Meeting the requirements of the NECB
The NECB 2015 provides three paths for compliance. These are:
- prescriptive;
- simple trade-off; and
- performance-based.
The prescriptive path provides specific performance criteria for each element in the building, including efficiencies for HVAC equipment as well as the maximum thermal transmittance values (U-values) for building envelope assemblies (i.e. walls, roofing, and fenestration).
The simple trade-off path applies to the thermal transmittance of above-grade building assemblies. With this path, the U-value of one assembly can be higher than permitted under the prescriptive path, while another is reduced—provided the overall U-value will not be greater than when following the prescriptive path. Vertical assemblies can only trade off with other vertical assemblies, and horizontal assemblies with horizontal.
The simple trade-off path thus allows for window-and-door-to-wall ratios (FDWR) to be increased, as long as the overall transmittance of the vertical assemblies does not increase as a result.
The performance path uses computer modelling to simulate the performance of the whole building. It requires the total energy consumption of the building not to exceed that of one constructed to the requirements of the prescriptive path.
Prescriptive path: Thermal performance
Figure 3 summarizes the prescriptive requirements for thermal performance of above-grade wall assemblies, extracted from Table 3.2.2.2 in the NECB 2015, providing maximum allowable U-values. It is more common for R-value or RSI to be the unit of measure used to evaluate thermal performance of building assemblies—the inverse of U-value.
Figure 4 provides an overview of the equivalent requirements for minimum overall thermal resistance, or the effective RSI required of wall assemblies. These are not nominal values, as would be assigned by considering only the frame-cavity thermal resistance.
Since the building industry is more familiar with R-values than RSI values, Figure 5 summarizes these values by climate zone. The imperial equivalent of RSI is as follows:
R-value (hrlsf F/Btu) = RSI X 5.678 (m2K/W)
It is important to restate that the RSI and R-values in Figures 4 and 5 are the effective values for wall assemblies, taking into consideration thermal bridging as discussed previously.
Using NRCan software called HOT2000, Figure 7 was developed to identify effective R-values of various wall assemblies with and without expanded polystyrene (EPS) as the CI. (Other software is available to calculate effective R-values as accurately, such as THERM—available at windows.lbl.gov/software/therm/therm.html.) These values include air films—an important factor, as indoor and outdoor air films on the surface of a wall marginally increase its thermal resistance. This table is by no means comprehensive, but is nonetheless a valuable tool to demonstrate the effect of framing on the overall thermal performance of the wall.
The yellow-highlighted row shows the effective R-value without stud cavity insulation and with only continuous insulation on the exterior. With 150 mm (6 in.) of EPS (RSI 0.68/25.4 mm [R 3.85/in.]), stud cavity insulation is not needed in Climate Zones 4, 5, or 6. This eliminates condensation risk in the stud cavity. This space could be used to run building services without concern of freezing or air barrier breaches (assuming the air barrier is on the sheathing, as would be required for EIFS).
In contrast to an un-insulated steel stud cavity wall, an insulated 150-mm (6-in.) steel-frame wall with R-20 batts does not meet the minimum required R-value of RSI 3.17 (R-18) for Climate Zone 4, and would not be acceptable for Climate Zones 5 to 8 either. This is because of the extent of thermal bridging through the steel framing—the effective R-value is only RSI 1.39 (R-7.92). However, by adding 57 mm (2¼ in.) of EPS, the minimum RSI/R-value requirement can be met for Climate Zone 4.
In the OBC—specifically, in SB-10, Energy Efficiency Requirements (and SB-12, Energy Efficiency for Housing)—there are tables of calculated solutions with CI for each of the three Climate Zones in the province. The tables provide prescriptive solutions to meeting the overall maximum U-values mandated by the code.
Figure 8 provides an excerpt of the insulation values contained in the OBC Tables SB5.5-5, SB5.5-6, and SB5.5-7, keeping only information pertaining to above-grade walls. CI is typically exterior insulation not interrupted by studs or other framing, including floor slabs. It can include, for example, EPS, extruded polystyrene (XPS), polyisocyanurate (polyiso), or other rigid insulation. Figure 9 summarizes R-values for EPS at varying thicknesses, using Figure 8 and Figure 9, it can be demonstrated R13 batt insulation between the studs, along with 95 mm (3 ¾ in.) of EPS, will meet the prescriptive requirements of steel-framed walls in Climate Zone 6.
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