I recently purchased a 12,000 btu window air conditioning for a 400 square foot room. The unit works great, until the late afternoon around 4pm, where it struggles to bring down the temperature to a comfortable level. The room has a very large west facing window which gets a lot of solar gain, so I decided to install an outdoor roller shade. This decreased the heat coming off that wall/window significantly, but didn't really help the a/c unit out in the later afternoon at all. I checked the doors and windows, and there doesn't seem to be any gaps where heat could be coming in, what is the culprit here? Should I install another unit to help cool down the room?

  • Have you cleaned the filters? West facing with minimal insulation that could be at the max for the unit. The blind should have helped but with the information provided a second unit sounds like it is needed, I would check the filters inside and look at the outside coils if a very dusty area even these can get plugged.
    – Ed Beal
    Jun 15, 2021 at 16:10
  • 1
    Does the sun beat down on the window unit?
    – JACK
    Jun 15, 2021 at 16:41
  • Where are you? What's the outside temperature in the afternoon? What kind of building is this? What floor is it on? What are the outside walls made of? That's not a tiny room but it's not a tiny A/C. It seems "about right". So various factors could mean you need more.
    – jay613
    Jun 15, 2021 at 17:33
  • @jay613 Southern California coastal town, temperature was low to mid 80's. I live in an older single story house, built in the 40s Jun 15, 2021 at 17:45
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    What color is the roof? Is the entire attic inside or outside the insulation envelope? Jun 15, 2021 at 22:06

1 Answer 1


Get an infrared thermometer.

enter image description here

With this you can measure the temperature of your walls and ceiling, and you will find where the heat seeping into the room is coming from.

I also have a large window facing south, which turned the living room into an oven in summer. I installed a vegetal awning like this:

enter image description here

It makes a huge difference, it looks nice, and the leaves fall off in winter, letting the sun through.

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