Put 1x4's up every 2 feet horizontal and fasten to them over the shingles...That gap lets heat escape...
This is what we did over 20 years ago. The electricity usage for A/C went down by half. We used the silver shiny Galvalume that reflects heat very well. The 1X4s should not be treated wood because the treat starts corrosion and the warranty would be void is what I was told. We have about 50 inches of rain a year and so far so good.
