Builders who run the other way when someone suggests using fiberglass batt insulation may want to rethink their point of view. A study released by an industry trade group says that one type of wall insulation performs as well as any other as long as it is installed correctly and the wall has been air-sealed.
That conclusion comes from an updated Thermal Metric Summary Report compiled by the Building Science Corporation and released this week by the North American Insulation Manufacturers Association (NAIMA), a trade group representing manufacturers of fiberglass, rock wool, and slag wool insulation.
“R-value is R-value,” NAIMA said in reporting the latest results of the multi-year study. “When walls are constructed to the same R-value and properly sealed, all insulation types perform essentially the same.”
The findings aren’t new; they were reported by GBA in a September 2012 article called “Air Leakage Degrades the Thermal Performance of Walls.” In that article, senior editor Martin Holladay wrote, “If the walls were sealed and there was no air flow through the walls, all of the R-13 walls behaved the same. As Gertrude Stein might have put it, an R-13 wall is an R-13 wall is an R-13 wall. ‘When the nominal R-13 walls are sealed and tested, they have the same heat flow, plus or minus 4%,’ said [Chris] Schumacher. ‘They all perform roughly the same.’”
A preliminary version of the report was released almost two years ago, NAIMA said, comparing fiberglass batts, cellulose, closed-cell spray foam, and foam sheathing. But open-cell spray foam wasn’t included because of defects in a test wall. The updated version of the report, dated June 2015, also includes test results involving R-15 fiberglass batts and R-15 blown-in fiberglass insulation.
NAIMA said that R-15 fiberglass batts performed better than a wall insulated to R-13 with open-cell spray foam and better than R-12 worth of closed-cell foam.
“The results of this research clearly demonstrate that there is no single ‘best’ insulation material and that all insulation materials perform equally well when properly air sealed and installed, debunking the claims that spray foam insulation performs better than other insulation types and provides air sealing for the whole building.”
Search continues for an R-value replacement
The industry-funded research is part of what’s turning out to be a very long process aimed at replacing R-value with another metric for describing thermal performance.
Joseph Lstiburek, a principal at Building Science Corporation (BSC), announced in 2007 that it was time for the R-value to be retired and replaced with a metric that took into account more complexities of building assemblies and site conditions. As GBA’s Martin Holladay reported nearly four years ago, BSC built a new test rig called a double-guarded hot box with the aim of measuring heat and mass flows. But testing dragged on for years without any firm conclusions.
As the report notes, BSC joined with six companies in the industry — including NAIMA and Icynene, a manufacture of open-cell foam — to develop a replacement for R-value.
“The Thermal Metric Project was initiated in response to renewed focus on building system performance and increasing use of a broader range of building materials and systems,” the report notes. “These factors highlight the shortcomings of the dominate thermal performance metric, namely R-value. Contemporary insulation materials and systems are more or less sensitive to thermal bridging, workmanship (i.e. quality of installation), internal convention and through convection (i.e. infiltration, exfiltration, wind washing and re-entrant looping).
“The impact of such ‘anomalies’ and ‘defects’ is not captured in the standard (label and installed) R-value metric,” it said.
Researchers still have more work to do: “At this stage of the research project, the industry partners have not proposed a new metric for quantifying the thermal performance of insulation beyond conventional R-value,” NAIMA said, “and the report does not include any recommendations for a new metric.”
But the study did yield a number of observations:
- Unless a continuous layer of insulation is installed on one side of the wall, all wood-framed walls are subject to thermal bridging, regardless of what type of insulation is used in the stud cavities. Thermal bridging through the framing lowered thermal performance by about 15%.
- Commercially available heat transfer models do a good job of predicting thermal bridging, as did the ASHRAE Handbook of Fundamentals.
- All types of insulation show changes in thermal performance with changes in temperature.
- No matter what type of insulation is used, all wall assemblies show some loss of thermal performance when air is permitted to move through them.