[Greenbuilding] Full coverage Ice & Water Shield

[Alan Abrams]

If you do the full Ice & Water Shied it will stop moisture from being 
driven in from the outside.
But since you have a vented attic I don't think it will matter either way.
The biggest problem would be warm moist air leaking from the house into 
the attic and condensing on the cold roof sheathing. If you get snow you 
can get ice on the roof sheathing, and ice dams from the snow melting 
and refreezing at the eves.

In our mixed humid climate (Wash DC area)--when we build conventional
attics--particularly with an airhandler and ductwork, we apply icynene
directly to the underside of the roof deck, with no external ventilation.
There's a legacy report, and some proposed (adopted?) IRC changes that
approve this method.  Unvented roof decks are a shingle warranty issue, but
not insurmountable, plus we minimize the practical impact by using light
colors.  We also must comply with codes related to exposed foam plastic.

We assume some moist air will be drawn up into the attic from the basement,
or will infiltrate or otherwise accumulate, so we provide a slight positive
pressure in the attic, by means of one or more scattered supply registers.  

The reality is hips, valleys, skylights, and other common features make
conventional soffit-ridge venting impractical.  Plus why have HVAC systems
operating in superambient conditions (ice cold in winter, blistering hot in
summer)?  Nor does blown fiberglass deal with infiltration, as does icynene.
And it makes the ice and water shield concern a nonissue.

So it's a no brainer to us.

Alan Abrams