I have a 2×4 rafter construction home in Northern Wisconsin. I have heat loss through the ceiling. I can see snow melts and frost melts showing every rafter and spots in-between the rafters are bare from frost in fall and spring and snow loss in the winter. On the slanted part of the upstairs, there is fiberglass insulation probably 3 inches, then sheet rock. What is the best solution to reinsulate the slanted ceiling to gain a bit more insulation without losing too much head room?
Your question is not an easy one to address. I can only speculate on the details of your construction. Is the vaulted ceiling part of the roof or is there an attic space above it? If there is, and you can get into it or reach into it, installing a radiant barrier across the attic side of the ceiling will improve the thermal performance of the ceiling. If you can install the radiant barrier on the roof rafters, that would be much the better. If not, use a perforated radiant barrier to lay on top of the ceiling.
If the vaulted ceiling is the roof structure, which would be unusual indeed, the only way to improve without completely reconstructing the ceiling/roof is to add a radiant barrier in the 2×4 cavity. This might not even be possible — again I am hampered by not having more details. Another approach from the inside would be to install firring strips on the drywall and attach a radiant barrier to them, then finish with new drywall.