My bedroom is upstairs in the attic and much of the interior walls/ceiling are the underside of the roof.

I believe there is a couple inches of hard foam between the drywall and the sheathing/roofing. The walls/ceiling of the room can get quite warm in the summer as the sun beats down on the dark asphalt tiles outside.

I'm trying to size an AC for the room, but see that most ACs suggest a square footage rule of thumb. I presume this square footage assumes a more typical mix of interior/exterior walls.

Is it worth considering my situation differently than just buying the AC that matches my room's square footage?

  • Is this a window-mounted A/C you're looking at?
    – JPhi1618
    Commented Apr 4, 2019 at 19:31
  • @JPhi1618 Yes, I'm looking at window AC Units
    – John
    Commented Apr 4, 2019 at 21:09
  • I agree with the answer from Joe. The sqft recommendations on window units are theoretical performance based on their size and power. They are usually a little less efficient than they let on and they're going to make your window leak more air/temperature than a normal closed window. With that and the warm walls, get a bigger one.
    – JPhi1618
    Commented Apr 5, 2019 at 2:37

1 Answer 1


Do not go by square footage alone. When calculating heat gain/loss, exterior surface (walls, ceiling and foundation) are the main determining factors. In your case you need to consider all the extra heat gain from the roof and also that cool air falls. In an attic you'll get all that extra heat gain plus all the cool air from the air conditioner is going to drop down effectively pushing the warmer air up. You can offset that effect by keeping your door closed when using the A/C. Assuming the same amount of exposed ceiling square footage as wall square footage I would triple the "recommended" size based on floor square footage.

This answer is just a guideline based on limited information. It is by no means an accurate calculation.

  • Thanks. Any advice on how to calculate my heat gain and loss for my room?
    – John
    Commented Apr 4, 2019 at 17:42
  • It's really hard to do without lots of information and it's time consuming. I'm not trained on proper methods but I could guess about a 18,000 btu unit (1.5 ton) would be sufficient for a typical attic room in Toronto where I live.
    – Joe Fala
    Commented Apr 4, 2019 at 17:50

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