Technical Discussion

Understanding Vapor and Moisture Permeance in AAC and Applied Coatings

A durable coating over AAC must, besides the usual qualities such as adhesion and resistance to fading and weathering, also have low capillary water suction but at the same time offer maximum vapor permeability to insure the wall is dry. The release of humidity contained in the wall is important not only during the drying out period after construction but also as the building ages. Over time vapor can build in a wall due to two factors. The release of moisture from interior living conditions can pass vapor into the wall and elevate humidity. Also, water penetration through the coating, cracks, damaged areas, or failed sealant and flashing, can occur which raises the humidity inside the wall. For these reasons it is important that a highly permeable coating exist on the exterior side of AAC installations.

Simply speaking, the release of humidity from "drying out" must be greater than the absorption of water when it rains in order to have external walls which are dry. The demand that the release of water vapor must be greater than the absorption of water leads to requirements for the water absorption and vapor permeability of the coating. As a result, coatings can be formulated to provide these qualities. Based on the research at the Institute for Bauphusik in Stuttgart, the basis for standards for the properties of coatings over AAC is as follows:

The Kunzel Criteria:

Requirements	Exterior Plasters	Exterior Direct Applied Acrylic Coatings
Water Vapor Permeance (metric)	Perm > 1.8 Sd < 2 m	Perm > 1.8 Sd < 2 m
Capillary Water Absorption (metric)	W < 0.2 lb/ m2 h0.5 W < 1.0 kg/ m2 h0.5	W < 0.1 lb/ m2 h0.5 W < 0.5 kg/ m2 h0.5
Kunzel Criteria the ratio of W ¸ Perm Kunzel Criteria the product of Sd x W	< 0.01 < 0.1	< 0.01 < 0.1

When these requirements are met a coating is normally considered to be resistant to wind driven rain. This however does not consider the additional effects of water penetration through areas other than the coating surface and occurring from leaks from damage or fault area. For these reasons it is advisable to utilize coatings well within these limits.