# insulating living space over garage



## Chokingdogs (Oct 27, 2012)

Going to threadjack, slightly, so as not to start a new one with a similar topic.

I also have an attached garage with living space over it, and I know for a fact the ceiling ( floor over garage ) has no insulation. 

Question is - the joists have 2X4 "X" bracing, on either end of the spans, about 10 or so feet in from the walls, a total of about 22 to 24 sets of "X" braces. I want to use Roxul.....will/should I cut the Roxul carfully, with little pieces too I assume, to fit the X snugly, or can I leave the X's open, devoid of insulation? If I leave them open, that'll mean a cavity of say 16" X 7.5" X 7", roughly 2/3rd of a square foot of empty ( un-insulated ) space.

Also, the lumber is old, not the current standard size. What's there is best described as a 2X8, but rather than being 7.5" they're 7 1/8". Roxul for a 2X6 frame is too small, a double of 2X4 insulation would get close, but would still leave an air gap of around 3/8"? 

Total ceiling area is about 25' X 17', with the joist bays running the long span. 

Thanks


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## Gary in WA (Mar 11, 2009)

No problem, started you your own thread!

IMO use Roxul, 3-1/2" thick in US- R-15. Cut triangles using a pattern for that area. Use foamboard under the 5/8" thick Type X drywall required per minimum safety code. http://www.roxul.com/residential/products/roxul+comfortbatt%E2%84%A2

Fig.7; http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces/

Gary
PS. Thanks, Joe!


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## Chokingdogs (Oct 27, 2012)

Thanks....got "busy" yesterday afternoon....hockey then NASCAR, so didn't get a chance to re-post.

I forgot to mention the ceiling in the garage is covered already, plaster over gypsum, so I'd be filling the voids between that and the subfloor in the room above. Also going to construct kneewall storage since there's so much unused space due to the long ( 25' span ) span and high-ish roof pitch, with the batts extending to the end of them.

I did mess around on Roxul's site and see they have R30 batts which are 7.25" thick, those would fill the voids with an extra 1/4" or so to compress....would those be a better alternative than the R15 which leaves a void of around 3.25"?

Since the garage ceiling is finished, and I'm not looking to tear down 400 or so sq ft of plaster and hang drywall, could I lay strips of foil backed foam board in the joist cavities with the R15 Roxul on top, similar to the figure 7 in the link you gave?


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## Nailbags (Feb 1, 2012)

Chokingdogs said:


> Thanks....got "busy" yesterday afternoon....hockey then NASCAR, so didn't get a chance to re-post.
> 
> I forgot to mention the ceiling in the garage is covered already, plaster over gypsum, so I'd be filling the voids between that and the subfloor in the room above. Also going to construct kneewall storage since there's so much unused space due to the long ( 25' span ) span and high-ish roof pitch, with the batts extending to the end of them.
> 
> ...


My question is this how are you going to access the joist cavities? you have a floor over the top and sheetrock on the bottom? explain to me because I don't understand how your going to run batts in there with out tearing up something?


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## Chokingdogs (Oct 27, 2012)

Nailbags said:


> My question is this how are you going to access the joist cavities? you have a floor over the top and sheetrock on the bottom? explain to me because I don't understand how your going to run batts in there with out tearing up something?


I'm going to rip out the subfloor in the room, in stages, lay the insulation, then put down new subfloor. I have to, basically, in order to air seal the couple of ceiling boxes and put in a fan box.


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## Nailbags (Feb 1, 2012)

Chokingdogs said:


> I'm going to rip out the subfloor in the room, in stages, lay the insulation, then put down new subfloor. I have to, basically, in order to air seal the couple of ceiling boxes and put in a fan box.


I would just do the Rock wool and what your plan is and you should be fine.


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## Gary in WA (Mar 11, 2009)

Sorry on the late return. "I did mess around on Roxul's site and see they have R30 batts which are 7.25" thick, those would fill the voids with an extra 1/4" or so to compress....would those be a better alternative than the R15 which leaves a void of around 3.25"?

Since the garage ceiling is finished, and I'm not looking to tear down 400 or so sq ft of plaster and hang drywall, could I lay strips of foil backed foam board in the joist cavities with the R15 Roxul on top, similar to the figure 7 in the link you gave?"-------- Yes, forgot the *2 times of 3-1/2" *in my answer, but I was thinking it, lol. Yes, the f.b. (PIC) and canned foam to fill the 1/8-1/4" gap all around will keep the cavity insulation dry by warming the condensing surface of the foam preventing condensation. With all the moisture from the vehicles being freshly parked and warm engine/brake pads/hood/exhaust system, would put warm moisture vapor in the air rising up onto the plaster ceiling, diffusing through- right to the sub-floor plywood/wood to condense there. Board would also decouple from cold garage slab/vehicles similar to article. Vapor wouldn't harm Roxul as it would f.g.(R-value), but why add moisture there when f.b. will completely stop it...

Gary


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