Sorry for any confusion. Side note... we don't want stiff, we want flex.

You want to avoid trapping any thin air cavity that is less than 2.5"  So installing a hat channel directly to a SIP panel, existing drywall wall, cement wall, etc is a bad thing. Any air cavity will resonate. Deeper cavities will resonate at a lower frequency than shallow air cavities. It is critical to drive that resonance point down as low as possible.

See the attached document. Data is from the NRC in Canada. Note how intruducing a small air cavity to a solid wall (in their example it is concrete, but the analogy is appropriate) makes things marginally better at some frequencies but much worse lower? The lower frequencies are where 95% of noise complaints are. So in practice, it is a very poor trade.

And just to add as another note, Ted...

It may not be much of a sacrafice anyway. Consider a 6 1/2" thick polyurethane SIP, with around say, an value of R42 and 3/8" drywall on the inside. Now take a 4 1/2" polyurethane SIP with R26 (both are sizes and numbers used by Eco-Panels), with a 2X4 wall on the inside(R-12 insulation and 3/8" drywall).  This is the way I drew up the prints for my house (though I did use the 6 1/2" wall).

We're looking at a 6 7/8" R42 wall vs. an 8 3/8" R38 8 3/8" wall. Now, you do get slightly less thermal insulating value and slightly more loss in floorspace, but you have accomplished two other things as well:

1. Presumably, less noise transmission through the wall (in all spectrums?)

2. Ease of renovations. I don't know if this is a big deal with anyone else or not, but I really like the thought of just being able to throw an electrical box in a new location if I happen to have a wall ripped apart for whatever reason.

Of course, on the other side of things:

1. More cost/time/space used up
2. Less insulating value
3. Potentially more work for drywall returns and whatnot
4. Necessity of moisture barrier? I'm not sure about this one. According to my building inspector, to meet my local codes it would not be necessary. I don't know that I understand why.


I suppose it's another one of those things where I won't know what I will do until I do it. It's nice to have a tight, warm house...but if you can have almost the same, except quieter...?