Of all of the commonly used types of insulation — including cellulose, rigid foam, and spray polyurethane foam — fiberglass batts perform the worst. As typically installed, fiberglass batts do little to reduce airflow through a wall or ceiling assembly; rarely fill the entire cavity in which they are installed; and sometimes permit the development of convective loops that degrade insulation performance.
Knowing this, why would any builder choose to install fiberglass batts? The answer is simple: because fiberglass batts cost less than any other type of insulation.
Before we totally dismiss all fiberglass batt installations, however, it’s important to note that there is a big difference between the typical fiberglass batt installation and a best-practice installation. If a conscientious builder installs fiberglass batts carefully, it’s possible — although not easy — to get the best of both worlds: adequate thermal performance at a relatively low price.
Lots of sloppy installations
Study after study has shown that most fiberglass batt jobs are sloppy. In 2002, the California Energy Commission contracted with researchers Marc Hoeschele, Rick Chitwood, and Bill Pennington to conduct a study of new California homes. In its March 2003 issue, Energy Design Update reported, “The performance problems uncovered by the study were particularly disappointing in light of the fact that all 30 houses [studied by the researchers] were enrolled in programs promoting building-envelope improvements and duct tightness.”
The article went on to note that “the California Energy Commission’s ‘envelope protocols,’ which include standards for air sealing and insulation installation, were widely ignored. … Not a single builder managed to implement any of the following standards:
Even builders enrolled in the EPA’s Energy Star Homes program have struggled to achieve good thermal performance in fiberglass-insulated homes. An article in the April 2005 issue of Energy Design Update, “Fiberglass-Insulated Homes Are the Leakiest,” discussed the findings of Bruce Harley, the Conservation Services Group’s technical director for residential energy services. “Harley assembled airtightness data on Energy Star homes (including single-family and multifamily homes) completed in 2004 in Massachusetts and Rhode Island. All of the homes were blower-door tested after completion,” EDU reported. “Harley found that houses with walls insulated with spray polyurethane foam were significantly tighter than those houses with walls insulated with cellulose, and that houses with walls insulated with cellulose were significantly tighter than those insulated with fiberglass.”
What’s required by code?
Many builders don’t realize that sloppy fiberglass installation is a code violation. For example, section 102.2 of the 2006 International Energy Conservation Code requires that “All materials, systems and equipment shall be installed in accordance with the manufacturer’s installation instructions and the International Building Code.”
This provision is relevant because the installation instructions provided by fiberglass batt manufacturers are widely ignored. Harley has written, “Most installation instructions require fluffing insulation to the proper thickness, covering continuously, filling cavities completely, and fitting products around all obstructions, such as wiring, plumbing, and framing.”
CertainTeed instructs installers of its fiberglass batts to follow recommendations of the North American Insulation Manufacturers Association (NAIMA). NAIMA recommends:
- “When insulating side walls, place the insulation in the cavity and check to be sure it completely ï¬lls the cavity, top to bottom.”
- “It is important that insulation be correctly sized for the cavity and ï¬t snugly at the sides and ends.”
- “Even the smallest openings between framing members should be insulated.”
- “Junction boxes for wall switches and convenience outlets at outside walls should be insulated between the rear of the box and the sheathing. Place insulation behind the junction box and if necessary, cut insulation to fit snugly around it.”
Doing it right
We’ve all seen sloppy fiberglass jobs — installations with a wavy surface that include sections of insulation recessed from the studs. Typical fiberglass jobs often have gaps at the edges of batts. The insulation is often pinched by wiring, and batts are rarely trimmed neatly around electrical boxes. (For a great selection of photos showing typical fiberglass batt installation problems, see Allison Bailes’s blog, A Visual Guide to Why Fiberglass Batt Insulation Underperforms.)
Moreover, many installation problems — for example, gaps behind electrical boxes — can’t be seen unless batts are lifted for inspection.
In a good installation job:
- Insulation should be installed without gaps, including at cavity corners.
- Batts should be trimmed about 1 in. oversized so they fit the cavity snugly.
- The entire framing cavity should be filled, without any air gaps between the insulation and the drywall.
- Batts should not be folded.
- Batts should be delaminated where necessary to fit around wiring. There should be no gaps behind wiring, electrical boxes, or pipes.
- Attic insulation should be installed over wall top plates.
- Batts installed in kneewalls and skylight shafts should be protected on all sides by a rigid air-barrier material like drywall, OSB, or Thermoply sheathing.
- If the roof assembly includes ventilation channels, insulation must not block air flow.
Defining sloppy installation
Gaps in fiberglass insulation have a disproportionate effect on thermal envelope performance; in other words, a 5% insulation gap in a wall lowers the wall’s R-value by much more than 5%.
The Residential Energy Services Network (RESNET), a national association of home energy raters, has long struggled with the question of how to estimate the R-value of walls that vary widely in performance depending upon the skill of the insulation installer. Eventually, RESNET developed a useful rating system for insulation installation quality. The system is described in an article published in the January/February 2005 issue of Home Energy magazine, “Insulation Inspections for Home Energy Ratings,” by Bruce Harley.
The RESNET rating system recognizes three levels of insulation installation quality: Grade I, Grade II, and Grade III. “In order to qualify for a Grade I rating, insulation must … ï¬ll each cavity side to side and top to bottom, with no substantial gaps or voids around obstructions (that is, blocking or bridging), and it must be split, or ï¬tted tightly, around wiring and other services in the cavity. In general, no exterior sheathing should be visible through gaps in the material,” Harley wrote. “Compression or incomplete fill amounting to 2% or less of the surface area of insulation is acceptable for Grade 1, if the compression or missing fill spaces are less than 30% of the intended fill thickness (that is, 70% or more of the intended insulation thickness is present).”
The standard for a Grade II installation is lower. “A Grade II rating represents moderate to frequent defects: gaps around wiring, electrical outlets, plumbing, other intrusions; rounded edges or ‘shoulders,’ larger gaps, or more signiï¬cant compression. No more than 2% of the surface area of insulation missing is acceptable for Grade II.”
Grade III installations are the worst: “A Grade III rating applies to any installation that is worse than Grade II.” For further information on the RESNET grading system — including illustrations of good jobs and sloppy jobs — see “Assessing the Quality of Insulation Installed in New York Energy Star Labeled Homes.”
It’s hard to do it right
Although the steps required to install fiberglass batts well are easy to describe, they are fairly difficult to achieve. It is the nature of a fiberglass batt to want to be installed sloppily. Unlike cellulose or spray polyurethane foam, a fiberglass batt doesn’t volunteer to fill a cavity completely; on the contrary, it tends to fight an installer’s attempt to make it fit snugly.
It’s interesting to compare the structural performance of a commonly used construction technique — stick-frame construction — with the thermal performance of a typical fiberglass batt job. Stick-frame construction is redundant and forgiving. Even done sloppily — with a few missed nails or split plates — wood framing rarely experiences structural failure.
By contrast, almost every fiberglass insulation job fails to live up to the R-value promised on its label. Unlike stick framing, the system is unforgiving and totally lacking in redundancy. To achieve the R-value shown on its label, a fiberglass batt must be installed perfectly in a wall or ceiling cavity enclosed by a six-sided air barrier.
Knowing this, builders who want their installed insulation to achieve decent thermal performance have two choices:
- Learn how to install fiberglass batts perfectly.
- Choose a more forgiving insulation system.
Last week’s blog: “Getting More Efficient, But Using More Energy.”