Rockwool/vapor barrier and open ceiling.

[Catch_22]

I have tired to research this as much as I can but would like some input from garage specific minded people.
I'm currently building (having built) a detached stick built 24*26 garage.

I plan to insulate it myself before/during the winter and intend to install a mini-split in the future. I would only heat and cool it when in use and mostly during extreme temperature swing months or extended use. I live in Tennessee so temperatures swing to 95+ in summer and as far as the teens in winter but mostly hang around 30s. Humidity is fairly high.

I want to use mineral wool in R15 in 2*4 walls and R23 or R30 in the ceiling 2*8 joists. I have read varying opinions about the need for a vapor barrier with rock wool in the South. Some say you always need a vapor barrier, others say its a norther climate requirement that's ideas bled in as a standard. Thoughts?

Secondly I have read varying opinions about venting behind the the sheathing on the roof for the soffit/ridge-vent. If required is there any opinions on accuvent cathedral vents? Or is there a better way?

Thanks for any contributions and ideas.

 

[ForceFed70]

When you say "South" where do you mean?

Generally, you are right. In Canada a vapor barrier is needed. Whereas in Florida you're probably better off without one.

 

[Catch_22]

When you say "South" where do you mean?

Generally, you are right. In Canada a vapor barrier is needed. Whereas in Florida you're probably better off without one.

I live in Tennessee

About there.

 

[ForceFed70]

About there.

Lol.

 

[BadgerBoilerMN]

The first question is; what vapor.

If you don't create humidity you won't produce vapor pressure. Vapor barriers--most retard rather than bar--are primarily intended to control unwanted frost, where the vapor meets dew point inside the wall stack up. The end result can be mold.

Rockwool will not support mold but your studs will. I would add a half inch of Thermax to the exterior walls to increase your R-value and kill infiltration--the main load in many poorly built structures..

Thermax the rafters for an effective radiant barrier.