To get the best performance from insulation, either the framer
has to build an airtight shell or the insulation contractor has
to install an airtight product. It's a proven fact that
insulation R-values are compromised by uncontrolled air leakage
into the building shell. Convective air currents moving through
wall and ceiling bays can bypass poorly detailed insulation
almost as if it weren't there at all, with consequences that
can come back to haunt the builder.
As a GC who also specializes in diagnosing home energy
performance, I receive a steady stream of calls from
homeowners, property managers, and other builders requesting
help with energy-related issues. Surprisingly, most of these
problems occur in new or newly-renovated homes. Common problems
include drafty interiors, frozen pipes, high heating bills, ice
dams, and comfort complaints. And yet these buildings also have
code-approved levels of insulation, tight windows and doors,
and high-efficiency heating systems. What's going on?
What's happening is a breakdown in the building sequence at
the point where the framer stops and the insulation contractor
starts. At that point, at least when air sealing is involved,
the typical construction plan becomes vague or incomplete. This
lack of continuity increases the likelihood that thermal and
air bypasses will get built in to the project. Any space, large
or small, that comes between the air barrier and the insulation
is a possible conduit for leakage.
Same Old Frame, Almost
To eliminate this disconnect, my company handles both framing
and insulation in-house. Our framing methods are identical to
standard practice in every way: 2x4 or 2x6 studs on 16-inch
centers, plywood sheathing, dimensional or engineered lumber
joists, and stick or truss roof framing assembled in typical
fashion. Our goal is not to make the building absolutely
airtight — an elusive goal at best — but to
reduce the aggregate amount of leakage and to manage the flow
of air entering and exiting the shell. Our typical target is a
building with a passive rate of exchange of about 1/10 ACH per
hour (.1 ACH). After we insulate a job but before the drywall
is hung, I perform a blower-door test to measure how tight the
insulated shell is (see Figure 1).
Figure 1.A blower door is a calibrated,
variable-speed fan used to depressurize a house, creating a
pressure difference between indoors and out. By measuring air
flow through the fan, the total volume of air leakage through
the building envelope can be determined.
Over time, the test results have shown us where to concentrate
our air-sealing efforts during framing. Fortunately, what's
required adds little time or effort to the standard workflow.
Instead of ordinary sill sealer, we use a rubber (EPDM) gasket
between the foundation and mudsill (Figure 2).
Figure 2.EPDM rubber building gaskets, such as
this sill sealer (bottom), are designed for heavy structural
loads. The author uses several different configurations to seal
various framing components. Between the rim joist and the
mudsill, a continuous bead of subfloor adhesive provides an
effective and inexpensive seal (top).
The 5 1/2-inch-wide gasket is designed to provide a positive
seal under a heavy structural load; it also serves as a
moisture barrier between the concrete and wood. After setting,
stringing, and squaring the rim joists, we run a bead of
construction adhesive over the joint between rim and mudsill.
We've tried EPDM gaskets under the rim, but they tend to get in
the way. Adhesive is less expensive and nearly as effective.
It's important to take this step before rolling the floor
joists into position; otherwise, rim sealing becomes a long,
hard slog. Glue-and-nail subfloor installation has become
standard industry practice. However, not everyone glues the
edge of the rim joist on the assumption that the walls hold the
subfloor down at the edges. In fact, this is a common point of
air entry, which we handle with a continuous bead of adhesive.
We frame and sheathe the walls on the deck before standing them
on a rubber gasket tacked around the perimeter of the deck
(Figure 3). I buy my gaskets from Resource Conservation
and Denarco (269/435-8404). These companies carry a good
selection of gaskets designed for sealing a variety of framing
Figure 3.Bottom plate gaskets may be tacked to the
deck or the underside of the plate before sheathing and
standing exterior walls.
Because I'm convinced that air doesn't move through plywood, I
sidestep the whole housewrap debate. Instead, shortly before
installing the siding, we tape all the horizontal seams and any
other cuts in the sheathing that aren't backed by framing. We
use either DuPont Contractor Tape (DuPont Tyvek, 800/448-9835,
www.tyvek.com) or 3M
Builder's Sealing Tape (3M, 888/364-3577,
www.3m.com) or both (Figure
Figure 4.Instead of using housewrap, the author
relies on plywood seam tape to block wind penetration. Tape
installs easily just prior to siding.
Gaps and Penetrations
Once the shell is tightened and weathered in, we turn our
attention to interior air gaps and passages. Plumbers and
electricians create their share of holes between bays and
between floors. Expanding foam caulk does a good job of sealing
around wire and pipe penetrations. If left open, these holes
become interconnected convective air conduits to the roof
soffits and attic.
Packing fiberglass around the jambs of a window or door isn't
particularly effective at stopping drafts, especially when the
gap's too narrow to stuff. Expanding foam doesn't move with the
frame's expansion and contraction cycles, and runs the risk of
distorting the jambs on installation. Instead, we use another
EPDM gasket configuration made for the task. The soft rubber
slides into narrow cracks without undue force and provides a
positive seal that moves with the framing (Figure 5). Cracks
too narrow to slide a gasket into get caulked with
Figure 5.A compressible dual-tube rubber gasket,
forced into the shim gap around windows and doors with a
drywall knife, provides a highly effective air
It's a good idea to size rough openings slightly larger than
standard to ensure that the gasket has sufficient space to
expand and seal.
Sealing the Ceiling
Balloon-frame configurations and open plumbing and mechanical
chases provide prime conduits for cold attic air to enter the
heated space. In new construction, these types of
through-the-floor gaps are required to be fire-stopped with
some form of noncombustible material, typically sheet metal.
But fire stopping alone isn't so tight that moisture-laden air
can't find its way into the attic. So we seal all the seams
between fire stop and flue or chimney with a high-temperature
Ceiling strapping, a typical framing detail in New England,
can create a common air bypass, and a big one at that. The
3/4-inch-wide gap introduced by the strapping between batt-type
insulation and a drywall ceiling can allow cold soffit air to
travel between the two, effectively short-circuiting the
insulating layer (Figure 6).
Unblocked Ceiling Joists
Figure 6.If ceiling bays aren't end-blocked,
soffit air can travel below the ceiling insulation and
neutralize its function. Piling on more insulation does nothing
to correct a cold ceiling complaint (top). Rigid foam blockers,
cut to fit around standard insulation baffles, direct
ventilation above the ceiling insulation and prevent cold air
Even in ceiling configurations that aren't strapped, batt
insulation is unlikely to lie in absolute contact with the
ceiling board and can permit the same kind of infiltration. The
result is a cold ceiling and a customer comfort callback. In
this situation, adding another layer of insulation to the attic
does little but waste money. To reduce soffit air infiltration,
we direct incoming air above the insulating layer into the
attic, using custom-cut soffit blockers of 1-inch rigid foam
board that fit snugly between the ceiling joists or rafters.
The top edge of the baffles is contoured to fit a typical foam
insulation baffle, and the seams are sealed with expanding