Hello all, I am a frequent visitor / reader but this is my first question. I’ll try to answer the anticipated “details, please!” stuff, but let me know if I’m leaving something out…

I’m a licensed contractor, but primarily cabinetry, built-ins, light remodeling. So I know my way around most of this trade, and know to ask when I don’t! We are renovating / updating an attic “bedroom” — I use quotes because, with only a 78″ ceiling height I can’t call it habitable space. It was only 74″ or so but I tore out a central section of roof to widen an existing 4′ wide shed dormer on the north side to 8′, and match it with one facing south. This is over the kitchen “ell” in a 1910 farm house in the mid-Willamette valley in Oregon, so wet and cool from Nov-Apr, warmer and mostly dry from May-Oct. The side walls are 46″ tall, with a 9:12 roof pitch, so we have a decent amount of the room with the 78″ ceiling height (and my wife and I are both well under 6′!). The horizontal ceiling plane will be insulated with fiberglass batts both between and over (across) the 2×8 joists that span across the newly framed dormer walls; I’ve already installed duravent baffles as necessary to ensure air washing of the sheathing / airflow to the roof vents. Once we get this all closed in, there will be no access to the very small “attic” that will be left, so I want to know it is “survivable” once the interior paint goes on! I am fine with sacrificing some energy efficiency for that — in any case it will be better than the hollow walls and *maybe* R-5 in the ceiling we had before we started!

Because we don’t have the budget to really tear down to studs both in and out, I chose to tack on 2″ strips to the existing studs to get to 2×6 wall thickness, (or, in the case of the all-new gable end, frame with 2×6’s) and will seal – caulk and/or foam – and insulate (again, fiberglass batts) in accordance with guidance I’ve found here. (I know the limits of fiberglass in terms of installation quality, but as I am doing it myself, it WILL be a quality installation!)

Where I get hung up is in the original rafter cavities, which are also 2×4, 24″ OC. (Two rafter bays on either side of the dormer, so 4 times about a 45″ x 75″ surface area each.) The rafters are capped with skip sheathing (old, generous 1×4) and then OSB / felt / asphalt shingles over that. (The re-roofed / re-sided part got a real test immediately in Jan with storm after storm, and I am confident the exterior shell is sound in terms of water intrusion.) I would like to not tack on material to the rafters, or at least not much, in order to preserve as much headroom at the sides as I can. So, I want to max out my R-value per inch in those cavities, while preserving ventilation for the underside of the sheathing(s).

I will have two airtight, IC recessed lights for general lighting, and will ensure they are completely sealed at installation, so there should be no inside air getting into the attic space from this room, but the roof meets the main bay of the house and the two attic spaces are open to each other by way of a couple main rafter bays, so if there are places elsewhere in this old house where interior air is getting to the attic, it could certainly migrate. No moisture issues observed when opened up, so I just want to make sure I’m not adding any new ones!

My “plan A” — unless someone can show me it’s not a good one — is to install a series of strips of 1/2″ XPS parallel to the rafters, probably a couple 6-7″ wide, with 1/2″ between them, (and rafters), to allow movement past the skip sheathing, then fill over those, tight between the rafters with sheets of XPS or ployiso to get to the face of the rafters, sealing the edges. I am also seriously thinking about adding an unbroken layer of 1/2″ on the inside face of the rafters before drywall as a thermal break.

My questions:

– Where, if anywhere, in this assembly should I look to incorporate a layer of foil-faced polyiso?

– Should I leave any air space between the last layer of foam and the back of the drywall? As I understand, I really should if I’m using foil-faced polyiso as that last layer. How much air space will reap the benefits of using that? I would face the foil towards the interior, right?

I welcome any other ideas besides spray foam. Just not going there.

Thanks!