Here you go JP. This is the way I did my roof.
First 2 layers of the Damp Pro.
Next 2 coats of something Second Skin Called "Firewall" I don't know if they still make this product.
It is used for both heat and noise reduction. Here in the Southern Calif desert where the temps
routinely hit 115 or higher in the summer, heat reduction was very important to me.
The stuff really does a job. I would say it keeps my cab 20-30 degrees or more
cooler than prior to installation. So the A/C doesn't have to work so hard.
This stuff is called Luxury Liner Pro. It's a closed cell foam noise barrier.
The combination of all this stuff reduced the in cab road noise a great deal.
Not as much as I had hoped for but a significant amount.
One I suggestion. Wait until the temps warm up a bit. I'd say about 65 to 70 degrees minimum.
This isn't really necessary, but because the Damp Pro is so thick, it's easier to work with at the warmer temps.
Also cut it into about 6" wide srtips. Again easier to work with that way. IMO