| I am sure the vibration dampening products have R value (they transfer heat slower and with greater losses then the metal skin of the vehicle so yes they have value but I do not think the time and money has been invested to quantify it). Brown bread, Dynamat and a couple other products are always being promoted as automotive wunder- insulation. And they are... to a point. With cars a BIG problem is structure borne noise propagation and only Dynamat-like products will address that. This is called vibration dampening and is effective from 5 Hz to around 500 Hz. My complaint is that above 500 Hz the brown bread like products are very inefficient (that means costly and HEAVY to use effectively). Solution: defense in depth, use several products/ techniques to attenuate frequencies thus minimizing cost and weight.
I will talk about Thinsulate, it is a 3M product that absorbs noise and converts it to heat it is similar that way to fiberglass. Both have good R values but the Thinsulate is lighter, hydrophobic (rejects water and molds, and non-itchy.), has a better R value. Fiberous products are lousy at stopping structure born noise but very good at stopping noise from ~800 Hz to 20,000 Hz. BTW fiberglass and Thinsulate are MUCH cheaper and lighter than Dynamat or Brown bread
So my answer to your question would be to use a defense in depth strategy (use more than one insulation type) for that quiet and warm ride. | 
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