Velux is very specific on installation. I recommend you read the instructions on their site.

Roof35 - thank you. The problem is that the installer said that the existing 25-year-old curb fits with Velux requirements. I am trying to guess how to pinpoint where the problem is actually coming from. Is it the old curb itself, is it the fit of the skylight onto the old curb, or the flashing, or?

I called Velux and they told me that the curb installation is between the contractor and the customer, and not Velux, and that Velux will only guaranty and instruct regarding the skylight itself, but not the curb.

I rented a thermal camera from Home Depot. What it shows is that everything is a cool temp except for the perimeter/border of the shaft just where the skylight attaches to the curb. In that perimeter it's showing bright red in the thermal camera. Everything else - the glass itself, the shaft, the ceiling - are green or blue in the camera. Outside temps at the time were about 80-82.

I showed it to the skylight contractor, and he said he cannot explain the red perimeter. Do any of the experts here have an explanation? I am wondering if the size of the skylight and the curb don't match, or there is some kind of sealing problem between the skylight and the curb. I am hoping for some expert advice from this forum. TIA.

"I showed it to the skylight contractor, and he said he cannot explain the red perimeter."

Either he's lying to you, or he's not fit to be a contractor. It needs insulation around the curb. They should insulate it with foam board and cover it with some EDPM membrane.

Thanks Geoffrey - I suggested to the contractor that it looks like it needs insulation around the curb, though I had not yet seen your post. He said he didn't want to mess with the roof membrane which is also why he left the old curb there. Does insulating with foam board and covering with EDPM membrane mean altering the roof membrane in any way?

You need to get a guy that does EDPM

That sounds good Geoffrey, but I have no idea how to find one. I tried googling, but about 100 roofers came up, no idea which ones do skylights. When I combined the search with the word skylight, no contractors came up.

The owner of the company has gotten back to me. He wants to fix the problem, which is really great. He says he will remove the roofing and the outside plywood cladding at the skylight curb, then replace the existing (old) fiberglass insulation with new spray foam insulation which has a much higher R-value, and install new plywood. Does this sound like it will correct the problem? The red area on the thermal camera pics was at the perimeter where the skylight attaches to the curb, and a few inches below that, then was cool for the rest of the skylight shaft.