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There are many posts on this subject with only bits of information that don't always seems to add up or is just a bit bitty. So I hope this helps with a little detail.

This generally applies If the housing "does not" have "dry" air inside, and has a higher humidity level, i.e particularly an issue in the tropics.

POLY carbonate housings often have more front port glass fogging issues than aluminium housings, the poly case doesn't condense much moisture onto its inner surfaces as it a poor conductor of heat, the inner surface inside the housing therefore remains warmer and any moisture that is present inside, doesn't readily condense on the housing walls, resulting in more vapour staying air borne longer which finds it way to the glass port. The glass port generally being the coldest surface, the moisture condenses, similar to your windows at home.

A METAL housing inner surface temperature is normally cooler as the case conducts heat way into the cold water, and the inner surface temperature is therefore much closer to the cooler water temperature, (i.e water cooled) then acts like a "moisture sponge", moisture condenses on the housing sides, this may not be visible, resulting in less moisture being "air borne", (the air is now a bit dryer) resulting in less moisture vapour reaching the glass port window, therefore less fogging issues. This process has a similar effect as putting silica gel packs inside the housing that help soak up vapour like a sponge.

In either housing type, a hot working camera (i.e perhaps when videoing), heating up the inside of the housing can in some circumstances re-evaporate some moisture, that is (if any) off the case inner sides to some degree.
So if a glass port is clear with the camera off or just taking pictures, Start to video or use flash a lot on the camera, in some
circumstances can cause fogging to occur.

As you dive deeper the water temperature often cools and you can find fogging occurs more often at depth and can also clear nearer the surface where it warmer.

The answer to all these issues is to have dry air inside your housing at all times.

Ways to achieve this is to always assemble camera in a air conditioned room.
Carefully blow dive cylinder dry air into housing as you close it.
Put open prepared camera housing in large plastic bag and fill with dive cylinder dry air, close it whilst in the dry atmosphere.
Least reliable option is using silica gel packs inside the housing.

If you would like to know about the physics, Google "dew point" and "relative humidity".

To help you with this, remember these following points..

Water condenses on a glass port when the RH reaches 100% at that point.

An air conditioned room may have a RH of about 40%.

When vapour condenses on a glass surface, it has reached its dew point temperature.

Dew point of dry dive air is in the minus figures. i.e Extremely dry.

As an example: an air conditioned room at 25 degrees C, with RH=40% has a dew point of 10 degrees C. Seal up the camera and trap this air inside,  condensation will not occur on the port until the water temp reaches 10 degrees C.

This is always going to stop fogging in the above example.

Condensation in underwater cameras housings Part 2

This section looks just a little further for the nerds or anyone just interested.

To offer  more information on how to accurately predict potential condensation rather than just a gut feel or just experience. We need to better understand (not fully) humidity and its associated dew point.

We also do not need to know how much (mass) water vapour is actually present, just that it is there and at point does it cause us an issue.

It is worth having a grasp however that the warmer the air is, the larger quantity (mass) of water vapour that can be held. Keep in mind this does not directly effect us, we just need to know if it may condense inside the camera housing.

Looking at a typical example may help this awareness.

Consider diving in tropical waters with a top side air temperature of 30 degrees Celsius,
the water temperature is 25 degrees Celsius.
We can just say it is hot and humid today so there is a risk of camera fogging issues.

If we have a value of the humidity, we now need this to go further. Consider this to be 80%, this would be considered  high.
We need to apply this information to the chart shown.      

If you follow the 30 degree air temp line up to the 80% humidity line (grey) then along left  to the dew point, you can read off approx 26 degrees.

This tells us condensation can occur on the housing port window, when the sea temperature is or below 26 degrees.

In our example the water is 25 degrees.

Therefore it is likely to be a issue, especially at depth when the water temp is lower than 25 degrees.

Entrapping dry air in the housing

If we assemble the camera in a air conditioned room, and trap this dryer air in the housing.

Let's say for example the room is 20 degrees Celsius, let us assume for this illustration, the humidity is lower, let's say 50%. ( you can measure it if you really wanted too)  Now apply this information.

Using the graph above

Reading 20 degrees, follow  up to the 50% line (green line), then across to the dew point, this reads about 10 degrees Celsius..
This means the vapour trapped in the housing will not condense until the port glass or case reaches 10 degrees.
This is unlikely to be the case. This means no fogging on the housing port glass.  

Low humidly is our friend.

Video below shows one way to ensure dry air enters the housing, Ideally leave hosing in the bag for a few minutes.

You can equally just blow dry air into the rear of the housing with the door just ajar.

Housing Condensation

These are my personal explanations based on
my experience and training as a engineer.