How to Best Insulate Garage Ceiling

rwbil

I need to upgrade the insulation throughout my house, but I am hitting the worst areas first. There is living space above the unconditioned garage, which has virtually no insulation in the ceiling.



The garage ceiling is comprised of 10” joist spaced 12” O.C. There is also some 9” Fiberglass batt insulation, but no sheet rock or other covering, so it is not enclosed (thus allowing air movement).



Spray foam is too expensive. So here is what I am thinking about and wanted some feedback.



First I want to stop all air infiltration. So I am thinking about caulking around the Band Board and then cutting and placing in 2” Rigid Foam next to the band board and then using the Great Stuff to seal that.



Then I am thinking about blowing in Cellulose insulation. From everything I read Cellulose is a superior product for air flow and R Value over fiberglass.


When I do some research I keep reading the weak point are the joists. They have an R Value of 1 per inch. Should I worry about this? I was thinking I could run 2x2s parallel to the 2x10 joist, then only the spots where the 2x2s hit the 2x10 would I have a low R value. Would the improved insulation value be worth all the work?


I want to dense pack the cellulose as best as I can. I was thinking about installing a house wrap as an additional air barrier. And I could then make holes in it to dense pack the cellulose. I was planning to install OSB on the ceiling. I could make holes in the OSB to dense pack, but thought that would look bad. I might add 1/2" Rigid foam to dense pack if the house wrap does not work.




Would I need to seal the joints in the OSB? How best to do that and make it look right?


Do I need a vapor barrier and should it be on the garage side or the room above side. I am located in Jacksonville Florida. I read the vapor barrier should be on the hot side during the winter, but the vapor barrier on my outside walls is on the outside.



Any thoughts appreciated
Robert

Windows on Wash

Are you just talking about the garage kneewall areas?

Foam blockers that are spray foamed will stop the air. No need to dense pack and you won't be able to effectively do it. Stopping the air is about 85% of the problem.

Spray foam seal the sill plate and any wall penetrations.

Back the wall with another layer of fiberglass or rigid foam.

No vapor barrier required.

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rwbil

No knee wall.

I sprayed Cellulose before and built a rig to dense pack. Now can I gurantee I met the actual dense pack requirement, No. But it was defineately more densely packed as I heard the machine whine and I slowly moved the pipe down. I dense pack to avoid settling and I do not think there was going to be much settlement after I dense packed.

I would love to seal the SIL plate, because that is another area for leak penetration, but there is sheetrock on the walls and T1-11 on the outside. Any suggestions on how to do that?

I was thinking about installing 1/2" Tuf-R. It has a silver side that I think is a vapor barrier, so does it matter if I have a vapor barrier? Again it is just the ceiling I am insulating.

You mentioned, "Back the wall with another layer of fiberglass or rigid foam". I was not planning on insulating the walls. Sheetrock is on the walls and I doubt there is insulation in them. The garage door has openings everywhere and I was not planning on trying to insulate the garage itself. Should I??


Also there is some Fiberglass batt insulation already installed, but from what I read it is nearly impossible to install that properly. Too Short, Too Long or around pipes and wires all problems. I had two thoughts. Keep the fiberglass in cavities with no wire and etc. or move it to the attic above the joist, where it might work better.

Windows on Wash

I think I am misunderstanding what you were doing.

Shoot me a picture. I am a bit slow today.

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MT Stringer

This morning, a company came in and blew R38 cellulose into my garage ceiling. However, the attic is low so no possibility of having anything above the ceiling anyway. Glad to have it insulated finally. I already had a contractor insulate the outside wall when they changed out our siding two years ago and the garage overhead door was replaced with an insulated energy efficient model.

rwbil

Attached 2 photos, showing living space above garage and garage ceiling with partial insulation.

Attached Thumbnails

   

Windows on Wash

Block, seal, and spray foam that outside ribbon board/band joist.

Typical fiber insulation (high density fiberglass or roxul) will work inside that joist bay fine. If you want more R-Value, rigid foam cut to fit works as well.

Air tight drywall and you are done. If you wanted to go an extra step, you could thermally uncouple the joists by covering them with rigid foam and then dry-walling but that is up to you. That will also help with garage noise.

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rwbil

Maybe I am being overkill, but I keep reading about people insulating their garage ceiling and the room above still feels cold or hot.

From what I read no matter what you do you can not seal around electric boxes and etc. with batt insulation and stop air flow. Thermal imaging always seems to show cold or hot spots around those penetrations.

I guess I could put up 1/2" rigid from, cut a hole near the boxes and spray foam around the boxes to get an airtight seal. Is there a better way?

High Density fiberglass and thick ridgid foam seem expensive, which is why I liked cellulose. I can buy it pretty reasonalbe from a local distributer. 1/2 the price for what HD and Lowes sells it for. Plus it has better R-Value than Fiberglass and seals better around electrical boxes, wires and etc.

I saw a video of a builder who built two simiiar houses. One he used batt fiberglass and the other one blown in insulation. They did the door test and there was not comparison. The blown in cellulose results blew away the batt fiberglass. And he took caution to seal around the batt where it hit electric boxes and etcs.

Is there a reason you do not favor it?

Windows on Wash

You will always have a slight thermal weak point as those locations because of the lack of insulation depth in those locations (i.e. electrical boxes, etc).

As long as they are air sealed, you are fine.

Seal around the boxes and a Roxul will be much easier to install than dense packing the ceiling.

Stop the air flow at the outside wall and you are good.

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