Detailing Rain-Screen Siding

by Gary Katz and Bill Robinson

A Ventilated Air Space

Rain-screen walls, sometimes referred to as "pressureequalized assemblies," solve these problems because they provide a ventilated air space that defeats the air-pressure difference. Properly detailed, they also provide a drainage plane - a way for any water that does get behind the siding to escape. And compared with the cost of replacing trim and siding that have failed prematurely, building a rain-screen wall is by no means prohibitively expensive.

The requirements for an effective rain screen are

• a minimum 3 .8-inch air gap between the back of the cladding and the drainage plane;
• ventilation to assist in drying and to partially equalize the pressure on the cladding;
• drainage at the bottom;
• a rigid drainage plane and air barrier.

In essence, a rain-screen wall - a ventilated gap or cavity providing drainage and a capillary break - works by promoting air circulation.

Prepping for Trim

As with most construction projects, the first step in building a rain screen is establishing the correct sequence. On the recent remodeling project shown here, we began by wrapping the wall, making sure that all window and door penetrations were properly flashed and sealed (see "Flashing a Flanged Window," page 198). The housewrap or drainage plane must be air- and watertight to prevent liquid water and air from passing through from outside to inside.

Rather than furring out the windows, which would create an offset in the drainage plane, we installed them against the sheathing and housewrap, then applied furring for trim and siding on top of the flashing (Figure 10, page 186). By leaving the trim . simple flat stock . slightly proud of the windows, we avoided creating a potential dam in the drainage plane.

To match the window trim, we chose to install a door with no brick mold; we added plinth blocks and flat casing afterward. This allowed us to run the drainage plane right onto the face of the door frame (Figure 11, page 186). Since we were using 1 . 2-inchthick furring strips for the siding and furring blocks for the trim, we had to add 1 .2-inch jamb extensions to the doors before installing the casing. We opted for the individual furring blocks instead of a continuous furring strip because we thought they would provide better air circulation behind the trim.

We covered the pressure-treated furring blocks with flashing or housewrap to isolate the trim from the blocks (Figure 12, page 187). We also sealed every cut and notch in the trim with two coats of primer. The height of backing blocks above the doors and windows was determined by the upper trim detail, where head flashings were later installed (Figure 13, page 188).

Once the door and window trim was installed, we turned to the water table, where scant clearance to grade required special attention. Before wrapping the house, we applied a self-adhesive membrane at the bottom of the wall to protect the wood sheathing, then used PVC trim — a rotproof, nonreservoir material — for the water table.

After attaching the water table, we carefully sliced through the housewrap and inserted a custom-bent PVC flashing.