Until OSHA implements a standard specific to heat or heat-related illness/injury, the General Duty Clause, Section 5(a)(1) would apply but doesn't stipulate an exact temperature to maintain in facilities. Some states have adopted standards that cover temperature hazards not addressed by federal OSHA standards. When the heat index is 80°F or higher, serious occupational heat-related illnesses and injuries become more frequent, so OSHA could be prompted to conduct inspections and interventions when temperatures begin to exceed this. So, this could be a good indicator for you for confined spaces as well.
OSHA highlights in their National Emphasis Program – Outdoor and Indoor Heat-Related Hazards the National Weather Service (NWS) classifications of environmental heat exposure using their heat index as a guide for inspectors. These are:
Employers may also utilize wet bulb globe temperature (WBGT) to gauge dangerous heat levels. The WBGT indicates heat-related stress on the human body during work in direct sunlight while factoring in humidity, cloud cover, wind speed, and sun angle. Both OSHA and NWS have apps and online calculators that can help determine WBGT.
WBGT readings are determined in accordance with WBGT sampling procedures and the performance of workplace assessments per OSHA Technical Manual (OTM), Section III, Chapter 4.