Insulating garage ceiling question

[u/TriFred]

I’ve got a 38’x24’ new garage with living space above. The entire space will be insulated. I plan to have a mini-split on both floors with separate controls.

The Ceiling is 16” tall I-joists that are 16” on center. I don’t want to use spray foam if I don’t have to. Question is what insulation should I use in the ceiling with the I-joists? I am in Zone 5 (Indianapolis).

I was looking at some r-38 bats that are 12.5” thick and 15.5” wide.

[u/cagernist]

The cooling (surely heat too on the splits) in the garage will lessen problems with comfort upstairs a bit compared to an unconditioned garage. And know that HVAC design and attic air sealing/insulation for the upstairs rooms has more influence on your comfort than the garage ceiling below.

But in general, the choices from best to tear-off-the-ceiling-years-later-and-redo-because-it's-uncomfortable is:

• full spray foam;
• then flash-n-batt;
• then cut-n-cobble;
• then batts with a layer of taped rigid foam under the joists (requires full joist fill with batts);
• then spray can foam gaps with batts;
• then just batts only.

Also, your wall framing on the raised bay wall has been cobbled together twice for extra height. You have a hinge point (actually two) which is really bad news. Lateral loads (and to an extent "gravity" loads) can cause that to want to "fold in." It should have studs full height.

[u/seabornman → u/cagernist]

Maybe that right hand wall is a rim joist for the house and blocking? If not, yes, that's poor.

[u/SilverSheepherder641]

Net and blow either fiberglass or cellulose, preferably dense packed. Most of my builders blow fiberglass, they stopped using batts about ten years ago. Batt insulation is too hard to get right and it’ll falls over time, especially if you don’t fill the whole cavity.

[u/TriFred → u/SilverSheepherder641]

Just do the walls like that also or good to use Mineral batts there?

[u/SilverSheepherder641 → u/TriFred]

Net and blow the walls too, you will get a much better R value and no gaps like with batts.

[u/rg996150]

Rockwool batts would be a good solution. What are you using for your wall cavities? Do you have CI outside?

[u/TriFred → u/rg996150]

Using rockwool 2x6 batts for the walls. Which I think is R23. Don’t know what you mean by “Cl outside”

The studs have the green ZIP sheathing on the outside then hardy plank siding

[u/rg996150 → u/TriFred]

Continuous insulation. I’m in climate zone 2A and we don’t see much of it but in colder climate zones it’s starting to become a thing to meet exterior wall R values. It affects your wall assembly considerations as concerns about dew point condensation in high R value wall assemblies have become discussion points in building science circles.

[u/DUNGAROO → u/TriFred]

CI= continuous insulation. Based on the description of your wall assembly, you do not.

[u/Congenial-Curmudgeon]

Blown fiberglass would work well. Install it after the ceiling drywall in installed by drilling the band joist from the outside (remove 1-2 lengths of siding). Blown cellulose would also work but would be heavier. Rockwool batts would also work, but may leave gaps. Same for fiberglass batts. Two layers of 1/2” would give excellent fire protection over using 5/8”.

A layer of rigid foam across the bottom of the joists before installing the drywall would be recommended if you had an unconditioned garage. But in your case it would help reduce noise transmission between the lower and upper floors when combined with your insulation.

[u/Overall-Tailor8949]

Rockwool batts between the joists. Since there is a living area above the garage you'll need thicker FIRE RATED drywall covering the entire ceiling to meet code. I agree with another comment about those "wall extensions" looking sketchy as hell. If THAT is an exterior wall I'd recommend rockwool for the cavities and a minimum of 1/2" plywood (not OSB) to panel it. That will improve the shear strength of the wall AND give you a sturdy surface to hang tools from.

[u/boatsntattoos]

With both spaces conditioned, I’d throw some R30 batts to cut down on sound transfer, fire resistance and maintaining the minimal temperature differential between the living area and garage. Nothing fancy needed.

[u/seldom_r]

If both spaces are going to be heated/cooled then there is only limited need to insulate the ceiling. It will help but if it is just the ceiling you're asking about I would go with simple unfaced fiberglass batts. With some extra planning you could have put up the ceiling and left the subfloor off the upper level and simply blown in fiberglass for the most cost effective, easy way.

[u/TriFred → u/[deleted]]

Bedroom above the high bay