I have an RV in my workshop that had a landing incident that required all front and rear belly skins to be replaced.
This aircraft is only 3 years old and has only done 140hrs of flight time.
The builder elected to use Foam insulation throughout the aircraft including the firewall. He did a masterful job of sealing every nook and cranny of the lower skins.
The foam was a closed cell type that you can buy at any hardware store.
The foam was so well applied that it sealed in any moisture preventing the normal evaporation of condensation. Compounding this problem was that no skins or ribs were primed or treated with alodine. This was just bare alclad against alclad.
The accelerated corrosion was heartbreaking.
This aircraft is lucky that it had a landing incident that allowed us to replace all lower skins and ribs. In the Eastern side of Australia where I live it is hot and humid so alodine conversion treatment and priming parts is the norm.
This same insulation was all over the firewall !
A simple burn test showed that this stuff burns and smokes with only simple ignition. I'm not sure what was worse, the thick black smoke or the fire !!
So please seriously consider where and when to use foam insulation in an aircraft. At the very least, do a simple burn test and if you are worried about noise, just use a good pair of noise cancelling headphones.