If you are using the camera with a housing in an environment where lens is fogging up, you can use anti-fog inserts designed specifically to alleviate this issue. The anti-fog inserts will help prevent your camera's housing lens from fogging, even in cold temperatures/humid environments. The kit includes 12 reusable anti-fog inserts, and retails at $19.99. Each set lasts 4-5 uses and can be dried in an oven (300°F for 5 minutes) and used again.
If the color in the circle on the Humonitor card that is included in the anti-fog inserts turns pink, then there is an issue with the seal on the packaging and the anti-fog inserts should be dried out.
Please note, if you are using the camera in conditions where waterproofing is not needed or where high wind noise is not an issue, then you should use the open back door for the housing. Using the open back door for the housing will ensure that the temperature inside and outside of the housing is equalized, thus preventing fogging.
QuikCapture (accessible in the camera's Settings/Preferences menu) is also a great feature to help prevent fogging. This feature will have the camera automatically fully power off when stopping recording. When the camera is powered off, it will not be generating heat which can contribute fogging.
For those using a HERO5, HERO6 or HERO7 Black, which does not require a waterproof housing, we recommend removing the Protective Lens and allowing the unit to dry in an air conditioned and/or low humidity environment (<50% Humidity). Once traces of fogging has cleared, simply re-attach the Protective Lens.