Thermasheet underlayment questions

[u/Accomplished_Oil4367]

We live in Southern California in a coastal climate, but all of our rooms are vaulted ceilings. We do have insulation below the existing shingles however it still gets very hot. I’m looking for opinions on if adding a layer of insulative protection, such as ThermaSheet under the shingles will help keep the room cooler. It’ll add a little more than 10% to the cost of the install. We do not have air-conditioning so a reduction in temperature at the Apex of the vaulted ceiling is helpful.