Compatibility Assessment of Exterior Finish Coats for Insulated Walls

Abstract

New approaches are emerging in contemporary wall constructionas a result of improved understanding of building materials andtheir behaviour. Not so long ago, the accepted practice was to createimpermeable exterior walls by using moisture-proof and vapour-prooflayers in their sectional compositions. However, any failure, such as tinycracks in any one of these impermeable layers, causes accumulation ofentrapped moisture which could not escape by evaporation from the wallsurface (Hughes, 1986; Massari and Massari, 1993; Richardson, 2001). Thisresults in a decrease in the lifetime of building materials, visible defects onwall surfaces, such as discoloration, cracking, scaling and flaking on finishcoats, and unhealthy interiors (Bochen et al., 2005). The concept of the"breathing wall", therefore, gained importance in the last few decades andexternal wall compositions, allowing the passage of water vapour back andforth through it, were started to be constructed.Along with this, energy efficient buildings and improvement ofconstruction technology in this regard became a current issue incontemporary buildings. In addition to installing more efficient fuelburningequipment, the use of both thermal insulation layers andlightweight 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-weightporous masonry needs to be protected from rainwater by means ofwatertight protective coatings and/or by water repellents (Andolsun et al.2006; Kuş, 2004). For these reasons, the exterior finishing systems consistingof under- and finish-coats, having low water permeability but high watervapour permeability properties are necessary.The matter which has not yet been foreseen, even overlooked, for multilayerconstructions is "compatibility with neighbouring materials". Thecompatibility of finishing/complementing layers with the porous masonry, in fact, has vital importance for contributing to the long-term durabilityand thermal performance of masonry wall structures. However, thebasic performance and compatibility properties of those layers, such aswater 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 informationso that the performance expected of such systems in providing healthyinteriors can be improved.Materials are considered to be compatible with each other if theyhave similar characteristics in terms of some physical, mechanical andcompositional properties (Sasse and Snethlage, 1997; Fassina et al., 2002;Andolsun et al., 2005, 2006; Karoglou et al., 2007). The two importantparameters of compatibility are water vapour permeability and modulus ofelasticity (MoE):-o What is required of the finish coat is to permit water vapourtransmission while resisting droplet penetration from rain or surfacewash; in other words, being essentially watertight (Kuş, 2004;Harderup, 1996; Cerny, et al., 1996). It is also necessary to ensurecontinuity in this vapour transmission property throughout allthe layers making up the wall section in order to avoid interstitialcondensation.o The compatibility assessment of a layer with its neighbouring layers interms of MoE is still under discussion. The MoE is defined as the ratioof stress to strain and indicates the deformation ability of a materialunder external forces (Timoshenko, 1970). According to studiesdiscussing this subject, the MoE of coating layers should not exceedthat 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 thanthat of the base material which is in contact so as to prevent mechanicaldamage in any of the weaker intermediate layer(s). If not so done,failures-especially in the form of tiny cracks-are liable to develop onthe fine coat and/or on sub-layers, which is often followed by flakingand scaling.Here, a number of proprietary exterior finish coats produced in Turkeywere examined in order to determine their compatibility for insulatedmasonry walls with an emphasis on their water vapour permeability andmodulus of elasticity characteristics (Örs, 2006) (1). It was expected toreveal not only their individual material properties, but also to developawareness in architects, builders and manufacturers about the significanceof compatibility in attaining an integrated building envelope.