Bulk water

Bulk water, such as rain and snow, is kept out of buildings with roof membranes and wall cladding systems. Initially, roofs were created for this purpose. Keeping out bulk water consists of watertight detailing that is typical for any roof assembly. For facilities aiming at high levels of efficiency, including air tightness in conjunction with water tightness is a vital detail. Often, the roof membranes will serve as the air barrier, so proper detailing at penetrations and perimeters is vital. This includes tie-in with exterior wall assemblies to ensure a continuous air and water barrier.

Air-transported moisture

Air-transported moisture, as the name implies, is carried into or out of a building by air that infiltrates or passes through the building envelope. There are several types of air conveyance that can occur through the building envelope. Air infiltration is where exterior air enters the building through gaps in the exterior enclosure. Intrusion refers to the interior air that enters a roof or wall system but does not exit to the exterior. Interior air can be problematic where interior activities cause raised humidity levels in the air, one such example is in a gym, which are often located in both schools and hospitals. Large congregations of people exercising and exhaling increase moisture levels in the air; not to mention that shower facilities are usually adjacent to gyms, which also increases air moisture levels. Where uncontrolled moist air from gyms or showers enters into the roof assembly, condensation can form, often developing within the roof assembly. Condensation within the roofing assembly is not likely to evaporate, and this can lead to wet insulation, roofing components with lowered R-values, water accumulation in the roof deck, and the possibility of fostering mold.

Air exfiltration occurs where interior air exits from the building, also through gaps and inconsistencies in the building exterior. Exfiltration can lead to inefficient HVAC systems, where the building heating and cooling systems have to work harder to maintain interior temperatures, which ultimately leads to higher energy bills.

Image courtesy of GAF

Air transported moisture.

Contrary to popular belief, air-transported moisture is much more critical to prevent than water vapor that enters a building by diffusion; condensation is likely to occur any time there is uncontrolled air movement. Controlling air leakage, and the associated moisture that air contains, can be of added difficulty when there are sources of moisture from the interior of the building, such as with gyms or shower facilities, which are often both located in schools and hospitals. Because of the amount of potential moisture carried by air, the code requires air barriers.

Vapor drive

Schools and hospital buildings are commonly maintained at set temperatures year-round. Depending on the climate zone, during summer months, the warm, moist outside air wants to move inward toward the cooler, drier interior. In the winter, this drive is reversed—the warm, conditioned interior air exerts pressure towards the cold exterior. The direction of the vapor drive experienced by the building dictates the best placement for vapor barriers, if recommended. Generally, the location of the vapor retarder should be located on the warm side. Oftentimes, the roof membrane acts as the vapor retarder and an additional vapor retarder is not required in the assembly. However, an analysis should be performed to determine if a dedicated vapor retarder is required and the most appropriate placement.

Managing Efficiency at the Roof: Air Barriers

Air moving through a building presents all kinds of hazards. First, uncontrolled airflow obstructs building conditioning. Building owners pay a lot for conditioned air. Air infiltration and exfiltration make up 25-40 percent of total heat loss in a cold climate. Air infiltration and exfiltration make up 10-15 percent of total heat gain in a hot climate. Losing one-third or more of conditioned air has a significant impact on the operational costs of a building. A 2005 study funded by the National Institute of Standards and Technology (NIST) simulated infiltration and exfiltration reductions in buildings and reported that, “Predicted potential annual heating and energy cost savings… ranged from 2-36 percent, with the largest savings occurring in heating-dominated climates…”

However, conditioned air loss is not the only concern. Uncontrolled air raises serious issues because of the moisture that air carries. Moisture in buildings can be damaging; moisture that has accumulated within the roof insulation will reduce the effective R-value. Saturated insulation has an R-value nearing zero, which is similar to having minimal insulation or no insulation at all. Dealing effectively with controlling airflow not only allows a building to save energy but also to mitigate moisture accumulation. Air barriers are systems of materials designed and constructed to control airflow between conditioned and unconditioned space, improving energy efficiency and comfort.

In 2012, the International Energy Conservation Code (IECC) published air barrier requirements, stating that a “Continuous air barrier shall be provided throughout the building envelope…” The purpose of an air barrier is straightforward: first, to minimize the loss of conditioned air from within a building, and second, to reduce energy loss and increase building energy efficiency.

It is important to distinguish that, the air barrier is often a system of materials that controls air leakage and convective heat flow through the building enclosure from the interior to the exterior. The air barrier is, critically, not one material, but instead is an integrated system of many different materials and components. As such, the connection of air barriers at interfaces between interfaces, such as the roof to exterior wall interface, is critical.

Roof air barrier code compliance

Designers should always consult local code to determine whether a dedicated air barrier is needed for a particular project. Under the widely adopted IECC 2018 and ASHRAE 90.1 2016, a building enclosure is required to function as an air barrier for all new construction except in climate zone 2b. Chapter 1 Scope and Administration, Section 101.4.3 references additions, alterations, renovations, or repairs. It reads as follows:

“Additions, alterations, renovations or repairs to an existing building, building system or portion thereof shall conform to the provisions of this code as they relate to new construction without requiring the unaltered portion(s) of the existing building or building system to comply with this code.”

ASHRAE Climate Zone Map.

Properly installed roof membranes can be used as part of an air barrier system. Membranes can be considered as both the water and air control layer in a roof assembly. Roof membranes that are automatically considered by code to be suitable for use in an air barrier system are:

• Built-up roofing membrane.
• Modified bituminous roof membrane.
• Adhered single-ply roof membrane.

Single-ply membranes that are mechanically attached are also considered to be included, so long as the manufacturer can provide a data certificate confirming that the material has an air permeability of no greater than 0.004 cfm/ft² (0.02 L/s · m2) under a pressure differential of 0.3 inches water gauge (75 Pa) when tested in accordance with ASTM E 2178. In practice, most manufacturers have this readily available on request. But note that the IECC states an important caveat, that materials shall be deemed to comply, provided that joints are sealed, and materials are installed as air barriers in accordance with the manufacturer’s instructions.

Roof membranes can be part of an air barrier system and their use will be considered code compliant only so long as they are correctly installed and tied into the wall air barrier layer.

There are two additional means of achieving compliance that are acceptable:

• First, assemblies of materials and components (sealants, tapes, etc.) that have an average air leakage not to exceed 0.04 cfm/ft² under a pressure differential of 0.3 in H₂O (1.57 psf) when tested in accordance with ASTM E2357, ASTM E1677, ASTM E1680, or ASTM E283. The following assemblies should meet these requirements:
• Concrete masonry walls that are fully grouted or painted to fill the pores.
• Second, certification via whole building testing, where the air leakage rate of the completed building can be tested and confirmed to be ≤ 0.40 cfm/ft² at a pressure differential of 0.3 inches water per ASTM E779, ASTM E3158, or equivalent method approved by a code official.