Air and vapor migration
This article has discussed how the key to thermal efficiency is the control of temperature and moisture. Up until now the focus of this has been on temperature, but clearly that is only part of the equation.
Because heat, air, and moisture transfer are coupled and closely interact with each other, they should not be treated separately. In fact, improving a building envelope’s energy performance may cause moisture related problems. Evaporation of water and removal of water by other means are processes that may require energy. Only a sophisticated moisture control strategy can ensure hygienic conditions and adequate durability for modern, energy-efficient building assemblies. Effective moisture control design must deal with all hygrothermal loads (heat and humidity) acting on the building envelope – ASHRAE Handbook – Fundamentals (2013)
While limiting the infiltration of liquid water is essential for the health and sustainability of a building, any discussion about the control of moisture in regard to energy efficiency is really about the control of air and water vapor.
The design and installation of appropriate and comprehensive air barriers are mandated by Section C402.5, “Air leakage–thermal envelope (Mandatory),” of IECC and must be continuous “throughout the building thermal envelope.” As the name suggests, an air barrier system’s primary purpose is to reduce the flow of air between the interior and exterior. However, it may also serve a secondary purpose of restricting the flow of water vapor.
Providing a comprehensive and contiguous air barrier “throughout the building thermal envelope” is a complex undertaking, and particular care should be taken in its design. Large-format details showing the continuity of the air barrier across changes in the thermal envelope should be developed, including at the field of the opaque wall assembly and its interface with:
- transitions in materials and assemblies;
- changes in plane;
- fenestration; and
- roofs.
The barrier must be designed and installed to resist forces that may deteriorate the assembly, particularly at seams and transitions. Additionally, entrances may require vestibules depending on their size, use, and climate zone location.
Standards for air barrier performance are very rigorous, and installed barrier systems, which include fenestration, must undergo testing to see they do not admit more air leakage than is permissible. Materials used in the opaque wall assembly must comply with ASTM 2178, Standard Test Method for Air Permeance of Building Materials, and “shall be deemed to comply…provided joints are sealed and materials are installed as air barriers in accordance with the manufacturer’s instructions.” Doors, windows, and skylights must conform to ASTM E283, Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen. Together, opaque walls and fenestration must form a uniform air barrier providing the requisite protection from leakage, even across joints and transitions.
While the requirements for the control of air through the building enclosure are stringently mandated, the requirements for vapor control are less so. This may seem counterintuitive, but there is a compelling reason. Energy efficiency and vapor control strategies are ineffective without a comprehensive air barrier system to restrict airflow. However, the extent to which a vapor control strategy is needed and how it should be designed and installed depends on several variables, including climate, building use and construction, and the potential and the extent to which moisture sources other than interior water vapor exist. ASHRAE 160, Criteria for Moisture Control Design Analysis in Buildings, is a recognized standard for evaluating the need for and placement of vapor retarders.
Additionally, consideration should be given not just to vapor control, but to the need of the building enclosure to effectively dry after it gets wet. This may require a ‘semipermeable’ vapor retarder, while in other cases, systems with a very low permeance may be appropriate. Care should be taken during design to avoid the potential to trap moisture through the inadvertent use of multiple vapor retarding layers in an assembly.
When vapor retarders are required, their placement relative to the insulation layer of the wall assembly is extremely important.
The retarder should be at or near the surface exposed to higher water vapor pressure and higher temperature. In heating climates, this is usually the winter-warm side – ASHRAE Handbook – Fundamentals (2013)
In other words, the vapor retarder is often installed on the ‘warm side’ of the insulation.
As always, proper installation is as important as proper design. In the case of vapor retarders, a significant increase in permeance can occur as a result of very small holes in the material.
