Part 5—Moisture & Roofing

Moisture has numerous opportunities to impact roof systems across the project life-cycle phases. Roofing systems must be designed and installed to address the many sources of moisture that can affect a building across the project phases. The primary root causes of moisture in roofing come from the following sources:

• Construction-related moisture
• Occupancy generated moisture
• Exterior leakage of water and air

Early in the design process, roofing professionals need to identify and plan for moisture from construction-generated sources, anticipated occupancy-related moisture, and moisture for discontinuities in the air and water control of roof systems.

CONSTRUCTION-RELATED MOISTURE

During the construction process for roof systems, moisture has a direct opportunity to find its way into areas where it was never intended. Temporary moisture protection plans discussed earlier in many green rating systems, are a good start but do not fully protect a building from experiencing excessive construction-related moisture.

Most construction practices release some amount of moisture into the building space. These can be as straightforward as wet-applied processes such as drywall installation and painting, although these are typically relatively short-term. However, some specific construction-related practices can release large amounts of water over a considerable time frame into the building. Concrete floor slabs, concrete roof decks, and temporary heating during construction are significant sources of moisture. Figure 8 demonstrates the relative construction moisture impact of these practices.

Figure 8. Examples of construction moisture sources.¹⁸

Concrete in new construction can appear dry but is rarely dry enough. Water takes a long time to diffuse out of a 4-inch-thick slab. Earlier when discussing complexity and interconnectivity, concrete decks in low-slope roof systems were used as an example. In the example scenario, the inherent moisture in the concrete structure is exacerbated by new concrete mix designs and the cascading adhesion problems that can occur in roof systems that rely on “rules of thumb” for roof design.

The flooring industry is well aware of the challenges that come with installing some flooring materials on freshly placed concrete slabs and have guidelines in place to use the building’s mechanical equipment to manage this moisture prior to installing the flooring finishes. Regardless of the type of concrete, significant amounts of water remain after curing is completed. In concrete roof decks, there is very little correlation between cure time and the amount of water remaining. Guideline “rules of thumb,” such as not installing the roof system until a minimum of 30 days after pouring and forming, are not particularly effective at reducing or eliminating issues.

The Single-Ply Roofing Industry (SPRI) has previously issued an advisory bulletin¹⁸ on construction-related moisture and provides the following guidance:

“...buildings under construction should be adequately ventilated during concrete hydration and other high moisture-related construction activities. Temporary, high-volume ventilation systems are commercially available and should be used during construction. These high-volume air-handling systems include dehumidification that is essential to remove large amounts of moisture from the air. HVAC systems designed for temperature control of finished buildings are not sufficient to remove construction-generated moisture.”

When measures are not taken, construction-related moisture can delay the installation or shorten the lifespan of installed building systems that are moisture sensitive.

OCCUPANCY GENERATED MOISTURE

Understanding the building occupancy is a good way to think about the main building-use scenarios. Occupancies can be categorized into small and large groupings, relative to the amount of occupancy-generated moisture anticipated during the operation of the building.

Buildings with Small Amounts of Occupancy-Generated Moisture. These are the most common situations, covering office, retail, and warehouse spaces, for example. In this scenario, using two layers of insulation with staggered joints can help prevent the migration of warm, moist air up to the underside of the roof membrane. Warm air and moisture generally move upward, so drying out the roof system during warmer/sunnier days is a slow and difficult process. In northern climates, regardless of the membrane type, condensation can occur between the membrane and the insulation. If a single layer of insulation is used, install cover boards, especially a high-density polyiso board, and use staggered joints. Both of these approaches use the staggered roof components to limit uncontrolled interior air intrusion that could by-pass the insulation and result in condensation.

Buildings with Large Amounts of Occupancy-Generated Moisture. This category includes paper mills, laundries, buildings with indoor swimming pools, and the like. In this scenario, the building's air handling and ventilation systems should be carefully specified to take into account the significant moisture loading. Also, the entire building envelope needs to be designed and constructed in such a way that damaging condensation and moisture build-up do not occur. It is prudent to include a building science professional experienced in these types of building occupancy and designs be involved.

By being aware of the use of the building, the roof can be designed and installed to accommodate the scenario. It’s important to take the occupancy-generated moisture into account when designing a new space or changing the use of an existing building.

EXTERIOR LEAKAGE OF WATER AND AIR

Bulk water (i.e., rain and snow) is kept out of buildings with roof membranes and facades of all types. Air-transported water, as the name implies, is carried into a building by air that leaks through the building enclosure. Water vapor enters a building by the process of diffusion of water molecules through an envelope material. There is an order of priority for the prevention of water intrusion. Bulk water is most critical, then air-transported water, and finally, of least concern—although still important—is vapor diffusion.

The design, manufacturing, and construction industries are quite good at keeping bulk water out of buildings. Only recently has there been a focus on the importance of stopping air leakage in buildings. That is why the International Energy Conservation Code (IECC), since 2012, requires all new buildings to include an air barrier. Including an air barrier prevents conditioned air from escaping and exterior air from infiltrating, saving energy. Preventing air-leakage across building enclosures also keeps the moisture in the air from being deposited as condensation in roofs and walls as it passes through. In a warm climate, air transports 10x more water than diffusion, and in a cold climate, air transports 100x more water than diffusion. This is why air-transported moisture is much more critical to prevent than water vapor that enters a building by diffusion.¹⁹

Vapor retarders do just as they are named—they reduce vapor diffusion, but not all vapor retarders are equal. There are 3 classes of vapor retarder materials, as shown in the figure. The lower the perm rating, the less diffusion occurs through a material. Most roof membranes are Class I vapor retarders as shown in Figure 9. Remember, perm ratings are material ratings; the full system needs to be designed and installed correctly for proper functionality.

Figure 9: Three classes of vapor retarders.

The roofing industry continues to use the term “vapor retarder” as a shorthand, but the discussion should be focused on the continuity of the building’s air barrier to managing energy and condensation moisture in buildings. All vapor retarders block air, but not all air barriers block vapor diffusion. Not all vapor retarders in roof systems are installed as part of the building’s continuous air barrier; the caveat is that the vapor retarder needs to be sealed at all perimeters and penetrations to act as an air barrier, and tied to the wall air barrier so it is continuous to prevent air to from bypassing the vapor retarder layer. So, practically speaking, the vapor retarder in the roof assembly can be part of the building’s continuous air barrier if it is installed to block the passage of air.

A roof assembly configuration that manages moisture potential from both air leakage and vapor diffusion is shown in Figure 10. An adhered roof system is shown with multiple layers of insulation (with board joints offset and staggered) over a vapor retarder that is connected to the wall air barrier, contributing to the building’s continuous air control layer. When installing an adhered membrane with a water-based adhesive, make sure the adhesive will not reliquefy in the presence of moisture. This roof design configuration helps lower the risk of condensation from diffusion and air leakage from occurring inside the roof assembly. The end result can be a roof system with increased longevity, thermal performance, and improved energy efficiency for the building.

Figure 10: A roof design that improves longevity and thermal performance.

ROOFING MOISTURE SUMMARY

The vast majority of roof systems provide trouble-free performance for many years. When problems do occur within a few years of installation, it could be due to construction-related moisture, occupancy-generated moisture, or leakage of air and/or water. Roofing manufacturers provide a wide range of material choices so that the design professional can configure the optimum system for each building.

It is important to install building systems, like roofing, in conditions that allow it to perform the full duration of its intended useful life. This may require addressing roof designs, sequencing, and site conditions that are evolving along with the building industry. As with buildings that have occupancy-generated moisture, care must be taken to ensure that these are specified and installed correctly. It is key that when a vapor retarder is utilized to prevent moisture accumulation, it should also be designed and installed to be sealed at deck-to-wall joints, at gaps, and around roof penetrations to limit air leakage.

Locating the air control layer in the roof towards the inside of the roof insulation in the assembly prevents warm moist air from entering the roof system and potentially condensing on a colder surface beyond the insulation. Also, using multiple layers of insulation (minimum of two) and cover board, each with staggered joints helps prevent moist airflow from the interior of a building up into the roofing system assembly. Importantly, make sure professionals who have experience with the specific construction types are involved early in the project process. These could include structural engineers, building science experts, roofing installers and concrete suppliers.