Insulation Between Roof 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.

Insulation between roof rafters.

This limits the maximum thickness of the insulation between the rafters to between 50mm and 175mm. 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. Common rafter depths range from 100mm to 200mm. 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.

Whether you need to insulate your roof rafters will depend on what you intend to do in the space directly beneath the rafters. A vent baffle should be installed between the insulation and the roof decking to maintain the ventilation channel. 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.

Modern u value targets mean installing insulation between rafters only is rarely a viable option. Once up at roof level the between rafter boards can be cut accurately to fit the rafter spacings. Being lightweight they do not impose significant additional loads on existing structures. Consider using high density r 30 batts which are as thick as r 25 batts but fit into 2x10 framing.