HomeMy WebLinkAboutbocc.pc Smith, Matthew - Existing Building Thermal Envelope Policy
MEMORANDUM OF POLICY
Building Codes
FROM: Jeffrey Erickson, Chief Building Official
DATE: January 28, 2026
SUBJECT: Existing Building Thermal Envelope Policy
_____________________________________________________________________
Application of IECC R503.1.1 Exception #3 – Alterations to Existing Dwellings
Purpose and Authority
Section R503.1.1, Exception #3 of the 2021 International Energy Conservation Code
(IECC) states:
Alterations exposing or demolishing more than 75 percent of the building’s
thermal envelope area shall comply with the energy requirements of Section
R407 (new dwelling requirements).
The intent of this policy is to provide clarification and establish consistent documentation
and review requirements for alterations to existing dwellings in order to determine when
the 75 percent thermal envelope threshold is exceeded.
Construction Document Requirements
Pursuant to Section R103.2.1 of the 2021 IECC, construction documents shall include a
depiction of the building thermal envelope.
Given the critical role of the thermal envelope and the alignment of the air barrier and
thermal barrier systems in overall building energy performance, the design professional
shall clearly demonstrate the following on the construction documents:
● The total building thermal envelope area (in square feet), and
● The total area of the building thermal envelope proposed to be altered, exposed,
or demolished (in square feet).
This documentation shall include existing conditions and, where applicable, the
combined existing and new construction.
The construction documents shall be prepared in a manner that allows the Pitkin County
Building Department to clearly verify the total percentage of the building thermal
envelope that is being altered.
When is a Thermal Envelope Depiction Plan Required?
Building Thermal Envelope Depiction Plans are required for all alterations affecting an
existing building’s thermal envelope, except for permits limited to exterior veneer, siding,
window, or door replacement only.
Where the Chief Building Official determines that the scope of work is minor and does
not materially impact existing wall assemblies, detailed thermal envelope calculations
may be waived pursuant to the exception to R103.1 of the 2021 IECC.
Change Orders and Threshold Exceedance
Any project that was previously approved under the 75 percent threshold and
subsequently exceeds the threshold as a result of a change order shall be required to
comply with the most recently adopted Pitkin County Energy Code, including new
dwelling requirements.
Definition of Building Thermal Envelope
Per Chapter 2 of the 2021 IECC, the building thermal envelope is defined as:
BUILDING THERMAL ENVELOPE. The basement walls, exterior walls,
floors, ceilings, roofs, and any other building element assemblies that
enclose conditioned space or provide a boundary between conditioned space
and exempt or unconditioned space.
Clarification of Thermal Envelope Disturbance
Removal of interior wall finishes that expose wall and ceiling cavities even if existing
framing remains shall be considered disturbance of the building thermal envelope and
shall be included in the thermal envelope depiction and calculations
Siding, Window, and Door Replacement
Replacement of exterior siding, veneer, windows, and doors only does not, by itself,
trigger new dwelling requirements or require exposure of interior wall cavities. However,
Section R503.1.1.1 shall apply, which states:
Where all the siding and windows are replaced on one or more sides of a
structure, the building envelope shall comply with Table R402.1.3.
To determine the required level of continuous insulation, the applicant shall
demonstrate the R-value of the existing cavity insulation.