Insulation and Moisture Control Layer for Finished Shed
I have a customer wanting to insulate and drywall a TuffShed in climate 4c. It’s going to be part time living situation, just sleeping/hanging out, no bathroom or kitchen.
The siding is LP panels with no exterior WRB, all 2×4 framing. After speaking with a few people and looking up some information I’m planning on kraft-faced R15 in the walls. For the ceiling I’m thinking I’ll fill the cavity with rigid foam, 2” and 1 1/2”. I’ll most likely be installing mold and moisture resistant Sheetrock and telling the customer to run a dehumidifier after it’s done .
Does this seem like a reasonable assembly to prevent moisture from building up in the walls and ceiling?
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Since you don't have any WRB or sheathing, here for the proper way to insulate the walls:
https://www.greenbuildingadvisor.com/article/managing-water-and-insulating-walls-without-sheathing
Since this is not a full time building, you can take some shortcuts though. In this case, you can staple some house wrap (tyvec or felt) behind the siding in each cavity and insulate with safe and sound batts. These are only 3" thick and would give a small space behind the siding to allow for airflow and drying. Make sure not to push them in against the siding, try to keep it flush on the drywall side.
You don't need any facing for the batts on the interior, painted drywall or osb is fine.
For the ceiling there is not much point for the cut and cobble foam. It doesn't increase the assembly R value and since it is not above the roof deck, it doesn't help with moisture. Lot of work that doesn't get you much benefit and sometimes can even cause moisture issues.
The simplest is standard batts and install some soffit/ridge vents. Since the place is not occupied full time, you don't need to go overboard on venting, just need a bit to help with drying from night time condensation. Make sure to keep a gap between the batts and the roof deck.
Dehumidifier is definitely a good thing.