# Insulating Garage Ceiling



## Nealtw (Jun 22, 2017)

I am not sure you get full value if you compress it but it would be fine to fill the space. 
The subfloor above is considered the vapour barrier.


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## Bud9051 (Nov 11, 2015)

Converting a garage attic to livi8ng space has safety issues and needs appropriate permits.

No, compressing two layers of insulation into a 7.25 inch space does not yield the total of the two before compression. Insulation is an r-value per inch so 7.25 x 3.5 is closer to what you would end up with. That would be about r-23.

You will also have an issue with insulating the sloped rafter area.

Pull your permits and talk to your local officials about what is required.

Bud


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## 3onthetree (Dec 7, 2018)

A little bit of info missing on the specifics of what you are doing, but in general:

compressing batt insulation (mineral wool or fiberglass) negates its value
the R23 you have is 5.5" thick and made for 2x6 cavities
the R49 you want is 13.5" thick and is not intended for cavity placement, but for laying on top of an attic ceiling
if you are capturing space above a garage for a bonus room, 2x8 (nominal) floor joists are on the light side, a span nearing about 12' is probably inadequate


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## Nealtw (Jun 22, 2017)

Squeezing 11" into a 7 1/4" space don't seem cost effective. 
New houses here that have the garage below bedrooms have 2x10 joists filled with fiberglass, they could use more insulation than the space allows.


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## jukeboxpunk (Jun 9, 2012)

3onthetree said:


> A little bit of info missing on the specifics of what you are doing, but in general:
> 
> compressing batt insulation (mineral wool or fiberglass) negates its value
> the R23 you have is 5.5" thick and made for 2x6 cavities
> ...


My only option is to insulate with R30 for 2x8 joists. 

For the 2x8 spans being 12 ft, would sistering them beef them up?


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## 3onthetree (Dec 7, 2018)

jukeboxpunk said:


> For the 2x8 spans being 12 ft, would sistering them beef them up?


Generally speaking, yes. But there is so much more to think about, like the beam holding up the joist overlap, whether you have a pool table up there . . .


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## HuckPie (Nov 15, 2020)

jukeboxpunk said:


> Hello all! Hope you all had a great Thanksgiving.
> 
> I am finishing off my garage upstairs; however in order to do so I need to finish off the garage below and fireblock areas, add vapor barrier then rock.
> 
> ...


For 2x8 you'll need doubled-up R-15 fiberglass,
or just use a single layer the R-23 rockwool.
The only way you can get better than R-30 at this point is either add
rafter extensions, or use closed-cell expanding spray foam, or use rigid foam.


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## Knucklez (Oct 21, 2007)

my garage was originally designed for minimum code which allowed for 13' span using 2x6. when i told engineer i wanted to use the 2nd floor as heavy duty storage he changed the plan from 2x6 to 2x8 machine stress rated 24"OC. 

i had similar issue with insulating roof. what i did was rip the batts in half so i could layer it up without squishing. dunno if that was good or bad, but i can tell you that the hydronic heating system is barely on and it maintains the temp all day/night in Canada winter. i need to do a thread on this hydronic system someday...


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