Utilizing airspace to provide an R-value in building assemblies is not straightforward. Air, by its nature, does not tend to stay still. Instead, it moves into, out of, and through assemblies. Traditionally, airspace R-values have been assumed by the building code to be ideal. That is, they are absent of essentially any air leakage to or from the airspace.
This becomes problematic for air-spaces located in very leaky conditions, even though they may be considered enclosed. For example, airspaces ‘enclosed’ behind cladding can experience air leakage rates of as much as 400 air-changes-per-hour. In such a case, the airspace likely provides very little R-value, even though it supplies ventilation benefits to the wall and cladding. The illustration at right shows air-flow behind cladding in relation to its ventilation/drying effect and impact on airspace R-value.
The above concern has received increased focus in recent years as airspaces are turned to as a means to help meet newer energy code requirements. In particular, use of reflective airspaces (airspaces that are enclosed with a reflective surface or material on at least one side) are known to provide thermal value. However, these benefits may vary substantially depending on the nature of the airspace and other conditions of use. The key is the degree to which the airspace is enclosed or sealed on all sides to reduce the flow of air through the space.
If the airspace is essentially sealed, then it complies with the R-value determined, based on ideal conditions as required by the ASHRAE Handbook of Fundamentals as referenced by the FTC R-value rule. Alternately, if the airspace is located behind typical claddings, then the use of ideal airspace R-values is invalidated and such airspaces (reflective or not) must be tested to determine an appropriate R-value using an ASTM C1361 (hot box) test method of a full-scale assembly.
It’s important to note this test method does not include a means to induce the actual air pressure differences that may drive air-change rates in the real world, as shown in the illustration. C1361 must be modified to induce air pressure differentials and air-exchange conditions.
Fortunately, these issues are more clearly addressed in the ASHRAE Standard 90.1 energy provisions for commercial buildings (which will be released soon), based on a lengthy discussion to arrive at a consensus position on this matter using the available data.
The available research, test data, code requirements, and FTC R-Value rule requirements are defined, analyzed and discussed in a newly released research report entitled, “Air Space R-Value.” The research report documents what is currently known about airspace R-values on a scientific basis, and makes recommendations on how to properly qualify air-spaces and determine R-values for airspaces that commonly differ from ideal conditions.
In August 2020, ASHRAE published a Research Report, "Thermal resistance of ventilated air-spaces behind external claddings; definitions and challenges," to provide additional data toward the development of an appropriate test methodology and criteria (read more here). Only after airspaces are appropriately characterized, designed and installed will they be able to truly help building designers meet newer energy code requirements.