Roof insulation thermal resistance (depth) at truss heel (roof perimeter) should be equal to or greater than the thermal resistance of the exterior wall.
While you may think you know how and where to install insulation, it requires experience and attention to detail to install correctly.
From a building efficiency standpoint, adding insulation that’s installed properly is one of the single most cost-effective energy efficiency upgrades. There’s also a trend in building codes to require increased levels of insulation in walls, attics and foundations.
Insulation
Most insulation is made of low conductivity fibers – fiberglass, paper and rock wool – that attempt to hold air in tiny pockets, providing most of the insulating properties. In this article, we’ll focus primarily on how to properly install insulation made of these fiber-based insulation materials.
Fibrous insulation is air permeable, and slows or retards airflow, but it’s not an air barrier and so does not stop airflow. The key is to prevent airflow through the insulation because this displaces the air meant to be contained in the tiny pockets. Missing this critical step can diminish overall thermal performance of the insulation, occupant comfort and energy efficiency.
Batts or roll insulation are the most common choices to insulate walls, roofs and attics across all climate zones. Batt insulation is low-cost, readily available and can be installed with little, if any, specialized equipment. It’s found in millions of homes and can be very effective when installed correctly.
However, in order for batts (fiberglass, mineral wool or other materials) to perform to their specified R-Value, they must be installed correctly. Holes, joints and other mechanical penetrations should be air-sealed before the insulation is installed.
It’s also critical that the insulation be surrounded by airtight materials that touch the insulation on all six sides, called encapsulation. The six sides in a stud bay are: top plate, bottom plate, vertical cavity studs, exterior sheathing and interior drywall.
Poor attention to detail during installation can result in compression, which displaces the air that should remain trapped in the pockets. Incompletely filled cavities can induce convective airflow through the insulation. A few cuts, gaps or cracks left between batts and the R-value or thermal resistance can be diminished.
Blown fiber and spray foam insulation can provide a more foolproof system, but many of the same rules apply, including the necessity of encapsulation. For the highest performance, you may want to use several types of insulation. For example, tight spaces or concealed spaces may be easier to insulate with air impermeable products, such as spray foam or a combination of cut to fit rigid foam sealed into place.
How and where to install insulation batts
When installing batt insulation, cut batts ⅜ to ½ inch longer than the cavity. Standard batts are pre-cut about ½ inch wider than a standard wall cavity with either 16 or 24 inch on center (OC) framing. For non-standard cavity widths, cut the batt ½ inch wider.
Place the insulation batt so it fills the cavity from top to bottom, side to side, and front to back. Cut the batt to fit around electrical junction boxes and split around wires or pipes in the cavity. Don’t just push it behind obstructions.
Knee walls should be treated as an exterior wall and to achieve six-sided encapsulation, there should be sheathing or an air-tight material installed on the attic side of the wall.
Air sealing
An air barrier as an assembly is rarely just one material and is typically an assembly of materials installed to control air flow between the conditioned space and unconditioned outdoors.
In a wall system, there are various materials with air retarding or barrier functions. From the inside out, they are:
• the interior typically contains drywall that can be used to control airflow
• cold climates may require a vapor retarder
• insulation
• exterior sheathing
• weather-resistive barrier
• cladding
An area of air leakage that’s often missed from an air barrier perspective is knee walls or walls between the house and attic area. Knee walls should be treated as an exterior wall and to achieve six-sided encapsulation, there should be sheathing or an air-tight material installed on the attic side of the wall.
How and where to install insulation in attics
In most attics, there’s a good deal of blown insulation placed on the attic floor. With flat ceilings or traditional ventilated attics, it’s important to maintain a consistent depth of insulation to achieve the required R-value for the specific climate zone. Typically, we’re seeing R-38 insulation as the standard for warmer climates and between R-49 and R-60 for colder climates.
Maintaining control of air leakage from the house to the attic is critical. Common areas of air leakage are around can lights and electrical boxes, as well as where walls adjoin the attic (both exterior and interior walls).
Making certain the truss heels are high enough to place adequate insulation over the top plate area of exterior walls is also an important consideration. A good rule is to choose trusses that allow at least 80 percent of the required attic insulation R-value be installed over the top plates of exterior walls.
For example: In an R-38 ceiling the minimum amount of insulation over the top plate should be at least R-30 and for R-49 at least R-38. Equally important is to provide a baffle to deflect any air from the vented soffit up and over the insulation.
Customer expectations
Regardless of how much insulation is required by code, builders should take into consideration the customer’s expectations for comfort.
Comfort levels are impacted by a number of factors, including the surface temperatures of surrounding objects, air movement or drafts, humidity and even personal characteristics, such as an individual’s metabolic rates and clothing selections. Insulation can help control air temperature and surface temperatures, and if combined with air sealing, reduce or eliminate drafts.
The basics of installing insulation
• Air seal before you insulate. Adding insulation without air sealing may not produce the desired results.
• Verify you have the recommended R-value for the building and climate zone.
• Consider vapor retarders in colder climates.
• Encapsulate – air permeable insulation should be surrounded by airtight materials touching the insulation on six sides.
• Leave no voids or gaps.
• Allow no (or limited) compression of the insulation.
• Properly cut and fit insulation around wires and mechanical obstructions.
• Be sure insulation is compatible with surrounding materials.
• Be sure spray foam insulation is properly mixed and applied safely.
• Area being insulated should not be subjected to constant wetting cycles.
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