As a contractor during the1980s energy crisis, I directed my
energies toward building the beta version of the
super-insulated home. I experimented and struggled with my
share of "cutting edge" strategies, with mixed results. It
became quickly apparent that some of the experimentation
— like site-built skylights — was less than
successful. What was not so obvious at the time was that, in
spite of beefing up the insulation levels in everything I
built, I wasn't really getting the thermal performance I would
However, as more building diagnostic tools became available, I
was able to see the flaws in my previous insulation strategies.
I also saw a business opportunity: troubleshooting the
all-too-common efficiency and comfort problems in
Over the last couple of decades, energy prices have continued
to rise and energy codes have gotten stricter, but many of the
same flawed insulation strategies are still being used in the
field. Meanwhile, the consumer's demand for increased comfort
often goes unanswered. Many of the calls we get involve
existing homes that have new replacement windows and are
already insulated. So what else is left to do?
Fortunately, advances in building science are helping the
industry take a fresh look at how buildings really perform and
what types of improvements are most practical and effective.
The critical first step is to understand how a building loses
heat. This may sound obvious, but there are still many
Probably the biggest misconception involves where a house loses
most of its heat and what types of details can stop these
"hidden" drafts. In most building configurations, fiberglass
batt insulation — or even loose cellulose blown into an
attic — doesn't do a good job of controlling air
movement. Many studies have shown that air moves through
fiberglass batts and degrades their R-value. And while
cellulose can stop air movement when it's blown into closed
cavities at densities above 3.5 pounds per cubic foot,
loose-fill cellulose blown in an attic will not stop air
Most of the homes I visit have attic insulation, but they also
have many air leaks from inside the house into the attic
— at mechanical penetrations and plumbing chases, along
partition-wall top plates, at framing intersections like
soffits or other changes in ceiling height, around chimneys,
and so on. In a typical leaky attic, upward air pressure from
the stack effect — the tendency of warm air to rise
— can replace all of a home's heated inside air with cold
outside air in just two or three hours.
Homes with knee walls or side attics have the further
complication of horizontal heat loss and infiltration between
heated floor cavities and the eaves (see Figure 1). In many
cases attic ventilation only makes this worse, by connecting
the interior to the outside and allowing wind to move far into
the heated space.
Figure 1. Even though they're often
insulated, attic knee walls are a common thermal problem area.
Because there is typically no blocking below the knee-wall
bottom plate, cold air from the attic or outside air moving
through the insulation at the eaves chills interior ceiling
cavities, drawing heat away from the living
New Tools, Better Results
These air leaks seldom get addressed during traditional home
improvements because they're not visible from inside the
conditioned space; they're concealed inside framed cavities,
behind drywall or plaster. But with modern diagnostic tools,
it's possible to find them and to complete an accurate,
detailed heat-loss assessment of a home in just a few
Using an infrared thermal camera in conjunction with a blower
door, an energy auditor can identify leaks in the air envelope
and insufficient or missing insulation in the thermal
On a typical job, I first set up the blower door and
depressurize the house to 50 pascals, relative to the outside.
This approximates the range of pressures a building would be
subject to on a very windy night and helps establish a
consistent benchmark for comparing the leakiness of one
building with that of another.
The blower-door fan draws outside air in through penetrations
in the shell and exhausts it through an entry door. The
calibrated fan measures the cubic feet per minute of airflow
required to maintain the 50-pascal pressure difference between
inside and out. The draftier the shell, the higher the flow in
cubic feet per minute. Simple math helps us convert the cfm
reading to a whole-house air-exchange rate, stated in air
changes per hour.
With the blower-door fan running, I make a visual inspection of
the home, looking for any obvious leakage spots (Figure 2). A
smoke pencil — a handheld device that emits a stream of
chemical smoke — helps in tracing drafts.
Figure 2. With the blower door running,
the author uses a smoke pencil to detect leaks in the house's
air boundary. The leaks shown here — at a window sill
(top left), at a plumbing penetration in a base cabinet (top
right), and at one of several can lights (bottom) — were
the result of a kitchen remodel that left the home feeling
draftier than before.
After the blower has been running for a while, drawing in cold
outside air, I take another tour through the house, this time
using the infrared camera to scan for hidden air leaks and
thermal bypass problems. The thermal image viewed through the
camera reveals the radiant temperatures of the surface scanned.
Since the R-values — and therefore the resulting
temperatures — of an insulated bay and the adjacent wood
framing members are different, it's simple to identify the
framing details as well as weak spots in the insulation (Figure
3). Thermal scans often reveal areas where the insulation has
settled or is not dense enough, and wall and ceiling bays that
were completely missed when the insulation was installed.
Figure 3. The darker spots in these
thermographic images indicate the presence of cold in framing
cavities behind the drywall surface — areas where cold
air has infiltrated or heat has been lost through conduction.
At top (A), the framing around a Palladian window lacks
insulation, while the ceiling area above suffers from
wind-washing at the eaves. An interior soffit (B) is cold
because the batt insulation does not make good contact with the
drywall; also, the gap around the ceiling register is drafty.
The uninsulated wall around an interior fireplace (C) is
chilled from cold attic air dropping down through the vertical
chimney chase between the framing and the masonry.
Because the blower door is drawing in cold air from the
exterior or attic, the scan can identify and measure areas of
air infiltration, recognizable by a plumelike thermal pattern
Figure 4. Wind-washing — air moving
under and through the ceiling insulation at the eaves —
degrades insulation performance, as is evident from the dark
areas in the ceiling next to the exterior wall (top left).
Leaks in window framing (top right) and around an exterior door
(bottom) show up in a typical plumelike pattern.
The scans also show areas where the wall surface has become
chilly, indicating that the cold air is moving through or
beneath the insulation. These spots are common on ceilings near
the eaves, due to the effects of wind-washing (outside air
moving into the eaves and through the fibrous insulation), but
they can also show up in interior locations where you might not
expect to find them (Figure 5).
Figure 5. The infrared scan revealed an
unexpected cold area in this interior partition wall, which was
traced to air leaking into the attic along the top plate, shown
here being sealed (bottom).
Because framing details, air leakage paths, and insulation
quality vary from home to home, there are no boilerplate
solutions. Sealing air leaks into the attic is usually the most
cost-effective improvement, but it can also be the most
challenging. Fiberglass batts and loose-fill cellulose do not
stop air leaks, so simply covering the leakage points with
insulation doesn't work. Neither does sealing cracks in the
attic floor above the insulation: Since the insulation is air
permeable, you have to seal the leak below the
It's important to keep in mind that the drywall ceiling is
discontinuous — it's interrupted by interior partitions
and framed chases that enclose leaks. While those partitions
and chases may appear airtight from inside the house, viewed
from the attic they are full of penetrations for electrical,
mechanical, and plumbing runs (Figure 6). Even the long
intersections between the edges of the top plates and the cut
ends of the ceiling board provide air paths into the
Figure 6. Moving attic insulation uncovers
a common cause of leaks: wire holes through partition-wall top
plates (top). The double line of soot stains indicates upward
air leaks between the drywall and a partition plate (middle
left). Fiberglass stuffed into a plumbing wall (middle right)
was no match for the air rising from below; note the spray foam
at left in the photo — the solution. A double layer of
fiberglass did not prevent air movement through an interior
soffit into the attic (bottom right).
The first step in the sealing process is to move the existing
attic insulation to expose the air leaks. These include
penetrations in the middle of a ceiling, like air supply
registers and light fixtures, as well as the top plates of all
Two-part foam. For sealing the leaks, most
weatherization contractors use two-part polyurethane foam,
which comes in cardboard containers in various sizes with an
attached 30-foot hose and spray nozzle. The foam sticks well to
dirty surfaces, and the high-pressure applicator makes it easy
to spray in hard-to-reach places (Figure 7).
Figure 7. Because it sticks even to dirty
surfaces, two-part urethane foam works well for sealing around
attic penetrations (left). A worker seals beneath an attic
ledger (right), after first chinking the gap underneath with
Small penetrations, like the gaps around light fixtures, fan
boxes, and duct boots, can be sprayed directly. With larger
bypasses — wet walls, unblocked balloon-framed cavities,
the space under an attic knee wall — the opening can
first be loosely stuffed with fiberglass, then sprayed. For
extremely large openings like drop soffits and large chases,
it's best to fit a piece of plywood or rigid insulation board
over the hole, then seal it in place with foam and screws
Figure 8. Rigid foam board is ideal for
sealing large openings like this oversized framing chase. After
cutting the board to fit, the worker beds it in wet spray foam
(left), then seals the edges (right).
Fire-code sealant. One place where you can't use foam
— due to fire codes — is around chimneys and flues.
Because of code-required clearance to combustibles, it's not
uncommon to find leakage areas of 3 square feet or more around
a masonry chimney. Here it's best to seal the gap with sheet
metal sealed with an ASTM 136 high-temperature caulk.
Once penetrations have been sealed, the attic is ready for an
additional layer of blown insulation, which fills gaps in the
fiberglass batts and also covers the tops of the ceiling
joists, reducing conductive heat loss.
Knee walls. Field and lab testing has confirmed that
batt insulation works best when it's enclosed in an airtight
space. This is rarely the case with attic knee walls, which are
typically open on the attic side. So I often recommend adding a
layer of rigid foam to the back of knee walls and taping the
joints. This prevents air movement into the wall cavity from
the attic side, and cuts conduction through the studs. Another
method is to add a second layer of fiberglass horizontally
across the backs of the knee-wall studs and secure it with a
layer of housewrap — again, to reduce infiltration and
conductive loss through the studs.
If for some reason an attic wall has not been insulated, I
recommend securing housewrap across the studs and blowing the
cavities with loose-fill insulation.
Some knee walls contain ductwork or pipes that make them
impossible to insulate effectively. In these cases, it makes
sense to move the thermal and air barrier to the outside roof
slope by adding a code-approved rigid board to the underside of
the rafters and blowing in cellulose. Where the budget can
handle it and for spaces with limited access, an approved spray
foam also works well.
Don't forget exterior walls. Although this article
focuses on attics, it's worth noting that we often recommend
additional blown-in insulation in the exterior walls below. I
say "additional" because many of the homes I visit already have
some type of wall insulation, but the scans frequently show
that it's settled or insufficient. Be sure to have the
insulation contractor blow in dense-pack cellulose at such
spots; otherwise, cold air in the walls will find its way along
floor and ceiling cavities and make the house cold and
With energy costs increasing, the demand for weatherization is
also on the rise. If you're a remodeler, this might be a good
time to consider providing your clients with a more
comprehensive approach to thermal upgrades, in addition to the
traditional home-improvement services.
After the first energy crisis, we learned that simply adding
insulation is not enough — you have to address air leaks
as well. Hopefully that lesson is not lost, and with a new
generation of diagnostic tools on hand, there's no excuse not
to get it right.Bruce Torrey is an infrared thermographer
and building consultant. His company, Building Diagnostics,
provides technical support and training to builders,
architects, and insulation contractors.