Insulation Compression / Damping

So I'm about to order my insulation and understand the concept that one really should add strapping to the back of the studs such that the insulation will be under slight compression once the first layer of rock is added. I do however have a couple of very basic questions however, (being as I've never so much as even held a piece of insulation yet): 1) If the insulation is listed as 3.5" is it actually a bit thicker to allow for this, or do I need to order 4" insulation for regular 2x4 framing? 2) Is the concept the same for SAFB? Would I want less compression given that it is much more dense of a product. Many Thanks in advance, Andy p.s. My friend who works for a drywall contractor says they also make pins that you glue to the studs that hold the insulation in place also.

Okay so I just got off the phone with the local building supply place here and confirmed that 3.5 inches is exactly that (weird eh?). So... now my question is this: Steve had mentioned that we should be looking for about 20% compression of the (complete fill) insulation to create damping. In my case the cavity is 9 1/8 inch. The nearest insulation depth they make is 12" which is more like 30% compression. Is that going to be too much? At what point will I be coupling my ceiling to the floor above? Thanks again, Andy

How about using three 3½" batts? That's 10½" total. Compressed in a 9 1/8" bay, that amounts to about 13% compression. --Keith :mrgreen:

7% under, 10% over - which is worse I wonder? It's a good thought keith, although three R11 batts will almost triple my cost - I've got a lot of ceiling to fill.

30% of compression is a bit too much pressure on your ceiling panes. The change of them curving is at hand. I'd look for a less thick blanket of wool, 10" for example, you don't need to fill the whole cavity. It's merely an option, if you can afford it, fill it. If not, don't. There's also the weight of the wool that'll do some pressure on your ceiling and you don't even need to strap the pieces (gravity will do it for you) Walls are another matter tho...

I got it! Two R21's @ 5 1/2 = 11inches - perfecto! The only problem is finding the stuff around here. All the building supply places carry is R11 R13 etc..