to prevent the slab of the garage from sticking to the block walls. In the event that the floor settles due to poor prep and compaction of gravel the floor will drop rather then stick to wall and crack.

necessary. If you don't have experience with this process you can damage your house or your self, but don't be afraid it is really only dirt. You can use mild dish soap just a little and a brush on a pole, then hose off. Bleach is to harsh for the paint if you are not going to paint right away.

straps at joists. The type of pipes and cause of knocking could change the answer.

are actually inefficient, but because they used infrequently you save by not constantly heating hot water, if used for a normal household there is a point of use where they use more energy. I opted for a heatpump water heater and put it in eco mode so the heating strips never come on, so it is operating as efficiently as possible.