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

Image 2 (Pg. 34).R-value vs. Temp & Humidity
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.October2015Rockford 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

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  1. It is important to point out that the reduced R value referred to in your article is applicable to polyiso only and NOT to closed cell spray polyurethane insulation. The article did not describe the R value reduction of semi rigid mineral fibre due to the effect of convection currents around and through the insulation. Please comment on the importance of proper installation and the need for the semi rigid (or rigid insulation) to be in permanent and intimate contact with the substrate.

    1. Hello Michael,

      The reason why I did not discuss “convection currents” in mineral wool insulation is due to the fact that there is minimal reduction in R value caused by “convection currents” (see BSCI website).

      As for the reduction in R value due to “convective currents” around mineral wool insulation……it can happen to any board stock insulation, however mineral wool can conform to irregularities where as board foam cannot.

      I agree, ALL insulation types need proper care in installation to ensure specified performance.

  2. Hi Rocky, Based on your reply I assume you understand and agree that the cool temperature reduced R value of polyiso is NOT applicable to closed cell spray foam. Also the key to installing any insulation is to make sure it is in intimate contact with the substrate. Spray foam does this naturally. Board stock must be installed with adhesives and pressure to achieve this.

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