Abstract:
New approaches are emerging in contemporary wall construction
as a result of improved understanding of building materials and
their behaviour. Not so long ago, the accepted practice was to create
impermeable exterior walls by using moisture-proof and vapour-proof
layers in their sectional compositions. However, any failure, such as tiny
cracks in any one of these impermeable layers, causes accumulation of
entrapped moisture which could not escape by evaporation from the wall
surface (Hughes, 1986; Massari and Massari, 1993; Richardson, 2001). This
results in a decrease in the lifetime of building materials, visible defects on
wall surfaces, such as discoloration, cracking, scaling and flaking on finish
coats, and unhealthy interiors (Bochen et al., 2005). The concept of the
“breathing wall”, therefore, gained importance in the last few decades and
external wall compositions, allowing the passage of water vapour back and
forth through it, were started to be constructed.
Along with this, energy efficient buildings and improvement of
construction technology in this regard became a current issue in
contemporary buildings. In addition to installing more efficient fuelburning
equipment, the use of both thermal insulation layers and
lightweight porous masonry blocks and/or panels for its walls proper,
should be incorporated within the compositions of the building envelope.
However, due to their high water absorption characteristics, light-weight
porous masonry needs to be protected from rainwater by means of
watertight protective coatings and/or by water repellents (Andolsun et al.
2006; Kuş, 2004). For these reasons, the exterior finishing systems consisting
of under- and finish-coats, having low water permeability but high water
vapour permeability properties are necessary.
The matter which has not yet been foreseen, even overlooked, for multilayer
constructions is “compatibility with neighbouring materials”. The
compatibility of finishing/complementing layers with the porous masonry, in fact, has vital importance for contributing to the long-term durability
and thermal performance of masonry wall structures. However, the
basic performance and compatibility properties of those layers, such as
water vapour permeability, water impermeability, thermal resistance,
dilatation, modulus of elasticity characteristics, are as yet not well known.
Comprehensive studies are, therefore, needed to derive this information
so that the performance expected of such systems in providing healthy
interiors can be improved.
Materials are considered to be compatible with each other if they
have similar characteristics in terms of some physical, mechanical and
compositional properties (Sasse and Snethlage, 1997; Fassina et al., 2002;
Andolsun et al., 2005, 2006; Karoglou et al., 2007). The two important
parameters of compatibility are water vapour permeability and modulus of
elasticity (MoE):–
• What is required of the finish coat is to permit water vapour
transmission while resisting droplet penetration from rain or surface
wash; in other words, being essentially watertight (Kuş, 2004;
Harderup, 1996; Cerny, et al., 1996). It is also necessary to ensure
continuity in this vapour transmission property throughout all
the layers making up the wall section in order to avoid interstitial
condensation.
• The compatibility assessment of a layer with its neighbouring layers in
terms of MoE is still under discussion. The MoE is defined as the ratio
of stress to strain and indicates the deformation ability of a material
under external forces (Timoshenko, 1970). According to studies
discussing this subject, the MoE of coating layers should not exceed
that of the underlying masonry (Caner, 2003; Fabbri and Grossi, 2000;
Kovler and Frostig, 1998; Sasse and Sneathlage, 1997). This means that,
any compatible layer should be expected to have MoE not higher than
that of the base material which is in contact so as to prevent mechanical
damage in any of the weaker intermediate layer(s). If not so done,
failures–especially in the form of tiny cracks–are liable to develop on
the fine coat and/or on sub-layers, which is often followed by flaking
and scaling.
Here, a number of proprietary exterior finish coats produced in Turkey
were examined in order to determine their compatibility for insulated
masonry walls with an emphasis on their water vapour permeability and
modulus of elasticity characteristics (Örs, 2006) (1). It was expected to
reveal not only their individual material properties, but also to develop
awareness in architects, builders and manufacturers about the significance
of compatibility in attaining an integrated building envelope.