Insulating above unconditioned garage

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mikeyitis
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Insulating above unconditioned garage

Post by mikeyitis » Sat Oct 07, 2017 6:56 pm

Hi Shannon,

I have a question about insulating the ceiling of an unconditioned garage with a bedroom above it. I am going to insulate the floor joist cavities above the garage space and was wondering if you could help me decide if I should use R-30 or R-38 insulation. The thickness of the cavity is 9 inches and I want to get the highest R-Value in the space as possible. I have been doing some research and found if I compress 12 inch thick R-38 down to 9 inches it will give me an R-Value of roughly 32. The other type of R-30 insualtion that I am looking at is 10 inches thick and would result in about a 28 R-Value if compressed in the 9 inch cavity.

Would you suggest that I go with the R-38 and compress it down 3 inches or go with the R-30 and compress it 1 inch? I live in Minnesota, so it does get cold in the winter. Thank you so much for any advice.

- Alex


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A. Spruce
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Re: Insulating above unconditioned garage

Post by A. Spruce » Sat Oct 07, 2017 10:10 pm

How do you intend to retain the squished insulation?
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mikeyitis
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Re: Insulating above unconditioned garage

Post by mikeyitis » Sat Oct 07, 2017 11:09 pm

I am going to button it up with 5/8" fire rated drywall.

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A. Spruce
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Re: Insulating above unconditioned garage

Post by A. Spruce » Sat Oct 07, 2017 11:14 pm

Well, I can tell you that if the insulation bulges so much as an inch out of the bays, hanging the rock will be quite the experience, it is really hard trying to smash 32 square feet of insulation at once, let alone overhead and trying to install fasteners without blowing them through the drywall.
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Shannon
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Re: Insulating above unconditioned garage

Post by Shannon » Sat Oct 07, 2017 11:19 pm

I do not like the idea of "squishing" batt insulation at all. I'm not sure where you got R numbers for compressed insulation but compressing it that much is not really wise IMO. Why not use a combination or R12 and R20 batts That would fit a 9" cavity and give you a total of R32 with no real guessing because of compressing it?
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mikeyitis
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Re: Insulating above unconditioned garage

Post by mikeyitis » Sat Oct 07, 2017 11:26 pm

Yeah that sounds like a good option, I will look into that. Thanks.

I got those insulation compression figures from this chart:

http://www2.owenscorning.com/literature ... lletin.pdf

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Shannon
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Re: Insulating above unconditioned garage

Post by Shannon » Sat Oct 07, 2017 11:39 pm

interesting, I have never seen that chart before.
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diytexas
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Re: Insulating above unconditioned garage

Post by diytexas » Sun Oct 08, 2017 1:58 am

A. Spruce wrote:
Sat Oct 07, 2017 11:14 pm
Well, I can tell you that if the insulation bulges so much as an inch out of the bays, hanging the rock will be quite the experience, it is really hard trying to smash 32 square feet of insulation at once, let alone overhead and trying to install fasteners without blowing them through the drywall.
That would be awful. Drywalling ceilings by itself is already unpleasant enough

I certainly would rent one of these, even if help is available. Save the frustration

https://youtu.be/OWj32SZFY-M

mikeyitis
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Re: Insulating above unconditioned garage

Post by mikeyitis » Sun Oct 08, 2017 10:30 am

Yeah, I definitely would rent a lift. I have done it before without one and it isn't easy. It is worth paying the $30 to rent one from Home Depot for 24 hours.

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A. Spruce
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Re: Insulating above unconditioned garage

Post by A. Spruce » Sun Oct 08, 2017 11:52 am

A lift will certainly help, but it will still be quite the bugger if you are trying to mash insulation with the sheet of drywall. Pushing insulation in with your hand is one thing, that's less than 1 square foot of resistence, trying to push 32 square feet of resistant is quite another. Think of it like trying to carry a sheet of plywood in a wind storm.
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mikeyitis
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Re: Insulating above unconditioned garage

Post by mikeyitis » Sun Oct 08, 2017 1:02 pm

If I go the R-38 route and compress it, I will probably install insulation ties to keep it compressed to make it less of a hassle to put the drywall over it. The reason I am liking the R-38 batts is because I can get it at Menards for a great deal after sale price and rebate. The sale ends today though, so I am trying to make up my mind. I might just buy it and return it later if I change my mind.

However, I might just put in two different sizes to get the R-32 value like Shannon suggested, so I don't have to compress it as much or at all. Not sure yet.

The asbestos test results for the old drywall and insulation that is in place right now won't be in until next Friday anyways. I figured I would get it tested before I start dropping a bunch of dust on top of me from building products installed in 1966.

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A. Spruce
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Re: Insulating above unconditioned garage

Post by A. Spruce » Sun Oct 08, 2017 1:15 pm

Good choice to have the existing stuff tested! 8-)
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mikeyitis
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Re: Insulating above unconditioned garage

Post by mikeyitis » Sun Oct 08, 2017 9:02 pm

I ended up getting 500 sq ft of R-11 kraft faced fiberglass rolls and R-21 unfaced batts. The R-11 is 3.5" and the R-21 is 5.5", so it is a nice exact fit at 9".

Now I am just hoping that the joists are mostly very close to 16" on center so I don't have to contend with anything that has a gap wider than 15". The R-38 I was looking at was 16" wide and the rolls and batts that I got are 15" wide. Not sure if having a space that is slightly wider than 15" will matter or if I would have to fill the space with a sliver of extra insulation to prevent a gap on the side?

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Shannon
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Re: Insulating above unconditioned garage

Post by Shannon » Sun Oct 08, 2017 11:12 pm

If your joists are spaced properly you should have no problems. If not you will want to cut some slivers .
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