Moisture control is one of the most important functions provided by a building enclosure. It is vital to building durability and longevity, energy performance, and occupant health. Yet it remains one of the most challenging aspects of building design.
There are four key pathways for moisture to move through a building enclosure. Arranged in order of significance, these are 1) bulk water leakage, 2) capillary movement, 3) air leakage, and 4) diffusion. In general, the building industry has learned to control bulk water leakage using a large variety of materials and products, but relying on a few basic principles such as slope and drainage gaps, redundancy, and proper overlap of layers. Capillary movement can generally be controlled with rainscreens, drainage gaps, and capillary breaks installed in key locations.
Often failure to control bulk water and capillary drive is quickly evident, which is one reason why the building industry has been able to learn adequate methods of control for these wetting pathways. However, diffusion-driven wetting and especially air leakage remain difficult for the building industry to properly control. Modern enclosures incorporate less moisture storage and higher insulation values, which translate into less available heat to drive off excess moisture. Increasingly moisture-sensitive materials may also be selected. As they become less tolerant to moisture, modern enclosures need to be paired with improved methods of air leakage and diffusion control. Better approaches to enclosure design, accessing enclosure expertise, setting air leakage targets, and testing performance have all been shown to improve enclosure durability, energy performance, and moisture safety.
A preferred method to control all four wetting pathways is through the use of an enclosure scheme called the Perfect Wall. This design approach can be safely used in any climate and for any part of the enclosure (wall, roof, slab on grade), as well as above or below grade. It also lends itself to easier installation and quality assurance inspections for the four essential control layers: water control, air control, vapor control, and thermal control. The Perfect Wall approach requires that all of these control layers be positioned outboard of the structure regardless of whether that structure is concrete, wood frame, steel frame, or other. Conveniently, the water, vapor, and air control layers can often be provided with a single product installed on the outside face of the sheathing or outermost structural surface. In this position, it can be installed continuously and inspected more easily than enclosure approaches using multiple layers installed variously inside, outside, and/or within the structure. The thermal control layer is applied outboard of these control layers, protecting them from damage functions and keeping them and the structure warm and dry as well. This outboard system may also be easier to repair or replace during the building’s lifespan. More information on the Perfect Wall is listed below under Additional Resources.
This Perfect Wall design approach is known as an inverted or protected membrane roof when applied to the top of the building. The waterproof roofing membrane is placed on top of the roof deck, providing air, vapor, and water control in a single layer. All of the insulation is placed on top of this, typically as rigid foam board, and weighed down and protected with gravel ballast or pavers. Although the roofing membrane is more difficult to inspect and access after construction is complete, it is also longer lasting because it is protected from sun, heat, ice, and physical damage. This type of roof design doesn’t create a cold side vapor barrier or double vapor barrier as most other roof designs do, leading to much greater drying potential and moisture safety. It also eliminates common air leakage pathways and associated moisture issues in truss roofs with dense pack insulation. Building enclosure consultants have found dense-packed truss roofs especially problematic due to the difficulty of creating a continuous, robust air barrier in the ceiling plane.
Adopting the Perfect Wall approach to enclosure design eases the requirements for conducting qualitative and quantitative moisture analysis, as these are considered less likely to have moisture issues. It will also help achieve the demanding air leakage requirement of 0.25 cfm/sq. ft. at 75 Pa, though use of this scheme is just one step toward achieving that target.
Project teams may wish to hire a building enclosure consultant as well, particularly if the project team or contractor has never designed a building that has to achieve this level of airtightness. Building enclosure consultants can help project teams design and construct an air barrier system that maintains continuity even at locations that have traditionally been problematic, such as parapets, windows, and projections like balconies and overhangs. They can suggest products, materials, and approaches that have been proven to perform better in the field or are easier to install in an air- and watertight manner. Building enclosure consultants may also suggest conducting preliminary performance tests around installed components such as windows and doors, or testing the tightness of certain wall sections or building zones. Finding and remediating discontinuities in the air barrier is easier and cheaper early in the construction process (rather than during or after the final blower door test).
Mechanical engineers and contractors also should be aware of the air leakage requirements of these guidelines. Specifying higher quality louvers and dampers that can close fully during normal operation has been shown to significantly improve commercial-scale buildings’ air tightness results. A reduced level of air leakage may also impact heating and cooling load calculations and improve mechanical air distribution and pressure management.
WUFI modeling for compliance with the quantitative moisture analysis should be done by experienced professionals. Currently, there are no widely-followed certifications for this particular modeling skill, though many building enclosure consultants and firms do employ people with WUFI modeling experience. It is highly recommended to follow expert guidance for WUFI modeling techniques. Thorough guidance can be found in the Building Technologies Office Strategy Guideline: Modeling Enclosure Design in Above-Grade Walls, 2016.