Insulation Between And Below The Rafters – Design Guidance
Condensation risks that are present will require the use of a vapour barrier. The vapour barrier is attached to the finished site of the insulation. The two layers of insulation will also need to be making sufficient contact to prevent the movement of air between gaps. A vapor resistance of 0.25MNs/g or less will be required of the underlay. The underlay can easily be draped over the corner battens or rafters. Ventilation beneath the underlay is not required, but proper air movement between the batten space is needed.
The installation of insulation under the rafters should be continuous. This is important when insulation is installed where the planes of the roof intersect at hips, valleys, and ridges. Spray foam insulation can be used to seal up junctions.
Ballytherm insulation needs to be cut properly to fit snugly between all the rafters. Metal tape needs to be used to seal joints where the boards meet. Avoid walking on boards at a job site to prevent damage to the product. They should also not be used as a platform for walking.
a. additional insulation to avoid thermal bridging b. insulation between inner face of wall 23 and joist