Your blackout curtains are a good start.
- They will prevent sunlight from entering the apartment and heating up the floors, counters, and furniture, all of which will store that solar heat and radiate it back to you when you're trying to stay cool.
- Much of the sunlight that comes through the glass will be reflected off the light colored backing of your curtains and back out the window.
- However, some of that solar radiation is still making it inside.
A better way to stop it coming in is right at the window glass. Since you can't change your windows, you might consider a window film designed to block solar radiation.
While not a permanent change, you should take care to not apply these films in the wrong situation:
- don't apply them to the inside of double-paned windows, for example, and
- don't apply them to windows that already have films applied.
But applied to the outside of the glass in hot climates on otherwise uncoated windows, these films can stop solar radiation from entering the apartment at all.
Seal cracks around windows
Sunlight coming through the windows is not the only way heat is entering your apartment, though.
As you suggest, warm outdoor air may be coming in through gaps around windows, as well as floors, doors, vents, plumbing holes, and electrical switches and outlets.
- Weatherstripping windows and doors,
- using gaskets on the back of outlet covers, and
- sealing cracks around baseboards and window trim with clear caulk will help keep the conditioned indoor air inside and the unconditioned air outside.
Done neatly, these changes should be unobjectionable to your landlord, and may even go unnoticed.
In apartments with central air conditioning, you don't have much control over the system except at the thermostat.
- Raising the temperature setting a little is the first change to make.
- You can make a warmer apartment feel cooler by using small table fans to circulate air around you.
The combination of a higher AC setting and a personal fan will keep you just as cool, while using less energy.
Depending on your building and the landlord, you can also advocate for regular servicing of your AC unit, and make sure the ducts are installed correctly and not leaking conditioned air into a basement or service areas.
Consider an evaporative cooler
In hot dry climates, there is an alternative to air conditioning that may work for you.
Evaporative coolers, sometimes called "swamp coolers" work by converting liquid water to water vapor, a phase change that sucks heat out of the surrounding air.
- They use less energy than an air conditioning unit, and don't contain the chemicals used by AC units that contribute to global warming.
- However, they do add humidity to the air, which in excess can become a problem in a home.
- And since air conditioners also dehumidify the air, you should not use an evaporative cooler at the same time as the AC, since it will just make your air conditioner work harder!
But depending on your building and how much space you're trying to cool, turning off the AC and switching to an evaporative cooler might be a viable option for you.