Water causes all kinds of problems for buildings. When rain leaks into walls through a poorly flashed window sill, or when the humidity in summer air contacts a cold water pipe and condenses, mold or rot can easily develop.

One possible way to handle localized leaks or intermittent humidity spikes is to build with hygroscopic materials that provide hygric buffering and hygric redistribution. To say the same thing in simpler terms: installing building materials that can absorb and store water may help handle moisture events.

Hygroscopic materials are materials that readily take on moisture. Examples include brick, framing lumber, and cellulose insulation. Green builders who use natural materials sometimes point out that in a house with thick walls made of rammed earth, straw bales, or logs, these hygroscopic materials act as a hygric buffer.

Hygric buffering

A material that acts as a hygric buffer can store (and sometimes redistribute) moisture. Those who sing the praises of hygric buffers compare these materials to thermal mass. Just as thermal mass (for example, concrete) that is in contact with a home’s indoor air can act as a thermal flywheel, helping to even out spikes and troughs in the indoor temperature, so hygroscopic materials can act as a flywheel for humidity.

In other words, if a house has enough hygroscopic materials in contact with the indoor air, these materials can smooth out episodes of high and low indoor relative humidity. When the air is very humid, the materials can absorb moisture; and when the air is very dry, the materials will release moisture into the indoor air (by evaporation).

Hygric redistribution

In addition to their hygric buffering function, hygroscopic materials sometimes provide useful “hygric redistribution” — in other words, these materials can take on water and move the water away from a wet…