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What’s new with the 2024 IECC

An update on thermal bridging, fenestration and air leakage

The 2024 update to the International Energy Conservation Code (IECC) is entering its final stretch. A first draft underwent public review in the fall, and the main committee and subcommittees are reviewing the received comments. I represent the Façade Tectonics Institute (FTI) on the Envelope and Embodied Energy subcommittee, which is chaired by National Glass Association code consultant Tom Culp.

While we cannot predict what new requirements will be in the final version, the following summarizes the current state.

Thermal bridging

For the first time in the IECC, language is added to the prescriptive, component trade-off and performance paths to recognize and mitigate the worst thermal bridging at certain building intersections such as balconies, parapets, and window and wall intersections. Thermal bridges create paths of least resistance for heat to flow through a building envelope, degrading thermal performance by up to 30 percent and potentially causing condensation issues inside walls. While previous IECC versions addressed some aspects of thermal bridging (for example, continuous insulation) they did not address all.

As currently written, the 2024 IECC requirements apply to climate zones 4 through 8, just like the new ASHRAE 90.1 thermal bridging addendum (av). The drafted prescriptive compliance language represents good construction practices using proven techniques.

For example, the prescriptive provision for fenestration mandates the glazing plane, or the frame thermal break when metal frames are used, be aligned with the continuous insulation (within +/-2 inches). This is accepted good practice.

For opaque cladding, using highly conductive continuous metal Z-girts is prevented in the prescriptive path by requiring linear supports be offset from the structure with attachments allowing the continuous insulation to pass behind the cladding support element.


In 2022, FTI completed a cost-effectiveness study to support changes to the stringency of fenestration U-factor prescriptive compliance requirements. The analysis included payback calculations for energy cost savings and the social cost of carbon, using model medium office and apartment buildings.

FTI submitted a U-factor change proposal based on this analysis during the second public review. The proposed changes are shown in the table below and affect only the fixed fenestration U-factors. The subcommittee just recently approved these changes, which will now move to the full committee.

Fixed Fenestration Prescriptive U-Factor

The analysis, based on curtain wall (considered the worst case for costs), identified the average cost of adding several efficiency measures to improve fenestration performance:

  • Argon gas fill
  • Warm-edge spacer
  • Replacing double-silver low-e with a triple-silver low-e
  • Room-side low-e coating
  • Increasing frame thermal performance
  • Replacing aluminum pressure plates with non-metal versions
  • Moving to triple-pane infill

This was then used to estimate the lowest upfront investment needed to reduce U-factors from their current values in each climate zone to a lower value. Generally, the lowest U-factors showing positive energy cost savings using IECC guidelines were chosen for the proposal.

Air leakage

The envelope air leakage section is being overhauled to improve clarity. The first draft also includes an increase in stringency for air leakage (0.35 cfm/ft2 at a pressure of 0.3 in water, reduced from 0.40 cfm/ft2) and expanded verification testing requirements.

The final 2024 IECC should be available later in 2023.


Helen Sanders

Helen Sanders

Helen Sanders, Ph.D., serves Technoform Glass Insulation on its North American market team. Sanders develops strategic markets and relationships, and industry-leading fenestration products, while guiding and supporting the company's continued focus on energy performance and sustainability.