i'm wanting to use rigid foam board instead of cellulose or fiberglass.   what is the best way to go about this.   i'll have trusses on 24" centers  and a 9'4" ceiling.   could i screw 2x4's perpendicular into the trusses then use long SIP screws  and plastic foam washers to screw through the foam into the 2x4's.   we're talking about 12" of foam board.      

I think I need a picture...

What is going to be the finish ceiling below this foam, and how do you intend to support it? (14" screws?)

Posted By Dana1 on 29 Aug 2011 05:33 PM
I think I need a picture...

What is going to be the finish ceiling below this foam, and how do you intend to support it? (14" screws?)

A photo would help,rather picturing what did you do. :)

haven't done anything yet so there are no pics. i was thinking acrylic stucco as a finish.

Is this what you are after? Ridiculously easy, if time consuming. Nail up temporary furring strips. Cut xps to fit. Wedge it in there. Foam the top, caulk the bottom. Repeat. I have four inches between 2x4 trusses. Dunno how you would get more because the cross bracing on trusses prevents you from switching directions. I blew cellulose over the foam. Took the furring strips down. No problems. Check with your building inspector before spraying acrylic stucco. It may not meet fire code.

Make that "...WILL not meet fire code." Half-inch gypsum does though.

Methinks he's talking about 12" of foam board screwed to the underside of the truss chords, not 2-3" between joists.