A.Ice dams can be controlled in
two ways: Ensure that the entire roof surface
remains cold so that continual melting and
refreezing do not occur, or build a roof that can't
leak if an ice dam forms.
To prevent warm indoor air from reaching the
roof sheathing, make sure you have a continuous air
barrier separating the living space from the
underside of the roof. Sealing all pathways like
recessed lighting, wires, pipes, and seams may be
difficult in an existing house, however. So have
plenty of ceiling insulation as well. In cold
climates, you should have a minimum of R-40 in a
cathedral roof. The insulation must be installed
uniformly with no voids or compression gaps.
A less important but usually helpful measure is
to incorporate roof ventilation, though in your
case, I don't think that providing a vented valley
is the answer. Adding roof venting to a cathedral
valley is at best extremely difficult and usually
not very effective. There is simply no good way to
provide an unimpeded air stream under the roof
sheathing short of building a raised roof, as you
suggest. Some builders drill holes through adjacent
rafters, but air doesn't move sideways through a
series of holes well, and this method can also
compromise the roof structure. Others drop the
valley rafter below the roof surface to provide a
connected pathway between the valley jacks and
ridge vent. But the resulting convoluted air stream
is not dynamic or effective.
Unfortunately, there are no easy solutions.
Perhaps the most sensible approach is to remove the
existing ceiling finish and insulation in the
valley area; fill the rafter bays with
foam-in-place or dense-pack cellulose to achieve
the desired R-value and develop an airtight roof
structure; then refinish the ceiling. It would also
be a good idea to remove the roofing in the valley
area and provide complete coverage with a
peel-and-stick membrane as a backup to prevent
leakage.
Paul Fisette is director of Building Materials
and Wood Technology at the University of
Massachusetts at Amherst and a contributing editor
to The Journal of Light Construction