As far as we could tell with my buddies chebbie Dynamat doesn't have anything but reflective insulation cabability (heavy aluminum foil cover) - its mostly a sound deadener. Made his stereo easier to hear with the Flowmasters barking - but didn't help the temp much at all. He just put the factory style carpet padding back over it, so that's all the insulation he really has. When sound proofing - which goes along hand in hand with floor insulation - you need at least two out of four of the following features: deadening, insulation, isolation, - or a barrier. Because insulation does both, it usually becomes number one on everyone's list.
But because deadening needs to be applied directly to the tin - it goes on first. Dynamat and Fatmat are two known brands of the stuff - both have a heavy plastic/rubber compound that sticks down on a steel, plastic or cardboard panel and changes its vibrational frequency. Since it is basically a "weight", thicker sheets seem to work better - I've been using the Fatmat product called "Rattletrap" which is half again thicker than the other stuff. there are other products that work well as a deadener - this isn't an advertisement. On the web you will find some articles about sound deadening small aircraft - they are very weight concious people these airplane guys - have a lot in common with race car guys. Advice from them - only use deadener where it counts mosts - don't sheet the whole floor, just use it in the center of open panels. I can vouch for this - it worked well on my Z28.
For a second layer - apply an insulator. A lot of rod shops and internet sources sell an automotive padding - about 3/8 thick with mylar aluminum facing. It works good for the price - cut easily with old shears and glues down with 3M upholstery cement. These fiber type insulators are just like house insulation - thickness rules in order to get R-factor. There are foam type insulators that are more effective per unit of thickness than the fiber - also lighter. Worth checking them out - but pay attention to their fire ratings.
Good Luck finding the right stuff for your job! There are a lot of products out there - just don't try to rely on a single solution.