Answer: Canada's national building code requires 2.5" of ventilation between top of insulation and underside of roof sheathing. If you have a low-sloped roof, insulation can touch the underside of the roof sheathing at the eaves and block ventilation of attic. In this case, a solution like Raft-R-Mate® attic rafter vents is needed to ensure the required ventilation space of 2.5 inches.

If the slope of the roof is high enough that insulation near the eaves has a minimum of 2.5 inches of clearance between the top of the insulation and the underside of the roof sheathing, then they are not required

Raft-R-Mate® attic rafter vents are especially helpful when blown-in insulation is used because it is hard for even the best installer to get the specified R value all the way to the edge of the attic without blocking the air flow up from the soffit to the ridge or other vents. Fiberglass batts can be placed more carefully but there is still a risk that the remaining air space will be less than required by code and some of the air flow will be blocked.