When a building owner would like to leave the wood trusses exposed, insulating to follow the roof pitch is a good option. The thickness of insulation used for this option is dependent on the placement of the purlins. They can be installed either flat or on edge. This determines the depth or thickness of the cavity. It is important to fill the cavity, making sure the fiberglass touches the roof panel leaving no air space. This will aide in the prevention of condensation.
2×4 purlins are commonly installed with the 4″ face flat against the roof panels. In this case, insulation blankets are run perpendicular to the purlins, in the same direction as the top chord of the roof truss. It is important to know your chord depth. If your chord is a 2×6, it is recommended that a 8″ R-25 blanket is run filling the 5 1/2″ chord depth as well as the 1 1/2″ flat purlin depth, eliminating an airspace and helping to prevent condensation. The fiberglass insulation blankets are secured with steel banding installed perpendicular to the top chord.
Purlins Placed On Edge
Purlins are typically constructed out of 2×4’s. When placed on edge, this allows and limits for 3 1/2″ of depth for a single layer system. The insulation blankets are commonly run parallel to and in between the purlins. Given the 3 1/2″ of purlin depth, a 3 1/2″ R-11 or a 4″ R-13 fiberglass blanket is run between the purlins, filling the cavity. The insulation blankets are then secured with steel banding running perpendicular to the purlins, staples, or roofing nails with washers. *Note: If 2×6 purlins are used,R-19 insulation should be used to fill the cavity*