What is a drawback of using rigid board insulation as an air barrier?

Using rigid board insulation as an air barrier does present some challenges due to its lack of flexibility. When it comes to installation, the rigidity of the material can complicate the treatment of joints and penetrations, which are critical to maintaining an effective air barrier. The inflexibility may lead to difficulties in achieving a continuous air barrier when the insulation needs to adapt to irregular shapes or when connecting to other building elements. If joints are not properly sealed or aligned due to the stiffness of the rigid board, air leakage can occur, undermining the performance of the air barrier system.

Considering the other options, while cost-effectiveness may vary between materials, rigid board insulation is not necessarily less cost-effective in all situations. The R-value of rigid board insulation is generally high, so lower R-value is not typically a drawback associated with it, and the absorption of water can depend on the specific type of rigid board used. Many rigid boards are designed to resist moisture absorption effectively. Thus, the primary drawback associated with this material, especially in the context of air barriers, is indeed its rigidity, which can complicate the sealing of joints during installation.