Moisture Management

Managing moisture involves multiple strategies to successfully combat bulk water, condensation, and water vapor. The first line of defense is to drain the water by draining and directing bulk water away from enclosure. This includes minimizing locations where water can sit for extended periods while working its way through weaknesses in the assembly, such as at joints or through capillary action in porous materials. The second strategy is to avoid allowing building materials to fall below the dew point temperature, defined as the temperature at which liquid air condenses out of an air sample. The more water is in the air (higher absolute humidity), the higher its dew point temperature. As materials cool, they become prone to moisture condensing on their surfaces because the air temperature on the surface dropped below the dewpoint of the higher temperature surrounding air. The key here is to keep materials above the dew point temperature of the air with which they interact. The final moisture management step is to manage diffusion. Diffusion is the process of moisture moving through materials themselves, not through interfaces; it allows for moisture to move through an assembly in at least one direction based on a material’s permeance. The process of diffusion is slow and should be the last strategy implemented for moisture (vapor only, not bulk water).

Case Study

In the scenario shown here, an architect's strict adherence to circular geometry led to significant performance issues. The architect incorporated natural light into a deep educational building's corridor through a series of linear skylights.

Copyright 2018 From Integrating Building Performance with Design by Elizabeth J. Grant. Reproduced by permission of Taylor and Francis Group, LLC, a division of Informa plc.

Adherence to a design goal, without weighing potential pitfalls, led to unexpected consequences in water management within this building.

The quarter-circle skylights, intended to brighten the space, also introduced leaks. Leaks developed at the awkward reglet joint with the slab above, where the quarter-circle becomes dead-level, and at the insufficiently sloped curb below, where the skylight meets an occupiable deck.

Copyright 2018 From Integrating Building Performance with Design by Elizabeth J. Grant. Reproduced by permission of Taylor and Francis Group, LLC, a division of Informa plc.

Complex intersections led to water intrusion.

Despite attempts to fix these leaks with tape and caulk, the problems persisted, causing water damage and mold growth below the skylight, concealed by a vinyl banner.

Copyright 2018 From Integrating Building Performance with Design by Elizabeth J. Grant. Reproduced by permission of Taylor and Francis Group, LLC, a division of Informa plc.

Over time, the water facilitated biological growth and deteriorated building materials.

The architects hired to correct these issues violated the original designer’s circular motif with monoslope replacement skylights, removing areas that were insufficiently sloped, a decision made to preserve the building's integrity. This example underscores the importance of considering a building's integrity and future needs over a dedication to aesthetic consistency.

Copyright 2018 From Integrating Building Performance with Design by Elizabeth J. Grant. Reproduced by permission of Taylor and Francis Group, LLC, a division of Informa plc.

Replacement skylights eschewed the original circular design to create better moisture management and preserve the building from further damage.

A Note on Bulk Water Management

Halting liquid water from infiltrating the building enclosure at any point is of first concern when thinking about control layers. Uncontrolled bulk water within a building enclosure is extremely deleterious and renders the other control layers futile. General strategies for bulk water management include sloping to locations where water can be managed and providing capillary breaks between materials where drainage occurs.

On a low-slope roof, for example, the membrane (which can be a single sheet, multiple plies, fluid application or a combination of these approaches) is the first line of defense to prevent the intrusion of bulk water. Water that hits the roof system must then be managed. The phrase “slope to drain and then drip” offers a good first strategy summary. If the roof deck itself does not provide slope, solutions such as tapered insulation should be considered to allow this to happen. [For further information on this topic, please see “Go with the Flow: Tapered Insulation Fundamentals”.] The next step is to ensure continuity at all interfaces, providing properly lapped flashings and membranes such that water cannot get caught on a lip or edge and allowed into the enclosure by way of capillary action or suction at a weakness in the bond between materials.

The roof-to-wall interface is one location where two separate systems must come together to prevent bulk water from entering the building. To do this, a water control membrane (likely a flashing or another accessory product) should be continuous under the coping or edge metal, with the coping sloping inward. This allows water to be directed to internal roof drains. Drip edges on the vertical face of the edge metal are vital to send bulk water out and away from the building, preventing capillary suction from bringing water under the cap.

As is true with all of these barriers, the water control layer’s effectiveness can be greatly reduced by openings and penetrations, even small ones. These discontinuities can be caused by poor design, poor workmanship, damage from other trades, improper sealing and flashing, mechanical forces, aging, and other forms of degradation. Paying attention to the details during the design phase is important as problems are notoriously hard and expensive to fix after the fact, as shown in the skylight detail example.

Planning for Air and Vapor Movement

One of the primary purposes of a building enclosure is to keep moisture out of a building. What makes this difficult is that moisture comes in many forms and can take many paths into a building, not just by way of bulk water. Water vapor can enter a building carried by air and by way of diffusion. The amount of moisture vapor available is dependent on the temperature: warm air is able to hold more moisture than cold air. When warm air cools, it is no longer able to hold as much moisture, so the relative humidity increases, along with the risk of condensation on surfaces. While water vapor itself is not frequently a cause of an issue, it does become an issue when it is allowed to condense at a location where it cannot be managed.

Examining the Visible Impacts of Air

It is important to differentiate between air leakage and vapor diffusion. Air-transported moisture is a bigger issue than vapor diffusion because of the comparative amount of moisture transported during each process. Air-transported water vapor, as the name implies, is carried into or out of a building by air that infiltrates or passes through the building enclosure. Gaps and discontinuities in air barriers within the building enclosure, including at the roof, allow air along with its associated moisture to enter the building (see Figure 8).