hidden in your walls, no
moving parts to fix—is
a material you probably
spend precious little time
thinking about. Then along
comes subzero (or sweltering!)
weather, a three-digit
utility bill or chilly
drafts, and you start thinking
about it a lot! At home,
you ponder whether it's
worth the time and
expense to add it to your
ceilings, walls and basement.
At the store, you
ponder which type, thickness,
width and density to
buy. And when you install
it, you wonder just how
good is “good enough.”
We asked insulation
installers which questions
they field most—and
which blunders they see
most. Following are six
things they (and we) think
you should know.
Seal up air bypasses before adding attic insulation (or you'll be wasting money)
Attics are one of the easiest and
most cost-effective places to add insulation.
But you'll increase the effectiveness
of insulation substantially if
you first seal up air bypasses (Fig. A)
around chimneys, plumbing vents,
wires, interior walls and exhaust
fans—places where warm, moist interior
air escapes into the attic.
Heat has a natural inclination to
both rise and migrate to colder
areas. Combine these two tendencies
and you can see why air bypasses
can reduce the effectiveness
of attic insulation by 30 to 70 percent.
You can track down bypasses
by lifting existing attic insulation
and checking for dark patches of
moisture or dust. Or head to the
attic on a cold day and feel for pockets of warm air or use a stick of
incense to check for drafts.
Before installing (or adding) attic
insulation, take these steps:
- Use caulk and expanding foam
sealant to close air gaps around
pipes, ducts and electrical wires
where they enter the attic. Use the fire-blocking type.
- Cut and fit strips of 24-gauge
sheet metal between the masonry
chimney and the surrounding
wood framing. Use high-temperature
caulk to seal the flashing
where it meets the chimney.
- Install weatherstripping around
the perimeter of the attic access
opening, then use screw hooks to
pull the hatch tight against the
weatherstripping. Glue rigid
extruded foam insulation to the top of the hatch.
NOTE: After air-tightening your
home, always test your carbon
monoxide detectors, or install
them if you don't have them.
Figure A: Seal Attic Bypasses
Seal attic bypasses
to stop air leaks and increase
the effectiveness of new or existing
Careful installation increases the insulation's R-value by over 20 percent
Leaving 5 percent of a wall uninsulated
will reduce the entire wall's
R-value (resistance to heat flow) by
20 percent. It doesn't take much
more time to install fiberglass insulation
properly (Fig. B). The most
important steps you can take:
- Fill the stud cavity from top to
bottom and side to side. To avoid
guesswork when insulating walls
built from standard 92-5/8 in.
long studs, purchase and install
precut fiberglass batts that are 93
in. long and 15 in. wide. When
you have to custom-cut batts at
wall corners and other places, cut
the batts 1 in. higher and wider
than the cavity you're filling.
- Split your insulation so half goes
in front of and half goes behind
electrical wires. Compressing
insulation reduces its R-value.
- Around electrical boxes, notch,
rather than compress, the batt—then tuck the cutout behind the
box. To help prevent frozen pipes,
insulate only on the cold side.
Figure B: Wall Insulation
Install wall insulation so it completely
fills each cavity side to side and top to bottom.
But remember, packing and compressing insulation
reduces its effectiveness.
You probably don't need kraft-faced insulation
Kraft paper—the asphalt-impregnated
facing available on insulation—is rarely called for these
days. When insulation was
first developed, it was only an
inch or two thick and the
attached kraft facing was stapled
to studs to keep it from
sagging. Insulation today is so
“full-bodied” and fills stud
and joist cavities so completely
that it resists settling—even when walls have been purposely
vibrated in tests.
Kraft paper does act as a vapor
retarder to slow the movement of
interior moisture through the wall
cavity and insulation. But for a
thorough job, especially in bathrooms
and other high-moisture
areas, a continuous 6-mil plastic
sheeting vapor barrier is much more
effective. Kraft paper still serves the
purpose of temporarily holding
insulation in place on horizontal or
sloped surfaces. And when you're
retrofitting insulation in the floor of
a crawlspace (Fig. D), installing the
kraft-faced insulation paper side up
provides an adequate vapor retarder.
Kraft paper and its underlying
asphalt adhesive are flammable and
should always be covered with drywall
or other fire-resistant material.
Another important point: Except
in hot coastal regions, insulation
should be installed with the kraft
paper (or plastic vapor barrier) on
the interior side of the wall. In hot
regions, vapor barriers are often
eliminated or positioned toward the
outside of the stud wall. If in doubt,
consult a local building inspector.
High-density insulation can pay off
Insulation is rated according to its
R-value, or resistance to heat loss:
the higher the R-value, the higher
the insulating value. Standard
fiberglass insulation has an R-value
of about 3.5 per inch of thickness
(Fig. C); this provides an insulating
value of R-11 for 2x4 walls and
R-19 for 2x6 walls. But if you're
serious about energy savings, you
can buy better-performing products.
High-density types of fiberglass
insulation, with more fibers
and air spaces per square inch,
offer R-values of up to 4.25 per
inch. Some provide R-15 for 2x4
walls and R-21 for 2x6 walls.
There's also high-density insulation
for ceilings and attics. You'll
wind up paying more and in some
cases you'll need to special-order
it, but high-density insulation
delivers up to 35 percent more
insulating value per inch.
You can also pack 5-1/2 in. thick
insulation into a 3-1/2 in. thick
wall to increase its R-value, but the
compressed R-19 batt will only
yield an R-value of about 17, and it will make it more difficult to get drywall flat against the wall.
Standard vs. high-density
Figure C: High-Density Insulation
The high-density insulation on the right, with
more fibers and air spaces, can deliver up to 35 percent
more insulating value than standard insulation.
How much insulation is enough?
The first 3 in. of insulation you add
to a bare ceiling or wall will yield
huge savings. Adding another 3 in.
will increase energy savings, but not
to as great a degree. For charts
showing the Department of Energy's
optimal amounts of insulation to
install, check the insulation manufacturers association at naima.org. Their
recommendations are based on climate,
fuel costs and other factors.
Adding more will give you a diminishing
return on your investment.
The payback period for retrofitting
insulation varies greatly, but
studies show that added insulation
usually pays for itself in saved energy
costs within 5 to 10 years. In Minnesota, a typical payback period might be five or six years. Payback in more
temperate areas takes longer, but
remember that insulation reduces
air conditioning costs too.
Back to Top
When you insulate attics and crawlspaces, you've got to vent them too
Since insulation changes the way attics and crawlspaces
“breathe,” it's critical that you maintain or
install proper ventilation (Fig. D). At first, it seems
odd to add insulation for warmth and then purposely
create ventilation “holes” for cold air to
enter. But if you don't do this, you're setting yourself
up for moisture problems.
When you add insulation to attic rafters and
attic floors, it's important to maintain at least a
1-in. continuous air space between insulation and
roof sheathing, from eaves to ridge. This air space
flushes out moisture. It also helps maintain a
“cold roof,” which prevents ice dams from forming
in the winter and excessive heat from damaging
shingles and increasing cooling costs in the
summer. The biggest mistake homeowners make
with installation is to install it so it blocks the flow
of air at the eaves. The best way to avoid this
problem is to install inexpensive air chutes to
keep the space open.
Newly insulated crawlspaces also need proper
ventilation. The standard procedure is to insulate
the ceilings of unheated crawlspaces and the walls
of heated crawlspaces. To reduce moisture
migration if there's no slab, a 6-mil polyethylene
vapor barrier should be laid on the ground in
both cases. At least 1 sq. ft. of ventilation should
be installed for each 1,500 sq. ft. of floor area. For more information, see How to Install a Vapor Barrier in a Crawl Space.
There are regional differences and various
codes and interpretations for insulating crawlspaces
and attics. Contact a local building official
to make sure your house stays code-compliant.
Attic and crawlspace insulation
Figure D: Attic and Crawlspace Insulation
Venting attics and crawlspaces
is critical to reduce moisture problems and allow
the insulation to perform right.