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Q.After stripping the original siding from a 30-year-old house, I'm being asked to re-side with 4x8 and 4x9 fiber-cement panels designed to look like stucco. The installation instructions permit caulking at the vertical joints, but call for Z-flashing at the horizontal joints. I'd rather not use the flashing because it will make it obvious the siding isn't really stucco. Instead of using housewrap, I'm thinking of covering the wall sheathing with a waterproof synthetic roofing underlayment so that any water that does get behind the siding won't go any further. Is there anything wrong with this approach?

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 Underlayments," 5/06.)

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 to dry.

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 seams.

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/

346-7655, www.benjaminobdyke.com), 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.