Bathwater loses much of its heat through the walls of the tub. This is more common in bathtubs made of steel and fiberglass, which do not hold heat as well as those made of cast iron. The best way to help prevent this loss of heat and save money on the cost of hot water in the process is to insulate your bathtub.
Spray Foam Insulation
Spray foam is the safest and most efficient insulation for a bathtub. Because it expands, spray foam insulation fills the spaces between the tub and outer walls more efficiently than other insulation. To reduce the risk of harmful mold and mildew buildup, make sure the plumbing is tight before insulating. The outer walls of the tub should be coated fully with spray foam to prevent air gaps where condensation can form.