Siga Majrex T&G Ceiling – Drywall or Not?

This is a homeowner forum question, not a news story, so there's no program or policy update to report. The person posting is finishing an attic ceiling in a cold climate and asking other builders for advice on their own project.

The homeowner is insulating a sloped ceiling under a vented attic. They plan to use rockwool batts plus blown-in cellulose for insulation, and a product called Siga Majrex as the air barrier — a membrane that stops drafts while still letting some moisture pass through the ceiling. Over that, they want a tongue-and-groove pine finish. Their original plan was to add a layer of 5/8-inch drywall between the Majrex and the pine, mainly for fire safety and some sound dampening. Now they're considering skipping the drywall and instead building a service cavity — a gap held open by strapping — behind the pine, which would protect the air barrier from being punctured by nails and wiring, but would add several more layers of material and weight to the ceiling.

Their question, posted to other builders and building-science readers, is whether cutting the drywall is a reasonable trade-off or a mistake — specifically whether it does enough for fire protection or sound control to be worth keeping. At the time this was captured, no one had replied yet, so there's no answer to pass along, just the question itself for others weighing a similar ceiling assembly in their own attic project.