Help with insulation please UPDATED!

[tome14]

Buy on PC of 2" XPS, lay it across 3 rafters, and see if the sag bothers you. Seams taped, it would make a big difference. Honestly, just poly would make a difference, so I guess reflectix would too, but 2" of XPS a lot more. Don't use anything with a flame inside garage. An electric resistance heater might cost several bucks to run each session, but may be worth it. (I wince that my fitness center only costs me 75¢ a day, but the gas for car is over a gallon for a round trip.)

After a year if sag bothers you add a ceiling joist between each pair. BTW are the jousts supported at all from roof rafters?

I was going to use 4x8 foam boards with a foil facing towards the garage floor, but I keep getting told it’s not to code..

RMAX Pro Select R-Matte Plus-3, 2 in. x 4 ft. x 8 ft. R-13.1 Foam Insulation Board 637898 - The Home Depot

Rmax R-Matte Plus Polyiso Rigid Foam Insulation Board is perfect to use with wall sheathing applications in new residential and commercial buildings.

www.homedepot.com

this has been frustrating because it’s the only thing holding up my build

 

[racecougar]

I would tape the seams yes.. there are soffits but there aren’t any holes for insulation… there also appears to be wood on the inside of them

The issue is, if you go forward with creating an attic space, you don't have any lower venting. Hence the suggestion to add soffit vents.

 

[tome14]

The issue is, if you go forward with creating an attic space, you don't have any lower venting. Hence the suggestion to add soffit vents.

Oh ok that makes sense.. the Door would open at times since it’s only a part time simulator.. if I leave the Foil a couple inches short on the perimeter to allow air to flow to the box vents and would still see some benefits Or is that a bad idea?

 

[billconner]

The issue is, if you go forward with creating an attic space, you don't have any lower venting. Hence the suggestion to add soffit vents.

I agree it's not to code and does present a small fire hazard. I was guessing you might not be pulling a permit and could get by. The foil faced polyiso is much less flammable than EPS or XPS, so good. It's also requires a much higher temp than even wood to ignite. I wouldn't worry about it if it were me.

As far as venting, I don't believe you'll build up much moisture based on your intended limited use and heating. It will have days at a time to dry. Worst case if you see condensation on the roof deck underside, add a gable vent or a roof vent.