Skylight Insulation // Vancouver Island BC, CZ 4C – GreenBuildingAdvisor

First time posting on GBA, I must give everyone here a giant thank-you for all the effort put into this website and the wealth of information it provides.

We are in the process of resheathing our existing cathedral roof and replacing the concrete tiles with a standing-seam metal roof. We took the opportunity to reframe rafters/headers for two 36×36″ curb-mounted skylights, and we are now realizing how cautious we should be with insulating the lightwell and 6″ curb. The skylights are near the ridge of this 15ft cathedral ceiling, so getting up there frequently to wipe off moldy corners is high on the “to avoid” list! Our plan is for this to be our forever home, so we are more than happy to put in the effort needed to create a bright, healthy, low-maintenance, and low-risk home. 

Roof Assembly: (8:12 pitch) 

– (new) 24 gauge ProLok standing seam metal roof

– (new) Full Peel & Stick Roofnado HT Underlayment

– (new) 1/2in Plywood Sheathing

– (new) Vertical 2×4 strapping laid flat for ventilation gap

– (existing) Horitontal 1×4 strapping (previously used as battens for the concrete tiles)

– (existing) 2×6 rafters 24OC

– (new) R22 Rockwool ComfortBatt Insulation

– (existing and just tightened up) 6-mil Poly interior vapor/air barrier.

– (existing) 5/4″ Tongue and Groove Cedar Interior Ceiling (potentially also main structural roof deck?)

– (existing) Exposed Structural Beams/Trusses, about 55″ OC

We have been researching and asking various people about adequate insulation requirements in this climate zone to mitigate condensation and mold growth in the lightwell. We have received all kinds of conflicting advice, going from “nothing needs done, just slap drywall on the framing, and you’ll never have issues” to “you must insulate both the exterior of the curb and the interior of the lightwell w/ XPS or you will have condensation and rot issues”.

We have noticed how an active (and inspiring) member of this community named Malcolm Taylor lives nearby and seems very experienced with our local cold and humid climate, so we are hoping he or others experienced with skylights in Climate Zone 4C could provide some guidance on the amount of insulation needed for a condensation-free skylight and lightwell for this assembly.