Vent over roof over parallel cord roof trusses

This is a technical forum discussion, not news about a program or rebate, so there's nothing here that calls for homeowner action. A homeowner in Wisconsin planning a new house asked whether a particular cathedral-ceiling roof design would work for a highly insulated, energy-efficient home. The design used steeply sloped trusses packed with dense cellulose insulation, with a ventilated air gap built above the roof deck to let any trapped moisture escape, plus solar panels on the roof.

Building science experts who responded raised some concerns. They noted that the vent channel material the homeowner planned to use would block moisture from drying out, and suggested a more vapor-permeable material instead. They also said cellulose insulation is very hard to blow into this kind of roof cavity from the ends only, especially across a long span, and that a different insulation product, often installed from inside before the drywall goes up, would likely work better. One expert pointed out that local building codes do not allow this type of permeable insulation to sit directly against the underside of the roof deck in this climate zone, so the plan as described would need to change to meet code.

For most homeowners, the takeaway is simply that cathedral-ceiling and vaulted-roof insulation jobs are tricky to get right, and small choices about materials and ventilation can affect whether moisture problems develop over time.