REScheck and Residential Energy Code Compliance: Choosing the Right Path

Every new home and addition in most US jurisdictions must show that it meets the residential energy code, usually a version of the International Energy Conservation Code (IECC) with local amendments. The code offers several ways to do that. REScheck, the free compliance tool from the US Department of Energy, is the most widely used, but it is not always the best fit. Choosing the right path early can be the difference between an easy approval and a late redesign.

The four compliance paths

• Prescriptive — every component meets the code's table of minimum R-values or maximum U-factors for your climate zone. Simple, but no flexibility.
• Total UA alternative — the overall heat loss of the envelope must be no worse than a code-compliant version of the same house. This is what REScheck calculates.
• Simulated performance — an energy model of the proposed home must use no more energy (or cost) than a code reference home. This path can trade envelope against equipment.
• Energy Rating Index (ERI) — a HERS-style rating must meet the code's index target for the climate zone.

How REScheck works

REScheck compares two numbers. For each assembly — walls, ceilings, floors, windows, doors — it multiplies area by U-factor and sums them to get the house's total UA. It then calculates the UA the same house would have if every component just met the prescriptive table. If the proposed UA is lower, the home passes, and REScheck reports by how much.

That is the power of the tool: you can trade between components. A home with large windows can compensate with better wall or ceiling insulation. A cathedral ceiling that cannot reach the prescriptive R-value can be offset by better glazing. As long as the total heat loss is no worse than code, it passes.

What REScheck does not do

REScheck checks the envelope. It does not trade envelope performance against more efficient heating, cooling or water heating equipment, and it does not model solar gains or air tightness beyond the code's requirements. If a design depends on a high-efficiency heat pump to make up for a weaker envelope, the simulated performance or ERI path is needed instead. DOE now directs users to the web version, REScheck-Web, which supports recent IECC editions.

Mandatory requirements apply to every path

Whichever path you choose, the code's mandatory provisions still apply. These typically include:

• Air sealing and a blower-door test meeting the maximum air leakage rate for the climate zone;
• Duct sealing and duct leakage testing where ducts are outside conditioned space or required by code;
• Equipment sizing to ACCA Manual J and Manual S;
• Mechanical ventilation, programmable thermostats, duct and pipe insulation, and high-efficacy lighting;
• Certificate posting and insulation labelling.

A REScheck report that passes on UA but ignores the blower-door target still fails inspection.

Additional efficiency requirements

Recent IECC editions go beyond envelope and mandatory items. The 2021 IECC requires homes using the prescriptive or UA path to also select an additional efficiency package, such as a better envelope, high-efficiency HVAC, efficient water heating or reduced duct leakage. The 2024 IECC develops this into a credit system. Check which edition your jurisdiction has adopted, because a REScheck pass alone may not be enough.

When to move to the performance path

The same logic that applies to commercial buildings applies to homes. Consider the simulated performance or ERI path when:

• Glazing is extensive and the UA trade-off cannot absorb it;
• The design relies on high-efficiency or all-electric equipment to compensate for the envelope;
• Ducts are inside conditioned space or the home is very airtight, and you want credit for it;
• A programme such as ENERGY STAR or a utility incentive requires a HERS rating anyway.

Common reasons for rejection

• Wrong climate zone or code edition selected;
• Areas that do not match the drawings, especially window areas;
• Window U-factors entered from a brochure rather than the NFRC label of the specified product;
• Insulation R-values entered as nominal values for assemblies where the code requires a continuous insulation component;
• Missing mandatory items or an unsigned report.

The takeaway

REScheck is fast, free and ideal for conventional homes where the envelope can carry compliance on its own. When the design depends on equipment efficiency, unusual glazing or air tightness, a simulated performance or ERI analysis gives the flexibility REScheck cannot. In every case, the mandatory requirements and the Manual J that sizes the equipment come along for the ride.

This article is general guidance and reflects information available at the time of writing. Compliance requirements depend on the IECC edition and local amendments adopted by your jurisdiction — always confirm the code that applies to your permit.