Cover the vent
They sell magnetic vent covers at nearly all hardware stores. Buy one, and cover the vent when it's not being used. This will prevent moist air from entering the vent, though it may not stop water if it pools around the vent.
Install an exhaust fan
Install a properly sized exhaust fan in the bathroom. Let it run during showers, and for about 20-30 minutes after showers.
WARNING: Make sure you vent the exhaust properly.
Sizing an exhaust fan
Exhaust fans are sized using Cubic Feet Per Minute (CFM), so you'll have to first figure out how many cubic feet your bathroom is. To do this, measure the length, width, and height of the room.
- Length = 10 ft.
- Width = 8 ft.
- Height = 8 ft.
To find the cubic feet, simply multiply these numbers together.
10 ft. * 8 ft. * 8 ft. = 640 ft.3
Next we'll divide the cubic feet by 60, to figure out how many CFM is required to exchange the air one time in an hour.
640 ft.3 / 60 Minutes = 10.66 ft.3/Minute.
It's recommended that the air in a bathroom be exchanged 8 times per hour, so you'll have to multiply our CFM value by 8.
10.66 ft.3/Minute * 8 = 85.33 ft.3/Minute
So in this example, we'll need an exhaust fan with at least an 86 CFM rating.
Quick formula: (Length * Width * Height) / 7.5 = Required CFM rating.
Making sure the fan runs long enough
When installing an exhaust fan in a bathroom, it's a good idea to put the fan on a timer. This allows you to run the fan for the recommended 20-30 minutes after a shower, without having to worry about remembering to turn it off.
Warning: Make sure you get a timer that is rated for motor loads.
Best Answer
Whether for heating or cooling, your goal is to limit the amount of heat going through the building envelope, it's just the direction the heat is moving is reversed. Cooling loads are complicated by radiant heat gain which is not much of an issue for heating loads. Thermal imaging can identify areas of high heat conductance, indicating where more insulation is required.
Other than that, simple inspection can indicate where the heat is getting in (or out). Are there any gaps in windows and doors allowing air to leak in or out? If the windows are singled glazed, that will be the largest single cause of heat gain. How much insulation is in walls and ceilings? What's good for heating is good for cooling. In addition for cooling, how is the sun allowed to radiate heat into the building. Lack of shading devices, window tints, draperies or shades, etc. all contribute to heat gain.