spray-on method is easier to apply around unusually shaped or
curvier shells. Some builders subcontract the work out, because
it’s somewhat messy and toxic.
The flatter option:
Above, extruded polystyrene panels are used to insulate the outside
of a “chill pool,” which must always remain at cooler
temperatures. Because they are flat and fairly unyielding, they
make most sense in rectilinear applications, such as geometric
pools and spas, at right.