DIYbanter - View Single Post - insulation after drywall for ceiling? or before?

Astro

wrote:
I am about to drywall and insulate my detached garage ceiling. Instead
of stapling the kraft faced pink fiberglass insulation to the ceiling
joists, I was thinking I could save a step by sheetrocking the ceiling
first and then dropping in the insulation (paper side down) and save
myself the stapling. Will I have a vapor barrier issue, or is this a
bad Idea?

This is a bit off topic to your question, but since you mentioned vapor
barriers, I figured it an appropriate thread to inject a comment.

Is the garage going to be a "conditioned space" and have moisture
sources? Will you be using the attic for storage or will it be totally
isolated from the downstairs? Do you live in a hot moist environment or
a cooler one?

A vapor barrier in a garage doesn't make any sense to me. Typically, a
vapor barrier is to prevent moist air from getting into a cold space,
condensing on the cold surfaces, and leading to moisture issues.

In a garage, there's not much of an air envelope so the inside air
would appear to be no different than outside. Perhaps with the doors
all closed, the garage will experience less temperature and humidity
swings than the outside, but overall, it's very different from a house
that is filled with moisture sources that will drive humidity through
the walls/ceiling.

That said, I've not seen many garages that have air sealing between the
attic space and the main garage. Hence a vapor barrier on your ceiling
will do nothing since the humidity throughout the garage would be
relatively uniform.

If I were in your shoes (and I sort of am because I'm just about to
build a detached garage), and I wanted to use the attic space for
storage (which I do), I would put a perforated radiant barrier directly
under the roof deck then insulate the upper cavities so that the outer
shell of the garage is insulated.
It sounds like you're further ahead on the project however. In that
situation, I'd still use a radiant barrier under the roof, then blow
cellulose into the attic space and avoid the pink stuff altogether.