In the NECB navigator the reference constructions are created via the ‘Generate Reference’ action further down the navigator workflow.
The ‘Generate Reference’ action creates a series of project specific reference constructions based on the construction makeup of the associated proposed model and the following NECB rules:
· Reference envelope thermal transmittance is determined from NECB 2011 Part 3 Building Envelope Table 3.2.2.2, Table 3.2.2.3, Table 3.2.2.4 and Table 3.2.3.1.
· NECB 2011 Section A-3.3.4.9(4)(d) states that the reference building envelope assemblies should follow the layer structure of the proposed building’s envelope assemblies (type and order) but the insulation thickness should be varied to match the maximum overall thermal transmittance as referenced above.
· NECB 2011 Section 3.3.4.10(5) states that the thermal mass shall be set to "light-weight," which corresponds to a framed exterior wall or a 51 mm concrete floor slab (150 kg/m2 of floor area). NECB 2011 Section 8.4.4.5(1) also states that the thermal characteristics of the reference building's building envelope assembly shall be modeled as being identical to those of lightweight construction. Thus it can be concluded that the 'light-weight' requirement applies to external walls, ground contact floors and roofs.
Based on these rules the following methodology is used to create the reference constructions:
Opaque
The proposed constructions are copied and assigned to the reference model as per the proposed assignment
The copied constructions are renamed ‘proposed name_reference’
If applicable, the thickness of the non-insulating layers are edited to achieve the thermal mass target value for the construction type
Next, the insulation layer is identified and its thickness is varied to get the correct thermal properties as per Table 3.2.2.2, Table 3.2.2.4 and Table 3.2.3.1.
Roof solar absorbance is set to 0.7 in reference roof constructions
Vertical Fenestration
The proposed constructions are copied and assigned to the reference model as per the proposed assignment
The copied constructions are renamed ‘proposed name_reference’
The thermal performance of the reference model construction is adjusted to get the correct thermal properties as per Table 3.2.2.3
Skylights
The proposed constructions are copied and applied as above
If the skylight area in the proposed model is greater than 5% then the skylight area in the reference model is reduced to 5% as per Section 3.3.4.10.
In this case the thermal performance of the reference model skylight is adjusted to meet the values shown in Table 3.2.2.3
However, if the reference model skylight area (total across the building) is less than 2% then the construction is adjusted to give 3.4 W/(m2·K)
