Insulation Between Roof Rafters

Once up at roof level the between rafter boards can be cut accurately to fit the rafter spacings.

Insulation between roof rafters.

This limits the maximum thickness of the insulation between the rafters to between 50mm and 175mm. This roof type will require a breathable membrane like kingspan nilvent to be. As this requires fitting insulation above the rafters the roof covering will have to be removed in order to fit the insulation. To create a warm pitched roof insulation such as kingspan kooltherm k7 pitched roof board should be fitted between and over the rafters.

Common rafter depths range from 100mm to 200mm. 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. If your attic is going to be converted into livable space then you will need to insulate the roof rafters. Being lightweight they do not impose significant additional loads on existing structures.

Modern u value targets mean installing insulation between rafters only is rarely a viable option. 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. 7 staple sheets of 6 mil plastic over unfaced insulation to create a vapor barrier. Then feed the rolled insulation under wires and pipe if either is present.

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. Unfaced insulation has no tabs and stays in place with friction between the fiberglass and the rafters. Make sure it is flush against the wood as even a 1 inch gap can cause a 20 percent heating loss. For rolled insulation place it between roof rafters.

Place the plastic sheeting on top of the rolled insulation. Install ventilation baffles into the space between the rafters before you install the insulation usually by stapling into place. We would always recommend having a breathable membrane in place between the tiles and the rafters too to allow the roof to breathe.