by Ed Weinmann
Many conflate R-value with good thermal performance or insulating value. The belief is the higher the R-value, the better the thermal performance. However, as a blanket statement, this could not be any further from the truth. (An earlier version of this article appeared in a 2015 issue of SMART|Dynamics of Masonry [vol. 2, no. 4] with co-author Brendan Quinn. Visit www.dynamicsofmasonry.com.)
The International Energy Conservation Code (IECC) defines R-value as:
the inverse of the time rate of heat flow through a body from one of its bounding surfaces to the other for a unit temperature difference between the two surfaces, under steady-state conditions, per unit area.
R-value is determined through a test performed in a guarded hot box where a steady-state condition is established. The environment is completely controlled with no variances except the initial passive absorption of heat into one side of the tested materials and the release of said heat on the other side of the materials until a steady state (constant heat at 38 C [100 F]) exists on both sides of the material prior to testing.
Guarded hot-box R-value testing and assessments do not account for dynamic conditions like fluctuations in sunlight, temperature, humidity, pressure, or wind. The test bypasses thermal mass, thermal lag, response to solar heat, and thermal flywheel effect. Since we do not live in a steady state and the test bypasses several real-world physics principles of heat transfer, it lacks accuracy and provides a false view of thermal performance. Simply put, it is the wrong test for masonry or high-thermal-mass material.
Would one measure air volume with a liquid measuring cup? Would one design a rocket to go to the moon without accounting for gravity? So why use R-value when it comes to designing buildings in the dynamic world? Why use R-value to assess the performance of masonry when thermal mass and thermal lag are bypassed?
According to the R-value testing, glass has about double the R-value of masonry per inch of thickness. If higher R-values alone equated to better insulation performance (i.e. resistance to heat loss or gain), then glass should be a better insulator than masonry. However, anyone who has gotten into a car on a sunny summer day knows how hot it gets. The greenhouse effect is what occurs through car windshields and through every southwest-facing window on an office building when the sunlight hits. The glass, with a higher R-value than masonry, is doing a wonderful job of amplifying the heat and a terrible job of insulating against heat intrusion (i.e. it is neither lowering energy usage nor improving the indoor environment).
Masonry, on the other hand, has high thermal mass. Although it has a low R-value, high-mass materials properly placed and protected tend to decrease both heating and cooling loads in a given building, thus saving energy. (For more, see National Concrete Masonry Association [NCMA] TEK Note 6-16A, Heat Capacity [HC] Values for Concrete Masonry Walls.) Mass can store, slow, and dissipate heat. It will not amplify heat with the greenhouse effect of glass.
The National Concrete Masonry Association (NCMA) and other code-contributing groups may argue thermal lag and thermal mass benefits are taken into account by requiring lower R-values for masonry in the code. However, this author sees R-value as a standalone narrative to be an incomplete method of setting the design parameters for a building’s thermal performance. It is clearly a piece of the energy efficiency puzzle, but there are other pieces that may have greater effect and even greater importance to complete the picture.
The myth surrounding continuous insulation
That continuous insulation (ci) is a mandatory requirement remains a longstanding misunderstanding of IECC. Unfortunately, while clarifications and explanations are issued repetitively, many architects and designers remain under the impression the energy code requires masonry or ‘mass’ walls to have ‘continuous insulation’ at all times. The confusion regarding the code is generally that of the readers rather than IECC itself—continuous insulation for mass walls is only a requirement of prescriptive design methodology.
As covered in NCMA FAQ 12-14, the energy code allows three different methods to be used to show compliance with minimum energy efficiency requirements: prescriptive, trade-off or system performance, and whole-building energy analysis. A project need only comply with one of these methods, not all three. (For more, visit multibriefs.com/briefs/ncmaorg/073114_FAQ_Continuous_Insulation.pdf.)
The idea of having continuous insulation over every inch of masonry seems to be a good idea in order to prevent any unwanted heat loss or gain through the walls. However, glass windows and doors are big holes in the continuous insulation. Buildings with glass envelopes or floor-to-ceiling windows do not require continuous insulation even though they have very low R-value due to their thickness. (Glazing also adds other detrimental effects on thermal performance like the aforementioned greenhouse effect, amplifying heat in a room or losing heat from a room.) Why does glass not require continuous insulation?