Why won't my AC cool the house when it's extremely hot outside?

What people tried

Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.

1. 1
   adding window AC units to specific rooms
2. 2
   using thick blackout curtains
3. 3
   sealing drafty doors and windows
4. 4
   shading windows from the outside
5. 5
   hosing off the outdoor condenser unit
6. 6
   Replacing or cleaning air filters
7. 7
   Adding insulation, radiant barriers, and air sealing
8. 8
   Using solar mesh screens, window film tinting, or foam boards in windows
9. 9
   Running a dehumidifier to make higher temperatures feel cooler
10. 10
    Keeping doors and windows closed to minimize heat gain
11. 11
    renting an insulation machine to blow in cellulose insulation oneself
12. 12
    sealing penetrations, vents, and HVAC ducts with HVAC tape
13. 13
    replacing seals and weatherstripping, caulking, and fixing windows
14. 14
    adding powered attic fans to existing ventilation systems
15. 15
    Trying window fans switched to exhaust mode
16. 16
    Consulting with HVAC professionals for custom solutions
17. 17
    closing window blinds
18. 18
    using circulating fans
19. 19
    putting tinfoil or outdoor shades on windows facing direct sunlight
20. 20
    installing an attic fan
21. 21
    adding more attic insulation
22. 22
    running an additional dehumidifier
23. 23
    spraying a light mist of water on the roof

In their words

Unedited, most upvoted first, each linked to the thread it came from.