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 is that sealed walls of the same R-value perform equally well regardless of the type of insulation used. Dubbed the Thermal Metric Project, BSC researchers, including principal Dr. Joseph Lstiburek, initiated the study as a fact-finding mission in the face of increased use of a variety of building materials and systems. With issues like thermal bridging, effective installation, and infiltration and exfiltration impacting new insulation systems and practices, the research team says R-value should be just one of several factors considered in wall construction.
"We're trying to engineer smaller and smaller changes and improvements into walls - 5% here or 10% there - so properly assessing the thermal resistance of wall and roof assemblies is necessary," Lstiburek says. You can argue about R-1 or R-2, and that does have to be considered in calculating energy trade-offs, but those small changes don't matter much in walls that don't have a lot of insulation. When you're talking about high-performance walls - walls that are designed to be R-20 and higher - you need to be realistic and honest. If you're going to be off by 10% in an R-30 wall, that's much more significant than in an R-13 or R-15 wall. At the end of the day, sealing holes is really more important."
New Tests & Equipment
To launch the study, six industry partners joined BSC in funding the development of a new thermal performance metric and its associated test method, including the design and construction of an improved "hot box." Dow Building Systems, Honeywell, Hunstman Polyurethanes, Icynene, North American Insulation Manufacturers' Association (NAIMA), and Greenfiber contributed to the funding.Lstiburek says he was impressed by how well the competitors worked together.
"These companies are very competitive in the free market, but all of the technical folks were very cooperative, collegial, and supportive when it came to the physics," he says. "I never fail to be impressed by that fact. This research will benefit all of their businesses and the entire industry."
The new equipment, which Lstiburek calls "the most complicated piece of machinery" he's ever worked with, allows researchers to test higher R-value assemblies, expose assemblies to realistic temperature differences, and measure the impact of imposed airflow across assemblies. "When you're dealing with high performing assembliesm you have to be very accurate," he says. "Until very recently, we didn't have the measurement tools to be able to make that happen. You're not going to see lots of walls tested this way from a ratings and standards perspective. This is a research tool and we're just beginning to learn its capabilities."
In 2010, baseline tests were conducted on a set of walls developed within the partner group. The set included six walls using 2x4 construction, one with 2x6 construction, five different stud cavity insulations, and one exterior insulation application.The wall testing resulted in a set of reference values for use in future research and testing.
After five years of continued testing, the study's returned some significant findings, including:
- Walls that are constructed with the same installed R-value in the stud space, and are air sealed both inside and outside (creating effectively zero air leakage through the assembly), they exhibit essentially the same thermal performance regardless of the type of insulation material used.
- All the wall assemblies experienced a loss in thermal performance due to air movement through the assembly, regardless of the type of insulation material.
- Conventional energy models may over-predict the negative energy impact on walls that have a significant interaction effect (e.g. air moving through insulation).
Lstiburek says these results show that, in a small but measurable way, walls act like heat exchangers. "The argument is that inside air that is leaking out is heating up the insulation, so some of that heat is being recovered," Lstiburek explains. "In that respect, leaking air is bad, but not as bad as where big holes are concerned. Most of the air leakage in a house doesn't happen through walls. It happens through big holes where you're not getting the benefit of that heat exhgange, so there's a huge benefit to air-tightening your buildings."
So what are those big holes? Lstiburek lists fireplaces on exterior walls, soffits dead-ending into an exterior wall, and cantilevered second stories as examples of constructions rife with the possibility for legitimate air leakage.
Other results from the study included:
- 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.
- Commercially available 2D and 3D 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% decrease in thermal performance regardless of the type of insulation material used in the stud space.
On its face, Lstiburek says the research confirmed some things that most of the industry already knew: That air sealing is important from both inside and outside, that all insulation is good and more insulation is better, and that installing insulation according to the manufacturer's instructions is important. The key, he says, is that the study allowed researchers to get a handle on those specific phenomena and actually measure how the physics performs.
"We were also able to establish magnitudes of performance," Lstiburek says. "We determined that publicly perceived "bad walls" didn't perform as badly as people thought, and publicly perceived "good walls" didn't perform as well as people thought. It also doesn't take much to fix a bad wall or to screw up a good wall. We knew there would be a spread between "good" and "bad" wall performance, but it wasn't as large as we were expecting."