I have a home office, which has 2.5 walls that border the outside of the house, and a floor that is over the garage. It's a nasty combination. It gets quite hot during the part of the day where the sun shines on one of the outside walls. The room is easily 5 degrees warmer or more than the rest of the house.
I can feel it, too. The floor itself is warmer than the floor inside the rest of the second story. The walls that border the outside are noticeably warmer to the touch than the other walls.
This house was built in 2006, so it's relatively new construction. And yet I question whether it was insulated correctly. Once we added a ceiling fan and had to open up the wall above the light switch -- there was no insulation there. That part of the wall does not border the outside, but that same wall DOES on the other side of the room.
I found this which is helpful for determining if there is insulation:
Determining if there is insulation behind a wall
So my question is really two things:
If the wall behind my switch is not insulated, does that mean they probably didn't insulate the entire wall? Is it common to only insulate the part of a wall that borders outside and not the rest?
Are there any DIY techniques to insulate the walls further that are relatively cheap and easy? If it does have to involve tearing open the walls, how big of a deal is it? What kind of cost would you think for a room that's about the size of a typical modern house's extra bedroom?