- Q.We were recently called in
to make siding repairs on a five-year-old home. The wall system
consisted of 1/2x6-inch beveled cedar siding fastened over
1-inch foil-faced polyisocyanurate foam, 2x4 studs with R-11
unfaced fiberglass batts, and an interior poly vapor barrier
covered by 1/2-inch drywall.
The siding was cupped towards the interior of the building,
and the stain originally applied to the siding was peeling off
in many areas. When we removed the siding, there was no visible
moisture on the back. What could have caused the siding to
fail, and how can the problem be corrected?
A.Henri de Marne responds: Siding applied
directly over rigid insulated sheathing has a history of
failures. The consensus seems to be that rigid-foam sheathing
is so impermeable that moisture absorbed by the siding (from
direct rainfall or "splash back" at horizontal surfaces) cannot
be buffered by temporary absorption in the underlying wood
framing elements. The rigid insulation also prevents the wall
framing from absorbing any of the heat generated by exposure to
Depending on the action of the rain, sun, and temperature,
the siding cups as moisture is driven back and forth through
the siding. In this case, the peeling of the latex stain
indicates that a solid-color stain was used. The surface film
of stain lifts off the siding when underlying moisture tries to
escape by migrating through the surface of the siding.
Inward cupping results when the interior surface of the
siding is drier than the exterior surface, and the back of any
siding exposed to direct sun will often be "baked dry" when the
foam sheathing blocks heat transfer to the underlying
The first line of defense against these types of problems is
to thoroughly back-prime the siding. Field-cut ends should also
be coated with stain or preservative. The siding should be
installed over furring strips, creating what is called a "rain
screen" cladding system (see illustration, above). The
resulting air space allows both heat and moisture to dissipate.
This buffering effect can be increased by venting the air space
at top and bottom.
Henri de Marne is a consultant in Waitsfield, Vt.,
specializing in moisture-related construction problems.
Rain Screen Siding