With one very notable exception, the model energy codes used by the State of Texas included a provision that exempted historic buildings from compliance with that code. No longer. The State of Texas has adopted the 2015 International Energy Conservation Code as the state-wide energy code and it includes an important new provision for historic buildings.
In a nutshell, historic buildings shall now comply. The critical qualification to that compliance, one that I think makes these new requirements fair and workable, is found in Section 1201.2. In it, a provision is made for a design professional or a historic preservation official having jurisdiction (SHPO, or local preservation officer) to prepare a report detailing how compliance with any provisions of the code would damage the historic integrity of the building. In such instances of damage, the building would be exempted from compliance with that requirement. I find this to be a very reasonable requirement, one that requires that renovations and restorations make what improvements they can while protecting historic fabric.
The actual text of the code reads as follows. Always check for local changes/amendments.
Buildings that are listed in or eligible for listing in the National Register of Historic Places, or designated as historic under an appropriate state or local law.
N1107.6 (R501.6) Historic buildings: No provision of this chapter relating to the construction, repair, alteration, restoration and movement of structures, and change of occupancy shall be mandatory for historic buildings provided a report has been submitted to the code official and signed by the owner, a registered design professional, or a representative of the State Historic Preservation Office or the historic preservation authority having jurisdiction, demonstrating that compliance with that provision would threaten, degrade or destroy the historic form, fabric or function of the building.