Abstract: A thermal boot apparatus for enclosing a test port and cap for a power house precipitator comprising an inner and an outer stainless steel mesh sleeve each having one end enclosed, a layer of insulating material sandwiched between the inner and outer sleeves, and a mastic coating on the outer surface of the outer sleeve and end. Also included is the combination of a test port for a power house precipitator, a removable cap on the end of the test port and the thermal boot apparatus with a stainless steel band including a sealing clip encircling the coated outer sleeve to secure the thermal boot over the test port and cap.