- Q.I'm in discussions with an
architect regarding the pros and cons of using Grace
Ice and Water Shield or a similar membrane material as
a housewrap. These materials are more expensive than
Tyvek or 15# felt but can't be beat when it comes to
the wind-driven rain we get here in the Florida
Panhandle. My architect has cited Paul Fisette's
articles on felt, housewraps, and flashings as his
reason for requiring felt ("Making Walls Watertight,"
12/95; "Housewrap vs. Felt," 11/98). But Paul Fisette
works in a "heating" climate, not a "cooling" climate
like ours. Would he recommend different details for a
hot, humid climate?
Paul Fisette replies: Your observation is
correct: Climate has an impact on the specification
of a weather barrier system. I would not recommend
using a membrane like Grace's rubberized product
over wall sheathing in a heating climate. It is
impermeable, and it forms a powerful cold-side
vapor barrier. If water or vapor leaks into a wall
cavity that has a tight "rubberized" covering on
the exterior cold surface and an impermeable vapor
retarder installed on the warm interior surface of
the wall, it will be trapped there.
But in your hot, humid climate, a low-perm
membrane applied facing the warm exterior could
work. It is important to build the wall so that
it's able to dry toward the inside when conditions
dictate -- in other words, don't apply a poly vapor
barrier or vinyl wallpaper to the interior.
However, given the expense of rubberized
membranes, I think I would choose 30# felt. It's
stiff and a bit difficult to work with, but it's
more forgiving if you get water on the wrong side
of the membrane. If you are building in a very
exposed location where "sideways" rain is common, I
would consider a rain screen design, which balances
air pressure and creates a drainage plane. This
option is not cheap, but I think it's more
effective for severe exposures.