Gypsum and silicate boards: Moisture conditions characterised by electrical resistance (NT BUILD 441 – Edition 2)

Abstract

SCOPE

This NORDTEST method describes a procedure for in-situ characterisation of the moisture conditions in gypsum and calcium silicate boards, e.g. bathroom walls. This is expressed by the relative humidity of the boards, which is important in relation to gluing properties, strength, formation of cracks etc. The measurements are carried out by means of electrical moisture meters. The method is based on the observation that the electrical resistance between two electrodes punched into a piece of board is a function of the moisture content of the board. Note: Different brands of gypsum boards contain different moisture contents at a fixed meter reading. On the other hand, at a fixed meter reading, all the gypsum boards are – by and large – in equilibrium with the same relative humidity (% RH) of the surroundings – cf. Appendix A.

FIELD OF APPLICATION

The test method is applicable to different kinds of gypsum boards used in for instance bathrooms and to calcium silicate boards provided that proper calibration procedures are carried out. The method is relevant for determining differencies in the moisture conditions in the board and for locating areas where moisture has been absorbed by the board and where consequently there is a risk of deterioration. Note: This is a simple in-situ measurement method where an instrument intended for measuring the humidity in wood and calibrated on wood can be used for measurements on other materials such as the types of boards described here. It should be noted that the method will leave two small penetrations in the surface, but in most cases this is of no importance. If the penetrations are believed to create path for water ingression the holes punched in the surface must be repaired.