How does this detached garage ceiling insulation plan sound?

[vanvanvanvan]

I've read a bunch on here and gotten some helpful advice that has lead me to a plan for insulating a garage ceiling. But before I buy the insulation, I wanted to double check that it isn't mistaken.

Garage details:
Size: 20 x 20 ft, four sided low pitch roof that is about 4" above the ceiling joists at the center. Full sizes door on one side, regular door and small window on the other
Type: Detached, one wall abuts neighbors garage
Region: Chicago
Intended use: Workshop + 1 parking space. It has electricity, don't yet intend to actively heat it in a major way.

The garage was built in 2020, and they included two square-ish roof vents at the peak, but the eaves are wood and have no ventilation. So I bought 9 aluminum vents and was planning to drill out 3 of them per side. I can only do 3 sides, because one wall is so close to the neighbors wall that it doesn't have an eave. As for insulation, so far I've sealed all the gaps and put R-19 batts in the walls, all of which are drywalled now (but not taped or finished.)

My current plan, based on advice here, is that I need a ceiling to close in the space, and then need to insulate that, while allowing airflow in the attic from the eaves to the top vents. To do that, I was thinking I'll run furring strips perpendicular across the 4 ceiling joists that currently span the width. Then, as a ceiling covering, I thought I could do two birds with one stone and lay down foam sheets across the furring. I'd need about 12 4 x 8 sheets, and I'm trying to decide between 1/2" in. R3/4 or 1" in. R6/7. Seems to be $12 for 1/2" in. and $20 for 1" in.

Then, on top of the foam, I'd roll out R30 fiberglass insulation. The best I've been able to price so far is to get 15" in. x 25" ft. rolls; I'd need three of them side by side in between each ceiling joist, and would have 5" ft. leftover from each roll. I'd need fifteen total, but with that excess, I think I could do 12 and then combine all of the 5" ft. leftovers into about 2.5 more pieces. I might be able to get that for $240.

What I'm looking at pricewise for the insulation then is $390~ if I do R30 on top of 1/2" in. foam sheets, and $480~ if I do R30 on top of 1" in. foam sheets.

If anyone has any criticisms or suggestions on this plan, I'd love to hear them. Thanks!

 

[DieselNut88]

The builder did not install vented soffits? If not you can install the vents. In the attic you need to install the plastic or foam chutes on the roof sheathing so the insulation does not block the air flow from the soffit. Put them in every other or so space. No need for furring strips and foam. I would put insulation in the ceiling joists and then go perpendicular the other direction with r30. You can buy metal strapping to hold the insulation up. Cheaper than buy foam.

 

[Mr.N]

Then, as a ceiling covering, I thought I could do two birds with one stone and lay down foam sheets across the furring. I'd need about 12 4 x 8 sheets, and I'm trying to decide between 1/2" in. R3/4 or 1" in. R6/7. Seems to be $12 for 1/2" in. and $20 for 1" in.

This is key... either use foam glue and glue every seam together or go with a 1/2" gab and use expanding foam. Your goal is to make an air tight seal. Either way, around the perimeter make a 1/2" gab and use expanding foam.
My vote would be for FOAMULAR 3/4 in. x 4 ft. x 8 ft. R-4 Tongue and Groove with glue. Then R-30 on top. Also chase any gaps...

 

[vanvanvanvan]

Thanks for the advice!

Yes, the builder did not install vented soffit, just those wood pieces with no venting, despite installing the roof vents. So there is now airflow in at the bottom, but there is at the top. I got the vents and am going to install them shortly.

I have the foam chutes and the every other tip is good; that'll cut down on the cost. The foam point about cost makes sense. But I did find a supplier a couple hours away that makes it tempting still: Discountfoaminsulation . com
They sell factory seconds and assuming they aren't garbage, $9 for 1" sounds pretty great. I'm considering making a trip there to get it all, and then I'd stick with R-30 on top like Mr. N suggested.