Compatibility assessment of esterior finish coats for insulated walls

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.