The prescriptive trade-off method developed by the National Energy Code for Buildings (NECB)8 captures this and allows a proposed assembly to be designed and constructed if equivalent or less amount of heat energy is lost by the proposed assembly then the prescriptive requirements for an elevation of the same dimensions at maximum permitted FWDR. Using the NECB prescriptive trade-off method, but with the net-zero targets of 0.15 W/m2 K (R-37.8) and 0.8 W/m2 K (R7.1) for opaque walls and for windows and doors respectively on the front elevation (Figure 11), the result of the calculation is a wall that loses 41 W/K of heat energy. This matches the prescriptive requirements for a 33 per cent FWDR wall with R-7 and R-38 opaque walls, but used R-3 triple glazed thermally broken windows and doors and requires only R-23.7 from the opaque walls.
The second step is accounting for thermal bridging of the opaque wall at horizontal linear transitions such as at-grade, floors, and roof transitions, at vertical transitions, at corners, and around windows and doors. Accounting for these bridging effects reduces the clear field value of the opaque block-brick veneer wall and provides a more realistic estimate of the thermal performance of the wall assembly.
When 203 mm (8 in.) of mineral wool is used as external insulation, providing a clear field R-value of the opaque assembly of R-37.3, the clear field R-value of the opaque wall is reduced by thermal bridging effects to R-24.6. However, the target R-value for the opaque wall assembly after trade-off (because of the low FWDR) on this elevation is only R-23.7, so the proposed opaque wall assembly is acceptable. Therefore, this concrete block wall assembly for the opaque walls has met passive house requirements for net-zero energy construction on the front elevation of this hypothetical warehouse/office building located in Calgary.
Thermal mass of CMUs
Thermal mass is a material property that describes how a material absorbs, stores, and then gradually releases heat energy.8 Buildings constructed with CMUs for the back up wall, the veneer, or both, have an energy-saving advantage because of their inherent thermal mass which allows CMU walls to absorb heat energy slowly and hold it for longer periods of time than less dense building materials.
Thermal mass results in fewer spikes in the heating and cooling requirements because the mass of the components slow the response time and better moderate indoor temperature fluctuations. In climates that experience large daily temperature swings, thermally massive buildings use less energy, in comparison to a low mass building of similar size due to the reduced heat transfer through the massive wall components. Thermal mass typically shifts energy demand to off-peak time periods. The thermal mass of concrete has the following benefits and characteristics:
In the U.S., the COM CHECK system permits thermal models to account for thermal mass of concrete masonry, providing a more accurate thermal resistance of the buildings constructed of the material. The NCMA’s thermal mass software9 is location dependent, using local weather data. This software can account for thermal mass, instead of only steady-state thermal models currently used in the NECB. For example, the effective R-value after accounting for thermal mass for a concrete block backup wall with R-20 exterior insulation (101.6 mm [4 in.] XPS) and 90 mm (3.5 in.) brick veneer in New York City is R-27.7. The steady-state thermal analysis methods required by NECB would estimate the effective R-value of the same wall (with embedded ties) at R-20.6.
