I am working on a new apartment project in Frederick, Maryland (Zone 4).
The project consists of several four story wood framed buildings (units only on levels 2-4).

We are interested in building above "code minimum" buildings because we are long term holders, want our structures to last, and think it is the wise thing to do.

I am persuaded by the benefits of placing insulation on the outside of the wall (the "perfect wall" as described by Building Science Corporation).

I have recently been looking at using Roxul ComfortBoard IS as an exterior insulation because of its beneficial properties including resistance to fire, resistance to bugs, resistance to water damage, and because it is vapor open.

The wall assembly we are considering would consist of:
Cladding (vinyl or hardie)
1x4 vertical furring strip (ventilated rain screen)
1.5" Roxul ComfortBoard IS (vapor open)
WRB (Drainwrap or Raindrop, etc...) (vapor open)
OSB (vapor open)
2x6 16" OC wall with either
Dense Pack or Damp Spray Cellulose (vapor open) or
Roxul Batt (vapor open).
Drywall
Paint

I am uncertain/unclear whether an "additional" vapor control layer (vs air control) is necessary.

I have been referred to "smart" vapor control membranes to be installed behind the drywall.

These products would be "vapor open" in summer when warm moist air might might make its way through the assembly and into the inside of the wall and "Vapor Closed" in winter when warm moist interior air may try to make its way into the assembly to cold(er) sheathing.

1. Do I need an "additional" vapor control layer like the smart membrane or is the totally vapor open wall assembly fine "as-is" assuming I have properly air sealed?
2. Is it a problem that this assembly would not result in the rule of thumb 2/3 of my R-value being outside of my OSB sheathing?
3. Any suggestions on good/affordable method(s) of air sealing this wall assembly?
4. Does anyone see any other issues with this assembly particularly in my climate zone?

Thank you in advance for your help.
Shane Pollin

Comfortboard IS:
http://www.roxul.com/residential/products/roxul+comfortboard+is

Smart Membranes:
Intello:
http://www.foursevenfive.com/images-ezpages/475%20Proclima%20INTELLO%20s...

MemBrain:
http://www.certainteed.com/products/insulation/mold-prevention/317391