Fiberglass remains the most popular insulation choice among home builders, with better than a 50% market share according to some studies. The advantages are compelling: competitive cost and a ready installer base. But like any fiber insulation, the performance of fiberglass depends on the quality of the installation job. Well-installed fiberglass is as effective as any insulation, R-value being equal, while poorly installed fiberglass underperforms its promised R-value rating. So it’s up to the builder to ensure a quality job.
Fiberglass gets its insulating quality not from the fiberglass itself but from the air that is trapped between the fibers. Wind pressure or even convective air pressures can make that air move, degrading the thermal performance of the batt. So to be effective, fiberglass has to be protected against air pressures. There’s a simple rule that’s not always followed: Align the air pressure boundary of the building with its thermal boundary. In practice, that means that fiberglass batts need to be installed in walls so that the batt is in full contact with the wall sheathing on the outboard side and with the drywall on the inboard side. The batts also have to be in full contact with the studs on either side of the stud cavity.
Housewrap on the wall is critical to the performance of this assembly. Wind blowing into the wall through cracks and crevices can degrade the performance of the insulation by 50% or more. Well-installed housewrap prevents this air intrusion and allows the insulation to perform at its rated value.
In the fast-moving world of the job site, insulation installers are under pressure to make time. The task often is not thought of as a craft where quality matters. But in fact, the insulation installer’s job is significant in the overall performance of the finished house, and there is skill involved. Insulation workers should be trained and supervised to do a quality job.
Batts should be carefully fit to the space. If the stud spacing varies from the standard 16-inch or 2-foot spacing, the batts should be trimmed to fit snugly, rather than jammed into the space. Batts should be cut around obstructions such as outlets or switch boxes. Where wiring interferes with the placement of the batt, the batt should be split and carefully fit around the wires. To make sure the batt is in contact with the wall sheathing, the installer should push the batt back against the sheathing, then pull the face side out from the wall to fluff the material up.
A Complete Air Barrier
To be effective, a home’s air barrier should be continuous and complete. This isn’t too hard to accomplish for most areas of the house, but there are locations in the building envelope where air barrier assemblies are sometimes neglected. These elements are called out in the Energy Star program’s Thermal Bypass Checklist. (See also the EPA’s Thermal Bypass Checklist Guide for detailed instruction on completing the checklist.)
One example is the location where a tub or shower unit is placed against an exterior wall. It’s common for the wall to be insulated, then the tub to be set without the wall being drywalled first. This results in the insulation being exposed to the air behind the tub, rather than protected and supported. To satisfy the Thermal Bypass Checklist, that wall should be drywalled or otherwise covered with a rigid sheet material before the tub is installed.
Fireplaces are similar to tubs and showers: They are often set against an insulated wall that hasn’t been drywalled. Here again, the Thermal Bypass Checklist calls for that wall to be covered with a rigid material that will resist airflow and support the insulation.
Another example is the wall between an unconditioned attached garage and the conditioned part of the house. While the garage wall may be drywalled, it’s often the case that the wall above the garage ceiling is left with exposed insulation. To satisfy the checklist, the interface between the house and the unconditioned space should be covered with a rigid airtight material.
Similarly, builders sometimes attach porch roofs to the main house without first sheathing the exterior wall, potentially leaving insulation exposed to the unconditioned air under the roof. To satisfy the checklist, this juncture must be protected with an air barrier material. The simplest way to accomplish this is just to sheathe the house wall before attaching the porch.
The Importance of the Stack Effect
Air movement through an air barrier requires a pressure difference and a hole for the air to move through. In cold climates, one of the important sources of pressure difference is the stack effect. Heated air in the building has a tendency to rise, creating a negative (outdoor to indoor) pressure at the bottom of the house and a positive (indoor to outdoor) pressure at the top of the house. There’s nothing you can do to prevent stack pressure from occurring. So to address the air leakage, the strategy is to seal up the holes.
In practical terms, this means that the priority air leaks in a two-story or higher house are the lowest and the highest holes in the air barrier. Pay special attention to low leak points such as the first-floor band joist area, and to high leak points such as the second-floor ceiling. Time spent sealing up attic locations such as can light penetrations, duct registers, and wall top plates will pay off in performance during cold weather.