R-value: They’re air sealants, not insulation. Both
are low-VOC, so a simple respirator is sufficient
392 Chapter 14
when applying the stuff. Both solutions are
aimed at the professional market—and priced
accordingly.
AIR-SEAlInG THE ATTIC
The “stack effect” of warm air rising makes an
unconditioned attic the place to start sealing.
Informed by your energy audit, suit up (gloves,
safety glasses, respirator, and long sleeves), then
go up. Wear a headlamp so your hands will be
free to work. If attic floor joists are exposed, take
along a short plank or two to sit on. Step only on
the plank, never on the ceiling between joists. If
the floor is covered with loose-fill insulation, take
a dustpan to move it aside temporarily as you
seal penetrations. Attics heat up quickly as the
sun warms the roof, so start early in the morning.
Start with the largest openings in the attic floor.
Is the attic hatch insulated? You can buy an
insulated cover or build one by gluing two layers
of 1-in.-thick rigid-foam panels to 1⁄2-in. plywood. When wall surfaces are exposed, it’s possible to spray an air-sealing caulk before insulating.
Around the edges, use sponge-foam or neoprene
Sprayable caulks were developed to improve the efficiency of fiberglass batts by stopping air
weatherstripping as a seal.
movement within walls.
A 2-in. gap between the chimney and framing
is required by building codes. But you can pack
the gap with a noncombustible material such as
rock wool, and cover it with sheet metal—metal
drip-edge works especially well. Apply a bead of
adhesive caulk to the framing around the chim-
ney; place the drip-edge in the caulk, with its
other edge snug to the chimney; screw the drip-
edge to the framing; then apply a bead of fire-
rated caulk to seal the metal to the chimney.
open framing above soffits and chases can be
a freeway for drafts, so cover large openings with
rigid-foam panels or drywall panels, apply caulk
or expandable foam to seal panel edges, then
screw them to framing.
old bath fan boxes can leak moisture-laden air
into the attic. Apply caulk or foam to seal fan-box
flanges to the ceiling; use mastic or metal duct
tape (not cloth) to seal holes and cracks in the
fan box. If any fan exhausts into the attic—rather The code-required gap between chimneys and combustible materials (framing) can be a freeway than to the outside—add a vent pipe that exits
for air leaks. Here, a prescored coil of metal stock is used to create dams around a masonry flue. It’s
through the roof or an exterior wall. Fans that
nailed to the framing and then caulked with fire-rated caulk to seal gaps. Some builders use
terminate in the attic can soak insulation,
leftover metal drip-edge as an air dam.
encourage mold, and even rot framing.
Recessed ceiling lights (ceiling cans) are big
P R O T I P
leakers, especially old, non-IC-rated cans that
can’t be covered with insulation (IC means insu-
If you have knob-and-tube
lation covered). Replacing non-IC-rated cans is
wiring that is still active (ener-
the safest and, ultimately, most cost-effective way
gized), do not spray it with foam
to deal with their energy loss. Enclosing non-
sealant or cover it with insula-
IC-rated fixtures in a sealed, insulated box is
tion. Instead, call a licensed
electrician to replace it.
sometimes suggested as a solution. But older
incandescent fixtures can get quite hot, and the
Energy Conservation and Air Quality
393
build-up of heat inside a box could be a fire haz-
ard. Replacing old ceiling cans with IC-rated
Air-Sealing
ones is a safer way to go.
ElEctRIcAl BoxES
IC-rated cans are either airtight or not air-
tight. Airtight cans don’t need sealing, although
When air-sealing holes in an electrical box,
some installers caulk can rims to drywall. Non-
make a quick pass with the spray foam. Don’t
airtight IC cans are rated for direct contact with
inadvertently fill the box with foam—which
insulation. Expanding foam is a type of insula-
could be an electrical code violation if it
tion, so apply spray foam to holes in can bodies
to seal them.
exceeded the box’s “fill capacity” (p. 280).
If using foam to air-seal IC-rated cans makes
Some installers recommend using fire-rated
you uneasy—or codes don’t allow it—there’s
foam to seal electrical boxes, but it doesn’t seem
another option: Cover cans with insulation pro-
critical; check local codes to be sure. “The
tection covers such as those manufactured by
National Electrical Code does not prohibit
Tenmat. Made of mineral wool, the covers physi-
installing Type NM-B [Romex] cable in spray
cally isolate the light cans. Tenmat’s fire-rated
foam insulation,” so presumably spray foam and
light covers are UL-certified.
cable sheathing are compatible.
Plumbing pipes, electrical boxes, and top
plates are simple to seal—if you can find them.
Vent stacks are easy because they stick up, but
water supply pipes, electrical cables and junction foot when looking for air leaks. Frequently, the
boxes, and the top plates of partitions are usually underside of batts or loose-fill areas will be dis-
buried under insulation. There’s no substitute for colored by dusty air. Remove insulation, apply
removing insulation and surveying every square
spray foam to fill penetrations, replace insulation,
and keep looking. Drywall seams and the top
plates of partitions are especially easy to over-
look; a schematic of the floor below, showing
partition locations, may help you find them in
The roof-sidewall juncture loses a lot of heat. Top plates have
the attic.
zzzzzz placing Rigid Foam along the Attic perimeter
low R-values (thermal bridging), and dropped soffits often
interrupt the drywall so there’s little to stop infiltration. And
Sealing the perimeter of an attic with sloping
vented eaves allow yet more air to blow through. Placing
roofs is tricky because the triangular space where
2-in.-thick rigid-foam panels between ceiling joists—and over
end plates—partially solves the air leaks, while holding back
roof meets wall is so tight; definitely wear a hard
loose fill from drifting down into vented eaves.
hat. With long arms and the straw applicator on
the spray foam can, you should be able to spray
Loose-fill insulation
where ceiling drywall abuts exterior walls. The
top plates of exterior walls are especially big
energy losers because wood is a poor insulator.
If the eaves are unvented, butt fiberglass batts
Drywall baffle
snug against the exterior wall, ideally covering
top plates in the process. If you’ll be insulating
the attic floor with loose-fill insulation, place
2-in.-thick foam panels along the attic perimeter
Ceiling joist
so that the rigid foam covers the top plates as
shown at left. Caulk panel edges with caulk or
expanding foam for a tight seal.
Caulk panel edges.
If the eaves are vented, pushing insulation
tight to exterior walls may block air coming up
2-in. rigid foam
from the eave vents. You’ll need to install some
kind of blocking to contain the insulation, but
that blocking may, in turn, keep you from cover-
ing the top plates with insulation. Perhaps the
Top plate
best solution in that case is installing a baffle/air
chute such as the AccuVent® product shown
Sheathing
Dropped soffit
on p. 400.
Lastly, if your finished attic has kneewalls, try
to air-seal and insulate the back side of the
walls—the side facing the exterior. Because rigid
394 Chapter 14
foam has higher R-values than fiberglass batts,
If blower-door or duct testing indicates a leaky
fitting foam panels into stud bays and sealing
fireplace damper or leaky register boots, attend
their edges with expanding foam will probably
to them, too. More on sealing ducts to follow.
reduce energy loss most effectively.
AIR-SEAlInG BASEMEnTS
AIR-SEAlInG wAllS,
If your basement is an unconditioned space, or if
wIndowS, And dooRS
you have a crawlspace, insulate the ceiling to
Correct flashing (chapters 6 and 7) is an essential retain heat in the floors above. Installing open-
first step to airtight windows and doors; weather-
cell spray foam insulation will air-seal and insu-
stripping (p. 124) is a good second step. Installing late joist bays simultaneously, while also creating
storm windows is a cost-effective alternative to
a semipermeable membrane that will allow mois-
replacing windows; it typically halves the energy
ture to migrate. Alternately, if you don’t have the
escaping through old windows. Then, start seal-
money to spray-foam the basement ceiling, you
ing air leaks you can easily access.
can insulate it with rigid-foam panels, using cans
of spray foam to air-seal panel edges.
Receptacles and switches on exterior walls can
Next, air-seal rim joists, which, along with the
be quickly sealed by unscrewing cover plates and joint between the mudsill and the top of the foun-
installing a compressible foam gasket. You can
dation, are the biggest energy leakers in the base-
also stop a major source of air leaks and save
ment. The most cost-effective way to seal rim
energy by replacing incandescent recessed lights
joists is to cut and fit pieces of rigid-foam insula-
with LED retrofit kits (p. 313), which typically
come with an air-sealing gasket.
If you’re planning to paint, that’s a good oppor-
tunity to seal room junctures that may be leak-
ing, notably wall–ceiling joints, along baseboard
trim, and around the perimeters of window and
door casing on exterior walls. A water-based latex
or acrylic caulk is the best material for the job:
After applying a thin bead of caulk, you can use a
fingertip to shape and compress it into the joint.
Once it is painted, you’ll never notice it.
Caulking a building’s exterior seams, especially
where siding meets door and window casing, will
also cut air infiltration. Use synthetic rubber or
modified silicone caulking. Exterior lights and
outdoor receptacles usually have foam gaskets
(check to be sure they’re still flexible); caulk
around outlet boxes to seal gaps in the siding.
Air-sealing window and door jambs is not usu-
ally part of an energy retrofit because the gaps
between jambs and rough openings are covered
by casing. But if you’ll be removing casing to
fine-tune old windows (p. 144), apply a bead of
To fill gaps around window and door frames, use a mild-expanding polyurethane foam; more
minimally expanding foam into the cavity around expansive foams can bow frames and cause a window or door to bind in the frame.
the jambs.
Under sinks and inside cabinets, DWV and
Rim joists lose a lot of heat in cold
water supply pipes often exit through rough cut-
climates. Here, a Connecticut
outs in the drywall. Look in upper cabinets, too,
homeowner cut and fitted XPS foam
where exhaust fans are frequently housed.
panels into joist bays and sealed
Expanding foam can work wonders around such
edges with expanding foam. Seal any
pipes and ducts. Investigate, too, shared walls
penetrations in basement walls.
with an attached garage. Code requires that
shared walls be firewalls constructed of 5⁄8-in.
drywall on both sides—with no through-holes,
such as back-to-back receptacles—but builders
don’t always get the details right. There, use fire-
rated caulks or fire-rated foams.
Energy Conservation and Air Quality
395
tion into each joist bay, sealing the perimeter of
AIR-SEAlInG duCTwoRk
each foam panel with expanding spray foam.
If it’s not possible to seal duct leaks while a
Spray-foam any foundation holes that go all the
smoke test is in progress, use a grease pencil or a
way through, too.
felt marker to mark leaks so you can go back and
To air-seal concrete or concrete basement
seal them later.
walls (assuming there are no moisture problems)
Ductwork is joined by various mechanical
caulk cracks and gaps, using masonry caulk or a
means. Rectangular ducts are fabricated to snap
compatible expanding foam. If you’d like to turn
together, so if a rectangular joint is leaking, sec-
the basement into conditioned living space, you
tion ends may have simply separated. With the
can air-seal and insulate basement walls quickly
aid of a helper, push sections together. Round
by installing EPS foam panels, as described on
ducts with crimped ends are friction-fit, with
p. 417. Note: Do not, however, install plastic (poly- three self-tapping, sheet-metal screws spaced
ethylene) sheeting as an air or vapor barrier on
even to hold them together. Insulated flex ducts
basement walls. It will trap moisture.
are joined to rigid ducts, floor boots, tees, etc., by
Inoperable basement windows should be
using two duct straps (zip ties), which are drawn
sealed shut with expanding foam. Windows and
tight with special tensioning pliers. Typically, two
doors that must remain operable for light, venti-
ties are needed because insulated flex ducts have
>
lation, or egress, in case of emergency, should be
an inner liner and an outer jacket to attach.
weatherstripped with tubular neoprene compres-
The size of gaps will determine which materi-
sion gaskets.
als you use to seal them. For best results, start
While you’ve got the spray foam can out, fill
air-sealing near the main plenum or the cabinet
holes around pipes and wiring rising out of
the basement to forestall air leaks to the floors
above. Lastly, if your HVAC system is in the
basement, now’s the time to seal ductwork, as
described below.
zzzzzz Sealing ducts
Living space
Rigid-metal duct (left) offers the least resistance to
Register boot
airflow; seal its joints and insulate the runs through
Fiber-reinforced
unheated areas. Flexible insulated ducts, such as Wire Flex
mastic
(center), and Alumaflex (right) don’t need fittings to make
turns, but Wire Flex can be punctured. Alumaflex offers a
good balance of strength and flexibility.
Main trunk
Fiber-reinforced
Screw and tape
mastic at swivel joints
duct joints and fittings.
Secure joints between duct sections and fittings
with at least three sheet-metal screws. Then wrap
the joints with aluminum duct tape—not fabric
To seal duct joints, apply a generous amount of water-
duct tape. Apply fiber-reinforced mastic to hand-
based mastic sealant. Correctly applied, the sealant should
snipped and swivel joints.
ooze through gaps; when it hardens, those tiny mastic
fingers will hold the seal in place.
396 Chapter 14
housing the system blower, where air pressure
class Iv (permeable) vapor retarders
will be greatest. Holes less than 1⁄8 in. wide can be have permeance levels greater than 10 perms.
sealed by brushing on mastic. If the hole is inac-
Class IV materials include housewraps,
cessible—say, atop a duct that’s close to a sub-
unfaced fiberglass insulation, cellulose
floor—you can smear on mastic with your hands
insulation, and unpainted drywall and plaster.
if you’re wearing disposal latex or nitrile gloves.
Vapor barriers are one of the most conten-
If the gap is wider than 1⁄8 in., first cover it with
tious topics in building science. Air moves back
self-adhering fiberglass tape, then coat the tape
Renovation 4th Edition Page 92
