Solution Strategy: Rigid Cover Boards

Recognizing the need for durability and protection in roofing, the National Roofing Contractors Association (NRCA) has recommended, for many years, the inclusion of a rigid cover board in all single-ply, commercial roofs. This best practice, which is growing to be more common, involves placing a rigid board between the roofing membrane and the foam insulation. While several different materials have been used in the past, coated fiberglass mat-faced gypsum cover boards are now preferred. This type of product is compatible with all types of roofing membrane installations, is a durable and rigid material, contains no organic materials that could contribute to mold, and has some built-in capacity to resist damage from water. It also has inherent fire resistance, can be cut and installed like other gypsum products, and typically weighs only 2.7 pounds or less per square foot.

Deciding to include a rigid fiberglass mat gypsum cover board into a roof assembly addresses at least the four following things:

Puncture/ Hail Resistance: Rigid gypsum cover boards form a solid substrate for the roofing membrane to help it absorb impact from forces such as hail, foot traffic, and equipment movement, helping to make the membrane less susceptible to punctures.

Insulation Preservation: Cover boards protect insulation from damage due to foot traffic and weather conditions. They also help preserve maximum R-value and energy efficiency by helping to provide an added barrier to water entering the insulation which would otherwise reduce the R-value. Nonetheless, in order to minimize the amount of water absorption in roof boards and contribute to a more resilient roof, it is recommended to specify roof cover boards that have been tested not to exceed five percent maximum total water absorption by weight.

Wind Uplift Protection: A roof assembly with a rigid cover board stands up to high winds that can produce damaging uplift pressure, preserving the integrity of the building and preventing future repairs.

Fire Resistance: Gypsum-based cover boards can also add fire-resistance protection when they are included in fire-rated assemblies, specifically those with a UL or FM fire-resistance rating. Gypsum board has been used for decades as a standard means to add fire resistance since it is inherently a mineral-based product that is non-combustible and has been shown to successfully slow the rate of fire damage. In many cases, a minimum thickness of ¼ gypsum cover board can help achieve a class A fire rating with unlimited slope.

Image: FM Global; courtesy of Georgia-Pacific Building Products

FM Global has identified different hail zones in the United States with the noted growth of Very Severe Hail Zones in recent years.

Rigid cover board products that can hold up to water intrusion, can maintain or regain their original strength, and continue to perform are obviously the most resilient and effective in protecting healthcare buildings.

Cost-Effectiveness

Of late, new technologies, new studies, and experiences both with and without cover boards have pointed to the fact that rigid roof boards are not only a logical choice but also an economical one. It is a common misperception that roofing cover boards are only needed on high-performance roofs or in areas where severe weather is common. As such, they are sometimes “value-engineered” out of a project.

Image courtesy of Georgia-Pacific Building Products

The cost-effectiveness of a roof cover board has been demonstrated by independent studies concluding the findings shown here.

The reality, however, is that they were studied by FMI in May 2020 and found to have a very strong return on investment (ROI). They conducted an independent survey of 74 organizations responsible for 112 single-ply roofs with an average size of 25,000 square feet. In addition to the ability to reduce risk exposure, they found that using a cover board as part of a single-ply membrane roof increased the median life expectancy significantly. The percentage of roofs that met their full 20-year life expectancy increased from 52 percent without a cover board, to 86 percent when a cover board is used—an overall median increase of four years in life expectancy. The study also found that annual operation and maintenance costs were reduced by an average of $1.40 per square foot or up to 65 percent for TPO membrane roofs. These significant savings and increases in life expectancy can only help with the ongoing operation and budgeting of healthcare facilities.

EXTERIOR WALLS RESILIENCE

Vapor drive and water intrusion are significant factors not just in roofing, but also through building walls when designing healthcare facilities. This is particularly important in light of the move toward tighter building envelopes with restricted airflow designed to improve energy efficiency. In theory, the tighter the building envelope, the less impact the vapor drive should have. But without proper airflow control through the walls, warmer air can easily move inside and outside, carrying moisture with it. That air movement can allow more moisture into the building and other assemblies, such as the roof. This can lead to vapor drives through temperature change and diffusion. Tighter building envelopes also make it harder for moisture to escape if it does manage to find its way in, making the ability to dry out quickly after exposure even more important. The key then, is to include effective air, water, and vapor control in exterior walls.

Water-Resistive Barriers (WRBs): This barrier is intended to do exactly what its name implies—resist bulk water from penetrating a wall assembly from the exterior side. WRBs are specifically required by the International Building Code (IBC) to protect the materials and components of a wall assembly from water that may penetrate past the exterior cladding. A properly tested WRB is typically called for behind the exterior cladding of a wall assembly on the face of the sheathing or similar surface. The WRB then needs to function as a water control layer by channeling water down its exterior face to drain harmlessly away from the exterior. In so doing, it reduces or eliminates potential water and moisture problems inside a wall assembly, particularly in cavity wall-framed construction.

Continuous Air Barriers (AB): This barrier has received a lot of attention in recent years since unwanted air infiltration has been seen as both a significant drain on energy performance and a means to transfer unwanted air-borne moisture into buildings. In particular, the International Energy Conservation Code (IECC) now has very specific, mandatory requirements for providing continuous air barriers in building envelopes aimed at restricting or preventing the passage of air to ensure minimum levels of code-required energy performance. Its purpose is to essentially “wrap” the building shell to prevent air from passing from the outside to the inside due to wind, building “stack effects,” or mechanical ventilation pressure differences. To achieve this in exterior walls, the AB is most typically located behind wall cladding on the face of sheathing or a similar surface, just like a WRB.

Vapor Retarders: While WRBs address bulk water and ABs address airborne moisture from one side of an assembly, there is also a concern for vapor penetration from the interior side of the assembly. Therefore, the IBC requires the use of vapor retarders to protect the building. Most commonly, a vapor retarder is required and should be installed on the interior face of an exterior wall, roof, or floor assembly in cold climate zones and the exterior in warm climate zones. The intent is to prevent warm, moist air from penetrating the assembly and condensing to form water that can become trapped and cause damage. The International Building Code indicates what type of vapor retarder is needed in different building and climate situations. A Type I retarder provides virtually no permeability of vapor while a Type II allows some permeability. A Type III vapor retarder slows the passage of vapor, or diffusion, but allows more permeability than either Type I or Type II. In this case, it is presumed that any vapor that enters will also exit.

Photo courtesy of Georgia-Pacific Building Products

The Samaritan Pacific Communities Hospital in Newport, Oregon, used an integrated fiberglass mat gypsum sheathing and waterproofing system to overcome construction schedule demands in a wet, coastal environment.

In light of the above, some gypsum manufacturers have developed, and now offer, the latest innovation in exterior wall technology, namely, incorporating both a WRB and AB directly into fiberglass mat gypsum sheathing. To ensure continuity in an assembly, they also offer a complete system of compatible liquid flashing materials to seal joints, seams, openings, penetrations, etc. This reduces the dependence on a field installation for effectiveness while taking advantage of the best properties of all products involved. Fiberglass mat gypsum sheathing with properly treated seams has the potential to dramatically reduce the risk of water and moisture intrusion and mold production.

These products may use different technologies, but they typically meet the code and performance requirements for use as a WB and AB provided the joints, sheathing fasteners, penetrations, openings, and material transitions are sealed per the manufacturer’s instructions. They also serve as a continuous drainage plane behind cladding to drain water away from the wall assembly. Fire resistance ratings can be achieved where needed based on tested assemblies using gypsum-based products. All of these claims are readily backed up by a variety of independent testing.