- Q.What is the best roofing and
insulation to use on a 20-year-old house with exposed rafter
cathedral ceilings? The framing doesn’t seem strong
enough for tile, which is common here in the Southwest, and the
roof needs substantial insulation to shield against an
extremely hot summer climate.
A. If the framing won’t
support the weight of concrete tile, my choice of roofing in a
hot climate would be fiber-cement shingles. These are becoming
more widely available and seem to provide a long-lasting roof
without adding much weight. Fiber-cement shingles weigh just a
little more than architectural-grade asphalt shingles
(approximately 400 pounds per square when installed).
There are two ways to handle the insulation problem. The
first method is to install a urethane foam insulation board and
a new plywood deck over the existing sheathing (see
illustration). In warm climates, a second possibility is to
install a new roof deck on 2x2 sleepers over the existing
sheathing, with a radiant barrier between the old and new roof
decks. This would prevent the building from absorbing a
significant amount of heat, providing the radiant barrier has
at least a 1/2-inch air space on either side. Use either two
layers of reflective foil insulation installed back to back, or
one layer of a "double-polished" reflective material, which is
shiny on both surfaces. The roof would then have about the same
insulating value as R-11 fiberglass against downward heat flow.
The space should be ventilated with continuous soffit and ridge
vents. This would not be a good solution in a cold climate,
where you would have to insulate against conductive heat losses
from inside the building.