[sit-rep]

You lost me man...

First you say that a 1900 era home holds the most moisture. Then you say a metal home holds 5% moisture. You say it is unfortunate that metal does not hold moisture. I thought the crux of the conversation was the reason there is mold is because of moisture??%&$#!

SR

[Restorer]

I don't blame you for getting lost. The theory is somewhat complex and to some extent counter-intuitive.

Mold growth occurs at the interface between a susceptible material (food source) and air, if the moisture content at that point is high enough and a spore is present (almost always is).

The relevant measurement of moisture content at that point is water activity, which is essentially the relative humidity on the surface, which can be very different from the relative humidity elsewhere in the area.

Walls are made up not only of studs, but also contain drywall, among other materials. Generally, the drywall is the most sensitive material.

A very tiny amount of water (less than an ounce) trapped in the wall cavity will raise the RH in the cavity above the danger point of .70 water activity, usually up to 1.0. If the cavity has wood studs, they can absorb a significant amount of this water without themselves reaching the danger point, bringing the RH on the surface of the drywall (water activity) back below .70.

If the cavity contains steel studs, with no ability to absorb water, the RH in the wall cavity will remain at 100%, which equates to 1.0 water activity, which allows mold growth to occur on the back side of the drywall.

I said it is counter-intuitive!