Common menu bar links
CONSTRUCTION INNOVATION, Wnter 1999
[ Table of contents | Subscribe ]
CCMC evaluates thermal resistance of low emissivity sheet material
For a number of years now, CCMC has been evaluating low emissivity sheet material (reflective insulation). This type of material, comprised of polyethylene air-bubble pack surfaced with aluminum foil, has always been evaluated as part of a wall system with air spaces.
Low emissivity material is used in wall assemblies as thermal insulation, although it has very little thermal resistance of its own. In order to provide any significant thermal benefit, it must be installed in conjunction with an air space. The low emissivity surface of the material increases the effective thermal resistance of the air space by reducing the heat transfer by radiation through this space.
WALL SYSTEM WITH LOW EMISSIVITY SHEET MATERIAL AND FURRED AIR SPACES
Evaluation findings
When the low emissivity sheet material is installed in the wall system (as shown in the figure), the effective R-value of this material in combination with the air spaces and the strapping material (furring) used to create the air spaces will account for about 26% of the thermal resistance of the wall, whereas the low emissivity material itself will account for only about 5%. (The RSI value of the material is in the order of 0.18.)
Most of the heat transfer through this assembly of low emissivity sheet material and furred air spaces is conductive; radiation contributes only about 10%. These percentages will vary depending on:
- the wall design
- the low emissivity sheet material used
- the way the material is installed, and
- the size of the air spaces.
The contribution of the assembly of low emissivity material, air spaces and furring strips to the thermal resistance of the wall system will also vary depending on:
- the temperature difference across the assembly, and
- the convective heat transfer mechanism within it.
Thus, the thermal performance of each wall system evaluated by CCMC is unique to that wall system, with respect to the test conditions and the thermal resistances provided in the CCMC evaluation report. These thermal resistances cannot be extrapolated to any other wall design without a complete engineering analysis of the wall system, using the detailed test results of the reference wall as benchmark data.
Similarly, each low emissivity sheet material needs to be tested individually to properly identify its physical properties (e.g., emissivity, thermal resistance, water vapour permeance, tensile strength, etc.).
Conclusion
CCMC's evaluation of low emissivity sheet material is focused on the effective thermal resistance of the wall system in which the material is installed. All other functions of the wall system need to be assessed separately to verify whether or not the system meets the applicable building code requirements, as it was not evaluated for other purposes (e.g., air leakage control).
The five-digit number that appears on low emissivity sheet materials should be used to refer to the CCMC evaluation report concerning the specific material and the performance of the wall system containing this material.
