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?
Best Answer
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.