5 Takeaways from the Building Science Corp Thermal Metric Study
The Building Science Corporation completed ground breaking research in 2015 – with an updated Thermal Metric Summary Report issued last June. Here's our take on it:
1. It's an Amazingly Useful Tool
In the executive summary BSC notes:
"Contemporary insulation materials and systems are more or less sensitive to thermal bridging, workmanship (i.e. quality of installation), internal convection and through convection (i.e. infiltration, exfiltration, windwashing and re-entrant looping). The impact of such ‘anomalies’ and ‘defects’ is not captured in the standard (label and installed) R-value metric."
Studying fiberglass, mineral wool, cellulose and foam insulated assemblies, in a multi-year effort led by Chris Schumacher, BSC designed and built a new "Thermal Metric Hot Box" tool that captures this missing information. We hope this new apparatus gets put to regular use going forward.
2. Yet Again, Spray Foam Is Unreliable
Even under controlled laboratory conditions, the initial application of open cell spray foam failed in the BSC study. To complete the analysis the open cell spray foam installation had to be redone. And the closed cell spray foam insulation, while technically not failing in the study, held on by its "fingertips". Wonder how that connection will hold-up over the years? With such problems in a laboratory setting we wonder how many quality control problems are out on the job sites. Helpfully the report does note another key deficiency with spray foam: "...spray foam insulations only seal areas where the spray foam is installed; significant leakage paths often remain at wood-to-wood connections." Good riddance.
Closed cell spray foam shows significant shrinkage in one of nine framing bays in Thermal Metric laboratory test.
3. Nomenclature Isn't Helpful
BSC reports that it developed "...a Novel Hot Box Apparatus for High-R Enclosure Performance Measurement..." but in its maiden voyage BSC didn't test high R-value walls. R13? R18? R21? Maybe in San Diego. Feels a bit like a bait-and-switch to us, or perhaps Orwellian linguistics we’d expect from heating fuel companies or the production building industry, but not from BSC. We hope BSC does actually test high R-value walls going forward as it is in the truly high R-value assemblies where this sort of analysis is even more important. (see #5 below)
4. Airtightness Is The Great Leveler
The report states:
"When walls are constructed with the same installed R-value in the stud space, and are air sealed both inside and outside (i.e. there is effectively zero air leakage through the assembly), they exhibit essentially the same thermal performance regardless of the type of insulation material used."
In a succinct reprise of this, Allison Bailes quoted Joe Lstiburek on twitter:
5. Higher Insulation Levels Demand Greater Airtightness
The most interesting and potentially impactful finding is one that isn't even called out in a summary bullet point, but is one the study's evidence never-the-less points to: that at higher insulation levels air leakage has ever greater negative effects. One can deduce this by looking at the heat flow results for the R13 and R21 walls used in the study. But writing about the preliminary report in 2012, Martin Holladay made this point clear, using a BSC summary graph that isn't in the actual report. Martin also quotes Chris Schumacher as saying, "The percentage effect is much larger on high R-value walls because the heat flows were lower to begin with...So with higher R-value walls, it’s more important to take care of air flow." So while none of this is news, as we build ever more highly insulated enclosures, it is worth repeating again, and again, and again, and again, and again.
Summary Graph from GBA article showing significantly higher losses from air leakage in higher r-value assemblies.