Existing situation –

We have a cabin and outbuildings built by my wife’s great grandfather in 1930. We like the character exactly as it is. This is a vacation home, used a few months in the summer, a few weeks in the winter. The building is not heated when it is vacant. We want to make the 20’x18’ garage space into two bedroom spaces with a high vaulted ceiling, and insulate the building. There are no interior walls or insulation. We would only add a toilet and sink. No shower, fan, nothing creating lots of moisture. We would not add an air conditioner, but would add electric wall heaters for the winter. In eastern Oregon, climate zone 5, in the summer it averages a high 85, low 45. In the winter high 40, low 25.

 For the proposed CEILING, we want to insulate the ceiling. Right now it has 2×6 rafters on 24 inch centers that that are exposed in the peaked roof. We want to keep the top and bottom chords and web exposed and have an open ceiling when we are done insulating. Now the roof has 1×8 board sheathing, felt paper, and 35 year old wood shingles. The old shingles will come off for a new architectural asphalt shingle roof.

 Insights in Martin Holladay May 10, 2019 article “Sandwiching roof sheathing between two impermeable layers” gives some good insight to put rigid foam on the exterior sheathing.  https://www.greenbuildingadvisor.com/article/sandwiching-roof-sheathing-two-impermeable-layers.   My proposed insulation from bottom to top,  I put on 1×5 pine paneling (R-0.96) directly onto the 1×8 sheathing, (but it seems like I need some type of air or vapor membrane here if we do it this way?), leave the current 1×8 sheathing on (R-1.25), then on the outside of the roof sheathing, put on 2.5” EPS (R-10), 2” EPS (R-8), then 1/2” plywood (R-0.63), roof membrane (type?), asphalt architectural shingles (R-0.44) = R-21.28.

 Since this is a stabilization of an existing building and it is only used a few months of the year, insulation requirements for a vaulted ceiling greater than 8 inches nominal rafter depth is R-21.  (We have 2×6 rafters) The article talks about R-49 for a flat ceiling in a new building. How should I approach an open vaulted ceiling and the R-value? It seems like putting the insulation on the exterior of the roof sheathing would meet this objective. Especially if we want to put ponderosa pine 1×5 bead board paneling on the vaulted ceiling. If we put the EPS insulation on the inside of the 1×8 sheathing, we would lose the visual effect of seeing the existing 2×6 rafters. The 20’x18’ garage has a simple pitched roof, but on one corner is an 8’x8’ attached outbuilding. It doesn’t seem like that should complicate the roofing detail too much? Your thoughts for a successful insulated ceiling?

 And the article by Martin Holladay, November 15, 2019, “Still fighting the same battles, 20 years later”, https://www.greenbuildingadvisor.com/article/still-fighting-the-same-battles-20-years-later says cathedral ceilings can be difficult to seal. And this definitely makes me NOT to want to use any closed cell spray foam in ceilings or walls, in case changes need to be made in the future, and these materials would be harder to change or remove. So the easiest might be to just make a flat ceiling instead of keeping the look of a cathedral ceiling?

 Condensation, air barriers, vapor barriers versus vapor retarders. I want to get this right for the ceiling.

 In the primary use period in the summer, the electric heater might be turned on a bit in the morning when there is a chill, but by 10, everything is warmed up. Should there be much concern about condensation on any of the interior ceiling material surfaces?

 By the fall, the water and heat is turned off, and the buildings are locked up for the winter.

 The garage will now be used for sleeping too when we have larger groups. A building that will now have insulated, walls, ceiling, and foundation. Sitting empty and unheated, it should get to be as cold as the surrounding in the winter, maybe 10 to 40 degrees. So when we arrive in the winter, we would go inside to heat up the space with electric wall heaters. That should warm it up and reduce the humidity, but that will push any moisture in the inside into the walls and ceiling? We want the moisture pushed to the outside so it doesn’t condense on a surface on the interior of the walls and ceiling? Then the space is heated for a week or two. At the end of this use, the heat is turned off, the door is shut, and the garage will sit unheated for a few months until spring or summer use again. We want the building so it can more easily be kept warm in the winter, and also be kept cool in the summer. And also don’t want condensation on the interior. How can we successfully do that?

 I would appreciate your comments.