(17): Why Never 100% RH in the Component?

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Joined: Thu Mar 03, 2005 2:35 am -1100

(17): Why Never 100% RH in the Component?

Post by support » Mon Mar 14, 2005 1:37 am -1100

I'm familiar with steady-state water vapor diffusion calculations (in particular, the Glaser method described in German standard DIN 4108). So I knew I had to expect more or less frequent dew conditions in the wall I was simulating. However, when I watched the WUFI film, I could never see the relative humidity reach 100%.

The usual building materials always have some moisture sorption capacity. This sorption capacity buffers changes in relative humidity inside the wall. If you define boundary conditions which would provoke instant condensation in a Glaser calculation, you may nevertheless not get condensation in a realistic case (such as simulated by WUFI).

That's because a relative humidity of 100% would correspond to a moisture content equal to free saturation of the material in question, and this amount of water must first be transported into the dew region. The diffusion flows do transport moisture to the location where dew conditions prevail, but the transported amounts of moisture are generally small, and the RH will only slowly rise from the initial value, say 80%, to 81%, 82% etc. It may take days or weeks until sufficient amounts of water have been transported to the dew region so that finally free saturation (i.e. RH=100%) is reached. Meanwhile, boundary conditions may have changed and there are no dew conditions any more.

The Glaser method, on the other hand, simply assumes that 100% RH are reached instantly, it doesn't consider the necessity to actually move water in order to reach the moisture content that corresponds to 100% RH.

Furthermore, real materials (as opposed to Glaser) usually have some capillary conductivity which tries to dispel any moisture accumulations. This effect actively works against local water build-up, so that 100% RH can't be reached easily.

Of course, you may get water accumulation in your building component if conditions are right (or wrong). But this will rarely be accompanied by 100% RH. If you see relative humidity approaching 100% somewhere in your component, it's probably much too late...