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Wednesday November 08 2006
Getting Quality from Fiberglass Insulation
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My company specializes in high-quality insulation work, in both new construction
and retrofit applications. We also provide energy consulting and energy audits
that include testing with a blower door, duct blaster, infrared camera, and
other diagnostic tools.
Experience has convinced us that blown-in insulation products are the way to
go if quality is the top priority. Correctly done, a blown-in job has inherent
quality control, making it the simplest and best way to eliminate heat-wasting
gaps or voids in exterior walls (see “Insulating
With Spray Cellulose,” 10/01). But blown-in insulation products cost
more than fiberglass batts, so most walls today are insulated with batts.
Unfortunately, most fiberglass installers don't give much thought to quality.
That's not surprising — batts are the budget choice to begin with,
and when the job goes to the lowest bidder, the only way the installer can make
money is to rush through the work. But the results are bad news for the home's
thermal performance. The more complicated the wall assembly, the harder it is
to get a good fit (see Figure 1). A 6-inch batt may have a measured R-value
of 19 at the factory, but a poor installation in an exterior 2x6 wall can degrade
the R-value by as much as 40%.
It doesn't have to be that way. Given a realistic budget to work with,
we've found that we can do high- quality work with fiberglass and still
make a profit. The keys are coordination between trades, quality control, and
knowing the right details.
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Figure 1. Irregular
stud cavities, wiring, and other obstructions in a wall like this make it harder
to cut and fit fiberglass accurately. Dark lines and visible stud shoulders
in this wall indicate a hurried job.
How Insulation Works
Fiberglass itself has little resistance to heat flow. The actual insulator is
the air trapped in the tiny spaces between glass fibers. The tiny air voids
slow conductive heat movement, while the glass fibers reduce radiant losses
and impede air movement to block convective heat flow.
Don't be fooled by so-called dead air spaces. Small air voids slow heat
flow, but large voids don't. A dead air space is one in which air does
not move — once a gap gets larger than 3/4 inch, convection kicks in and
overrides the insulating effect. Even though they contain air, uninsulated framing
cavities have little or no R-value.
Making contact. That understanding should
govern the way batts are installed in the field. They should make good contact
with wall and ceiling and nestle snugly against the sub-floor within floor cavities.
If the batts don't touch the inside face of the drywall or subfloor, convection
coupled with air leakage will seriously undermine their thermal performance.
We use only unfaced batts in exterior walls, because we've found that inset
stapled kraft-faced batts tend to create gaps between the insulation and the
drywall. (Using unfaced batts also prevents the drywallers from complaining
about the presence of stapling flanges on the surface of the framing.)
In other words, even though shoving batts into a stud cavity may seem like a
no-brainer, doing it right takes some care. If a batt is simply jammed into
place, its edges tend to drag along the sides of the studs on either side, which
often prevents the rear corners of the batt from coming into contact with the
exterior sheathing. To avoid this problem, I teach my crews to push each batt
into its cavity, then pull it out flush with the face of the framing (Figure
2).
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Figure 2. To achieve
the required tight fit, batts are trimmed to the required height and width in
place. Each batt is pushed tight to the sheathing, then pulled forward until
it's flush with the stud faces. On inspection tours, the author checks
for exposed stud shoulders and makes sure batts have been split around wires
as needed.
Fiberglass needs protection. Fiberglass batts insulate properly only
if they can trap still air; they aren't an air barrier material themselves.
Wind or even stack pressure in a heated house can readily push air through unprotected
batts. So framing cavities need to be closed in with cardboard or plastic baffles,
rigid foam, caulking, and the like, and walls should be protected with housewrap
or something similar. I recommend an inch of rigid insulation on the exterior
of buildings in addition to the fiberglass in the wall cavities.
In addition to keeping wind pressure from pushing air through or around the
batts themselves, the rigid foam also cuts conductive heat loss. Fiberglass
batts have an R-value of R-3 to R-4 per inch, but wood framing — which
makes up at least 20% to 30% of an exterior wall — has an R-value of 1
per inch, allowing heat to bypass the insulation by flowing through studs, plates,
and headers. The added inch of rigid foam can reduce this bypass heat loss by
half, significantly boosting the performance of the whole assembly. You'll
see the difference in equipment sizing and utility bills.
Quality Control
The market seldom rewards or demands much in the way of professionalism from
insulation contractors. Bad work gets rocked over within days or hours, and
unlike plumbing, roofing, and electrical defects, sloppy insulation rarely comes
back to haunt a builder.
Most installers get paid piece-rate, which rewards quantity, not quality. Around
here they get two or three cents a square foot. Can you blame them for blasting
through a job to get a bigger check? With piecework, it's practically impossible
to see that the job is done right. I pay my men hourly, and even then it's
essential to inspect the work and make sure that any errors are fixed before
the walls are closed in.
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Figure 3. Framers often
leave spaces blocked off that are hard or impossible to insulate. Left, the
intersection between an interior and an exterior wall can be reached only through
a small gap. On the right, a wood I-joist has been hung out over the top plate
to serve as a nailer for the ceiling drywall, completely blocking access to
the band joist. (For the correct detail in this area, see Figure 6, next page.)
In both photos, the kraft-faced batts have been jammed into place with no effort
to achieve a proper fit.
Seams and shoulders. When I walk onto a fiberglass
job, I immediately begin looking for dark seams or shadow lines that indicate
where an improperly cut batt stops short of the framing. Another detail I look
for is the shoulder or side of the stud. I want to see only the edge or the
face of the framing, not the side. If I see a lot of shoulder, I know the batts
have been compressed from hasty installation. I also lift up batts adjacent
to electrical receptacles and plumbing fixtures to make sure they have been
split around wires and pipes.
Working with subs. Good batt installation
requires the cooperation of the builder, framers, plumbers, and other subs,
because framing, plumbing, and other details can create large inaccessible areas
that often don't get insulated during assembly.
In infrared camera images, those empty voids show up clearly as hot spots or
cold spots on a wall. It's common to find inaccessible voids framed into
arched window headers, outside corners, wall intersections, and rim joist areas
(Figure 3). Draft stops are often missed, and many builders mistakenly think
that simply laying a batt over a chase will be adequate.
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Figure 4. Batts behind
manufactured fireplaces are strapped in place so they won't sag or droop.
Kraft-faced batts aren't used because the facings are combustible.
When a house has big built-in voids that the insulators can't reach, it
doesn't much matter how careful you are everywhere else. Ideally, the plans
should call out good framing and insulation details to ensure that no major
thermal defects will be built into the structure.
I advise builders to let us come out to a job and insulate before they seal
up cantilevers, install metal fireplaces, set fiberglass tub enclosures, run
ductwork, or do anything else that will make the installation of batts more
complicated later. When we insulate behind metal fireplaces, for example, we
strap up the batts so they are supported for the life of the building (Figure
4). Unfaced batts that are not strapped or wired off might slip or sag over
time. For the sake of fire safety, we don't use kraft-faced batts in those
areas. If timing is an issue, we encourage our GCs to take some fiberglass from
our warehouse so they can insulate special details themselves.
Insulating Flat-Framed Assemblies
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Figure 5. Flat-framed
assemblies pose a special challenge. Here, the wall between a cathedral space
and the unconditioned attic consists of a gable truss. The author fastens batts
across the back side of the truss using Simplex nails, but he notes that this
does not fill the voids between truss members.
Kneewalls & Skylights
The framing at skylights, knee walls, and ceiling transitions doesn't always
lend itself to batt installation. We often have to insulate a structure that
is framed on the flat, or that incorporates a truss. In such places, drywall
faces the conditioned space and the flat-framed back of the assembly is exposed
to the unconditioned attic. There's really no way to fit batts between
those flat 2x members. We usually end up fastening batts over the entire assembly,
framing and all, but that does not fill the 1 1/2-inch-thick voids between framing
members.
Here's how we make do: On skylight wells I recommend using a 5-inch Simplex
nail (Figure 5). The head of this nail will hold the batt in place, and the
large nail won't overly compress it.I don't think a 5/16-inch staple
provides enough insurance. Staples also compress the batts.
On conventionally framed kneewalls with full-depth voids between studs, I think
the best detail is a face-stapled kraft-faced batt. Inset-stapled batts can
lead to cold air leaking down from the attic between the drywall and kraft paper.
A face-stapled batt is more likely to withstand the test of time.
by Michael Uniacke
This article has been provided by www.jlconline.com. JLC-Online is produced by the editors and publishers of The Journal of Light Construction, a monthly magazine serving residential and light-commercial builders, remodelers, designers, and other trade professionals.
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