Installation procedure determines whether or not it lasts for a week. Luck and conditions determine how long it will last after that. Nothing compensates for its poor performance - it's just wishful thinking that has you believing it is performing as well as the Second Skin in your trunk. Here's what comparable ALF results look like for FatMat vs Dynamat Xtreme at 200 and 400Hz at 74°F:
FM 0.03 0.0349
DX 0.341 0.414
1.0 is a theoretically (but unattainable) perfect result. Dynamat Xtreme is an order of magnitude better than FatMat. Since there isn't a linear relationship between layers applied and performance, it isn't as simple as saying that it will take 10 layers of FatMat to equal the performance of 1 layer of Dynamat Xtreme (or Damplifier, V-Max, etc), but in my crude testing, 6 times as much FatMat still hadn't reached the damping performance of Dynamat Xtreme - it had just lowered the RF of the test panel to the point where I couldn't compare them with any confidence.
Since using that much more of an asphalt material negates any cost savings, the question of durability really doesn't matter any more. Some people will continue to insist that cost/ft² is the only relevant metric and choose asphalt. I'd urge them to search the boards for posts by Jacob from Sundown on the topic. He used to believe that asphalt could be installed reliably. His procedure involved carefully sized pieces of mat, heat and sealing the seams with aluminum tape. He has had multiple failures since then and now warns against using asphalt. Shon at ROE has reported similar results.
There was a time when using asphalt for sound deadening seemed like a reasonable choice. It was better than nothing in terms of vibration damping and many people were willing to risk the reliability questions. That time has long since passed. We have a much better understanding of why and how asphalt products fail in a car. We also know that asphalt is such a poor vibration damper that the perception of value is just wrong. The best way to save money is to use less of a quality butyl/aluminum foil product. If you planned to spend $100 on asphalt, instead buy $100 of a decent vibration damper and spread it out in a checkerboard pattern. It will take less work, add less weight, incur none of the risk, cost the same and outperform the "cheaper" solution.
One factor has confused people about the performance of asphalt. In those situations where applying a mat eliminates rattles by immobilizing adjacent panels instead of damping vibrations, asphalt will work as well as buyl/aluminum foil, as would a piece of duct tape, a 10 cent shot of silicone or even a piece of chewing gum.
You can't conclude anything about performance by comparing the FatMat in your doors and the SS in your trunk. To make a real comparison, you'd have to treat equivalent areas of the car and compare. Even then, it is tough to make a judgment in such a complex environment. Much better to do your testing outside the car, where you can control as many variables as possible. Anything else is anecdotal and really shouldn't be used as the basis for choosing or recommending.
