Understanding heat, air, and moisture control

Air
Both ASHRAE 90.1 and 2015 IECC have similar air control provisions calling for a continuous air barrier throughout the building envelope. The 2015 IECC also stipulates the air barrier can be placed on the inside or the outside, as further explored later in this article.

Beyond the presence of a continuous air barrier, there are further prescriptive provisions mandating the air barrier materials not exceed an air permeance of 0.02 L/(s.m2) @ 75 Pa (0.004 cfm/sf @ 1.57 lb/sf) when tested in accordance with ASTM E2178, Standard Test Method for Air Permeance of Building Materials, and assemblies not exceed 0.2 L/(s•m2) @ 75 Pa (0.04 cfm/sf @ 1.57 lb/sf) per ASTM E2357, Standard Test Method for Determining Air Leakage of Air Barrier Assemblies.

In lieu of compliance with the materials and assemblies air leakage rates, IECC stipulates the air barrier system, or whole building, be tested in accordance with ASTM E779, Standard Test Method for Determining Air Leakage Rate by Fan Pressurization, with a not-to-exceed rate of 2 L/(s•m2) @ 75 Pa (0.4 cfm/sf @ 1.57 lb/sf). The whole-building air performance is determined not only by the materials selected, but also by the constructed assembly or collection of one or more of those materials. A material itself may limit air transfer to extremely small amounts, but once this component is assembled with other materials, the acceptable air leakage rate is increased. Other industry standards, such as the U.S. Army Corps of Engineers (USACE) or the 2012 International Green Construction Code (IgCC), provide varying and more stringent thresholds for recommended whole building air leakage.

It is important to note all these code-referenced test methods quantify the amount of air leakage, but do not identify the location or specific source(s) of leakage. Therefore, if air leakage testing fails to meet requirements, then other standards (e.g. ASTM E1186, Standard Practices for Air Leakage Site Detection in Building Envelopes and Air Barrier Systems) would need to be utilized to identify deficiencies in the air barrier system.

The 2015 IBC also addresses air transfer in specific building assemblies in colder zones with certain operating conditions. For example, it limits use of air-permeable insulation materials in unvented cathedral ceilings and provides placement information for such materials.

Vapor
As of 2009, the code provisions related to vapor transfer were moved from IECC to IBC. During this time, there were more detailed vapor retarder classifications  also added. Historically, a vapor retarder had a permeance of 1 perm or less—a common example was a polyethylene sheet. In the 2015 IBC, the classifications for vapor retarders include:

  • Class I vapor retarder: 0.1 perm or less;
  • Class II vapor retarder: 1.0 perm or less or greater than 0.1 perm; and
  • Class III vapor retarder: 10 perms or less or greater than 1.0 perm.

Therefore, the traditional polyethylene sheet is now a Class I vapor retarder.

Common examples of a Class II vapor retarder are unfaced polystyrene or plywood, whereas a Class III example is gypsum board or some water-resistive barriers (WRBs).

To determine a material’s vapor permeance and subsequent compliance with code, there are two test methods within ASTM E96, Standard Test Methods for Water Vapor Transmission of Materials. It is important to understand which method may be applicable to the specific design because the results can vary. In the water method, the material sample is adhered to a test dish containing water so water vapor ‘flows’ from the wet side through the test specimen and into the dry chamber—a condition similar to winter with humidified interior space. The desiccant method is similar to the performance of vapor transfer in a heated, dry structure during rain; it measures the inward drive into the building.

Placement of a vapor retarder within an assembly becomes particularly important given that more than one material within an assembly may now qualify as some class of vapor retarder. Caution must be dedicated to these situations to not trap or allow unwanted moisture to migrate between layers of vapor retarders within a given assembly.

The 2015 IBC definition of a roof assembly hints at the intended location of the vapor retarder inboard of thermal insulation, but other provisions within that code are more specific. For exterior walls, the vapor retarder location is specified to be on the interior side of frame walls in Zones 5 through 8 and Marine 4. The code does permit an exception with accepted engineering practice for hygrothermal analysis. Special case assemblies, such as unvented cathedral ceilings, include provisions that actually limit the use of Class I vapor retarders, but require a Class II material with airtight insulation.

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4 comments on “Understanding heat, air, and moisture control”

  1. I had no idea that there were so many different types of air conditioners available. Based on what you said, a multi split unit might work best in my home since I have many smaller rooms. I’ll have to call a contractor and see what my best options are and how to install them. Thanks for the awesome advice and info!

  2. I am definitely not very knowledgeable when it come to heaters and such. We are looking at getting a different heater in our home, and this article definitely helped me better understand the basics of heaters. I will be sure to share this information with my husband, especially the part about thermal control and the difference between different heater types.

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