A better solution would be to use a thin layer (1/2” - 1”) of flexible high-density polyurethane insulation or an aerogel matt insulation.  Both are hydrophobic and the aerogels have R-values ranging from R10 - R25 per inch … the only thing better is vacuum panel insulation (~R50/in), but they are too fragile for most retrofit applications.

I plan to use aerogel when I redo both my ’66 Westy and the Hyena.

> On 15-May-2018, at 15:57, Dan N <dn92610@GMAIL.COM> wrote:
> 
> I use Fat Mat insulation and add a thin layer of  fiber glass insulation on
> top of it.
> 
> https://www.amazon.com/FatMat-Self-Adhesive-Sound-Deadener-Install/dp/B003TP4R16