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

September 9, 2016

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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:

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.


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:

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.

Exterior insulated sheathings provide more than just additional RSI value. Depending on the material properties, they can function as other barriers (air and/or weather) within the assembly.

Thermal bridging
One of the main reasons the language has changed is due to thermal bridging. Thermal bridging occurs when a conductive material (i.e. a wood or steel stud) creates a path for heat flow to bypass the insulation layer. This shortcut significantly reduces the RSI value of the insulation layer, lowering the overall performance of the assembly. For example, a 2×6 at 406-mm (16-in.) on-centre (oc) wood stud wall with RSI 4.23 (R-24) batt insulation has an effective RSI value of 3.25—a 23 per cent loss of RSI value. Adjusting the assembly to steel studs further lowers the effective RSI value to 2.11—a 50 per cent loss.

One can try increasing insulation between the studs, but this has little effect on increasing RSI value as the main issue of thermal bridging through the studs has not been addressed. To minimize this loss, an insulated sheathing material needs to be placed on the exterior side of the framing members. When exterior insulated sheathings are installed, they reduce thermal bridging in assemblies by lessening the transfer of heat loss in winter and heat gain in summer, decreasing energy consumption.

The goal in moving to effective RSI values is to ensure a portion of the insulation is placed outside the framing members. The obvious advantage is lessening of heat loss. However, there are supplementary benefits. Placing insulation outside of the framing member allows the space between the studs to experience warmer temperatures.

This increase in temperature has a twofold effect on the durability of the assembly. First, it increases the temperature between the studs, which moves the dewpoint from its traditional location (i.e. back side of exterior sheathing) to the outer surface of the exterior sheathing. At this location, moisture can drain with the aid of a properly detailed rainscreen. This limits the amount of moisture that sensitive materials, such as wood or gypsum board, may encounter. Further, small amounts of moisture from condensation that may form on the coldest days of the year will normally dry up as a result of the increased temperatures provided by exterior insulated sheathings. This means moisture does not have a chance to deteriorate the assembly. This leads to the question: how much insulation is necessary outside of the framing members to reduce condensation in a cold climate such as Canada? It all depends on geographic area, but most locations will need RSI 1.76 to 2.64 (R-10 to R-15) to limit condensation formation in the assembly.

Exterior insulated sheathings provide more than just additional RSI value. Depending on the material properties, they can function as other barriers within the assembly. The code states any sheet or panel type material that has an air leakage characteristic less than 0.02 L/(s·m2) qualifies to be used as an air barrier or part of an air barrier system. If the exterior insulated sheathing meets that requirement, the air barrier can be moved from the interior (typically, sealed poly) to the exterior surface on the insulated sheathing. Moving the air barrier from the interior to the exterior reduces air leakage as complex details are transitioned to simple details. Basically, it is easier to air seal a building from the exterior than the interior as there are less penetrations and fewer transitions between different materials. This decrease in air leakage plays two roles. Firstly, it reduces moisture transferred via air leakage, which can be substantial depending on exterior/interior temperature and relative humidity (RH). Removal of this moisture from the assembly means durability is significantly increased. As the residential construction industry moves toward a stated goal of net-zero buildings, construction practices and techniques will need to be affordable. The first measure to ensure these buildings are affordable is to conserve energy before generating it.

To conserve energy, it is important to limit the amount of conditioned interior air leaked to the exterior. Not only will this reduce the cost of generating energy, but the occupants will also experience a comfortable building as air leakage has been minimized and controlled.

The last supplementary advantage is that exterior insulated sheathing can function as the weather barrier. If the sheathing is hydrophobic and has a continuous closed cell structure, that material can perform as the weather barrier, keeping in mind accessory items such as caulking and tape are required to seal any joint that is not ship-lapped (i.e. tongue and groove). This aids in the construction of net-zero buildings being affordable as exterior insulated sheathings can perform multiple functions resulting in lowering of total construction costs.

An illustration of ‘nominal’ versus ‘effective’ RSI value terminology.

Simulating and modelling success
To help the industry with effective RSI value calculations, there is a data bank[3] of pre-calculated assemblies the user can search through to find a match for their preferred method of construction. Breakdown of selected assembly will include the thermal resistance of each component, effective RSI value of the entire assembly, and effective RSI value of the entire assembly with advanced framing.

A simulated durability analysis is also provided for each assembly. Two key items are captured in this analysis—a hygrothermal modelling of the assembly, and outboard to inboard ratio compliance as per building code requirements. The hygrothermal modelling comprises a comprehensive analysis on the selected wall assembly in each of the five climate zones found in Canada.

Interpretation of the results is simple and straightforward. Green indicates the wall performed well and is suitable for the selected climate zone, while yellow indicates the wall performed moderately and should be used with caution in that climate zone. For outboard to inboard ratio, ‘green’ indicates the wall meets the climate’s required minimum ratio, while ‘red’ indicates that the wall does not. Finally, the calculator provides advice on each assemblies ease of construction, affordability implications, and esthetics.

These helpful guidelines can aid in:

An alternative tool to the data bank is an RSI value calculator. This tool allow calculations for:

There is complete flexibility within the calculator to customize each assembly according to how one builds. Many options are available for framing types, spacing of framing members, various kinds of cladding, and several sheathing material options. The flexibility in these types of calculators allow the user to discover the optimal placement for insulation, thus maximizing effective RSI value of the assembly. A detailed report of the calculation can be downloaded providing thermal resistance for each component and the entire assembly. The full report can be printed and used during building permit application.

It is a substantial modification in the construction industry when altering language from “nominal” to “effective” RSI values. However, this change will allow construction of durable assemblies and the twofold effect of minimizing thermal bridging will lessen heating and cooling costs while letting occupants experience a comfortable building.

Tyler-Simpson---Headshot-2016Tyler Simpson is the technical manager for Ontario at Owens Corning Canada LP. Involved in the residential and commercial construction industry for seven years, his educational background includes a diploma in architectural technology from Mohawk College and a degree in civil engineering from McMaster University. Simpson also represents Owens Corning on the North American Insulation Manufactures Association (NAIMA) and Ontario Home Builders Association (OHBA) technical committees. Through extensive educational background and developing in-field knowledge, Simpson is able to provide detailed solutions for building envelope design, fire safety, and acoustical assemblies. He can be reached at[4].

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