Investigation of the influence of surface coatings on heat transfer in multilayer walls with air gaps

Air cavities that occur at interfaces in ETICS (External Thermal Insulation Composite System) assemblies (e.g., between concrete structural layers and thermal insulation due to adhesive mortar) can strongly affect heat transfer in walls, because radiative exchange across the gap may dominate under typical design temperatures. The present study investigates the potential of reflective–smoothing surface coatings applied to concrete to reduce surface emissivity in the far-infrared band and thereby increase the effective thermal resistance of wall air gaps. Seven commercially available coatings were applied to precast-like concrete specimens measuring 30 × 30 × 3 cm. Surface emissivity was determined using thermography under controlled laboratory conditions, while coating-induced changes in surface morphology were quantified via 3D optical scanning and areal topography parameters. The measured emissivity of uncoated concrete was found to be approximately 0.95. The most significant reductions in emissivity were observed with acrylic enamel (approximately 0.29) and aluminum spray (approximately 0.59). Several coatings produced only marginal changes in emissivity. The scanned surface meshes were incorporated into a validated computational model to quantify heat transfer across a 20 mm vertical air gap. A reduction in emissivity from 0.95 to 0.29 led to an increase in the air-gap thermal resistance from 0.189 to 0.311 m²K/W, and decrease in the overall U-value. The findings underscore the efficacy of emissivity control, which is associated with surface leveling and micro-facet geometry, in enhancing the thermal performance of multilayer wall assemblies with air gaps.