Tight makes right behind tubs and showers—because they can be *ruff* to retrofit
To get the Energy Star, you need to work through the Thermal Bypass Checklist. One of the items is walls behind tubs and showers.
Why it matters:
Because tub units are often installed before the insulation and drywall, they can become a thermal nosebleed. It’s important to get the sequence right, because it can be rough to retrofit.
Tubs can become a giant air chase that connects the living space with basements, damp crawlspaces, attics, and the outdoors.
Energy Star requires " ... exterior walls to be enclosed on all six sides, including a complete and continuous air barrier behind the tub."
The requirement is a little easier to meet if you are lucky enough to live in climate zones 1, 2, or 3.
How to do it:
Get to work before the tub goes in.
Some materials can insulate and seal against air leaks, like spray foam, or rigid insulation board that is loosely installed and sealed at the edges low-expanding foam.
Another way to do it is to fully insulate the tub walls and seal a rigid air barrier, like Luan plywood or Thermoply to the framing against air leaks.
Continuous insulation and an air barrier mean that this tightly-insulated tub space won’t be dogged by cold and mold.
2012 IECC: Section 402, Building Thermal Envelope