We’re not 100% sure why people think this technique works but please (pretty please) do not attempt to soundproof your room by nailing old mattresses to the walls. This concept is wrong for a whole host of reasons.
Yes, you read that correctly. Some people literally take old mattresses and nail them to the walls thinking they’re adding insulation. This would work if you were able to seal the mattresses together so that there were no seams, spaces, or open edges – but we’re not even sure that’s possible with the best soundproofing caulk on earth.
Besides, mattresses stink. They’re old for a reason. Throw them away. Why would you want to nail your dingy used mattress to the wall? If, by chance, you live in a place where this is your only option – go for it. But seriously, folks, if anyone tells you this is the way to go we suggest you turn and run in the other direction. Find a better sound proofing expert and do so quickly!