ing them. To avoid compressing the edges of
ing insulation, place new batts perpendicular
faced batts, staple facing flanges to the edges—
atop old ones.
The pros trim insulation in place: It saves time and ensures As you place fiberglass batts in each stud bay, plump
a tight fit without measuring. Here, an installer leaves the them up to full loft, and adjust them so they fill the batt folded because his 3-in. knife blade is long enough to space completely.
cut through a double thickness in one pass.
412 Chapter 14
If you intend to make the attic a conditioned
space, you can forego insulating the floor and
preventing cold SPotS
instead insulate the kneewalls and rafters. But
because insulated floors soundproof somewhat,
To prevent cold spots behind pipes and wiring,
you may want to insulate them anyhow. Before
split batts in two—sort of like pulling apart a
inserting batts into the rafter bays, staple air
sandwich—so that each piece is roughly half the
chutes to the underside of the roof sheathing. The
batt thickness. Slide one half of the batt behind
chutes create a 1-in space between the sheathing
the wiring or pipes, and place the other half in
and the insulation so that air can keep flowing
front. Where it would be tedious to split and slide
under the roof. If the rafters are spaced regularly
batt ends behind the obstruction in the middle
on 16-in. centers, unfaced friction-fit batts will
of the stud bay, instead slice halfway through
stay in place until the drywall goes up. But if the
the batt as shown; split the fiberglass at the cut-
rafter spacing is wide (24-in. on center or greater)
line, and fit the insulation behind the obstruction.
or irregular, friction-fit batts may sag or fall out.
Instead, you might (1) staple paper-faced batts to
the rafter edges or (2) have foam insulation
sprayed to the underside of the roof sheathing
after installing air chutes.
Another option: An unvented roof. The options
above assume that the roof is vented. Thanks to
the air-sealing and insulating properties of spray
polyurethane foams, unvented roofs (also called
hot roofs) have become increasingly popular.
Unvented roofs are an especially attractive
option when:
Complicated roof framing makes eave-to-
ridge ventilation all but impossible. Hip roofs,
valleys, dormers, and skylights, for example,
may preclude continuous ventilation flows
to a ridge.
A house lacks soffit vents—or lacks soffits
altogether. It may be possible to add eave or
drip-edge vents, but retrofitting ventilation to
an old house is rarely easy. At the very least, it
will require scaffolding so one can safely and
accurately cut vent slots along the eaves or
drill through blocking to add vent holes.
Moreover, such vents may clash with the
house’s architectural style.
Strong winds drive rain into vents or, in
hurricane-prone regions, so pressurize a house
that its windows blow out or its roof blows off.
Rafters aren’t deep enough to accommo-
date the code-required amount of insulation
and an air channel flowing over that insulation.
In any region, eave ventilation can be a two-
edged sword, admitting air and moisture into a
vulnerable area and one that’s difficult to insulate
well. A wall’s top plates, for example, lose heat
through conduction (thermal bridging), a prob-
lem made worse by cold air washing over them.
For more about vented and unvented roofs and
illustrations of each, see pp. 398–399.
Energy Conservation and Air Quality
413
InSulATInG An ATTIC
InSUlAtInG ARoUnd ElEctRIcAl outlets
wITH looSE FIll
Before installing loose-fill insulation, read “Air-
Electrical outlet boxes in exterior walls can be big energy losers because builders
Sealing the Attic” (p. 393). Loose-fill materials
often forget to insulate behind them. To stop air leaks, split insulation batts and
can blow everywhere, so it’s important to cover
slide portions behind the boxes (below left). In very cold climates, you might also
chases and other large openings so they don’t fill
want to install airtight outlet boxes. Be careful not to nick cables around boxes.
with insulation. Equally important is keeping the
Before insulating around any outlet (below right), disconnect the electrical power
material away from non-IC-rated recessed light-
to the area, and use a voltage tester to be sure the power is off.
ing cans, chimneys, and eave vents. You can blow
loose fill over existing batts, but first hand-pack
fiberglass into gaps between batts and joist bays
that blown-in insulation might not otherwise fill.
Working in the attic has its risks, especially
if it’s unfinished. If the joists are exposed, run
planks across them so you can move safely
around the attic. Stepping from joist to joist is
risky—if you miss, you could step through the
ceiling. Having enough light is essential, so set
up several work lights and wear a head lamp.
Dress the part: respirator mask, safety glasses,
work gloves (especially if blowing fiberglass),
long sleeves, and long pants. Lastly, determine
how deep the insulation should be when you’re
done, and staple cardboard depth gauges to the
side of joists so you’ll know when you’ve reached
that goal.
Insulating with loose fill is a two-person job.
Down below, spread a plastic tarp and place the
blower as close to the attic access door as possi-
ble (ideally, outside or in a garage). Loose-fill
materials are, well, loose, and they can make a
huge mess. Anyhow, the person downstairs (also
wearing a mask, etc.) feeds bales of cellulose or
fiberglass into the blower hopper. Feeding the
right amount of insulation takes practice, but
after a while, the machine’s whine tells you when
you’ve got a steady flow going.
Up in the attic, the person handling the hose
starts by blowing insulation into the farthest
reaches and gradually works toward the access
door. The trick is applying the loose fill evenly.
Angling the hose slightly upward and allowing
the insulation to “rain down” seems to disperse
the loose fill better than aiming directly at the
section you’re trying to fill.
An alternative: If the attic has a rough floor of
boards, pry them up in the center of the attic to
expose the joists. Then blow insulation into the
joist bays. It’s difficult to blow insulation much
farther than 4 ft., so remove boards every 8 ft. or
so and feed in about 4 ft. of the blower hose under
the floorboards on both sides of the opening,
gradua
lly withdrawing the hose as the bays fill.
When installing batt insulation, be careful not to block eave-to-ridge ventilation pathways along
the underside of the roof sheathing. The holes in the unfilled bays at the right are fascia vents.
414 Chapter 14
InSUlAtInG Floors ABovE BASEmEntS
And cRAWlSPAcES
When insulating a floor over an unconditioned crawlspace or basement, you’re fight-
ing gravity and moisture.
First, let’s deal with gravity. The easiest way to install fiberglass batts without
needing three hands is to precut a number of thin wood slats—1⁄4-in. fence lath is
light and springy—1⁄4 in. longer than the distance between the joists. As you hold the
unfaced batts in the joist bays with one hand, use the other to wedge the slats into
place, under the insulation. You can also buy precut wires, “lightning rods,” for the
same purpose.
If the subfloor area is damp or if there’s heavy condensation during warm months,
rigid-foam panels or spray polyurethane foam is a better choice than fiberglass batts.
Use a compatible construction adhesive or spray foam in a can to glue the foam panels
When blowing loose-fill cellulose in an attic, angle the
to the underside of the subfloor. If the joists are irregular, trim the foam panels a bit
blower hose slightly up so the loose fill falls evenly onto
smaller and use expanding foam to fill gaps.
the section you’re filling. Aiming the hose directly at the
spot you’re filling is more likely to create a mound.
dEnSE-pACkInG wAll CAVITIES
Equipment. Blowing insulation is dusty work, so
S a f e T y a l e R T
a tight-fitting respirator mask and safety glasses
There are several ways to dense-pack insulation,
are essential, as are work gloves when dense-
as noted on p. 408. In the sequence shown on
Rotating paddles of an insu-
packing fiberglass.
p. 416, insulation is blown in through holes
lation blower that are strong
Insulation blowers and hoses can be widely
drilled into the exterior, so interior finish surfaces
enough to break up compressed
rented, and some suppliers loan the equipment
remain intact. Dense pack materials are the same
insulation can also injure hands
free to any contractor who buys the insulation
and arms, so keep them out of
cellulose or fiberglass bales used in loose fill,
from them. It takes a powerful machine to dense-
the hopper while the paddles are
except that here they are blown into confined
pack walls, typically one wired for 240 volts or
turning. Instead, shut off the
spaces at higher pressures. Thus dense packing
120/240 volts. Almost all pumping units require
machine or use a stick to dis-
should be done only by trained installers. It’s not
two workers, one to feed insulation into a blower
lodge balky clumps of insulation.
a job for DIYers.
hopper and the other to operate the hose. Conse-
Before starting, review earlier sections on
quently, most machines have a remote on/off
sealing air leaks, correcting excess moisture, and
switch so the hose operator can stop the blower
keeping insulation away from potential ignition
as cavities fill. A remote switch also allows one to
sources. Knob-and-tube wiring that’s still ener-
shut off the blower at the first sign of a clogged
gized must not be covered with insulation. If
hose. Most machines have adjustable gates or air
such wiring is present, have an electrician
inlets that control the air-insulation mixture. The
replace it beforehand. Because it’s impossible to
equipment supplier should explain safe operating
know what’s hidden behind walls, thermal images procedures to both workers.
from an energy audit may be helpful to identify
cold spots.
Vermiculite: A PotEntIAl ASBEStoS RISk
Vermiculite is a small, pebble-like, gold or brown mineral fiber (colored rather like
mica) that at one time was widely used for insulation and soundproofing. A loose-fill
material, it was often poured in attic floors. But vermiculite often contains asbestos.
If you see it in your home, don’t disturb it. Only an experienced asbestos-abatement
contractor should remove it. Visit the U.S. Environmental Protection Agency’s (EPA’s)
website for more information (www.epa.gov).
Energy Conservation and Air Quality
415
Units typically come with 3-in. corrugated
plastic hoses whose sections join with steel
couplings. When filling wall cavities at low pres-
sure, installers often use a 3-in. by 1-in. reducer
to minimize the size of the holes they’ll need to
patch later. When dense-packing cellulose, some
pros prefer to duct-tape a 5-ft. to 6-ft. length of
1-in. clear vinyl tubing to the end of the 3-in.
hose so they can see if the insulation is flowing
freely. Because narrower hoses are more likely to
clog, the pros run an air-rich, low-insulation mix
through the vinyl tubing and blow just air inter-
mittently to minimize clogging.
In addition, installers need a drill and drill
Hoses often clog near the reducer. To clear a clog,
remove the reducing nozzle, insert the end of the hose
bit (or hole saw) to drill into exterior sheathing,
in a trash bag, and turn on the blower. If that doesn’t
a flat bar to pry up siding, and a shingle ripper
work, rap the sides of the hose and use a stick to dig out
or a reciprocating saw with a metal-cutting blade whatever you can.
to cut through nail shanks if there’s wood siding.
If there’s vinyl or aluminum siding, one needs a
zip tool to lift the tops of courses to be removed.
and use a voltage tester to make sure the power’s
If the exterior is stucco, it will take a tungsten
off. Or they turn off the service panel’s main dis-
carbide hole saw to drill through it.
connect and run the blower off a generator.
Important: To be safe, many installers turn off Insulating wall cavities. The task varies accord-
electrical power to the areas to be drilled and filled, ing to the type of siding. It’s possible to drill
through wood siding and plug it later; but it’s
preferable to remove the wood siding, drill
through the sheathing, and then replace the sid-
ing over the plugged sheathing. If there’s vinyl
or aluminum siding, it must be removed by
inserting a zip tool under the top of the course to
be removed and then sliding a flat bar under
nails holding the siding strip. After the insulation
holes are plugged, the siding is replaced. If there’s
stucco siding, the installer drills directly through
it; plugs will hold well and can be hidden easily
by textured patches.
Ideally, holes drilled should be roughly 1⁄4 in.
larger than the diameter of the insulation hose. If
the holes and the hose are the same siz
e, little air
will escape and the wall cavity will become so
pressurized that insulation can’t be blown in. If
holes are too large, insulation will blow all over
and patches will be more obvious.
When a wall cavity is almost full, it will
become so airtight that it will block the flow of
additional insulation, causing the blower to
whine. After filling a few cavities, an installer gets
a sense of how much insulation is needed to fill
each stud bay, and the blower’s whine becomes
familiar. At that point, the installer can use the
remote on/off to shut off the blower and go on to
the next cavity. Clear vinyl tubing taped to the
end of a hose can be inserted all the way into a
stud bay and then gradually pulled back as the
cavity fills and the insulation slows.
Blown-in installation is typically fed through a 3-in. hose that is reduced into
a 1-in. nozzle, which is inserted into holes drilled in the siding. The remote
Plugging holes. Insulation suppliers will have a
switch draped over the end of the hose enables the operator to shut off the
stock of beveled plugs or corks. After smearing
blower quickly should a clog develop.
416 Chapter 14
exterior-grade glue around plug edges and driv-
ing the plugs flush to the sheathing, an installer
Two Things to BE AWARE oF
can replace the siding, caulk the joints, prime,
and paint.
If dense-pack insulation is packed too densely, interior drywall can bulge. So as an
installer fills the first few stud bays, a second worker should monitor drywall or plas-
InSTAllInG EpS pAnElS
ter surfaces for nail pops, cracks, and bulges. Such excessive pressure isn’t common,
To CREATE A dRy BASEMEnT
but it can happen. Second, if a cavity takes forever to fill, either the hose is clogged or
In the last decade or two, building scientists have
the cavity has an opening, and cellulose may be pumping into a nearby cabinet or
changed their minds about the migration of
crawlspace. If insulation is flowing and the cavity’s still not filling, an installer should
moisture through exterior walls. Today’s best
shut off the blower, find the escape hole, and plug it.
thinking (www.buildingscience.com) suggests
that builders accept that a small amount of mois-
ture will migrate and choose materials that won’t
Renovation 4th Edition Page 97
