Moisture Balance in Painted Wood Panelling

Abstract: Over the last years thousands of houses have been damaged by wood rot in the exterior wood panelling due to excessive moisture conditions. This thesis is about moisture conditions and the way moisture enters and exits painted wood panelling structures.

The work started with a survey of damaged houses in order to obtain information on where the rot appeared and which parts of the panelling was most exposed to moisture. The existing test methods for measuring moisture protection of paints were found to be insufficient and a new full-scale method was developed. It provided reliable and more revealing information about the moisture protection ability of different paints etc. A large number of paint systems have been studied and coatings that consist of several paints (penetrating oil, primer and top paint) gave the best results. It was also found that treatment of the end grain is crucial.

The full-scale test method is cumbersome and time consuming to carry out and therefore more sophisticated tools were developed. These tools provide a means of simulating the moisture balance by using accurate material properties and knowledge of the boundary conditions.

A numerical computer program, designed especially for painted wood structures, has been developed for calculating moisture transport in two dimensions. Moisture flow is based on the fundamental flow potential, which has advantages for this specific application because it makes the calculation faster and more reliable. A qualitative model for the way moisture enters and exits a painted structure provided information on the boundary conditions.

A new technique is proposed to evaluate the transport properties of the wood (spruce) and paint under steady state conditions. A slicing technique provides a moisture profile through the specimen and the transport properties are evaluated according to Fick's law. This provides not only the material property for the paint film but also 1.5 mm of wood behind the paint film. This is more realistic because the penetration of paint into the wood is taken into consideration.

The last part of the thesis deals with simulations of moisture conditions in both single boards and panelling structures. The calculated and measured results are compared.