Prescriptive Energy Code or Performance Model for a Custom Home?

A performance path does not automatically preserve more design flexibility. Many jurisdictions limit which trade-offs are acceptable, and some won’t let the model compensate freely for weaker envelope components with equipment efficiency or extra glass.

The permitted documents still need to match the model’s assumptions closely, including window area and performance, insulation levels, HVAC efficiency and sizing, duct locations, and air-leakage targets. If those change during design or construction, the model may need to be rerun, not merely noted in the inspection file. I’d ask the building department in writing:

- Which software and compliance path they accept
- Whether envelope trade-offs have specific limits
- What model inputs must appear on the plans
- Whether blower-door or duct testing is mandatory
- Who verifies changes against the approved model

That answer may show that the model buys useful flexibility, or that it mainly adds documentation while the practical design constraints remain.

The possibility of rerunning the model when window area, insulation, or HVAC details change is an important cost consideration. I’d evaluate the performance path by pricing that coordination, including a reasonable allowance for revisions, against the value of the specific design change it enables.

For example, preserving a particular amount of glazing or avoiding a difficult assembly may justify the modeling effort. If the only benefit is a modest reduction in one insulation requirement, the consultant fee, plan-review coordination, and construction verification may outweigh it. The comparison should include not just the initial model, but also:

- updates when the construction documents change;
- coordination among the architect, HVAC designer, and energy consultant;
- any required blower-door or duct testing;
- a contingency for substitutions that no longer match the approved assumptions.

That makes the choice less about whether performance compliance is “better” and more about whether its flexibility has enough design value to pay for the additional coordination.

The revision allowance is easier to control if the jurisdiction offers an early review of the compliance approach. Has anyone confirmed whether the building department will look at a preliminary model, or at least review the proposed software, trade-offs, and key assumptions before the design is finalized?

A useful sequence would be:

1. Submit the proposed compliance path and major inputs for informal or pre-application review.
2. Get written confirmation of acceptable trade-offs and required testing.
3. Complete the model after the window schedule, assemblies, and HVAC design are stable.
4. Reserve the rerun contingency for actual design changes rather than discovering during plan review that the approach itself isn’t accepted.

That could make the performance path less of a blank-check coordination risk, especially when the main benefit is preserving a specific amount of glazing.

A preliminary review could reduce the risk of choosing an unacceptable software or trade-off, but I wouldn’t treat it as assurance that the final model will pass. The outcome can still change once the window schedule, assemblies, HVAC sizing, and leakage assumptions are fixed in the construction documents.

It’s also worth confirming what the jurisdiction means by “review.” Staff may comment on the proposed approach without formally approving the model, or may leave field verification to the inspector or a required testing report. I’d have the architect document which inputs are binding, who must approve substitutions, and whether a rerun is required before construction or only at final inspection. That distinction matters when budgeting the revision allowance.

The question of whether the model would preserve the glazing is easier to answer if the same finalized plans are checked both ways. A prescriptive comparison can show whether the design already meets the requirements, or exactly what would need to change. Then the model’s value is tied to a specific trade-off, rather than assuming the extra glass requires performance compliance.

A model shows compliance for its specified inputs; it doesn’t establish that the home as built matches them. Confirm how the jurisdiction handles differences in installed insulation or airtightness: does the approved path require testing and corrections, or allow a prescriptive alternative? That field-verification requirement is distinct from documenting the model and could affect the risk of change orders.