gas or electric? gas you need to be careful not to insulate near the pilot light or flue.
make/model? Manual should tell you if recommended or not.
do you have access to the HW pipes coming out? insulating them will help, again being aware of recommendations on your model of WH.
make/model? Manual should tell you if recommended or not.
do you have access to the HW pipes coming out? insulating them will help, again being aware of recommendations on your model of WH.