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Old 07-06-2018, 03:36 PM   #1
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Insulating Detached Garage Ceiling


Hello all. First post.

I have a detached garage in Southern California that I use as my pool hall. It's getting hot and I want to mitigate some of that.

The walls are insulated with R-13 and drywall, but the 15000 btu window ac can't keep up on hot days.

24" on center 2x6 joists (is that the right term) on a gable style roof. I want to put insulation in the joist cavities on the ceiling but I'm not sure what to use.

R-38 23"x48" is like 12" thick. Do I just jam that up in the space?

I'd appreciate any suggestions with how to proceed.
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Old 07-06-2018, 03:47 PM   #2
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Re: Insulating Detached Garage Ceiling


You need venting from the soffit to the ridge, after that you look at insulation.
Can you post pictures if peak inside and out and top of wall inside and out.
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Old 07-06-2018, 03:49 PM   #3
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Re: Insulating Detached Garage Ceiling


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Originally Posted by Nealtw View Post
You need venting from the soffit to the ridge, after that you look at insulation.
Can you post pictures if peak inside and out and top of wall inside and out.
I'll get some pics posted this evening. I was hoping to avoid having to deal with that kind of venting. If it helps I have an exhaust fan near the peak.

Thanks.
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Old 07-06-2018, 04:08 PM   #4
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Re: Insulating Detached Garage Ceiling


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I'll get some pics posted this evening. I was hoping to avoid having to deal with that kind of venting. If it helps I have an exhaust fan near the peak.

Thanks.
So that would work for the upper vent, you just need are to get in.
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Old 07-06-2018, 04:22 PM   #5
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Re: Insulating Detached Garage Ceiling


It's not realistic to think that detached garage's eaves and gable ends are sealed like a piece of vulcanized rubber. Just insulate and enjoy a poker game if that's occasionally available in your private pool hall.
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Old 07-06-2018, 04:26 PM   #6
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Re: Insulating Detached Garage Ceiling


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It's not realistic to think that detached garage's eaves and gable ends are sealed like a piece of vulcanized rubber. Just insulate and enjoy a poker game if that's occasionally available in your private pool hall.
That's my question though, what kind of insulation?

It's Southern California so we don't get the deep freezes, or tons of rain so I was figuring that simply insulating would do a good enough job.

That said, I don't want to throw good money after bad.
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Old 07-06-2018, 04:48 PM   #7
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Re: Insulating Detached Garage Ceiling


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Originally Posted by imtheomegaman View Post
That's my question though, what kind of insulation?

It's Southern California so we don't get the deep freezes, or tons of rain so I was figuring that simply insulating would do a good enough job.

That said, I don't want to throw good money after bad.
It is also about the heat of the roof and making the roofing last longer.


With out a ceiling you can feel the heat that comes from the room, that is why you have a fan. How hot would it get in a much smaller space above the ceiling?


The R value is just a measure of time. How long with it take for heat or cold to get to the other side. Even with AC in the space below, all the cold air goes straight to the floor and pushes any warm air to the ceiling where you need the cold to combat the radiant heat from the attic. Keeping the attic just a few degrees cooler makes a difference.
Then there is discussion about trapped moisture.
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Old 07-06-2018, 05:00 PM   #8
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Re: Insulating Detached Garage Ceiling


If I'm reading right the garage has 2x6 ceiling joists. If so I'd probably be looking at rock wool batts etc. to fill those 2x6 joists. In your area I wouldn't be concerned with a vapor barrier.
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Old 07-13-2018, 03:40 PM   #9
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Re: Insulating Detached Garage Ceiling


I'm not clear on your ceiling , do you have a finished ceiling that isn't insulated ? Or do you have no ceiling and when you look up you are looking at the rafters ? My 2 cents you need a finished ceiling ( drywall , metal ) and then add insulation . At a minimum go with Kraft faced 24 " on center from local big box store or lumber yard . R19 will be 5.5 " thick and will " fill " your 2 x 6 rafter cavities . The kraft face provides a vapor barrier . See how that works out , if you think it's better but still a little hot then you can add unfaced fiberglass rolls or blown in on top of the existing first layer . Ideally you would want about R38 - R40 . But there is no question you need good attic ventilation as mentioned by others .
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Old 07-13-2018, 05:11 PM   #10
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Re: Insulating Detached Garage Ceiling


Ok hopefully this makes things a little more clear:

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The one with the vent fan shows the peak of the roof and the fan of course. I ended up with some R-21 Batts.

I've been told to use some of that sheet foil insulation on top of that, not sure if that's worth the extra couple hundred it would cost.

Thanks for all the replies.
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Old 07-13-2018, 05:13 PM   #11
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Re: Insulating Detached Garage Ceiling


Also purchased an infrared thermometer. Was getting readings of 140 degrees on the surface of the ceiling.
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