Why is it misleading to label some materials as "vapor barriers"?

Labeling some materials as "vapor barriers" can be misleading because, in practice, these materials cannot completely stop vapor diffusion. While they may significantly reduce the passage of moisture vapor through building assemblies, they often allow some moisture to permeate over time due to the nature of their materials and construction.

This understanding is crucial in building science and construction practices, particularly when considering moisture control in attics and walls. A true vapor barrier would need to be a perfect impermeable surface, which is practically unattainable in most situations due to factors like material degradation, environmental conditions, and changing pressure differentials within the building envelope. Therefore, recognizing that "vapor barriers" are more accurately described as vapor retarders helps in setting appropriate expectations for moisture management in construction.