Insulation Between Roof Rafters

Unfaced insulation has no tabs and stays in place with friction between the fiberglass and the rafters.

Insulation between roof rafters.

This limits the maximum thickness of the insulation between the rafters to between 50mm and 175mm. Being lightweight they do not impose significant additional loads on existing structures. Once up at roof level the between rafter boards can be cut accurately to fit the rafter spacings. Then feed the rolled insulation under wires and pipe if either is present.

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. Place the plastic sheeting on top of the rolled insulation. 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 into. 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. As this requires fitting insulation above the rafters the roof covering will have to be removed in order to fit the insulation. Make sure it is flush against the wood as even a 1 inch gap can cause a 20 percent heating loss. If your attic is going to be converted into livable space then you will need to insulate the roof rafters.

For rolled insulation place it between roof rafters. Insulation can be installed between the rafters using a ventilated roof construction or an un ventilated roof construction. When you use solid insulation board to create a warm loft the key thing to remember is ventilation you must keep a 50mm gap between the roof and the rigid insulation board. To create a warm pitched roof insulation such as kingspan kooltherm k7 pitched roof board should be fitted between and over the rafters.

Modern u value targets mean installing insulation between rafters only is rarely a viable option. 7 staple sheets of 6 mil plastic over unfaced insulation to create a vapor barrier. We would always recommend having a breathable membrane in place between the tiles and the rafters too to allow the roof to breathe.