how to attach 3" of Iso foam board insulation? (Fasteners?)

[nereo]

Such an easy question, but I can't find an answer

Q: How do I attach the second layer of 1.5" polyisocyanurate to a wall?

Scenario:  I have an unheated garage that shares a wall with the interior living spaces (the garage wall is the heating envelope).  There's one section that was 'skipped over' during the initial insulation push.  5/8" OSB over 2x4 studs.  My plan is to put two layers of 1.5" polyisocyanurate foam board with staggered seams.  First layer is attached with 2" plastic-capped nails.  Problem is with the second layer.  My inclination is to use Liquid Nails to glue the second sheet to the first sheet, or possibly anpother adhesive.  but I know many adhesives melt dissolve foam board.
 
I can't find plastic-cap nails long enough to go through all 3" of foam plus at least 1/2" of the OSB (would need to be 3.5" minimum).  Even if I could I worry that with only 0.5" of a 3.5"+ nail it wouldn't be terribly secure.

I know lots of people put 4" (sometimes more!) of rigid foam... I'm just at a loss for what fastener they are using.

[Fishindude]

Use roofers screws and plates.   
Roofers deal with this on a routine basis when they install thick insulation to a flat roof deck.   They use and extra long screw and a 3"-4" round or square, light gauge, galvanized metal plate.
Just google it and you can see a picture of what I'm talking about.

I would also use an adhesive between the sheets.  Find something that won't melt the foam.

[Papa bear]

Long screws with plastic washers. Don’t overdrive them.


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[Dogastrophe]

You can use Loctite/Lepage PL300 Foamboard adhesive

[nereo]

Use roofers screws and plates.   
Roofers deal with this on a routine basis when they install thick insulation to a flat roof deck.   They use and extra long screw and a 3"-4" round or square, light gauge, galvanized metal plate.
Just google it and you can see a picture of what I'm talking about.

I would also use an adhesive between the sheets.  Find something that won't melt the foam.

Thanks - that's what I was looking for.

..and this...

You can use Loctite/Lepage PL300 Foamboard adhesive

Use roofers screws and plates.   
Roofers deal with this on a routine basis when they install thick insulation to a flat roof deck.   They use and extra long screw and a 3"-4" round or square, light gauge, galvanized metal plate.
Just google it and you can see a picture of what I'm talking about.

I would also use an adhesive between the sheets.  Find something that won't melt the foam.

Thanks!

[nereo]

Follow up:

While my local big-box stores only carry plastic-capped roofing nails up to 2”, they did have a box of hot-dipped galvanized metal plates hidden way up top near the roofing screws. Was pleasantly pleased with the price ($3.80 for a 1lb box... which appears to hold maybe 100 of these thin metal plates).

So basically I can mix/match screws/nails with these plates. Problem solved.

Also had a mini-stroke going through all the different types of construction adhesive/caulk.  First estimate is there’s more than 50 varieties.  Again, found PL-300 and it seems to work as it should.  I almost bought PL Premium which also said it could be used for rigid foam boards, but then saw a box of the PL-300 Foamboard Adhesive way up on the top shelf.

Trip to the store took me almost an hour, mostly searching for the nail caps.  Actually insulating the space (two layers of Polyiso - four sheets total) took me about as much time.  Go figure ;-)