The only explanation that I can think of is that BH5 has a large component of open cell foam structure, such that air permeates it to a surprising degree. Explanations?
When you add damping material to a box it slows down the air flow in the box and makes the woofer act as if it is in a larger enclosure. This is true with insolation, poly fill, and open cell foam.
While the barrier layer and the damper layer take up air space, the open cell foam counters that. Impedance measurements before and after installing BH-5 shows little to no change in tuning. These means that it does not take up box volume, at least according to the woofer.
Whispermat is cheaper but falls short of the performance of BH-5. The number one, most important function of BH-5 is damping the resonances of the enclosure. This is achieved by the first damping layer that is adhered to the cabinet wall. Whispermat lacks this ability as it has no damping layer. Whispermat only has an isolation layer and foam.
In some applications the barrier layer can be a negative thing if laid in too heavy because the barrier layers can mass load the driver like a little internal passive radiator. So 100% complete covered is not recommended.
I currently have the manufacturer of BH-5 working an a version with a heavier damper and a 1" open cell foam layer with a textured surface.
I'll let you guys know how it tests in a few weeks if you are interested. If I have it produced it will be a little less expensive than BH-5.
