Fiberglass Batts-Labeled vs. Installed Performance

Oak Ridge National Laboratory research shows that “perfectly installed” fiberglass batts lose 11% of their labeled R-Value, and that “commonly installed” fiberglass batts lose 28% of their labeled R-value.1 This study confirms tests conducted 20 years ago by fiberglass manufacturers, and reveals the surprisingly large disparity between the labeled R-value and the installed R-value of fiberglass batts.

The R-value results presented here are the clear wall R-values, which Andre Desjarlais of Oak Ridge explains, “includes the studs, top and bottom plates, sheathings and exterior facade…It does not include additional structural components around details such as corners, windows, etc.”

“The clear wall R-value… represents the area of the wall containing insulation and only the necessary structural member away from all interface details.” “To address the number one wall research need…whole wall performance was ranked by 270 private building industry contributors as the most important public sector R&D need to accelerate the development and application of energy-efficient building walls.”

Full-size walls were constructed and tested to determine their thermal conductivity. The highest tested R-value for “R-19” labeled batts was R-17.4 for batts before they were installed. From there, the test results dropped to R-17 and then R-13.7 “R-19” batts have an R-value of 13.7 when installed as commonly found in actual walls.

Fiberglass Batts Labeled vs. Installed Performance - Explained

Q: Did an independent laboratory conduct the tests? Who funded the tests?

A: Oak Ridge National Laboratory conducted the research. Oak Ridge is completely independent and funded by the US Department of Energy.

Q: Why were the tests conducted?

A: According to Oak Ridge, builders, architects, designers, and homeowners want energy-efficient walls. The best way to determine how insulation systems perform is to build and test full-size walls.

Q: Can’t R-values be used to compare insulation systems?

A: R-values are a good starting point – but they are the results of small, meticulously prepared laboratory samples and do not necessarily reveal how an insulation system performs once installed in actual buildings. Different insulation systems with the same laboratory “R-value” can deliver much different levels of comfort and energy efficiency.

Q: What did the researchers find?

A: The researchers found that fiberglass batts deliver far less than their labeled R-value in real walls.

Q: Where does the R-value go?

A: Technically, the “R-value” doesn’t change because it is based on specific laboratory test claims by the fiberglass manufacturers. However, the Oak Ridge research reveals the following:

• “R-19” labeled fiberglass batts have an R-value of 17.4 before they are installed.

• “R-19” fiberglass batts have an R-value of 17.0 when installed perfectly (the scientists installed the batts before installing the exterior sheathing to precisely fit the batts in place from both sides).

• “R-19” fiberglass batts have an R-value of 13.7 when installed as commonly found in actual walls.