The Theory of R-value Relativity: The impact of thermal conductivity on building enclosure durability

by Katie Daniel | October 27, 2015 9:53 am

Photo courtesy Roxul

By Rockford Boyer, B. Arch. Sc., BSSO
When designers are deciding which insulation products to specify for a given project, two questions come to mind: ‘What is the R-value?’ and ‘What is the permeance?’ The answer to both these queries is, ‘It is all relative.’ If Einstein had not been so focused on the speed of light, he would have had the time to establish the third theory of relativity—R-value relativity. Thermal insulation’s effective performance and permeance properties are relative and dependent upon project location—specifically, temperature and climate. In other words, a building assembly will perform differently in Victoria, than it would in Whitehorse. Therefore, it is essential to understand the dynamics of building products under regional climatic conditions in order to increase overall building durability.

Generalizations are not effective
Insulation types should not be treated equally, as there is a wide range of varying performance characteristics. However, some codes and standards organizations are pushing for equal testing on these types of products. Buildings are not constructed in laboratories, so why do these arguably semi-irrelevant tests—which have no bearing on real life—persist in-situ scenarios?

If buildings were built with the quality and precision accomplished in the laboratory, many failures would not exist. Obviously, since the industry spends millions of dollars annually on repairs, a better mechanism for relevant building components and assemblies testing needs to be in place.

Designers should ask manufacturers and product suppliers for tested data relevant for their applications and locations. It is understandable codes and standards are necessary for marketing value. However, project teams should consider additional questions and analysis outside the realm of standardized testing.


In lay terms, R-value is defined as a material’s resistance to heat transfer. One might assume the higher the R-value, the lower the thermal conductivity (k-value)––resulting in higher performance. However, the answer is again relative. Three mechanisms of heat transfer cumulatively determine the relativity of the performance:

In general, residential and commercial insulations use an ASTM standard test method to measure heat flow through a given medium. In the case of insulation, the standardized test for determining heat flow is ASTM C518, Standard Test Method for Steady-state Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus. This standardized approach is an accurate way to measure and determine the R-value of a product. However, the mean temperature to report R-value is stated at 24 C (75 F). It does not make sense to insulate buildings from room temperature, but using this mean temperature of 24 C results in higher repeatable accuracy. So, is it the intention of this test procedure to produce a more accurate result to the nearest third decimal place or to actually provide valuable and applicable information for designers to increase energy efficiency and envelope durability? As an industry, a great deal of knowledge and awareness are required to fully understand how temperature-dependent thermal conductivity (TDTC) impacts building and component durability. As a designer, one of the most important questions is to ask, ‘What the R-value is of a product for the project’s particular climate zone?’

Researchers, organizations, and educational institutions conduct research to understand how temperature affects insulation material’s thermal conductivity. Organizations such as the National Roofing Contractors Association (NRCA), Building Science Consulting Inc. (BSCI), RDH, and the University of Waterloo (Ontario), all invest time, money, and resources to promote the in-situ performance of insulation under realistic conditions. Research conducted by these various organizations determined R-values are, in fact, based on the temperatures with which they interact.

Depending on the insulation type (whether mineral fibre or foam plastic), three or four major variables could affect the material’s R-value performance. Conduction and convection exhibit a linear relationship between temperature and R-value, whereas radiation impacts R-value to the fourth power. Additionally, blowing agents used in certain foam plastic insulations are typically not taken into account but can have an adverse affect on R-value. These blowing agents have both a condensation point and a boiling point—meaning they have the potential to change from one phase to another based on ambient temperatures. If the temperature is low enough, condensation occurs in the blowing agent; this increased thermal conductivity of the liquid in the cell significantly reduces the R-value. Figure 1 illustrates heat transfer mechanisms and the effects of condensation in the cells. Moisture present within insulation can drastically reduce insulation’s efficiency.

Figure 1 (Pg 33) R-value vs. Outdoor Temp[2]
Figure 1: Building Science Consulting Inc. (BSCI) tested the apparent R-value on four polyisocyanurate (polyiso) manufacturers’ samples, including a sample of high-density mineral wool roofing material. Image courtesy Chris Schumacher

Theory vs. real world
Although lab tests can help determine the individual characteristics of a product, these characteristics can drastically change when installed or tested[3] as a system. In turning lab test theories[4] into understandable or real project situations, a well-known hygrothermal modeling software program, WUFI-ORNL/IBP[5], can assess the combined heat and moisture transfer in building components based on building type and local interior and exterior environments. In WUFI-ORNL/IBP, a user generates personalized assembly constructions from a wide range of default materials, climates, and conditions to predict their in-situ performance.

Image 1 (Pg. 34) - NRCA Min Max Ranges for Polyiso[6]
National Roofers Contractors Association (NRCA) examined R-value minimum and maximum ranges for polyiso samples. Image courtesy Mark Graham

Sometimes, data associated with generic or default materials can be inaccurate, assumed, estimated, or missing completely. This is the case with one of the generic polyisocyanurate (polyiso) materials located in the WUFI-ORNL/IBP database for North America. Figure 2 references the generic default material located in the North American database, whereas Figure 3 is derived from out-of-scope testing and resembles actual in-situ temperatures. The two graphs show a discrepancy between the temperature versus the R-value for the default value, as well as the actual tested value for a similar insulation material type.

The objective of this comparison is not to discredit the software program, but rather to educate users on encouraging manufacturers to input actual test data into the software database. Product manufacturers must take responsibility in providing accurate hygrothermal data to building professionals to ensure designs are as realistic as possible. Figure 4 depicts a realistic thermal prediction of an ASHRAE-compliant wall with the use of three common continuous insulation (ci) materials.

Thermal conductivity and building durability
How can R-value affect the durability of a building assembly? Primarily, it does so based on the condensation potential of the condensing plane (in the case of Figure 4, this would be the sheathing board). A higher potential for condensation can occur when low temperatures, high humidity, and low-permeable materials are present within a building assembly.

Air leakage, which ex-filtrates from the interior to the exterior, also has a negative impact on interstitial condensation. This is especially true if non-permeable components are used. Predicted maximum temperature swings for extruded polystyrene (XPS) and mineral wool insulation types are 3 C (7 F)—between 7 and 4 C (46 and 39 F)—whereas, polyiso has a maximum fluctuation of 6 C (11 F)—between 6 and 0 C (43 and 32 F).

When calculating the potential condensation hours for a given time period, one should utilize temperature-dynamic R-values, rather than static R-values. Otherwise, it is easy to severely underestimate the hours of condensation predicted, which can lead to such future durability issues as deterioration and mould.

However, it is not detrimental to the building assembly when a small amount of interstitial condensation occurs within it, as long as the drying outweighs the wetting. Research conducted several institutions, such as National Research of Canada (NRC), the British Columbia Institute of Technology (BCIT), and BSCI, demonstrated the drying potential of insulated sheathing when water ingresses or occurs behind/between a building’s insulated sheathing and structural sheathing.1 In addition, BCIT research revealed higher permeable insulated sheathing dries 50 per cent sooner than non-permeable insulated sheathing.

Another solution that minimizes moisture damage to building assembly components––even when using non-permeable insulation––is locating all insulation outboard of the air-, moisture- and vapour-control layers. Theoretically, this ensures the dewpoint occurs in the insulated sheathing material (i.e. sound building-science principles result in durable building assemblies). However, resilience is ensured when materials are placed in locations that allow for drying in both directions, minimize deformation, and provide resistance to moisture and fire damage. This even applies with unexpected failures and improper detailing.

Image 2 (Pg. 34).R-value vs. Temp & Humidity[7]
The University of Waterloo tests illustrate R-value of four common insulation materials at varying temperatures and relative humidities (RHs). Thermal conductivity measurements of building insulation using a guarded heater plate apparatus. Image courtesy J. Wright and I. Beentjes

Going forward
It is important to remember using static R-values and static temperatures to calculate dewpoints (through the Glaser method) can be helpful in tight situations. However, the potential result may include unexpected heat loss, durability issues, and mould issues (due to underestimating condensation hours). (The Glaser method is a simple calculation model to help assist design professionals in determining the dewpoint (condensation) within their enclosure designs. Calculations are based on static R-values, temperatures, relative humidity (RH), and permeance.)

Bulk water formed or directed in the building assembly does not always result in deterioration, provided stipulations are introduced in the system to account for deficiencies. Various provisions to minimize the potential for moisture damage, such as the specification of materials, also allow for continuous drying. Additionally, the location of materials to force dewpoint outboard of control layers is important, as well as adequate drawing and site review.

In addition to industry-standard testing, product manufacturers must be held more accountable to demonstrate how their products/systems perform in-situ. As for insulation, a building owner essentially purchases R-value, therefore, it is important to get the full value of the product.

The fundamental understanding of how the industry works and calculates R-value has to change. It has been documented that not all insulations are created equal; if treated as such, this can lead to durability issues as well as increased energy use. To ensure the enclosure is designed as intended, it is imperative all factors are taken into account. Designers should not hesitate to ask manufacturers the tough questions, since it is their name on the construction documents.

Rockford Boyer.October2015[8]Rockford Boyer, B. Arch. Sc., BBSO is the North American manager of the Energy Design Centre for ROXUL Inc. (Milton, Ont.) Rockford has been with the company for more than eight years, serving in the technical innovation department. Previously, he was a building science specialist with AMEC Earth and Environmental. He has a diploma in civil engineering, a degree in architecture (building science), and is currently in the process of completing his master’s degree in building science. Boyer can be reached at[9].

  1. [Image]:
  2. [Image]:
  3. drastically change when installed or tested:
  4. lab test theories:
  6. [Image]:
  7. [Image]:
  8. [Image]:

Source URL: