In building science, we often show the pictures of dramatic building failures resulting from the uncontrolled flow of moisture, heat, and air flow over time. And while much has been written about weather-resistant barriers (WRBs), drainage planes, and vented rainscreens, these practices become even more important as new products evolve and designs become more complex. Truly green, sustainable buildings must last a long time, and the No. 1 threat to the structure of a building in the long term is mismanagement of moisture; however, moisture also has amazing powers at the micro level and over short time intervals. Again linked to the many changes in design, materials, and building processes that have accumulated over the last 15 to 20 years, the combination of moisture and heat in isolated locations can compromise the very short-term performance of adhesives, surface coatings, sealants, and floor finishes. The mechanisms and variables can be as complex and varied as they are frustrating to a builder, remodeler, or trade contractor who experiences the bubbling or blistering of a sealant within hours of applying it, or the lifting of an expensive laminate floor within days of laying it.
Let’s start by reviewing a few changes the industry has experienced that may be at cross purposes when it comes to long-term vs. short-term moisture management and product performance. It will be of no surprise to anyone that to smooth out labor resources we are building consistently throughout the year. Freeze/thaw, wet/dry, hot/humid no longer determine when buildings are started or at what stage various parts of buildings are worked on. Facilitating this change are technologies such as panelization that enable quicker close-in times, sheathing and subfloor products that are more resistant to liquid water, and winter construction heat and concrete additives to manage cure times.
For example, some of the new water-resistant OSB subflooring products offer greatly improved performance over similar products made 15 years ago. The lower water absorption and greater stiffness of the new products helps protect them from exposure during construction and, in a longer-term view, offers builders significant savings in time and possible warranty claims resulting from swelling of joints, squeaky floors, and telegraphing of flooring problems. But short-term conditions could come into play. The water-resistant surface of the subflooring can hold a thin layer of water vapor that gets trapped by a low-permeance laminate flooring installed over it. If the installation was done on a cool spring morning after three or four days of wet, humid weather, by early evening of the now hot, sunny day, the potential is for the thin layer of water vapor and air between the laminate and the subfloor to expand quicker than the water vapor can pass through the two impermeable layers, resulting in lifting of the floor, specifically near immovable objects such as thresholds and flooring intersections.
Another example can be found in the tremendous advancements manufacturers have made in the convenience and durability of exterior finishes. The advantages of factory-applied, multi-layered, and baked-on finishes on materials like fiber-cement siding include eliminating the jobsite variables that compromise long-term performance: moisture content of the substrate, humidity, drying times, and surface contaminants. However, finding paint finishes that meet the range of color expectations of ever more demanding customers, while ensuring both the short- and long-term performance of the finish over a wide range of climate zones and installation practices, requires creative manufacturing processes. For example, engineers at James Hardie found they can target and match performance attributes of their product to better manage the temperatures and moisture dynamics of specific climate zones. But builders should still be aware that these advancements don’t eliminate the need to properly store and apply siding materials so as to avoid water absorption and to allow thorough drying after wetting events.