Fixing the Holes Where the Air Gets In,
Frozen pipes. Infrared
photography can reveal some surprises. The leakiest areas of a
building may not be the exterior walls but interior partitions
that conceal hidden air pathways to the attic. Ceiling drywall,
an otherwise effective air barrier, typically stops short at
either side of an interior partition, leaving the top plate
exposed to the attic space. Loose-fitting and air-permeable
attic insulation can allow cold attic air to flow past the
plate into the wall cavity through continuous narrow gaps
between the drywall and the framing (Figure 7).
Figure 7.Interior partitions can be leakier than
exterior walls, as cold attic air bypasses the top plate
through cracks behind drywall and wire or pipe penetrations
(drawing, top; photo, bottom left). In remedial work, the
author seals the plate with two-part expanding foam to prevent
convective looping inside the wall (photo, right).
A convective air loop results, with cold incoming attic air
replacing warm indoor air drawn through electrical outlets and
other drywall gaps. More than once, I've responded to seemingly
freakish complaints of frozen pipes within an interior
partition in a brand-new "well-insulated" home. In new
construction, we make sure that all interior and exterior
partitions are sealed at the top and bottom plates, using
either drywall adhesive or a continuous rubber gasket along
both sides. To correct an existing problem wall, we expose the
top of the partition in the attic and seal the plate with a
layer of expanding foam.
Rafter chutes. In a sloped
ceiling design, soffit-to-ridge ventilation is critically
important to ensure continuous removal of moisture-laden air
that finds its way through the ceiling insulation. We use
cardboard "chutes," purchased from insulation wholesalers,
rather than the ubiquitous polystyrene insulation baffles,
which don't fully cover the underside of a rafter bay or
prevent soffit air from moving under and through ceiling
insulation (Figure 8).
Figure 8.The author uses full-width
vapor-permeable cardboard chutes in cathedral ceilings rather
than rigid foam baffles. A bend in the bottom piece allows it
to act as a wind stop at the top plate.
The chute has a smooth, flat face and prekerfed stapling
flanges that automatically space the panel an inch away from
the sheathing as it's installed. The cardboard is
vapor-permeable but durable enough to be permanent.
Installation begins at the top plate as a soffit blocker, then
transitions to follow the slope of the roof to the ridge vent.
I've recently acquired a rapid-firing pneumatic stapler
makes chute installation a breeze.
Fiberglass batts are widely regarded as the most
cost-effective insulation. But it's difficult to properly
detail batts around obstacles like wires, plumbing, and
electrical boxes and in irregular framing configurations.
Chemical smoke testing also shows that fiberglass provides
little resistance to air movement. Expanding polyurethane or
icynene foams provide good solutions to these problems, but are
way more expensive. For us, the answer is cellulose; we've had
good results dry-blowing cellulose in both new and retrofit
work. Blown-in cellulose effectively fills very small voids and
hard-to-access areas at a competitive cost per square foot. Its
R-value is 3.5 per inch, and when installed at the proper
density of 3.5 pounds per cubic foot, it is highly effective at
reducing air infiltration. The fact that cellulose is a
recycled product (newspaper) makes it even more appealing to my
Settling not a problem.
Complaints that cellulose is prone to settle after installation
are based on a common misconception. Voids found in an existing
cellulose job are invariably due to faulty installation. If the
wall bay isn't filled at the minimum density, or is
incompletely filled, voids will occur, regardless of the
insulating material. Blown at a minimum density of 3.5 pcf,
cellulose is installed at a density greater than its own
natural settled density, which eliminates future voids.
The best way to blow cellulose in a new home or addition is to
treat walls and ceiling separately, at different stages. Flat
ceilings are best blown after the drywall has been hung.
Drywall provides containment, a built-in air barrier, and
unyielding support for a 16-inch-deep layer of cellulose.
For walls, we use a reinforced plastic membrane for
containment. In an ordinary wall installation, I use par/PAC
poly membrane. The membrane is tacked up, then stretched taut
over the edges of each stud and stapled to its inner face.
Stapling the membrane like this prevents the cellulose from
"migrating" across the stud face when blown, trapping lumps
that interfere with drywall installation.
Proper density. The
difference between good and poor cellulose installation is
about 15 seconds per stud bay and some basic technique. To
ensure complete, void-free filling, I use a rigid PVC wand
tubing at the end of the feeder hose. By inserting the wand
through a slit in the middle of each stud bay, I first fill
from the bottom, withdrawing the wand as the cellulose fills to
slit level. I then reverse direction and fill from the top of
the bay down (Figure 9).
Figure 9.Blowing the wall from the center down,
then up, ensures complete filling. A rigid extension wand
enables the author to blow to the extremes and gradually
withdraw as the cavity fills.
To blow the cellulose into narrow and hard-to-reach areas, I
switch to a smaller-diameter, flexible adapter, made from a
length of vinyl tubing. The membrane is designed literally to
take a beating. As the bay fills, I vigorously slap the
membrane to help condense the cellulose and flatten the face,
which otherwise bulges from the fill. With experience, you
develop an accurate feel for the proper fill density.
Upgraded wall system. My
preference, where the budget allows, is to sheathe the interior
wall, using 3/4- or 1-inch-thick foil-faced foam board, after
filling the cavity with cellulose. The foam board provides a
thermal break over the studs, reducing convective heat loss
through the framing. This application virtually guarantees that
there are no "cold spots" on the wall where moisture might
condense and support mold growth. The foil facing, with all
seams and fastener penetrations taped and sealed, creates an
effective air barrier that also retards the convective movement
of moisture into the wall cavity (Figure 10).
Figure 10.Foam board applied to the interior face
of the exterior walls breaks thermal bridging through the studs
and provides and effective air and vapor retarder.
To retain the cellulose, I use a less costly, vapor-permeable
membrane such as 100% polypropylene InsulWeb (Hanes Industries,
or MemBrain (CertainTeed, 800/233-8990,
a polyamide film whose permeability changes with ambient
humidity conditions. Both products claim to avoid the potential
moisture-trapping problems of conventional vapor
On sloped ceilings, we nail up 1-inch-thick foil-faced foam
board but leave a narrow "window" near the middle of each slope
for blowing access (Figure 11).
Figure 11.A narrow access strip in the middle of a
cathedral ceiling insulated with rigid foam allows the author
to view the bays as he fills them.
Before installing the board, we cover the window area with a
strip of containment membrane; elsewhere, the board holds the
cellulose in place. After we blow the rafter cavities, the
window is closed with foam board. All the seams and fastener
penetrations are then sealed with housewrap tape.
Intricate or hard-to-reach framing transitions like tray
ceiling perimeters, floors behind cathedral knee walls, and
cantilevered rim joists are difficult to properly seal and
insulate. We've had good success using a two-component
polyurethane foam marketed as Zerodraft (Canam, 877/272-2626,
especially in remedial applications where initial air sealing
was never properly done. The two-component pressurized system
is a rapid-high-expansion foam, packaged with a 30-foot hose
and applicator, with a 600-board-foot coverage capacity. The
foam cures in 45 seconds and makes it simple to seal otherwise
challenging configurations in short order (Figure 12). At
around $400 per pack, it's too expensive to use as the primary
insulation, but it's unbeatable for tricky areas and sealing
leaks. I typically get about four average houses out of a
Figure 12.Two-component expanding foam seals and
insulates in one step, making it an ideal solution for awkward
configurations like this overhanging second-floor rim
Tight houses require ventilation to avoid problems with indoor
air quality, condensation, and mold. Quantifying how much fresh
air is neededis a nearly impossible as well as somewhat
subjective task. The ASHRAE (American Society of Heating,
Refrigerating and Air-Conditioning Engineers) standard
recommends .35 ACH, or a complete replacement of indoor air
every three hours. Another standard states that for each
occupant, you need 15 cubic feet of air change per hour. This
provides a decent working rule of thumb for determining exhaust
ventilation rates. But because natural ventilation rates vary
widely with wind pressure and differences between outdoor and
indoor temperatures, mechanically assisted ventilation is
essential to ensure a continuous rate of ventilation under all
conditions. And, because the number of occupants and their
activities also vary, the ventilation system itself should be
variable. But that doesn't mean it has to be complicated.
I always install quiet high-efficiency fans in the bathrooms
or a remote-mount inline fan to provide whole-house exhaust
ventilation (Figure 13).
Figure 13.The author often installs a
remote-mounted inline fan (top) to provide ventilation for one
or more bathrooms, as well as for the whole house. A
programmable control is set to provide the correct flow based
on the number of occupants and the house's infiltration rate.
The author verifies the fan's rated capacity with a flow meter
There are many such fans available from a number of
manufacturers, including Aldes, Fantech, and Panasonic. I
typically install an Airetrak microprocessor control system
(Tamarack Technologies, 800/222-5932,
operate the fan. The programmable control operates the fan at a
constant low speed, moving the air at, say, 75 cfm for a family
of five (5 x 15 cfm = 75 cfm). But if one of the occupants
takes a shower or wants a momentary higher rate of ventilation,
pressing a "boost" button runs the fan at higher velocity for a
preset interval of 20 minutes or a longer programmable period.
As an option for clients who want a more hands-off system, I'll
install Tamarack's Humitrak control in the bathroom. This
automatically boosts the fan speed as the humidity level rises.
The manual boost option is still available. Finally, to make
sure the fan is performing at its intended capacity, I verify
its operation with a flow meter.
When I have a client who places a high priority on ventilation
control and energy efficiency, we'll install a heat recovery
ventilation (HRV) system.Bruce Torreyis a building contractor and consultant
on energy-related building problems in Sandwich, Mass.