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I have 2 skylights in my master bedroom and they cause the room to get much hotter than the rest of the house in warmer months. Today it is in the 90s outside. My IR thermometer shows the area just under the skylight glass is 134 degrees! My central AC is struggling to keep the house cool. What can I do about this? I would prefer to do something about the cause (skylight), instead of something like putting a window A/C in there.
Where I live (southern Arizona) skylights allow too much brightness and too much heat into the house. The solution is to install a sun blocking screen that lets in lots of light but very little heat. Big savings in cooling costs! The screen is stretched over an aluminum frame.
IR meters are not the best measurement tool for this but adding a shade inside or out will reduce light transmission. The only way to eliminate the warmer area is to create a dead airspace since the heat from the roof is what you are reading, even putting a piece of cardboard over the hole will help (on the inside) but I doubt this will reduce the heat load by much.
Get a sheet of ~ 1/2" thick polyurethane insulation and cut pieces out that can be wedged with an upward curvature into the openings on the inside. You'll get no light, but it will reduce the heat gain. You might have to make a stretcher frame to hold the polyurethane. You might have to get flexible rods, thin wood, spring steel or fiberglass, to prop it from the inside.
Alternatively make a frame that will just fit in the ceiling opening. Attach the polyurethane insulation on the top of the frame.
I agree with @EdBeal and @JimStewart , adding anything in the skylight opening will help.
My favorite skylight to use is Velux Skylights and they have a great website. You’re looking for “solar reflective” shades and you can get them custom made. Here’s their site:
https://www.veluxusa.com/products/blinds
There are energy tax credits for using skylight shades and they can reduce your “load” by 39% (if you believe their propaganda).
If you’re a DIY er, you can make skylight inserts using styrofoam, but they’re not easily taken down or put up...you loose the purpose of having a skylight, but it’s very effective AND if you cover them with fabric , they can be attractive and match your decor.
Installing a piece of glass or acrylic flush with the ceiling will keep out much of the heat. And a "room darkening" window shade above it ( the shade lays on the ceiling level glass). This is a cheap but effective improvement. It will reduce heat with the option of reducing light.
