What are these things made of?
This is at the heart of the matter. There are three basic compounds used for the adhesive layer of these products:
SBR(styrene-butadiene-rubber)/Asphalt
This is the least expensive material to use. It is predominantly asphalt with a latex polymer, the SBR part, added for stability. Asphalt has one virtue. It is cheap. They make roads out of it. Unfortunately, it has several negative features that limit its usefulness for our purposes. It melts at a relatively low temperature. It becomes brittle at a relatively high temperature. Both states are bad for us. Optimum viscoelasticity requires that the material not be liquid or solid. Basic mass loading requires that the material adhere to the surface we want it to. If it melts, it will fall off vertical or inverted surfaces. If it becomes brittle, it is susceptible to detaching under normal vibrations.
The rubber compound is added to the asphalt to stabilize it. SBR/asphalt melts at a higher temperature and becomes brittle at a lower temperature than plain or "neat" asphalt. This doesn't completely solve the problem. SBR is subject to long term deterioration at temperatures that are possible in a car.
When I started this investigation, one of my primary objectives was to figure out why so many people seemed to have trouble with their mat falling off. If you search the Web for sound deadener falling off, you will see lots of cases of asphalt mats failing and no failures that I have been able to find involving butyl based mats. It just isn't possible that user error would be so neatly divided.
I looked at the specifications for products developed for roofing, which many of the products in this category began life as, I saw the problem. Roofing materials are designed to withstand temperatures of 158°FX for two weeks without melting. Manufacturers supply butyl products for situations that will either exceed this temperature or duration. Extend the time and the maximum sustainable temperature falls. This explains why some people have had success with XasphaltX mats for a few years, only to have them fail in a subsequent year. Failure can be disastrous. If you're lucky, the mat will just fall off. If you're not, it will melt - pools of asphalt in your carpet or upholstery or window mechanism.
Another problem with asphalt mats is the smell. You should be fine during cool weather, but on a hot day, they tend to smell like, well asphalt.
Peel & Seal and Jiffy Seal are roofing products that fit in this category. Many people buy them for sound deadening. Of the products I tested, XB-Quiet Extreme, Brown Bread, Dynamat Original, eDead v1, eDead v1SE and FatMat are in this group. Many people report great success with one or more of these mats. They are certainly the cheapest option. If you know your car will never be exposed to extreme heat or cold, they may work for you.XI'm not suggesting that all of these products have a 100% failure rate under extreme conditions, but I do believe they are risky. Their narrower temperature range also means they will be less effective the farther they get from room temperature. I would not use any of these products in my car.