See http://www.nrcratings.com/nrc.html.

Aside from the acrylic/cellulose slop I'm putting on as a first layer, I'm
going to try to stuff the interior panels with fibreglas acoustic ceiling
tile. These aren't quite as good as what is listed above, but they also
provide thermal insulation. These have an NRC of 55% and Transmission
coefficience of 35%. That will make a big difference. And that's before I
install the cloth-covered interior surface panels. If there's still space in
the "walls" I'll stuff it with more fibreglas.

Nathaniel