The GoPro camera has trouble with the lens cover on the housing fogging up, as it is the most exposed part of the housing. While this does not make the footage that should have been shot completely invisible, it severely blurs it basically ruining the footage. These fog inserts completely remove that problem. Simply slide them into the side of the housing (push in as far as possible to prevent interference with the rubber seal of the housing door) and these will work for 4 uses of the camera.
There are 6 of these inserts included in the packaging (3 sets). They do their job exactly as described and work very well. A near must have for anyone in a cold climate or using the camera for anything to do with water.
NOTE: A lot of people think that after 4 uses the inserts will become useless. This is not true. After the 4 uses just put them in the oven at a high temperature (described on the packaging) for 5 minutes and you're good to go again. The inserts will last for hundreds of uses and there are 3 sets. Definitely worth the money. I would not bother looking for that chromotography paper. It costs 70 dollars and you end up with hundreds of inserts that you will never need having paid much more money.
ALSO: If you really don't want to spend on these inserts what will work - and are free - are beermats. Cut them up to the proper size with a scissors and they will work for ONE USE ONLY, but can be re-used by heating in the oven. On the bad side though they are made of very fine fibers. When frayed the beermat fibers will come loose and land either on the lense or around the housing. They become a bit messy. Also, are not designed to for the gopro and fit less cleanly in the housing
Anti-fog inserts? Definitely worth it.