are very vapor and water permeable and hence
rely on the skins to control the entry of these sources of moisture. While
almost all wet-applied monolithic plaster finishes are sufficiently air
impermeable to control air flow, their liquid water absorption are highly
variable and poorly known. The ability of an exterior plaster to absorb and
store rainwater is critical since this water can then be transported inward to the
strawbales by vapor diffusion and capillarity.
To aid in the control of rain penetration and absorption, water repellents and
sealers have been proposed as simple and relatively inexpensive solutions.
Manufacturer's and designers often do not understand the effect of such
products on liquid and vapor transport of moisture across the outer skin
because of a lack of material property information. Such information would be
very useful to guide strawbale builders in their choice of a climate-appropriate
The thermal resistance of the strawbales are important for the insulating
function required of modern enclosure wall systems. The thermal resistance
also affects the temperatures experienced by the skins when exposed to varying
temperatures over the day, and this greatly influences the flow of water vapor.
Thermal resistance has been measured by others, is similar to other cellulosic
materials of similar density, and the literature is being reviewed as part of
another EBNet project. Hence, the work reported here did not involve thermal
resistance testing.
Given the material properties described above interior and exterior
environmental conditions computer models can predict, with reasonable
accuracy, the temperature and moisture conditions within a wall system.
1.3 TechnicalBackground
The material properties described in this report are fundamental building
science properties that may not be familiar to all readers. Hence, the
terminology is reviewed in this section.
Moisture Properties of Plaster and Stucco for Strawbale Buildings
) is a fundamental material property that
describes the rate of heat flow across a unit area, through a unit thickness for a
temperature gradient of one degree. The symbol
is often used in Europe
instead of k.
: SI
W / m·K
Btu·in / (hr·ft
Conversion 0.144 Btu· in / (hr·ft
F) = W / m·K
Thermal conductivity often varies with the mean temperature and the
magnitude of the temperature differential. Standards normally specify these
values so that different materials can be compared.
: SI W / m
Btu / (hr·ft
Conversion 5.678 Btu / (hr· ft
·°F) = W / m
·K or
Btu / (hr·ft
·°F) = 0.176 W / m
Thermal resistance is the reciprocal of conductance.
Resistance = 1/C = thickness / conductivity = l/k.
: SI (m
·K ) / W (RSI)
·°F / Btu
Conversion R = 5.678·RSI or RSI = 0.176 R
Hence, R-value is valid for a specific wall thickness, temperature difference,
and temperature across the specimen. It is not uncommon for R-values to be
reported as R-values per inch, so that users can simply multiply the given value
by the thickness of the material to assess the total R-value.
It should be emphasized that the heat flow decreases with the inverse of R-
value, that is:
Heat flow = 1 / R-value * Temperature difference
1.3.1 ThermalConductivity
Thermal conductivity (symbol k or
Moisture Properties of Plaster and Stucco for Strawbale Buildings
Hence, the reduction in heat flow as the a walls R-value increases from 20 to 25
(a reduction of 0.01 in heat flow) is small compared to the increase in R-value
from R10 to R15 (a reduction of 0.033), the R-value increase achieved by
moving from a typical 2x4 wall to a 2x6 wall in many modern homes. Debates
over the difference in R-value of 25 or 29 are essentially academic, since the
heat flow is so small that little benefit would be gained by the increase.
. Vapour permeance
is a measure of the ease of vapour flow through a material layer, in units of
perms (equal to 1 ng/Pa s m
or 1 grain/(hr·in Hg· ft
)) and given the symbol
M. Permeability and permeance are analogous to thermal conductivity and
thermal conductance respectively. Imperial US perms can be converted to
metric perms by multiplying by 57.1.
Many codes define a vapor barrier as any material or system that has a
permeance of less than 1 US perm. This is an arbitrary value based on a limited
and questionable study conducted in the 1940’s. Vapor diffusion flow through
a wall may need to be controlled with vapor resistant layer in some special
cases, but plastered strawbale alls usually don’t need them, and often appear to
perform much better without them.
1.3.3 LiquidUptake
The water absorption coefficient is a measure of a materials ability to transport
capillary water. It is determined experimentally by measuring the rate of water
absorption when a sample is placed in contact with liquid water. The water
absorption coefficient so determined can then be used to estimate the liquid
diffusivity, the fundamental measure of liquid water movement within porous
There are no material property standard requirements for it in North America
although German stucco standards provide upper limits if a stucco is to be
called “water repellent”.
In general, water absorption is measured in units of kg/m
and given the
symbol A.
1.3.2 VaporPermeability
Vapour permeability is a material property, expressed independently of material
thickness, in units of ng/Pa s m, and given the symbol,
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