I will give a simple and long answer. (take your pic)

"SIMPLE ANSWER"
Thermal wrap voids most warranties because it can lead to premature metal fatigue.


"LONG ANSWER"
Thermal wrapping amounts to a layer of insulation (being what keeps the underhood cooler), so more heat stays in the pipe and it stays hotter over a longer distance.

That means that the pipe will expand slightly more,(SS has the largest amount of thermal walk) and that the cyclic thermal stresses (from dead cold through starting the engine up and running it hard and cooling back down to cold again) will increase a bit.

Expansion of two pieces of metal at different rates.

For example, weld attachments where thermal expansion or contraction of one or both pieces of metal results in tensile stresses (strain) on the tube interior and/or exterior. During operation, the resultant stress can be fixed or cyclic. Stressed surfaces are more susceptible to cracking and corrosion (stress-assisted corrosion).

A layer of external insulation allows this variable to happen on a consistant basis and also introduces the variable of moisture inclusion due to the external exsposure to the outside air.

The amount of stress that is put on the manifold is directly related to how the car is driven and will detemine how long the manifold will last in these conditions.

Stainless steels are sensitive to thermal fatigue due to the unfavourable combination of high thermal expansion rate and low thermal conductivity.
The stress raised during thermal cycling is proportional to thermal expansion coefficient, elastic modulus and temperature differences.

Nascar and top fuelers normally change there thermal wrapped headers every 1-2 races as those cars see extreme cyclic thermal stresses in short periods of time.

A year is not very long in the life of a header in a daily street driven car.

However if the plans are to race the car on a regular basis (weekend warrior) the manifold should see temps in the area of 1300*-1500*F introducing a...

I AM SO SORRY!!!!! I tend to ramble when I am tired.

/hijack

A candle burning on both ends is twice as bright but lasts half as long.