# Insulating Garage Ceiling w/ Access to Joists



## Windows on Wash (Aug 30, 2011)

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.


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## KpR (Dec 22, 2014)

Thanks.
I guess I shouldn't rule out a tear-down completely. Would you go dense-pack either way? Or batts if you went with a tear down?


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## Windows on Wash (Aug 30, 2011)

I would go either spray or rigid foam if I were tearing it down.


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## Gary in WA (Mar 11, 2009)

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


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## Windows on Wash (Aug 30, 2011)

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.


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## jogr (Jul 24, 2007)

Gary in WA said:


> 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.


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## Windows on Wash (Aug 30, 2011)

11" of dense packed cellulose is going to be super heavy.


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## Gary in WA (Mar 11, 2009)

Good catch on joist direction, jogr; thanks! I meant the joists were overspanned as well (but read it wrong). 1/2" drywall fastened 24" o.c. will carry 1.6# safely. Cellulose weighs (not dense-packing); http://www.greenfiber.com/step_one_-_calculate_your_need_how_to_install.html

Gary


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## KpR (Dec 22, 2014)

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.


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## Windows on Wash (Aug 30, 2011)

How are you on ceiling height?

Is it tight?


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## KpR (Dec 22, 2014)

I'm at 7'6" at the sheetrock now. Not too tight.

I have only about another 1" to come down from the current sheetrock though, before I'd hit the bracket for the garage door motor chain.


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## Gary in WA (Mar 11, 2009)

If you looked at Fig.7 in my #5 first link, XPS on the joist bottoms, then FG or whatever for cavity fill- leave an air space if you desire a warmer (to the toes) floor; http://www.buildingscience.com/documents/insights/bsi-064-bobby-darin-thermal-performance

Gary


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## Windows on Wash (Aug 30, 2011)

+1 to Gary's recommendation. Best of both worlds.


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## KpR (Dec 22, 2014)

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.


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## Windows on Wash (Aug 30, 2011)

Save the foil faced stuff. It won't serve any purpose here used in this assembly.


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## KpR (Dec 22, 2014)

Would you recommend any vapor barrier then? I thought the foil was what lowered the perm rating.


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## Gary in WA (Mar 11, 2009)

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


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## Windows on Wash (Aug 30, 2011)

I was referring to the radiant barrier portion of the ISO as noted by Gary.

I would stick with poly faced XPS. Should be cheaper and tends to be a bit more flexible as well. It will give you all the perm rating resistance that you need out of that assembly.


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## Ocelaris (Oct 9, 2012)

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.


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## KpR (Dec 22, 2014)

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.


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## Ocelaris (Oct 9, 2012)

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. 













































Finished


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## Windows on Wash (Aug 30, 2011)

Great work Ocelaris!!

Thanks for posting that up. Looks fantastic!


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