Insulation Between Roof Rafters

This limits the maximum thickness of the insulation between the rafters to between 50mm and 175mm.

Insulation between roof rafters.

Consider using high density r 30 batts which are as thick as r 25 batts but fit into 2x10 framing. 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. Unfaced insulation has no tabs and stays in place with friction between the fiberglass and the rafters.

A vent baffle should be installed between the insulation and the roof decking to maintain the ventilation channel. Once up at roof level the between rafter boards can be cut accurately to fit the rafter spacings. Whether you need to insulate your roof rafters will depend on what you intend to do in the space directly beneath the rafters. 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.

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. Common rafter depths range from 100mm to 200mm. You can also add rigid foam insulation under the rafters which adds r value and eliminates thermal bridging through wood rafters. Modern u value targets mean installing insulation between rafters only is rarely a viable option.