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Getting Quality from Fiberglass Insulation
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.
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).
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.
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.
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
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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