Intello Needed w/ Dense Pack Cellulose? – GreenBuildingAdvisor

Dear GBA Community,

We are in the midst of building a moderate-to-high-performance home in central Vermont.  Following extensive discussions with our architect, we had settled on the following wall assembly (from outside in):

* Siding (Everlast vertical board & batten, which has a tiny bit of rainscreen)

* Hydrogap (for a little extra rainscreen to get us up close to 1/4″)

* 1/2″ ZIP sheathing

* 2×6 wall with strips of 2″ ZIP-R nailed on the insides of the 2×6’s, filled with Rockwool batting

* Intello membrane

* 2×4 wall (with electrical and plumbing), filled with Rockwool batting

* Sheetrock

Today we met with a high-performance air sealing and insulation contractor with decades of experience, who strongly advised us against using Rockwool batting and instead advocated for rigorously dense-packed cellulose.  He argued that any batting will inevitably have gaps, leading to convection inside the wall and resultant problems.  He also stated that cellulose will not support convection at Delta-T’s less than 250 F.  He further argued that the borate fire retardant completely kills any mold growth, and is repellent to insects and rodents.  He is using some fancy high end cellulose product.

Even more surprisingly, he argued that vapor barriers are unnecessary in rigorously densely packed cellulose wall assemblies, are a product of people with “hydro-phobia,” and that real-world experience has shown that walls without membranes do just fine.  Based on our extensive reading of this forum and other sources, this seems to be contradictory to what is typically advocated in the building science community.

In trying to reconcile all of this information, one thing that occurs to us is that he emphasized the importance of rigorous air sealing (he uses the AeroBarrier system, which we aren’t particularly interested in paying for) because air leakage moves considerably more water than diffusion through solid materials.

Could it be that he is “right” about a vapor barrier being unnecessary, but only if the air leakage is reduced to essentially zero?  Perhaps it’s the case that in high-performance structures with only even a little bit of air leakage, a smart membrane is important?

We would appreciate any insights and/or advice on this enigma!