Although there are many different.
Insulation between roof rafters.
A vent baffle should be installed between the insulation and the roof decking to maintain the ventilation channel.
Being lightweight they do not impose significant additional loads on existing structures.
Where the insulation does not fill the total rafter depth the low emissivity foil facings can gain additional thermal benefit from the air spaces they face.
Consider using high density r 30 batts which are as thick as r 25 batts but fit into 2x10 framing.
Unfaced insulation has no tabs and stays in place with friction between the fiberglass and the rafters.
This generally would not be a sufficient thickness to achieve the u values required to meet building regulations and more insulation would need to be added.
7 staple sheets of 6 mil plastic over unfaced insulation to create a vapor barrier.
This limits the maximum thickness of the insulation between the rafters to between 50mm and 175mm.
It allows insulation to be installed in a watertight roof either after the roof covering has been fixed on a new build or without disturbing an otherwise sound existing roof covering as part of a renovation.
Once up at roof level the between rafter boards can be cut accurately to fit the rafter spacings.
Common rafter depths range from 100mm to 200mm.
For example if the space beneath the rafters is going to be used for a living area then you will need to insulate the roof rafters.