Insulation for the cabin ceiling? - Too HOT!!!!
If I can 'imagine' what you have, you have an ATTIC w/o a floor, if you know what I mean. In other words, you have a roof that ISN'T vented.
Yes, this is bad news. ALL that heat is now 'pushing' its way into the living space. There's only one way I know of to fix this problem, and it's the right way.
Since you have an 'exposed' ceiling, you need to add soffit or eave vents, then 'install' a true ceiling. After the ceiling is installed, the 'cavity' between the vents and the ridge will allow for ventilation. Yes, you'll need a Ridge Vent too. The Physics here is that air will enter in the Soffit/Eave Vents, rise up and cool the underside of the roof, and push out any hot air through the Ridge Vent. This process will happen naturally so you needn't assit it.
I am assuming that you have rafters on the underside of your roof, and that your roof is an A-frame type roof; not a flat one. A Professional Contractor can estimate the job for you and tell you what needs to be done.
If this won't work for you, post back w/more details. My best to ya and hope this helps.
Jay J -Moderator