Moisture in Masonry: A Key ConcernWords: Jennifer Frey, Atlas RWIPhotos: Atlas RWIAs building envelopes become tighter and more performance-driven, clarity around moisture management in masonry applications is more important than ever. In masonry wall assemblies, where water penetration is expected and managed rather than eliminated, the ability of insulation to resist water absorption and support drying becomes especially important. Central to that strategy is the inclusion of a drainage cavity, typically a minimum of one inch, and often recommended at two inches, between the cladding and the wall assembly. When properly designed, this space allows bulk water to drain and promotes airflow for drying, both of which are essential to long-term wall performance.What we know: it’s not about preventing water from entering the wall, because it will; it’s about creating a reliable path for it to exit. While masonry is sometimes perceived as waterproof, materials like brick, stone, and concrete tend to be absorbent and can be impacted by rainwater, vapor diffusion, and even groundwater. Why is the drain and dry principle of modern masonry wall design so important? If bulk water is not effectively drained and remaining moisture is not allowed to dry, problems can quickly compound.Choose the wrong material for your wall assembly, and a host of problems could show up on the project doorstep, including damage from a freeze/thaw cycle; staining and residue from salts; mold and indoor air quality concerns; corrosion of metal wall ties, anchors, or flashing; and wet insulation not performing as intended. A recent study comparing polyisocyanurate (polyiso) and mineral wool wall continuous insulation evaluated how each material responds to water exposure, absorption, and drying, key factors in masonry wall performance. The study was conducted following ASTM 209, the standard sorption test for closed-pore foam insulation; the test requires samples to be immersed in one inch of standing water for a period of two hours. Against this backdrop, material selection within the wall assembly becomes critical. Continuous insulation, in particular, plays a significant role: not just in thermal performance, but in how a wall handles moisture when it inevitably gets in.What the results mean for real-world wall performance: Mineral wool wall continuous insulation slabs absorbed 8-38 times more water than foil-faced polyiso (4-19 times more water than coated glass-faced polyiso), which translates to up to 78% of its weight in water, due to its open-pore, fibrous makeup. Whereas it was once believed that water would freely drain from the void structure and that water retained by mineral wool wall continuous insulation would dry quickly, it was instead proven that water is suspended by capillary forces and the network of fibers within the product.Within the mineral wool wall continuous insulation product bundles from the same company, the range of sorption values varied significantly between the individual pieces. When mineral wool wall continuous insulation was tested in accordance with ASTM 209 and then rewetted, it was found that the product increased water absorption by 130-190% and extended dry times by up to four days. Additionally, repeated wetting of mineral wool wall continuous insulation revealed moisture-holding capacities that vary due to changing pore and fiber structure. Foil-faced and coated glass-faced coupled with the closed-cell foam structure of the polyiso absorbed less than 4 percent water by weight and less than 0.13% by volume. Both polyiso products were effectively dry within 24 hours. These fast-drying times for the two polyiso samples were attributed to low water absorption coupled with the ability to rapidly release water vapor. When rewet, the polyiso specimens show no appreciable increase in water absorption and no significant changes in drying time. Polyiso sorption behaviors and dry times remained constant because the structure of polyiso is unaltered by wetting and rewetting. Drying results: mineral wool wall continuous insulation absorbs 8-38 times more water than foil-faced polyiso, and drying requires 2-6 days longer for mineral wool than polyiso. It’s clear that water absorption is directly linked to pore structure: open, fibrous materials (like mineral wool wall continuous insulation) absorb and retain significantly more water than closed-cell foam (like polyiso).Why do these results matter? If insulation is wet, it doesn’t perform thermally. Water performance is directly connected to thermal performance and makes selecting the right material that much more important. The implications for masonry wall assemblies are significant. In a drained cavity system, materials that absorb and retain moisture can slow drying, extend wetting cycles, and increase the risk of long-term damage and thermal inefficiency. Conversely, materials that resist water absorption and dry quickly help maintain the effectiveness of the drainage plane and support the wall’s overall drying potential, ultimately providing long-term thermal performance and energy efficiency.Polyiso continuous insulation has the potential to perform as the primary water-resistive barrier (WRB), or provide redundancy, when properly detailed, leading to a higher level of protection for the backup wall from moisture intrusion. This flexibility allows designers to tailor assemblies for both moisture protection and drying potential. Again, the purpose of insulation is thermal protection.For contractors, the takeaway is straightforward: moisture management failures are one of the leading causes of building envelope issues, and ensuring proper drainage and drying is a low-cost solution for thermally effective, high-performance walls when compared to the often-high cost of remediation. Well-designed assemblies can help maintain product and assembly performance, reduce callbacks, and help extend the life of the building. In masonry construction, success isn’t defined by the ability to keep water out of the wall assembly entirely; it’s defined by how effectively a wall can manage the water that will, inevitably, get in. Material selection plays a central role in that outcome. About: Technical Talks
