Installing insulation under addition.
First off I wouldnt mix the two insulation types. A Dewpoint could occur in the batt insulation and cause mold inside the home.
Secondly a vapour barrier does not stop all moisture. Getting rid of moisture out of soil is impossible unless you want to spend big bucks and its not mentioned anywhere in this post. Your soils ability to drain water through drain tile (if you have it) or proper soil materials to provide drainange is key to keeping an area dry.
Heres the easy solution and it avoids a crap load of work.
If you want this space heated run your ductwork in the joists (call a mechanical person to look at this for you)
Insulate the joist pockets w/ 4" of extruded, at perimiter joists, foam in place (or do the exterior side of the deck and clad it with something to hide the insulation. (I would insulate the pockets)
Staple (with 3/8" staples) your continuous vapour barrier at the u/s of the joists (overlap barrier joints 6" and tape all edges and seams) @ the edge of your deck run blueskin on the perimiter joists (4" on either side) Just far enough to overlap under your vapour barrier to seal the edges (you can always add a skirt board around the perimiter to hide the insulation and blueskin later) If you have a foundnation wall nearby tape your barrier to the concrete and finish by using blueskin over the vapour barrier and the concrete wall (for a good tight seal)
Run 2" of extruded under your vapour barrier tape joints. (tongue and groove insualtion works well) secure with galv. screws (With washers i forget the specialized product for this) to u/s of joist (tape all seams). Run second layer of 2" extruded perpendicular to the last layer and again secure it with some longer galv screws (just make sure you can get them) and tape all seams. Foam edge of insulation @ foundation if applicable (both layers). Some extruded boards come with built in furring for this type of application (check that out too).
If you absolutely have to put something under the deck to protect for fire protection denshield or some sort of exterior grade plywood might work here.
Finish off with your skirt board (chamfer/caulk top edge to shed water. Secure to perimiter joists. Foam edge of insulation and call it a day.
This saves you a lot of time and is quite easy to do. The extruded should not have issues with mold. I would advise against closing in the u/s of the deck area unless I did it properly from the start. Radon is not a joke google it. Your gonna spend more time goofing around in a tiny little space trying to work from underneath of the deck.
Just a thought
Last edited by Bldg 2 Spec; 07-01-2010 at 12:12 AM.