NBN ISO 16957:2021

This International Standard describes a method of measuring the thermal conductivity (diffusivity) of
a wet porous building material and a method of evaluating the measurement uncertainty.
While ISO 10051 is the current International Standard, based on a steady-state method, this International
Standard proposes a method that makes use of a non-steady-state method which uses a small temperature
change with a short period as an input. Along with the measurement, an evaluation of the measurement
uncertainty is described, which makes possible a simple and practical measuring method.
This International Standard intends to measure the apparent (effective) thermal conductivity, including
latent heat transfer caused by vapour movement. The situation in which moisture and/ or air movement
occur due to convection or gravity is excluded. The application of this International Standard to high
moisture content is excluded so that the gravity effect can be neglected. This International Standard
can be applied to a porous material heavier than about 100 kg/ m3, in which radiative heat transfer can
be neglected.
This International Standard specifies the following:
a) a non-steady-state method of measuring thermal conductivity
b) an approximation formula for the measurement uncertainty caused by moisture movement and
nonuniform moisture distribution (and, thus, a determination of the measuring conditions that
satisfy the upper limit of measurement uncertainty)
c) an estimate of the heat transfer caused by moisture (vapour) movement.