Every time an interior panel gets opened, I assess the condition of the existing fibreglass and if required, replace it with
Super Soundproofing from Aircraft Spruce.
I've found the 1/2" thickness with adhesive the most useful. A couple of times the 1/4" and 3/4" have been necessary. I've replaced a lot in the ceiling, around the windows, the side walls and the top half of the rear panel.
There's some good guidance in the "
Sound-proofing the Light Aircraft" pamphlet by Bill Nash.
Unfortunately, success depends upon the continuity and layering of the soundproofing material, which is difficult to achieve. It's slow, tedious work, but easy. It's a good job for an owner to do with the oversight of a licensed mechanic. I think I've heard a difference with the partial amount done to date.
I wouldn't install foil with bubble plastic without seeing proof of it's fire-resistance. The fiberglass batt is OK, but so .... 1960ish! You want a closed-cell foam that won't absorb or hold water like an open-cell foam. And speaking of water, be on the lookout for the black vibration dampening patches that may be glued to the inside of the skin. If they've seen any water there will be corrosion behind them. They will need to be removed, the surface cleaned and treated with anti-corrosion chemicals.
A weight and balance amendment may be required, and it's easy to lose track, because there are lots of small pieces. Rather than weigh every single piece, calculate the weight you've installed by weighing the un-cut sound-proofing material you bought. Put all the scraps in a bag, and when you're done, weigh the bag of scraps and subtract the weight from the new material. Similarly, put all the insulation you removed in a bag, and weigh that when you're done.