A.Paul Fisette, director
of Building Materials and Wood Technology at the
University of Massachusetts Amherst and a JLC
contributing editor, responds: Unless you are
located in a cooling-dominated climate, I would
steer clear of installing a synthetic roofing
underlayment in the manner you describe.
With a couple of exceptions, polyethylene- and
polypropylene-based roofing underlayments are
vapor-impermeable and would function as an exterior
vapor retarder. (See "Synthetic Roofing
If the existing wall already has an interior
poly or kraft-paper vapor retarder beneath the
drywall, any moisture trapped inside would be
unable to diffuse out in either direction,
resulting in a wall cavity that would be very slow
Any attempt to create a "barrier" system by
caulking panel seams is a strategy that will almost
certainly fail over the long term. Proper flashing
— rather than caulk — is the only
surefire way to keep water out of horizontal
And although it's common practice to caulk
vertical joints, I've found that panel sidings
routinely leak through these joints as well.
So you should plan on dealing with the water
that will inevitably penetrate the fiber-cement
panels in a more effective way.
The best approach would be to create a vented
drain screen between the back of the panels and the
face of the sheathing.
First, cover the sheathing with asphalt felt or
a nonperforated housewrap like Tyvek or Typar,
which will protect the sheathing from moisture but
also allow the wall assembly to breathe.
Then install vertical nailers over the wrapped
sheathing and attach the panels to the nailers,
which will create an air space between the panels
and the wall sheathing. Be sure to provide clear
drainage along the bottom of the vented space, and
install screening along the top and bottom of the
wall to block out insects.
Another option would be to install Home Slicker
(Benjamin Obdyke, 800/
a 1/4-inch-thick 3-D nylon matrix, between the back
of the siding and the face of the sheathing in
place of the site-built rain screen.