A cold roof is another roof framed over the top of your existing roof.
For a cold roof.
Tear off the existing roof, repair any deck damage and cover with underlayment, regular old felt is fine but you can use any underlayment you wish.
Lay 2"x4"'s over the existing rafter/trusses and secure them with nails or screws.
These 2"x4"'s that were just laid down will provide for 1 1/2" if clear space for airflow.
Sheet over the 2"x4"'s and proceed as usual.
For intake ventilation you can use some vented drip edge at the pitch transition.
Now the air will flow between the 2 roofs with no blockage.
Your original roof deck is now the warm side and the new roof deck that was framed over the old is a cold deck.
Any heat or humidity that emanates from your home will be vented between the 2 roof decks.
So in theory the new roof deck never gets warm enough to create ice dams.
Constructing a cold roof is usually pretty easy and straight forward.
Since 2" of height has been added to your roof there will be some facia work that needs to be done.
Instead of re-doing the facia sometimes you can just use a bigger edge metal to cover the extra 2", it depends on the home and what look you prefer.
This type of roof construction is ideal for cold climates.