We know that materials are divided into different classes based on their ability to allow water vapor to pass through. Class I materials are vapor barriers, they barely allow any vapor through. Class 2,3 and 4 are vapor retarders.
Understanding vapor retarding and barrier systems is crucial in building construction, as it helps control water vapor movement within walls and prevents potential issues like condensation and mold. Let’s explore the different scenarios and considerations for various wall assemblies to achieve optimal results.
Stud Cavity Insulated Walls: The common construction method in the US involves wood studs filled with insulation and finished with drywall. In cold climates, a vapor retarder on the interior side limits water vapor movement from inside out. In warm climates, the vapor retarder is placed on the exterior side due to outward vapor flow.
CMU/Concrete Walls: Exposed CMU or solid concrete walls, furred out with studs and insulated, require a vapor retarder behind the drywall in cold climates. In warm climates, a vapor retarder isn’t necessary as concrete acts as a natural barrier.
Brick/Stone Walls: These walls have an independent masonry layer, air gap, air barrier, vapor retarder (e.g., Tyvek or OSB), and drywall. High humidity conditions created by moisture-storing claddings require well-ventilated cavities to allow water and vapor escape.
Stucco Walls: Stucco walls consist of stucco on metal lath, an air barrier/vapor retarder, sheathing, insulated walls, and drywall. To avoid water issues, avoid using vapor barriers and consider continuous exterior insulation.
Vapor Impermeable Insulation: Using vapor impermeable insulation like XPS, polyiso, or closed-cell spray foam on the exterior requires no additional vapor barrier in cold climates, but may need one in warm climates.
History and Code Compliance: Over time, the approach to vapor control has evolved. Presently, it’s advised to prioritize vapor retarders over barriers, with exceptions for high interior humidity spaces. Code requirements differ based on climate zones.
Prioritizing Air Barriers: Focusing on air barriers is crucial to stop air infiltration, which can cause more moisture issues than diffusion. Continuous exterior insulation like Rockwool Comfortboard is recommended to reduce thermal bridging and prevent air infiltration.
HVAC and Moisture Control: To maintain optimal moisture and relative humidity levels in air-tight homes, mechanical ventilation systems like HVACs and HRVs are essential. Proper air exchange rates are vital for effective moisture control.
Adapting to the Ever-Evolving Industry: The discussion on vapor control is constantly evolving, and the industry currently leans towards continuous spray foam insulation, inoperable windows, and exterior insulation as air barriers. Time will reveal if this technique remains effective or requires further adaptation.
While navigating the intricacies of vapor control might seem challenging due to varying advice, staying informed and being adaptable are key to achieving successful, moisture-controlled building designs. As construction methods and products evolve, embracing the best practices that fit your specific needs will lead to long-lasting, efficient buildings.
