Here is a link to an article that explains the right way to build an unvented insulated roof assembly.
How to vent a roof without soffits.
For an attic without soffit vents you may have the option to use gable vents instead.
Be certain that the hole is between the rafters.
Place the roof vent over the hole.
Since your roof is plagued by ice dams however it s clear that your roof does not perform well.
By virtue of their design and location on the roof ridge vents are predominantly exhaust devices.
A ridge vent without a soffit vent doesn t work and here s why.
Install special roof intake venting products that work at the lower edge of the roof decking underneath the first course of shingles.
It s possible to build an unvented roof one without soffit vents or ridge vents that performs well.
Typically you would have a gable vent on each end of the attic.
Drill a hole inside the shape for the jigsaw blade.
The only way that i am aware to ventilate the roof is to still have the ridge and soffit vent system but you have long baffles in between the roof sheathing and the top of the insulation.
Smart vent a special roof eaves vent product that provides a 3 4 opening about 6 above the top of the roof drip edge.
Rose a research architect at the building research council at the university of illinois replies.
Remove the shingles in the area where the vent is to be installed.
In most cases all you have to do is remove about one inch off the top of the fascia the part that covers the end of the rafters on the roof install the drip edge finish with a vent that s on the top of the roof and you re good to go.
Venting solutions for roofs with no overhang or soffit.
How to build an insulated cathedral ceiling.
Determine the shape of the hole needed for the vent.
Draw it on the roof deck.
The venting drip edge seems to be the easiest and most common way to go.
Normal vented air likes to enter the attic lower down near the bottom third of the attic and as it passes through it will exit higher up near the top third of the attic space.