Get several quotes. They will vary significantly.
If adding initial insulation to the garage or lanai, you also need baffles along the soffit. Some quotes may not include them.
Solar attic fans may not cool the attic effectively. Often placed next to an existing open vent, it pulls air into the attic through what should be an exhaust vent, and then discharges it. Net effect close to zero. Be skeptical of pretty pictures with big blue and red arrows. Air flows the path of least resistance.
An electric powered attic roof fan moves a lot more air. The often cause the interior of the attic pressure to be a lot lower than ambient. That can pull air conditioned air from inside your house into the attic via cracks, gaps, outlets, exhaust vent holes, HVAC pipe holes. etc. That forces replacement air to come in from hot outside air. Net is a hotter interior. Every house is different.
R38 is now the standard. That is roughly 12 to 14 inches thick of blown in insulation. At that amount, adding more still improves overall, but it is a small amount. At today's energy prices, typically not cost effective. What tomorrow's cost will be are harder to know. It's known as diminishing returns. If R50 is better, then how about R80? Or R125? There is a point when the cost to do something is more than the benefits it causes.
Insulating a garage does not "cool" it. That is a badly worded conclusion from using incorrect terminology.
In the summer daytime, heat moving downward from the ceiling warms the space. Insulation in the attic prevents radiant heating in the day, and also blocks radiant cooling at night. Garage (without AC) will never be cooler than the outside temperature, except in the early morning when the quick rise in outside temperature is faster than the static temperature of all the stuff in the garage that cooled off overnight. Insulating the garage causes the inside temperature to stay more steady. The may or may not be cooler, depending upon the season and time of day.
If the garage door faces north, away from the sun, insulating it may do the unexpected. On a warm day, when the ceiling is radiant heating the garage (say to 100deg) from a hot attic, and the outside temperature is 85deg... then the garage door surface would be cooler (near outside air temp) than the interior garage temperature. That large cool panel is helping keep the garage cooler because it does not have insulation on it.
If heating or cooling a garage or lanai, insulate it.
Looking at temperatures with an IR thermometer sometimes gives false readings on metal or glass surfaces. They reflect similar to a mirror. That issue can be seen easily with a thermal image camera.
So beware of generalized recommendations that are not correct in all situations.
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