The standard pipe insulation process:
1. cut away the top and bottom 2x4 sill plates and subflooring so that there is an inch or so clearance around the pipe in every direction passing through floors and ceilings;
2. remove pipe anchors typically located at the subflooring;
3. install full length sections of armiflex where it is easy to access in the wall;
4. slide these full sections of insulation down until it passes through the subfloor, through the ceiling-floor cavity, and (where appropriate) through the ceiling below until it is visible and accessible in the opened wall of the floor below;
5. place an extra thickness of armiflex and install new pipe anchors to maintain a consistent R value of the insulation throughout (the R value goes down considerably when armiflex is compressed by the pipe anchors so the thickness of insulation should be at least double);
6. repeat the operation described in steps 3 and 4 but this time sliding the full section of insulation up into the ceiling-floor cavity above;
7. insulate the section of remaining pipe in the wall gluing and taping the butt joints.
(Bob Cowherd)