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Insulating Garage Ceiling w/ Access to Joists

6K views 22 replies 5 participants last post by  Windows on Wash 
#1 · (Edited)
I am trying to decide on the best way to insulate the ceiling of my garage/floor of the living space above the garage.
The ceiling of the garage is already drywalled, but I have access to most of the joists in the ceiling (see picture).
There is some pretty pathetic looking fiberglass in there now. And there are some joists supports in each joist about 10 ft from the accessible wall.

I'd prefer not to have to tear down the drywall. I was thinking of either A) fiberglass bats cut to size and somehow pushed in or B) blowing cellulose--but was unsure if I'd have to have dense packed done in order for it to be efficient.

I'm in central NJ, right on the cusp of Zone 4 & Zone 5.
The dimensions of the garage are 15 ft across (in the direction of the joists) and 22 ft deep.

Any thoughts? Thanks.
 

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#13 ·
#20 ·
We just went through the same thing last year, and were very happy with the results, R30 batts fiberglass batts in the ceiling, R13 fiberglass batts in the 2x4 walls, 1/2" foil backed Poly ISO, then 5/8" drywall on top. The air sealing with the rigid foam made a huge difference, garage is basically a conditioned space now. I can't remember why we went with foil backed Poly ISO, but I don't think at the 1/2" it was much more expensive.
 
#2 ·
If you don't want to tear it down, dense pack it. That is going to be a bunch of material given the depth of the joist but that is the only way to do it with leaving it intact (majority).

Be sure that the ceiling can handle that weight as I am going to bet you are going to have issues.
 
#5 ·
WoW, if he dense packs, wouldn't he have a lot of moisture from wet vehicles from rain/snow that would try to dry to the garage with the vapor retarder built-in from plywood glue? I second your second choice of foam/board, fig.7;http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces/ and it would decouple it from the garage floor.

I agree on the added insulation weight also with those 2x12? joists spanning 22' and only 16" on center spacing, I doubt they are SS grade. Check side of joist for an ink stamp, look for #2 on it; http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_5_par020.htm

Gary
 
#7 ·
WoW, if he dense packs, wouldn't he have a lot of moisture from wet vehicles from rain/snow that would try to dry to the garage with the vapor retarder built-in from plywood glue? I second your second choice of foam/board, fig.7;http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces/ and it would decouple it from the garage floor.

I agree on the added insulation weight also with those 2x12? joists spanning 22' and only 16" on center spacing, I doubt they are SS grade. Check side of joist for an ink stamp, look for #2 on it; http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_5_par020.htm

Gary
Gary, he said the 2x12 joists run the 15 ft length so the weight of the insulation is unlikely to be an issue unless the drywall itself can't support it.
 
#6 ·
Yes.

Not a fan of the dense pack option for multiple reasons.

Despite the building science objections to dense packing ceiling assemblies, I have pulled roofs that were fine over conditioned spaces that had dense packed roofs.

These were sloped composite shingle roofs. I would never do it over a flat roof and this more resembles that scenario except the cold side is reversed.
 
#9 ·
#10 ·
Thanks for the replies and the helpful insight.
The joists are 2x10 (actually measuring a hair over 9") and 16" OC.
I've decided to go with the full tear down and am trying to decide on insulation type.
I'm leaning toward rigid foam, but think I'd have to layer it pretty thick to fill the 9" of space, prevent a large air pocket and achieve any significant R-value. Is this thinking right?
Would rolled, faced, fiberglass be out of the question? Again though, a roll of R-30 FG is only 6.5" thick, and I'd have another 2.5" of air to take care of. Should I combine the two?

I'll be re-installing the sheetrock as well.

Any further wisdom or ideas? I'm trying to keep this DIY which is why I'm not considering spray-foam.
Thank you.
 
#15 ·
Thanks a lot.
I must have forgotten that I looked at figure 7 when you posted it the first time.
That's the way I'll be doing it--probably with 1/2" foil faced polyiso or XPS and unfaced r-30 rolls before putting 5/8" sheet rock back up--air sealing any gaps/holes/cracks before hand of course.
Thanks again for the guidance. I'll be back with more questions as I move through the rest of the house I'm sure.
 
#18 · (Edited)
Perhaps WoW was thinking that without an air space the foil wouldn't radiation decouple the floor from the concrete slab .... IMO, use the faced as it is a vapor barrier which is exactly what you want in your application. Hot exhaust, warm engine parked just after driving in rain/snow will deposit a large amount of moisture on the ceiling, you don't want that in your FG with only 1" of foamboard because of the higher perm rating (vapor open) than 2" XPS. As per article is fine, it mentions above a garage there as well.

Gary
 
#21 ·
Thanks again Gary and WoW.
I'll be price comparing foil and poly faced XPS tomorrow and will be moving on it by New Year's.
Ocelaris: thanks for commenting. I was reading through your thread from last year. I wanted to ask how the project turned out but haven't hit my requisite 15 posts for a PM. Good to know you're happy with the results.
 
#22 · (Edited)
I just dropped you a line with some links to pictures. One thing you might consider is putting some OSB behind the drywall in some places so you have something to screw into. It's very difficult to find a stud with insulation over drywall. On the inside walls we put Poly ISO, but on the outside walls without 1/2 poly iso we put OSB behind the drywall to attach shelves, hang ladders etc... Here's a quick run down, and my PM has links to more.










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