The walls, floors, and roof/ceiling compose the physical shell of the home, also called the building enclosure or building envelope. Within these assemblies are components that comprise the home’s thermal envelope (insulation) and air barrier. The air barrier layer or layers prevent the unwanted entry of outside air and escape of inside air. For the best performance, the home’s thermal layer should be fully aligned with (in full continuous contact with) the air barrier.
The air barrier can consist of any durable solid material that blocks air flow between conditioned space and unconditioned space, for example dry wall or rigid foam insulation can serve as an air barrier when they are are caulked or taped at seams and edges to form a continuous boundary. While ENERGY STAR recommends rigid air barriers, flexible air barriers such as house wrap are acceptable if they are fully sealed at all seams and edges and supported using approved fasteners.
The air barrier may be required on the interior side of the insulation, the exterior side of the insulation, or both, depending on the building component and the climate. For example, according to the ENERGY STAR Thermal Enclosure System Rater Checklist (Version 3, Rev. 6), in ceilings in IECC Climate Zones 1-3, the air barrier can be aligned with either the interior or exterior surface of the insulation but in Climate Zones 4-8, it must be aligned with the interior surface of the ceilings (i.e., mudded, taped drywall). In walls, the insulation should be aligned with the exterior surface (the OSB or rigid foam sheathing) in all climate zones and also at the interior surface of the walls (the drywall) in Climate Zones 4-8. Regarding floors, the insulation should be aligned with the interior surface (the plywood subfloor) in all climate zones.
While the thermal envelope (the insulation) is easy to see, it is sometimes difficult to determine what components comprise the air barrier, especially where one building component meets another such as at rim joists. This is a problem because, to be effective, the air barrier must be continuous around the entire building envelope. This can be addressed at the design stage by making a copy of the plans and drawing or highlighting the components that will form the air barrier in each subassembly.
All holes made in the building’s air barrier must be sealed.
The home’s thermal barrier of insulation must also be continuous for best performance. Insulation must be installed without misalignments, compressions, gaps, or voids in all exterior walls.
How to Install a Continuous Air Barrier
- Determine the location of the air barrier in the ceilings, walls, and floors, and specify it on the house plans. According to ENERGY STAR requirements the air barrier location is based on the home’s climate zone location:
* Ceilings - At interior or exterior surface of ceilings in Climate Zones 1-3; at interior surface of ceilings in Climate Zones 4-8.
* Walls - At exterior surface of walls in all climate zones; and also at interior surface of walls for Climate Zones 4-8.
* Floors - At interior surface of floors in all climate zones.
- Install the air barrier. An air barrier is defined as any durable solid material that blocks air flow between conditioned space and unconditioned space, including necessary sealing to block excessive air flow at edges and seams and adequate support to resist positive and negative pressures without displacement or damage.
ENERGY STAR recommends, but does not require, rigid air barriers. Open-cell or closed-cell foam shall have a finished thickness ≥ 5.5 inches or 1.5 inches, respectively, to qualify as an air barrier unless the manufacturer indicates otherwise. If flexible air barriers such as house wrap are used, they shall be fully sealed at all seams and edges and supported using fasteners with caps or heads ≥ 1 inch in diameter unless otherwise indicated by the manufacturer. Flexible air barriers shall not be made of kraft paper, paper-based products, or other materials that are easily torn. If polyethylene is used, its thickness shall be ≥ 6 mil. ENERGY STAR highly recommends, but does not require, inclusion of an interior air barrier at band joists in Climate Zone 4 through 8. All insulated vertical surfaces are considered walls (e.g., above and below grade exterior walls, knee walls) and must meet the air barrier requirements for walls, with the exception of adiabatic walls in multifamily dwellings. All insulated ceiling surfaces, regardless of slope (e.g., cathedral ceilings, tray ceilings, conditioned attic roof decks, flat ceilings, sloped ceilings), must meet the requirements for ceilings.
- Air seal all seams, gaps, and holes in the air barrier. For example:
* Use caulk and approved tape to seal seams in sheathing, subflooring, and drywall.
* Use caulk and spray foam to seal gaps around wiring, piping, etc.
* Cut pieces of rigid air blocking material like rigid foam or plywood and caulk in place to block larger air gaps.
* Use caulk to seal around electrical boxes or install boxes with built-in rubber gaskets.
* Use sheet metal and fire-rated caulk to air seal around flues.
- Use the ENERGY STAR Thermal Enclosure System Rater Checklist to verify alignment of the insulation and air barrier at all building components.
- Specific examples of air sealing details can be found in other guides under Fully Aligned Air Barriers and Air Sealing.
Figure 1 - The air barrier is continuous across several components of the lower section of wall, including the foundation, rim joist, bottom plate, wall, window, and header.
How to Install a Continuous Thermal Barrier
Follow RESNET Grade I Insulation Installation Standards when installing the insulation. ENERGY STAR permits Grade II installation for cavity insulation with ceilings, walls, or floor assemblies that have continuous rigid insulation sheathing. For such homes, Grade II installation is acceptable if the rigid insulation sheathing meets or exceeds the following levels: R-3 in Climate Zones 1 to 4; R-5 in Climate Zones 5 to 8.
Install insulation at levels that meet or exceed the component insulation requirements in the 2009 IECC - Table 402.1.1 (with some exceptions, see ENERGY STAR 2012).
Specific examples of insulating building assemblies can be found at Reduced Thermal Bridging.
Figure 2 - The wall behind the fireplace is an exterior wall and requires a thermal barrier that is continuous with the rest of the wall’s insulation.