Exterior coatings

Unlike interior coatings, exterior coatings are exposed to weathering. Given this different exposure, they are classified using different assessment criteria and therefore a different standard: DIN EN 1062-1. This standard classifies exterior coatings according to chemical type of binder, state of dissolution or dispersion of the binder in the coating material, gloss, dry film thickness, grain size, water vapour transmission rate, liquid water permeability, crack-bridging and carbon dioxide permeability. Of these, liquid water permeability and water vapour transmission rate are particularly important in defining the quality of an exterior coating.

Liquid water permeability

Liquid water permeability describes how well the surface is protected against rain and how much water is able to penetrate through the coating to reach the background surface in the event of contact with water. The result of this test is the so-called W value as measured in accordance with DIN EN 1062-3. The tested coating is applied to a lime brick sandstone surface and the back and facing sides are sealed to be watertight. The lime brick sandstone is very absorbent so that all the water that is diffused through the coating is taken up. The coated side is immersed in a water bath for 24 hours at a depth of approx. 1 cm.

Water-vapour transmission properties

The water vapour permeability describes how well moisture in the form of water vapour is able to diffuse from the background surface and penetrate the coating.

The V-value is measured in accordance with ISO 7783. For testing, the coating is applied to highly porous frits. The coated frits are placed on a plastic box and are sealed all around. This plastic box contains a special salt solution with a rel. humidity of 93 %.

The test is performed in a climate chamber at 23°C and 50% rel. humidity. Because of the partial drop in pressure from 93% rel. humidity to 50% rel. humidity, in the climate chamber the water vapour diffuses through the coating to the outside. Over time, the test specimens becomes increasingly light. The water vapour transmission rate is calculated from the loss of mass, the defined test area and the time of measurement.

The sd-value describes the  diffusion equivalent air layer thickness of a coating and is calculated from the V value, using a conversion factor.