OK, I've put this job off all year while it was warm. I need to insulate
the garage/shop attic. I have about 500 SF of attic space to insulate and
the joists are 2x12 on 16" centers. I'm trying to decide between cellulose
and fiberglass. From what I can determine, cellulose will be cheaper to
install and may perform better. In fiberglass' corner are built-in vapor
barrier and maybe easier to deal with overall. Here are the questions I
1. The manufacturer's instructions for cellulose only state the blowing
machine as a method of installation, but a few places on the net that I've
read state that if the rafters are exposed, you can just cut open the bags
and essentially dump the stuff in. Can anyone confirm one way or the other?
2. If I use cellulose, do I need to put down a plastic vapor barrier first?
Anything I'm missing?
The cellulose they put in my attic could not be "just emptied out". It
was more like a bale that the blowing machine chewed up and spit out.
The contractor said it was free to use with the purchase of the
cellulose insulation. I'm sure there was a deposit. It took one man in
the attic and one feeding the machine, which looked like a 55 gal.
drum with a blower and mixer inside.
From looking at it in the store, I get the same impression that it would
need to be mechanically separated as it appeared to be a compressed bale
like you said. But I've never really opened one up, so I wondered if it
just came apart when opened.
Blowing it in is a lot easier and quicker. The machine is normally free where
you buy the cellulose. (big box stores). You can do it by hand, but you have to
break it up as you do it and that is time consuming. You have to break it up so
it is nice and lose.
I just blew mom's attic. No way you are going to get that tightly compressed
celulose to work w/o the blower. Besides, Home Depot didn't charge for it as
long as I
brought it back with in 24 hrs.
It is common to break up cellulose insulation by hand and rake it into
place for very small areas around repairs. Perhaps a brick or two.
Like the other posts have mentioned it is not realistic for an entire
job and the end result will not have the loft and coverage like if you
use the machine. There will be large chunks and voids which is not
what you want. Additionally with the machine you can really minimize
your effort by being able to shoot it all the way out into corners
meaning you can hit large areas by standing in one spot.
With regards to the vapor barrier, it has been a highly debated
subject in the building industry but mainly with regards to spaces
that are conditioned in summer. I personally feel it is necessary
however depending on your situation it may be very difficult to
install. Typically if cellulose is to be used, prior to installing the
interior ceiling material you would tack up a poly vapor barrier. If
this wasnt done the only option would be to attempt to install it from
the attic which would likely be a nightmare and could pose some other
If your finished ceiling in the space is well sealed and painted, and
your attic is well ventilated you may opt to forgo the VB but you have
to remember that cellulose can hold a tremendous amount of moisture. A
few key things to remember are #1 Respirator, #2 a fan pulling from
the attic is handy to clear the air while your blowing, #3 Respirator,
#4 Dont block your air flow from the soffit vents into the attic (dont
pile insulation against the underside of the roof sheathing at the
outside wall), and #6 Respirator.
Here is a link you may find handy,
section 9.1.2 speaks to the VB
It sounds as if you want to put insulation between the rafters to
insulate the attic space above the garage. If this is correct, you
will want to use the FB Batts and staple them to the rafters. Or use
those aluminum faced insulation boards and tape to seal the space.:
The general idea with insulation is to wrap the conditioned living
space with a blanket that will slow the movement of heat - to the
outside in winter and to the inside in summer. If your conditioned
(heated and/or cooled) space is wrapped with insulation, then it is in
place to do its job. The insulation on the floor is slowing the
movement of heat to and from the rooms below. Insulation in the walls
of the new bedrooms is helping slow the movement of heat in and out of
those spaces. Insulating above the ventilated attic space won't help,
and keeping the roof deck well ventilated and dry will help it last
The original post states "the joists are 2x12 on 16" centers". It
mentions nothing of rafters. He is talking of a DIY cellulose
installation. This would not allow for insulating the underside of the
roof deck (between rafters) as there is no way for a DIY'er to blow a
The post sounds to me like this is a retro situation in that the
garage was built and put into use (finished ceiling installed) and the
insulation was left for cooler weather. This would mean there is no
way of stapling paper faced insulation to the joists. At best he would
be able to put the paper face down and live with the leaks in the VB.
Omitting the VB may be fine depending on other details not included in
Cellulose is likely a better fit.
Though possible this is a very difficult thing to do depending on many
factors. Framing accuracy, protrusions in the ceiling, mechanicals,
etc. It can often times be far less tight and will always be more
expensive especially with todays cost of ISO given petro costs.
I dont think there is anything in the post stating he aims to insulate
the underside of the roof deck (between rafters)
i>nsulation was left for cooler weather. This would mean there is no
You are correct. I might have made a poor choice of words in my initial
post by saying that I wanted to "insulate the attic". Indeed, I am just
putting insulation into the attic floor. FYI, I decided to go with rolls of
fiberglass, in part because I didn't want to mess around with hauling their
cellulose machine back and forth and having to deal with trying to keep
cellulose out of the the soffits. At this point, I'm probably 40% done.
Worst part is taking up the plywood that I had put down previously.
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