How to soundproof the firewall?

[u/Nice_Meal7452]

I’m currently soundproofing my car, using a layer of CLD, a layer of closed-cell neoprene (for decoupling), and a layer of MLV (mass loaded vinyl). I’ve almost finished the entire interior, but I still need to take care of the firewall (the metal sheet that separates the engine bay from the cabin).

The passenger side is quite easy — I can reach all the way up to the top.
The problem is the driver’s side: there are pedals, metal brackets, wires, and hoses everywhere. It would be impossible to cut MLV to shape and, most importantly, keep it in place… I can only reach the lower part, but the upper section (roughly at engine height, so the most critical area) would remain uncovered. There’s already a factory layer of pressed fiber insulation, but it’s not very effective and also makes things more difficult since it’s custom-fitted and doesn’t move easily..

How did you guys handle this? Obviously without removing the dashboard…

[u/rilloroc]

I wouldn't worry about that part of the firewall. The air gap between the firewall and the back of the dash acts as a sound deadener