Every residential project permitted under the Florida Building Code, Energy Conservation has to choose a compliance path — and for most projects, that choice comes down to R402 (prescriptive) or R405 (performance). They arrive at the same legal outcome — a code-compliant home — through very different processes, and picking the wrong one for a given design can mean unnecessary redesign work or a compliance report that fails to reflect the project's actual strengths.
R402 compliance works like a checklist. Every individual envelope component — wall insulation, ceiling insulation, window U-factor and SHGC, floor insulation, air leakage — has to independently meet a fixed minimum requirement for the applicable climate zone. There's no flexibility to offset a weaker component with a stronger one elsewhere; each item stands or falls on its own.
This makes R402 fast and simple for conventional home designs that comfortably clear every individual minimum without much design tension — a straightforward layout with reasonable window area and standard construction can often demonstrate R402 compliance with minimal back-and-forth.
R405 takes a fundamentally different approach. Instead of checking components individually, it uses whole-house energy simulation to compare the proposed home's total estimated annual energy performance against a reference home baseline — a hypothetical version of the same home built to prescriptive minimums. If the proposed home's modeled performance meets or beats the reference home (accounting for required credits and mandatory items), it complies, regardless of how any individual component performs on its own.
This flexibility is the whole point of R405: a home with a larger-than-prescriptive window package, an unconventional envelope assembly, or design choices that would fail R402 outright can still comply under R405 if the shortfall gets offset by strength elsewhere — better HVAC equipment efficiency, improved duct sealing and testing, upgraded water heating, or other measures that improve overall performance.
R405's flexibility comes with real costs: it requires approved energy simulation software, more detailed data collection (accurate specifications for every energy-relevant system, not just the envelope), and — critically — follow-through on any measures the model assumed, like duct leakage testing. If an R405 submission assumes tested (not default) duct leakage but the ductwork never actually gets tested and documented, there's a real gap between what was modeled and what gets verified, which can hold up final approval.
R402's simplicity comes with real limits: no flexibility means a design that fails even one prescriptive minimum has to be redesigned to meet it directly, with no way to compensate through overall performance.
A useful early gut-check: run the design's key envelope numbers against R402's prescriptive minimums for the applicable climate zone before committing to a compliance path. If everything clears comfortably, R402 is often the faster, simpler route. If one or more components fall short — particularly window performance or wall assembly R-value on a design with significant architectural glazing — R405 is usually the more practical path forward rather than forcing envelope changes that compromise the design.
Can a project switch from R402 to R405 partway through design?
Yes, though it typically requires starting the compliance documentation process over with the appropriate energy modeling software, since R405 requires substantially more input data than R402's component checklist.
Does R405 require more expensive HVAC equipment to compensate for a weak envelope?
Not necessarily — R405 allows offsetting through multiple measures, not just HVAC efficiency, including improved duct sealing, better water heating, and other energy-relevant system upgrades. The specific mix depends on what's most practical and cost-effective for the given project.
Is R405 required for additions, or only new construction?
Both R402 and R405 can apply to additions and alterations, though specific requirements and thresholds for additions can differ from full new construction — confirm with the local building department for the specific project scope.
Does choosing R405 mean skipping the mandatory requirements that apply under R402?
No — certain mandatory items apply regardless of compliance path chosen. R405's flexibility applies to the overall performance target, not an exemption from every individual code requirement.
Psychro.Pro supports both R402 and R405 Florida compliance workflows alongside Manual J, S, and D load calculations and duct design. Learn more.
