Abstract

Well-insulated building envelope systems are subject to alternating vapour pressure gradients. Therefore the installation of traditional vapour barriers or retarders to avoid interstitial condensation may have undesirable side-effects. Numerous moisture damage cases can be attributed to the fact that a vapour barrier is nearly impermeable in both ways, i.e. it does not allow any dry-out either. Some wall and roof assemblies are only durable if they can dry to the interior side too. The attempt to create a perfect seal is rarely successful and should be better replaced by controlled moisture management. Therefore, the transient hygrothermal behaviour of the building enclosure is investigated and the importance of moisture leaks is discussed.
Recently, adaptable vapour retarding systems have been developed in order to assure a sufficient drying potential. Two of these retarders are presented in this paper. The humidity controlled retarder reacts to local humidity conditions by increasing its vapour permeance when drying conditions prevail. The capillary active retarder relies on capillary suction to remove moisture from the interior of the envelope. By way of several field tests their performance has been evaluated and compared to that of conventional vapour barriers. The results clearly show a faster drying of construction moisture and diminished long-term humidity within the building envelope.
The improved drying potential through the application of adaptable vapour retarders increases the durability of insulated constructions because rot, corrosion and fungal growth are less likely to occur under dry conditions. Because these retarders become more permeable in the case of condensation or high humidity their application must be restricted to buildings with normal indoor air conditions. In the case of the capillary active retarder only non hygroscopic insulation materials may be employed. Otherwise the summer condensate will be absorbed by the insulation layer before it has a chance to reach the retarder and be wicked to the other side. Despite of the limits, it is worthwhile to consider the application of the adaptable retarders in practice because the durability benefits are significant.

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Adapted Vapour Control for Durable Building Enclosures

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  • 29 December 2014 Creation Date
  • 25 September 2019 Last Updated