Equal R-Values Do Not Mean Equal Performance.

Green building professionals of all stripes need to remember that two walls with equal R-values may not have the same thermal performance. Every time this fact is ignored, opportunities to maximize energy efficiency and sustainability are lost.
Here's an example. I have an ICF (Insulated Concrete Form) wall with an R-22, and an NRG Insulated Concrete Block wall with an R-22. Architects, engineers, and codes treat the walls the same. They are not.The ICF wall has isolated thermal mass, and the NRG wall has insulated thermal mass.
The ICF wall has high thermal mass, but the interior wall insulation negates the effects of the thermal mass. The NRG wall also has high thermal mass, and its thermal mass is exposed to the building's interior. Its high heat capacity and long thermal lag time act to moderate the indoor air temperature, and produce more energy savings than the ICF wall.
A more accurate measure of a wall's thermal performance is the thermal time constant, which takes into account the relative placement of the insulation. An NRG wall has a much higher Thermal Time Constant than an ICF wall. In talking to architects and engineers, it is clear that the majority are unaware of the benefits of insulated thermal mass verses isolated thermal mass, and perhaps this is why they keep putting the insulation in the wrong place.  
Does anybody have a suggestion as to how to incorporate the benefits of insulated thermal mass into the green building codes? 


mass walls below grade

David Butler's picture

On a similar note, one of my pet peeves is when a thermal mass wall manufacturer issues "effective R-value" energy calculation and comparison based on the total surface area of the wall, without deleting the below-grade portions from the wall take-off.

A mass wall installed below grade has no intrinsic thermal benefit over a low mass wall, except perhaps at the transition zone. Modeling software already considers the mass benefit of earth-coupled (e.g., below grade) walls, and expects the wall to be defined in terms of its steady-state R-value. Modelers who are unaware of this (such as HERS raters and HVAC designers) will incorrectly use the "effective R-value" provided by the manufacturer for below grade wall segments, which ends up dramatically understating thermal loading in below grade zones.

David Butler - March 20, 2010 3:56pm

Insulation R-values

mdewein's picture

Great comments on the relative performance of different insulation systems and details! We are hearing more often that that the traditional R- value measurement that we all use for code compliance and "measuring" insulation performance may not be sufficient any longer, becasue of what we've learned by measuring building energy use and performance over the years. Also, there are many new types of insulation materials and design approaches to their use that weren't available when the R-Value metrics and tests were developed and popularized. It has become clear to many in the energy efficincy industry that we need new metrics to evaluate insulation with.

One of the best treatises I have seen, and the best work being conducted currently is that by John Straube and Joe Lstiburek. I recommend you all take a look at John's paper, here; http://www.buildingscience.com/documents/reports/RR-0901_Thermal_Metrics... as well as all the insulation-related information on the Building Science Corp. website, here; http://www.buildingscience.com/

And watch for John and Joe's work on thermalmetrics!

mdewein - March 25, 2010 8:54am

Thanks for sharing

Eric Plunkett's picture

We'll be sure to check that out.

Eric Plunkett - April 5, 2012 1:50pm

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