A.Paul Fisette, director
of Building Materials and Wood Technology at the
University of Massachusetts Amherst and a JLC
contributing editor, responds: In a warm
climate, it would make sense to use impermeable
rigid foam to insulate the exterior of the
building, because it would place the vapor barrier
in the right position and make the mass of the
block wall available to help modulate internal heat
But in a cold climate such as yours, the most
economical approach would be to insulate on the
inside, particularly since you're reframing anyway.
It would be a good idea to apply masonry
dampproofing to the inside surface of the block.
Then space a 2x3 wall away from the inside of the
masonry wall by 3 inches (more or less depending on
your insulation and thermal goals; R-19 would be a
good target in your climate).
The best option would be to install
blown-in-place fiberglass or cellulose insulation
so that the fiber completely fills the void between
the block and frame walls as well as the stud bays.
This application will reduce thermal bridging
through the studs and increase the overall
performance of the walls.
But the job could certainly be done with
fiberglass batts, too, with the first layer
oriented horizontally between the framing and the
block wall, and the second layer conventionally
installed in the stud bays.
Check the insulation specs to ensure that the
density and depth of the insulation you choose
achieve your target R-value, and be sure to install
the vapor barrier on the warm side as usual.
On the exterior, an efficient low-cost option
— which would eliminate the expense of
layers of foam and sheathing — would be to
install horizontal strapping on the outside surface
of the wall in such a way that the on-center
spacing matches the required nailing pattern of the
shingles. You will have to calculate the exposure
for each course of shingles and then install the
strapping accordingly; window and door openings
will have to be strapped out around their
perimeters to receive trim.
Next, cover the walls with felt paper or
housewrap, positioning the wrap on top of the
strapping and extending coverage into the door and
window openings. (Windows and doors will need to be
either replaced or reinstalled to match the newly
established exterior plane of the wall.)
As with any type of wall, good water management
is essential, so pay attention to the details as
you install the felt paper (or housewrap) and flash
door and window openings. (For more on
water-managed wall assemblies, see "Detailing
Rain-Screen Siding," 3/06, and "Water-Managed Wall
Installing the strapping horizontally rather
than vertically shouldn't cause problems. If the
wrap is installed and flashed properly over the
strapping, very little liquid water should get
behind the wrap. Any water exposure that does occur
would be occasional and the wall design as
described should easily dry toward the outside.