In bathrooms and kitchens, what must be installed for permeable materials?

• A. A membrane not exceeding 18 ng
• B. A vapor barrier
• C. An insulation layer
• D. No membrane required

Answer: (A)

Moisture control in wet zones relies on a barrier layer between the substrate and finishes. When permeable materials are used in bathrooms or kitchens, a dedicated membrane with very low vapor permeability is installed to limit moisture diffusion into the structure. The specification of a membrane not exceeding a tiny permeability (like 18 ng) ensures the barrier is effective at blocking water vapor and protecting framing, insulation, and adhesives from moisture damage. This prevents problems such as mold growth, rot, and tile debonding over time. Other options either describe a general vapor barrier without the precise permeability standard, suggest insulation (which doesn’t address moisture control for permeable finishes), or imply no barrier is needed—which would leave the substrate vulnerable to moisture intrusion.