There are two reasons to insulate the walls behind a tiled shower -- sound dampening of interior walls, and to prevent heat loss and increase energy efficiency of exterior walls. Three types of insulation can be used for this application: fiberglass bats, rigid foam, and foam-in-place. The most popular insulation for this application is fiberglass batting, which has a paper and plastic backing on one side, comes in rolls, can be cut to fit, and is the least expensive. The scope of the project, budget, and personal preference will determine the type of insulation used. When installing a tiled shower, it is also necessary to install a vapor barrier between the insulation and the tile underlayment

Instructions
Things You'll Need:

* Gloves
* Paper face mask
* Fiberglass batting
* Tape measure
* Razor knife
* Straight edge (at least 2 feet long)
* Staple hammer
* Staples
* Vapor barrier
* Scissors

Installing the Insulation
1.
Step 1

Choose the type of insulation you'll use. The usual choice is fiberglass batting.
2.
Step 2

Measure the area between two studs to find the width measurement needed for this piece.
3.
Step 3

Measure the distance, from top to bottom, of the area between two studs to find the length needed for this piece
4.
Step 4

Unroll at least 10 feet of insulation, paper side down.
5.
Step 5

Measure the bat from the end. Place the straight edge on top of and across the bat at the desired length measurement.
6.
Step 6

Cut the bat with the razor knife, pressing down firmly, so the knife cuts through the paper backing.
7.
Step 7

Measure the width of the bat. If the bat will fit between the studs, no more cutting on this piece will be needed.
8.
Step 8

Observe to see if the bat is too wide to fit in the space between two studs; measure across the bat, at the end of the bat, and place the straight edge at the desired width measurement. Measure across the bat at both ends of the straight edge to make sure the straight edge is straight on the bat.
9.
Step 9

Cut the bat lengthwise, from one end of the bat to the other. Press down firmly with the knife and cut through the paper backing.
10.
Step 10

Push the bat into the space between the two studs, starting at the bottom, with the insulation side facing the exterior wall and the paper side facing the installer. A properly fitting insulation bat will fit snugly into the opening.
11.
Step 11

Staple the paper wings on either side of the bat to the sides of both studs. This will hold the bat in place until the wall can be finished. Do not let any material stick out of the wall past the outside edges of the studs.
12.
Step 12

Repeat these steps for each bay between studs until all have been filled with insulation.
Installing the Vapor Barrier Part 1
13.
Step 1

Measure the width of an end wall, from the inside corner -- where the shower will be -- out at least two studs past where the tile will end. Add 2 feet to this measurement. This will be the length of vapor barrier you'll need.
14.
Step 2

Unroll at least 10 to 15 feet of vapor barrier on the floor, and measure the material from the end of the roll.
15.
Step 3

Place a straight edge across the vapor barrier at the desired length, and cut it with either scissors or the razor knife.
16.
Step 4

Hold the vapor barrier up to the wall, with the bottom edge touching the floor (or the top of the tub, if applicable). Position the barrier so that the excess material extends beyond the corner, to at least the next stud on the adjacent wall. This step will require two people.
Installing the Vapor Barrier Part 2
17.
Step 1

Staple the barrier to each stud along the top and bottom of the barrier only. Be careful not to tear the barrier.
18.
Step 2

Repeat the process and install the next barrier on top of the first. Position the barrier so that there is at least 1 foot of overlap on the bottom barrier. When this step is completed, there should be 7 feet of vapor barrier measuring from the floor, up the wall.
19.
Step 3

Complete this process for the other end wall. It will be necessary to make holes for the plumbing fixtures once the vapor barrier is partially tacked in place.
20.
Step 4

Cut small incisions in the barrier and allow the plumbing fixture to come through. Cut only the amount necessary.
21.
Step 5

Overlap both corners when installing vapor barrier to the middle wall. The vapor barrier should overlap at least one stud on the adjacent walls.