Not in wall cavities. The dense stuff WORKS in wall cavities, but not as good as the lightweight fluffy stuff... so you might as well save your money! Unless you prefer to use paper money for wall insulation.. a LOT of people do! And the myth goes on and on and on! I have this explained fully in my doc, "How To Find How Much Isolation You Need". It's on my Resources page.
The rigid (dense stuff) can/should be used in these places only:
1. As thin absorption panels on walls where only limited absorption is necessary.
2. As a membrane-type front surface for deeper, fluffy-filled traps. The rigid creates a nice smooth finish to the fabric covered traps as well.