Study: Sealed Walls of the Same R-value Perform Equally Well Regardless of Insulation Type

Building science and architecture firm Building Science Corporation (BSC) has released a report detailing the results of a multi-year insulation research project. The most significant finding from the report, titled “Thermal Metric Summary Report”, is that sealed walls of the same R-value perform equally well regardless of the type of insulation used.

The study entailed a baseline set of seven test walls using various insulation types, including fiberglass, cellulose, spray foam and extruded polystyrene.

Among other findings of the report are:

  • When walls are constructed with the same installed R-value in the stud space, and are air sealed inside and outside (there is effectively zero air leakage through the assembly), they exhibit essentially the same thermal performance regardless of the type of insulation material used.
  • All of the reference test wall assemblies were subjected to significant temperature differences. Natural convective looping was not noted in any of the wall assemblies.
  • Conventional energy models may over-predict the negative energy impact on walls that have a significant interaction effect (air moving through insulation).
  • All wall assemblies experienced a loss in thermal performance due to air movement through the assembly. This is true for all of the assemblies tested, regardless of the type of insulation material used (cellulose, fiber glass, open cell spray foam, closed cell spray foam or extruded polystyrene.)
  • Commercially available 2-D and 3-D heat transfer models provided good predictions of thermal bridging in the assemblies tested, as did the parallel path method described in the ASHRAE Handbook of Fundamentals and other texts.
  • Thermal bridging through the framing resulted in a roughly 15 percent decrease in thermal performance regardless of the type of insulation material used in the stud space.

Energy cost and security issues have generated demand for building enclosures that exhibit higher levels of thermal performance. R-value has long been the industry standard for assessing the thermal performance of insulation materials. The findings of this report clearly demonstrate that sealed walls with the same R-value perform equally well regardless of the type of insulation used, whether fiberglass, cellulose, spray foam or extruded polystyrene.

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