While often considered a less impactful transfer mechanism due to the quantity of moisture moved, vapor transfer should not be ignored. Small amounts of moisture in the form of water vapor can pass directly through the exterior enclosure materials by a process called diffusion. The amount of vapor diffusion occurring in a building is partly determined by the force that pushes it, commonly known as the vapor pressure differential, as well as the material’s vapor permeance. The lower the vapor permeance (measured in perms), the less diffusion occurs through the material.
Similar to air transfer, vapor migration also moves from high to low vapor pressure. High vapor pressure can be caused by numerous things, including an increase in relative temperature or mechanical pressurization. Most materials cannot eliminate vapor diffusion, but there are options to significantly slow the process.
Codes and design
With respect to air, vapor, and thermal control for the exterior enclosure, many changes and updates have been incorporated into not only the 2015 I-Codes, but also the 2013 American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings. These standards and codes contain both prescriptive and performance paths for compliance. This article primarily focuses on the updates and additions associated with the latest prescriptive requirements, which can be more universally implemented. This discussion provides an overview of provisions typically referenced for enclosure design, but not all changes are included—one should also consult the codes for any changes potentially affecting the specific design.
Updated thermal control provisions are provided in the 2015 IECC and 2013 ASHRAE 90.1. Each document requires all envelope surfaces, both opaque and fenestration products, meet specific thermal values based on location and structure composition. Compliance for opaque thermal envelopes is shown prescriptively through R-values for insulation products or U-factors for assemblies, which were made more stringent in some instances in the recent code updates.
Some U-factors and Solar Heat Gain Coefficients (SHGC) for fenestration products were also tightened in recent editions. A few other notable changes in the 2015 IECC for the opaque wall thermal requirements include a method to determine the effective R-value for steel stud wall assemblies, as well as new criteria for mass walls.
While not new to the recent code updates, it is important to note a few limitations to R-value and U-factor testing. Opaque walls can be assigned R-values derived in accordance with ASTM C518, Standard Test Method for Steady-state Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus, or U-factors determined through ASTM C1363, Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of Hot-box Apparatus.
While both test methods involve measuring thermal transfer between cold and hot planes, ASTM C518 measures transfer through materials, whereas ASTM C1363 measures transfer through the assembly, including components that may produce thermal bridging. However, there might be deviations between test conditions and proposed built details, which the project team may need
For example, whether for opaque walls or for fenestration products, test methods are conducted with a prescribed set of boundary conditions—such as −18 C (0 F) exterior and 21 C (70 F) interior—that may or may not align with the conditions experienced by a given building. The project team must understand the test conditions to evaluate the results’ applicability to individual designs.