Generate the Reference Model NECB 2017
This navigator step automatically generates the reference model & assigns all relevant reference information created in previous navigator steps/stages e.g. constructions, space data, FDWR, etc.
The reference model can be reviewed by switching building types in the NECB view; the Query tool can be utilised to interrogate space data and in conjunction with the model decomposition controls to query surface, and sub-surface areas etc.
Creation of the reference model – vertical fenestration & doors:
NECB clause 8.4.4.3.(3) explicitly defines FDWR (as clause 3.2.1.4) for the reference building and states that the proposed glazing should be adjusted proportionally along each orientation to achieve the reference model FDWR. Clause 8.4.4.5 explicitly defines the position and dimensions of reference fenestration (centred, 0.9m sill, 1.8m high). It is not explicitly stated in Part 8 of the NECB, but for modelling purposes the distribution of vertical fenestration should ensure that solar gain per space (i.e. the fenestration on each orientation in a space) in the reference model is proportionate to the that in the proposed model. Thus:
a) The reference model utilises a scaling factor (see user guide example 8-2) that includes the area of roof monitors;
b) It is not explicitly stated in Part 8 of the NECB, but doors are replicated in the reference model and the reference door area scaled only when a space / orientation only has doors;
c) Fenestration is applied in the reference model per space / orientation; the fenestration area from the proposed model space / orientation is scaled accordingly and positioned as per clause 8.4.4.5 on the reference model;
d) If the reference fenestration in c) does not fit on the space / orientation (for example if the proposed building concentrates fenestration on a few spaces) the algorithm will attempt to adjust the reference fenestration to fit, first by adjusting sill & height, secondly by adding additional fenestration to fit;
e) If the algorithm fails to meet the requirements (for example the proposed FDWR is low and the proposed glazing is concentrated in a few spaces or there are roof monitors), the result will be reported to the user and the user given the option to re-distribute the shortfall in each space to un-fenestrated walls in each space. This option offers a reasonable work-around to edge cases not covered by the NECB and where there is no ‘correct’ solution;
f) In extreme edge cases if the algorithm and re-distribution option both fail to achieve a satisfactory outcome the user can manually edit the reference geometry in Model-iT (View menu > Model > select NECB Reference). The user should:
- Manually add / edit reference model fenestration only after the reference model is generated (re-generation will overwrite any changes);
- Manually assign constructions in NECB View to any new fenestration elements;
- Document any interventions.
Creation of the reference model - skylights:
It is not explicitly stated in Part 8 of the NECB, but for modelling purposes, the skylight-to-roof (SRR) ratio in the reference model is set to 2% because of clause 3.2.1.4, that limits skylight area to 2% of the roof area (in the Prescriptive Path).
The NECB Navigator generate reference model action will add the 2% skylights automatically to the centre of each space with a roof; where a square roof light does not fit within the room geometry the algorithm will insert a suitable shaped roof light centrally. Where proposed model rooms with roofs also have ceiling voids roof lights will not be automatically added to the reference model and the user will need to manually add roof lights to the reference geometry in Model-iT (View menu > Model > select NECB Reference). The user should:
- Manually add / edit reference model skylights only after the reference model is generated (re-generation will overwrite any changes);
- Manually assign constructions in NECB View to any new skylight elements;
- Document any interventions.
