Energy Code Requirements for Significant Alterations and Conversions

Last updated April 2, 2025
By Ian Story

This article discusses energy code requirements for residential buildings that are undergoing a significant alteration or change of use, with no addition of floor area, while still staying within the Residential provisions of the energy code. This article specifically deals with the Washington State Energy Code (WSEC). The example that prompted this article was a down-to-the-studs gut of a duplex and simultaneous conversion to a 4-plex (R-3 occupancy to R-2 occupancy).

First, check to confirm that the WSEC Residential provisions still apply. Per the WSEC, the Residential provisions apply to buildings that meet the definition of residential buildings, below. This definition indeed covers most residential projects. The two triggers likely to push a building into the Commercial provisions are 1) R-2 occupancy with internal corridors / shared stairs / lobbies and 2) R-2 occupancy with 4 or more stories above grade plane.

RESIDENTIAL BUILDING: For this code, the following building types are residential buildings:

1. Detached one- and two-family dwellings.
2. Multiple single-family dwellings (townhouses).
3. Group R-3 occupancy areas in buildings three stories or less in height above grade plane whose dwelling units are accessed directly from the exterior.
4. Group R-2 occupancy areas in buildings three stories or less in height above grade plane whose dwelling units are accessed directly from the exterior.
5. Accessory structures to residential buildings.

Group R-2 buildings with dwelling units accessed from interior corridors or other interior spaces are not residential buildings.

Requirements for Modifications

Per the Washington State amendments to the IEBC, modifications to existing buildings also need to apply with the relevant chapters of the WSEC (302.2). First, determine whether the work is an alteration or a repair. alterations are defined as follows:

Alteration: Any construction, retrofit or renovation to an existing structure other than repair or addition. Also, a change in a building, electrical, gas, mechanical or plumbing system that involves an extension, addition or change to the arrangement, type or purpose of the original installation.

Repair: The reconstruction or renewal of any part of an existing building for the purpose of its maintenance or to correct damage.

Requirements for Repairs

The relevant section of the WSEC for repairs is section R504.1. The requirements for repairs are minimal: essentially, the repaired or replaced components just need to continue meeting the energy code they were originally designed under (in practice, this means no worse performance than they currently provide).

While not technically required by the code language for a repair, it is common practice (and required by many reviewers and inspectors) to install the largest-available size insulation batts in any wall, roof, or floor envelope cavities exposed during construction.

Requirements for Alterations

The relevant section of the WSEC for alterations is section R503. The alteration requirements only apply to the altered portions of the structure. A project can mix and match repairs with alterations, and apply the relevant code requirements to each.

Generally, any new building components have to comply with the full energy code, and the combined structure needs to be no less conforming than it was prior to the alteration. The following are specific exceptions to the requirement to comply with the full energy code:

1. Storm windows installed over existing fenestration.
2. Surface-applied window films.
3. Existing ceiling, wall, or floor cavities exposed during construction. Instead of meeting the full requirements, 2×4 walls just need to be insulated with R-15 batts and 2×6 walls with R-21 batts. Exposed roof cavities can be insulated either below or above the sheathing.
4. If insulated cavities are not exposed, upgrades aren’t required.

If windows or doors are being replaced, the new windows or doors need to meet the code requirements on an area-weighted average basis.

New heating systems, service hot water systems, and lighting systems need to full comply with the code.

Requirements for Change of Use

The relevant section of the WSEC for change of use is section R505. Generally speaking, only spaces converted from nonresidential use to residential use, or from unconditioned space to conditioned space, need upgrades.

One line of this section is unclear: “Spaces undergoing a change in occupancy that would result in an increase in demand for either fossil fuel or electrical energy shall comply with this code.” It’s not clear from a first reading whether this would apply to subdividing an existing dwelling unit into two units. Based on our experience, projects that subdivided an existing house into a house plus attached ADU were not required to fully comply with the code for new construction, just the requirements for alterations.

Additional Energy Credits

Section R406 requires projects to select additional energy efficiency options from a list. This section only applies to new construction, additions, and changes of occupancy that require upgrades. An interior-only alteration or R-3 to R-2 change of occupancy would not trigger this section.

However, note that alterations require the building to be no less conforming under the WSEC. This means that any existing energy credit features (such as a heating system) need to be maintained or restored, and can’t be replaced with a version that provides fewer credits.