What happens to the R-value of insulation when it becomes wet?

When insulation becomes wet, its R-value, which measures its thermal resistance, decreases. This occurs because moisture within the insulation material affects its ability to resist heat flow. Most insulation materials, particularly fiberglass and cellulose, rely on trapped air pockets to provide their insulating properties. When these pockets become saturated with water, the air is displaced by water, which is a much better conductor of heat than air.

As a result, the insulation's capacity to resist heat loss is diminished, leading to a significant reduction in its effective thermal resistance or R-value. This concept is critical in the context of building performance, as wet insulation can lead to energy inefficiencies, higher heating and cooling costs, and increased risk of mold growth and additional damage within the structure. Knowing how moisture affects insulation can help in making informed decisions regarding building materials and maintenance practices.