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We investigated the characteristics of OSB boards in laboratory

12.11.2020

The University Centre for Energy Efficient Building CTU in Prague in cooperation with the company Kronospan performed laboratory measurement of diffusive resistance of chosen OSB boards. The data obtained will allow to better design envelopes of modern wooden buildings and thus ensure their long-term moisture safety.

Envelopes of modern wooden houses are more and more often realised as diffusion-open. This solution is based on the principle that a controlled amount of water vapour is able to enter from internal environment into the construction, without forming excess amount of condensate on its cold side and threating the moisture sensitive materials used. The advantages of such solution are less demanding application (construction does not contain vapour barrier membrane sensitive to workmanship) and the ability of fast moisture regeneration of the construction (e.g. faster drying of the initial built-in moisture).

Diffusion-open envelopes based on wooden framing often contain OSB board, which, together with stiffening of the framing, also serves as an air barrier (preventing the flow of humid interior air into the cold part of the structure) and vapour barrier (regulating the diffusion of water vapour from humid interior air into the cold part of the structure). The diffusion resistance factor of OSB boards, i.e. the material characteristic describing their diffusive properties, still isn’t mapped in adequate detail. The situation is also complicated by the fact that it isn’t a constant, but a moisture dependent function. This means that it decreases with growing moisture of the boards. Constant values usually used in engineering hygrothermal evaluations could thus be far from reality.

The Hygrothermal laboratory CTU UCEEB measured the diffusion resistance factor on a representative sample of OSB boards of the type Superfinish and Airstop produced by the company Kronospan. Measurements on both types of boards was performed at three levels of relative humidity, during both adsorption and desorption. The result is moisture dependent function of the diffusion resistance factor of OSB boards.

Its validity was verified on two diffusion-open compositions of external wall with OSB board in the role of vapour barrier. The verifying experiment was made in twin climate chamber, where both compositions were exposed to step changes of boundary conditions. The course of the twin climate chamber experiment was later with good agreement repeated using a software for dynamic simulation of hygrothermal behaviour of structures.

The research was supported by the Technology Agency of the Czech Republic under the research project MTS of the CAMEB National competence centre.