Correlation Between Water Vapor and Air Permeability of Building Materials: Experimental Observations

Two fundamental properties of building materials that influence the hygrothermal performance of building envelope systems are water vapor permeability and air permeability. The driving forces for water vapor and air transmission through building materials are, respectively, partial water vapor pressure and total air pressure differentials. The theoretical similarity of the driving forces would suggest the possibility of a relationship between water vapor and air permeability. During past two decades, researchers at the National Research Council of Canada ? Institute for Research in Construction (NRC-IRC) have compiled a database of measured air permeability and water vapour permeability properties of building materials commonly used in North America. This material properties database was examined to identify the degree to which air permeability and water vapor permeability of the building materials might be functionally related. The database was segregated into categories for different material types. An approximately linear relationship could, within some categories, be observed between water vapor permeability and air permeability. There were however approximately as many categories within which no relationship between the parameters was apparent as there were categories within which functional relationships between the parameters were apparent. Within categories where functional relationships were apparent, the form of the relationships generally depended on the mean relative humidity (RH) at which water vapor permeability was measured. In most cases, a definitive functional form of the relationship between water vapor permeability and air permeability could not be establishedLa perm\ue9abilit\ue9 \ue0 la vapeur d?eau et la perm\ue9abilit\ue9 \ue0 l?air sont deux propri\ue9t\ue9s fondamentales des mat\ue9riaux de construction qui influent sur la performance hygrothermique des syst\ue8mes d?enveloppe de b\ue2timent. Les forces motrices de la transmission de la vapeur d?eau et de l?air \ue0 travers les mat\ue9riaux de construction sont, respectivement, la pression partielle de la vapeur d?eau et la pression diff\ue9rentielle totale de l?air. La similarit\ue9 th\ue9orique des forces motrices donnerait \ue0 penser qu?il puisse exister une relation entre perm\ue9abilit\ue9 \ue0 la vapeur d?eau et perm\ue9abilit\ue9 \ue0 l?air. Ces deux derni\ue8res d\ue9cennies, des chercheurs de l?Institut de recherche en construction du Conseil national de recherches du Canada (IRC-CNRC) ont compil\ue9 une base de donn\ue9es des propri\ue9t\ue9s mesur\ue9es de la perm\ue9abilit\ue9 \ue0 l?air et de la perm\ue9abilit\ue9 \ue0 la vapeur d?eau de mat\ue9riaux de construction utilis\ue9s couramment en Am\ue9rique du Nord. On a examin\ue9 cette base de donn\ue9es des propri\ue9t\ue9s des mat\ue9riaux afin d?identifier le degr\ue9 auquel ces deux propri\ue9t\ue9s des mat\ue9riaux de construction pourraient \ueatre reli\ue9es au plan fonctionnel. La base de donn\ue9es a \ue9t\ue9 s\ue9par\ue9e en cat\ue9gories correspondant \ue0 divers types de mat\ue9riaux. Une relation quasi-lin\ue9aire \ue0 l?int\ue9rieur de certaines cat\ue9gories pourrait \ueatre observ\ue9e entre les deux types de perm\ue9abilit\ue9s. On notait cependant que les cat\ue9gories \ue0 l?int\ue9rieur desquelles aucune relation entre les param\ue8tres n?\ue9tait apparente \ue9taient \ue0 peu pr\ue8s aussi nombreuses que celles dans lesquelles des relations fonctionnelles entre les param\ue8tres \ue9taient apparentes. \uc0 l?int\ue9rieur de ces derni\ue8res cat\ue9gories, la forme des relations d\ue9pendait en g\ue9n\ue9ral de la valeur moyenne d?humidit\ue9 relative (HR) \ue0 laquelle la perm\ue9abilit\ue9 \ue0 la vapeur d?eau \ue9tait mesur\ue9e. Dans la plupart des cas, on ne pouvait pas \ue9tablir une relation r\ue9solument fonctionnelle entre les deux types de perm\ue9abilit\ue9.Peer reviewed: YesNRC publication: Ye