Journal of Building Physics

As a consequence of the reduced transmission heat loss, algal growth on external thermal insulation composite systems has given rise to a serious aesthetical problem over the last decade. Manufacturers of paints and rendering systems are competing to increase the algal resistance of their products. The high time investment of free-weathering tests and the lack of objective measures to quantify the growth, however, prevent a systematic and efficient product advancement. Within a multiannual study, the application of fluorometric and numerical analysis was evaluated for assessing the algal resistance of external thermal insulation composite systems. The efficiency of pulse-amplitude modulation fluorometry for directly quantifying the algal biomass on the facade surface was analysed within three weathering tests which comprised 33 different external thermal insulation composite system specimens. The results show that the IMAGING-PAM (imaging pulse-amplitude modulation) fluorometer of the company Walz allows to measure the algal resistance in the course of the weathering process objectively and efficiently. The measurements confirm the effectiveness of biocides and indicate a higher algal resistance of the mineral rendering systems compared to the organic systems. The options and limitations of using numerical simulation for the assessment of the algal resistance of external thermal insulation composite systems were evaluated using the software WUFI^® Pro 5.0 developed by the Fraunhofer Institute of Building Physics. Within selected parameter studies, an appropriate evaluation criterion was identified and the impact of varying material data and exterior boundary conditions was assessed. The integrated results emphasize the need to combine experimental and numerical analysis. The missing correspondence between the calculated and measured algal resistance for selected specimens of the weathering test is attributed to the simplifications inherent to the approximation of the hygric material functions and therefore emphasizes the need for further research.

Cavity walls are a widely used external wall type in north-western Europe with a good moisture tolerance in cool humid climates. In this work, a cavity wall configuration with a brick veneer outside leaf and a wood fibre board inside leaf is analysed with a newly developed coupled computational fluid dynamics–heat, air and moisture model. Drying of the outside or inside cavity leaf, both for summer and winter conditions was analysed. The new model was compared with a widely used simulation tool for building envelope analysis (WUFI^®) that uses a simplified modelling approach for the convection in the cavity. The study showed that the simplified model overestimated the drying and moistening rates of the cavity wall compared to the detailed model. For both models the drying of the outer leaf was mainly determined by the outside conditions, and the outside leaf dried out mainly to the outside and not to the cavity. For the inside leaf, however the cavity ventilation was of major importance in drying. The study revealed that the simplified model could not be used to evaluate the drying potential of a ventilated cavity because it overestimated the ventilation effect systematically. The simplified model would in such case indicate lower moisture contents than in reality and consequently lower risk for mould growth, wood rot or other structural damage. Only detailed modelling of the convection in the cavity, as in the new model, leads to a correct evaluation of ventilated cavity walls.

Infiltration heat recovery is the process that occurs when a building envelope acts as a heat exchanger for infiltrating air. This heat recovery process results in a reduced heat loss compared to predictions that use only flow rate and the total difference in enthalpy between inside and outside air. A series of experiments show the relationship between infiltration flow rate and heat loss in a test cell, with an emphasis on the high flow rate regime. A 3.5-m³ test cell was built with standard light-frame construction and one removable panel, to allow testing of wall sections with different engineered flow path lengths. Experiments were conducted with two different wall sections and at six different infiltration flow rates. Experimentally determined heat recovery factors are compared to computational fluid dynamics and agree to within approximately 15%.

This article highlights the need for an active role for building physics in the development of near-zero energy buildings while analyzing an example of an integrated system for the upgrade of existing buildings. The science called either Building Physics in Europe or Building Science in North America has so far a passive role in explaining observed failures in construction practice. In its new role, it would be integrating modeling and testing to provide predictive capability, so much needed in the development of near-zero energy buildings. The authors attempt to create a compact package, applicable to different climates with small modifications of some hygrothermal properties of materials. This universal solution is based on a systems approach that is routine for building physics but in contrast to separately conceived sub-systems that are typical for the design of buildings today. One knows that the building structure, energy efficiency, indoor environmental quality, and moisture management all need to be considered to ensure durability of materials and control cost of near-zero energy buildings. These factors must be addressed through contributions of the whole design team. The same approach must be used for the retrofit of buildings. As this integrated design paradigm resulted from demands of sustainable built environment approach, building physics must drop its passive role and improve two critical domains of analysis: (i) linked, real-time hygrothermal and energy models capable of predicting the performance of existing buildings after renovation and (ii) basic methods of indoor environment and moisture management when the exterior of the building cannot be modified.