I am finishing out the bonus room above my garage. Space is 530 sq.ft. with 7' ceilings, with full bath and closet, for a guest room. When used, the room will normally only have 2 people. I have put 10" insulation in flat ceilings and 6 1/4" (R-19) in angled roof ceilings with air baffles. R-15 in vertical walls and knee walls backed with 1/2" insulation board. I am planning to install a thru-the-wall a/c heatpump with backup elec. heat. From the a/c sizing charts, it looks like I will need approx. 12,000 to 13,000 BTU for a/c. Will a 12,000 btu be the right size or should I go up the next size to 14,000 btu? I am going with thru-the-wall because I can't run any ducting from the central a/c-heat from the main part of the house. I just don't want to undersize or oversize this unit. I have already run a new 30 circut for this. I live in central Illinois weather and the roof gets full morning sun on one side and afternoon sun on the other side. Thanks for you help or suggestions.