R-Values for Insulation Fail to Tell the Whole Story 1

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Friday, November 20th, 2015
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The constituent materials of different insulation products that share the same R-value can have a profound impact upon their efficiency under different climate conditions.

The R-value for insulation products is the standard benchmark for their ability to impede heat transfer and thus maintain comfortable indoor temperatures efficiently. Generally speaking, the higher the R-value of a given building product, the lower its thermal conductivity (k-value), and thus the better its insulation performance.

The Building Code of Australia (BCA), for example, outlines different R-values for the building fabric based on the climate zone of the country’s geographic regions, as well as the position of a site above the Australian Height Datum.

Building experts point out, however, that the use of a single value to determine the performance an insulating product can oftentimes be misleading, given that the transfer of heat occurs via a variety of mechanisms, which can result in variable performance depending upon the nature of the insulating material.

Heat transfer occurs by means of three mechanisms – conduction, convection and radiation. Conduction involves the transfer of internal energy by means of the collision of microscopic particles, convection is restricted to the collective movement of groups of molecules within liquids or gases, while radiation is the emission or transmission of energy in the form of waves or sub-atomic particles.

In Australia, the disparate means by which heat is transferred are addressed by two different R-value categories for building insulation – an “up” R value that addresses upwards heat flow (referred to as “winter” R-values) and a “down” R-value which addresses downwards heat flow (referred to as “summer” R values).

While these provisions acknowledge the importance of differentiating between different mechanisms of heat transferral, they do not account for the disparate responses of various building insulating materials to ambient factors, and the impact that this can  have on their efficiency.

The constituent materials of a given building product can profoundly influence the way it responds to or is affected by different heat transfer mechanisms, and thus its impact upon the built environment.

An outstanding example of this is insulated and tinted glass windows. While insulated glass units can reduce the penetration of solar energy by means of conduction, the tinting of glass windows instead leads to the absorption of solar energy that is re-radiated internally.

According to David Baggs, head of the Global Green Tag Green Product Certification Program, the increase in radiant temperature subsequently induced by such tinted windows can result in rooms that are stiflingly hot to human occupants, even though HVAC sensors may fail to detect a rise in ambient temperatures.

When it comes to wall insulation products, those that are susceptible to phase changes, condensation or the penetration of moisture can all undergo significant variations in their R-value depending upon external temperatures.

Foam plastic insulation types that contain blowing agents, for example, can suffer sharp declines in their R-values as a result of shifts in ambient temperature. The blowing agents can undergo phase changes because of their condensation and boiling points, affecting their resistance to heat transmission by means of conduction.

Once temperatures drop to low enough levels, the blowing agents undergo condensation, which raises their thermal conductivity due to the presence of liquid in the insulation cells and thus reduces their R-values.

The presence of moisture within insulation products will in general lead to a sharp reduction in efficiency levels because of the heightened propensity of liquids to transfer heat. For this reason, air leakage, humidity levels and the permeability of adjacent wall materials can all have an impact on the efficiency of insulating products lead to changes in their R-values.

Developers and owners who are mindful of building efficiency should thus keep these factors in mind when procuring insulating materials, instead of focusing solely upon the R-values of product options.

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  1. Trevor Dale

    Great article, really enjoy what your team puts out.

    Great work as always!