The presence of an airspace enclosed within a building envelope assembly is known to contribute to the overall thermal performance of the assembly. But, the actual R-value of an airspace can vary significantly depending on various conditions of use, such as the air-tightness of the assembly of materials enclosing an airspace.

This guide reviews current technical knowledge and regulatory requirements to assist the designer in R-value characterization and limitations of use for airspaces within building assemblies for code compliance.

Step-by-Step: 

Intro

Intro

  • Airspaces in building envelope assemblies are known to affect thermal performance.
  • The exact R-value of an airspace can vary significantly depending on factors such as air-tightness or the type of surrounding materials.
  • While conventional “mass” insulation materials also have variable thermal performance depending on conditions of use, the degree of variability is generally much smaller.

This guide will detail the two main steps to evaluating the air space R-value:

  1. How to determine the type of airspace in the wall assembly
  2. How to determine the R-value for each type of airspace

Step 1

First, determine whether the airspace is Ideal or Enclosed.

Per ASHRAE Handbook of Fundamentals (HOF) Chapter 26 Table 3, footnote ‘b’, an Ideal airspace has:

  • Uniform thickness
  • Bounded by plane, smooth, parallel surfaces
  • No air leakage to or from the space (‘sealed’)

Ideal airspace examples:

  • Enclosed and sealed airspace in a stud cavity

  • The space is considered enclosed (non-ideal) if it does not meet the criteria for an ideal airspace.
  • Determine whether the enclosed airspace is Case 1 or Case 2

A Enclosed Case 1 (minimized air leakage) airspace is, per ASHRAE 90.1-2013 with Addendum ac or ASHRAE 90.1-2016 Section A9.4.2:

  • Enclosed in an unventilated cavity
  • Located on the interior side of the continuous air barrier
  • Bounded on all six sides by building components to minimize airflow into and out of the space

Enclosed Case 1 example:

  • An enclosed but not sealed airspace inside a stud cavity

For airspaces with non-uniform thickness, use the average distance between bounding surfaces as the thickness.

Airspaces which do not qualify as ideal airspaces or which do not minimize air leakage (Case 1) are Case 2 airspaces.

Enclosed Case 2 example:

  • An airspace located behind or underneath cladding materials, which is subject to airflow

Step 2

Determine Airspace R-Value

  • Once the airspace type is determined, the airspace R-value can be evaluated.
  • Different evaluation methods are permitted for each airspace type per the ASHRAE HOF

For Ideal airspaces ½” thick or greater:

  • Determine the R-value for the airspace (including R-values for different directions of heat flow as applicable to horizontal airspaces) in accordance with ASHRAE Handbook of Fundamentals (Chapter 26), Tables 2 and 3.

Use Chapter 26 Table 2 to determine the emittance of the airspace surfaces:

  • For example, a 0.75” airspace bounded on all sides by building materials has an emittance of 0.8

Then use Chapter 26 Table 3 to determine the R-value

Unless use conditions dictate otherwise, the R-values shall be based on:

  • Mean temperature of 50oF
  • Temperature difference of 30oF

In our example, the ½” airspace bounded on all sides by building materials has an R-value of 0.90.

For ideal airspaces of less than ½-inch thickness, or otherwise not complying with ideal airspace conditions, there are two options to determine airspace R-value.

Ideal airspace <½” testing ASTM C 1363:

  • For reflective ideal airspaces, the application of the ASTM C 1363 test method shall comply with conditions and formula specified in ASTM C 1224 (e.g., 75oF mean temperature and 30oF temperature difference).
  • For horizontal airspaces, testing shall include upward and downward heat flow directions to determine R-values accordingly.

Ideal airspace <½” calculation:

  • The ideal airspace R-value shall be calculated in accordance with equations in Chapter 4 of the ASHRAE Handbook of Fundamentals for combined radiation and convection for heat flow directions and temperature conditions applicable to the end use conditions.

For an Enclosed airspace R-value, Case 1 (minimized air leakage) two methods are permitted.

Enclosed airspace, Case 1 prescriptive:

  • Determine the R-value in accordance with ASHRAE Standard 90.1-2013 (with Addendum ac) or ASHRAE 90.1-2016 Section A9.4.2.
  • Airspaces less than ½” thick shall have no R-value.

  • The R-value for 3.5-inch thick airspaces shall be used for airspaces of greater thickness provided the airspace thickness does not exceed 12”.

Enclosed airspace, Case 1 testing (ASTM C 1363):

  • For reflective airspaces, the application of the ASTM C 1363 test method shall comply with conditions and formula specified in ASTM C 1224 (e.g., 75oF mean temperature and 30oF temperature difference).
  • For horizontal or sloped airspaces (reflective or non-reflective), testing shall include upward and downward heat flow directions to determine R-values accordingly.

A single “effective” R-value for energy code compliance purposes shall be permitted to be derived based on weighting of the R-values for different heat flow directions by the relative magnitude of heating and cooling degree days within each climate zone of ASHRAE 90.1

  • Refer to Table A9.4.2-1 for benchmark example of this weighting procedure resulting in different airspace R-values for horizontal airspaces in different climate zones

For an Enclosed airspace R-value, Case 2 (uncontrolled air leakage), both prescriptive and testing methods are also permitted.

Enclosed airspace, Case 2 prescriptive:

  • The airspace R-value and R-value of any material to the exterior side of the airspace shall be taken as zero (0).
  • An exterior air-film R-value shall be permitted to be applied. (Table 1)

Enclosed airspace, Case 2 testing – ASTM C 1363 (modified):

  • The airspace R-value shall be determined in accordance with ASTM C 1363 modified with an air-flow entering the bottom and exiting the top of the airspace.

The minimum air-movement rate for testing shall be:

  • 3 cm/s for claddings installed in end use with bottom vents only including bug screens or other obstructions in the vent openings;
  • 7 cm/s for claddings with intermittent top and bottom vents including bug screens or other similar obstructions in the vent openings or in the airspace;

  • 15 cm/s for all claddings having continuous top and bottom vents or claddings that are air-permeable (e.g., distributed ventilation through ports or unsealed seams).

  • The air flow rate and temperature shall be monitored during testing and the enthalpy change of the ventilation air shall be incorporated in the analysis of the R-value of the overall assembly.
  • To determine the R-value of the airspace alone, an additional test shall be conducted on an assembly without the airspace, and the difference between the two may be taken as the R-value of the airspace (by itself or in combination with a specific cladding)

  • An ASHRAE research project is under development to provide a recommended testing approach and criteria for airspaces  with uncontrolled air leakage.

Conclusion

  • Many variables and conditions affect the R-value of airspaces.
  • Airspace R-values can vary substantially from those published in ASHRAE Handbook of Fundamentals, which are based on idealized conditions of uniform air space thickness, smooth surfaces, and no air leakage to or from the airspace.

  • Using technical data and regulatory requirements, one can categorize airspace types and determine appropriate R-values for each type of airspace, particularly in relation to air leakage