The dependable characterisation of the moisture buffer potential of interior claddings

The influences of interior humidity on the performance of building zones, building parts and building occu¬pants are strongly multifaceted and highly interrelated. The levels of interior humidity substantially influence the energy performance of a building zone, via the latent cooling and transmission loads, the appearance and stability of a building part, via the biological activity of moulds and fungi, and the health and comfort of a building occupant. For all these reasons and more, (some) control over the interior humidity levels is often desired. Such control can be maintained with active measures like HVAC, but recently the focus has shifted to the passive contributions of the interior enclosure's hygric inertia. While whole-building simulations or full-scale measurements may be applied to assess such contributions, these are expensive, both time and cost wise, and a simplified methodology is thus required. Such simplified methodology is presented in this paper and its companion paper [1]: this paper examines the dependable characterisation of the moisture buffer capacity (MBP) of single claddings, while the companion paper focuses on its generalisation towards entire room enclosures and its application for quantification of interior humidity variations.

Recently numerous proposals for the MBP characterisation of single claddings or finishing materials have been suggested. It will be shown that cyclic-step-change-(de)sorption based methods, as proposed in the Japanese Industrial Standard A 1470-1, the Draft International Standard 24353, and the Nordtest Moisture Buffer Value protocol [2], are most promising. The three proposals do though comprise different prescriptions for the cyclic (de)sorption measurements, and an analysis of their influence on the MBP characterisation is executed. To that goal, a numerical study of cyclic (de)sorption of seven different materials is presented.

Finally, it is shown that the resulting MBP characterisation indeed yields a dependable characterisation, by relating the MBP values to the dynamic response of a small room with each of the materials used in turns as finishing material. It is shown that an appreciably unique relation between the MBP and the dampening of the interior humidity variations – taken as measure for the moisture buffering effect by the cladding – exists, but only if there is a close agreement between the loading protocol used for the MBP determination and the production protocol applied in the room simulation. To account for the multitude of different real production protocols, the weighted average between a short term and long term MBP value is shown to be generally applicable.

In the companion paper, the presented MBP characterisation will be shown generalisable to the room level. The resulting room hygric inertia will finally be demonstrated to be applicable for the approximate quantifica-tion of interior humidity variations by simplified means. [1] Roels S, Janssen H: Characterisation of the hygric inertia of a room for a reliable prediction of interior humidity variations. Paper submitted for Nordic Symposium on Building Physics 2008. [2] Rode C, Peuhkuri R, Time B, Svennberg K, Ojanen T: Moisture buffer value of building materials. Journal of ASTM International 2007: 4(5).