The Language of R-values: Understanding differences between ‘nominal’ and ‘effective’

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By Tyler Simpson, B. Tech.
The 2015 National Building Code of Canada (NBC) brought about multiple changes. One of the most important from an insulation standpoint is the treatment of the language concerning RSI values. (Although most in the construction industry are more familiar with the term ‘R-value,’ this article will use the metric values RSI for insulation.)

The NBC now refers to “effective RSI value” where it previously stated “nominal RSI value.” This change in language will impact residential construction in Canada. Nominal RSI value takes into account the thermal resistance of the insulation layer only, which is typically batt insulation placed between the studs. Effective RSI value, on the other hand, takes into account the cumulative value of thermal resistance for all materials within the assembly. This transformation in language will have the construction industry asking:

  • how do you calculate effective RSI values?
  • what role does thermal bridging play in the loss of RSI value?
  • what does a durable, highly insulated assembly look like?

This change will allow the construction of assemblies with increased, properly placed insulation to ensure a durable and healthy assembly, while maintaining a comfortable environment for the occupants.

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Calculating effective RSI values
The NBC has chosen to adopt the isothermal planes (series-parallel) method for calculating effective RSI values. This method breaks assembly components into two groups during calculation—components which have parallel paths of heat flow (i.e. assembly containing both framing members and cavity insulation), and continuous layers of homogeneous materials included in series (i.e materials such as exterior/interior air films, cladding, air space, exterior insulated or uninsulated sheathings, and gypsum board). The calculation can be described as:

RSIeff = RSIE1 + RSIE2 + RSIE3…+ RSIEn + RSIparallel + RSII1 + RSII2 + RSII3 +…+ RSIIn

In this calculation, RSIeff is total effective thermal resistance of assembly, RSIE are the continuous layers to exterior of frame-cavity component, RSIparallel is effective thermal resistance of the frame-cavity component, and RSII are continuous layers to interior of frame-cavity component. The equation can be simplified by combining all continuous layers into a single term labelled RSIseries (RSIE + RSII). The final derived equation streamlines to:

RSIeff = RSIseries + RSIparallel

American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) Handbook of Fundamentals contains tables providing typical thermal properties of common building materials. Utilizing these tables, one can cumulatively calculate all RSIseries layers. For calculation of RSIparallel another equation is required:

RSIparallel = 100 / ([AF/RSIF] + [Ac/RSIc])

Terms in the equation are defined as:

  • RSIparallel is effective thermal resistance of the frame-cavity component;
  • AF is the percentage area of the assembly that framing occupies;
  • RSIF is the thermal resistance of framing member;
  • Ac is the percentage area of assembly that cavity insulation occupies; and
  • RSIc is nominal thermal resistance of cavity insulation component.

Calculating the effective thermal resistance of an assembly indicates its true performance in reducing heat loss. It allows the user to have a clear picture of how insulation performs in the assembly and the optimal placement of that insulation to maximize its RSI value.

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