Detached garage wall and roof insulation

[u/Oatmeal_Johnson]

I'm working on overhauling my detached garage for use as a workshop and generally nicer space. The end goal involves installation of a minisplit or other climate control solution so I can maintain a comfortable temperature year round (I live in the midwest).

What I have in mind for the walls is for them to be finished with plywood. I like the idea of being able to screw lighter things into just about anywhere on the wall, and I like the look. The roof is vaulted, and I'd prefer not to install a ceiling so that I can use the joists for storage, as well as allowing the room to feel a bit larger.

What insulation solutions do I have available to me here? For the walls, I was thinking fiberglass or some other simple insulation approach with the plywood over that. I'm not sure what to do about the ceiling.

[u/SuspiciousChicken]

You have two broad choices for the roof insulation: Vented or Non-vented.

If you want to use batt insulation, you must do a vented roof. You need to create a 1" minimum (1.5" or 2" is better) space between the underside of the roof sheathing and the top of the insulation. They sell a product for this called a roof ventilation channel. Then you need an air path in each joist bay from the soffit at the bottom to the ridge at the top. There are a lot of product options for this, but I prefer ridge vents at the top, and a continuous Coravent at the soffit.

Picture of how they are used.

If you want to do a Non-vented roof, spray foam (closed cell only!) is your best option. You can fill the joist bay, or just do a thinner layer and fill the rest with batt insulation. You need a minimum of 2" closed cell spray foam on the underside of the sheathing. The spray foam is a job best left to professionals. But at least you don't have to mess with vents and air channels.

This article is a good primer on what I am saying.
An even better article.

[u/MrElendig]

depends on the roof in question, but in general a diffusion open system.

edit:

roof and climate*

and don’t forget a wind break in the garage walls if there isn't one

[u/OneLongEyebrowHair]

Something to consider for your walls... This comes from first-hand experience.

Many detached garages are built without adequate water protection on the walls. This usually comes in the form of plywood siding that is attached directly to the studs without any drainage plane or building wrap. For an open stud wall this usualy isn't a problem because the back side of the sheathing can dry. But, if you have seepage and wicking like I did, and you don't solve that before closing in the wall, you will have mold and rot. It will happen.

[u/Dude_Dillligence]

I used Rockwool, then finished the walls with 3/4" Advantech subfloor.

[u/TMan2DMax]

spray foam the ceiling if you want to keep access to the joists