Pacific Northwest National Laboratory (PNNL)
Credit: Consortium for Advanced Residential Building (CARB)

One of the best returns on an energy retrofit project is to air seal the attic. Sealing the attic can significantly decrease the amount of air and water vapor that flow between unconditioned spaces and living areas. When done properly, sealing the attic can help in three specific ways:

  • Manage air movement from the inside of the home’s conditioned space to the outside (increased thermal control).
  • Manage air movement from outside to the inside of the home (increased thermal control and improved indoor air quality).
  • Reduce water vapor and condensation issues (increases durability, energy efficiency, and indoor air quality).

Of the many areas that need to be air sealed during the attic air-sealing process, recessed lights and chimney chases are often technically challenging to the contractor because they require special attention to reduce the risk of fire.

Recessed Fixtures

Recessed can lights are often very leaky, but they can be difficult to air seal correctly. If they are present, it is important to determine if they are rated for insulation contact (“IC-rated”). If so, they can be directly sealed with mastic, spray foam, or other sealant, and insulation can be placed directly over them.

If they are not IC-rated, a box enclosure may be made from fire-resistant material to keep insulation at a distance, so that the fixture does not overheat. Appropriate materials for this enclosure include fiberglass duct board or drywall. This can then be air sealed with caulk or mastic and covered with insulation. It is important to allow enough clearance (about 3 inches to each side) inside the box for heat to dissipate.

Chimney Chases

There is often a gap between a chimney or heating system flue and neighboring materials, resulting in significant leakage. These gaps can often be easily sealed, but chimneys require additional consideration due to requirements for clearance to combustible materials. Generally, a 2-inch clearance on all sides is required, but this may vary, so consult the relevant local codes before performing work on chimneys.

Sheet metal is often used to seal around a chimney, essentially as a barrier board, with any remaining gaps sealed with an appropriate high-temperature caulk. If blown-in insulation is to be added to the attic following air sealing, it will be necessary to add a vertical dam to a height that matches or exceeds the depth of the insulation to maintain the required clearance from the chimney. Alternatively, rock wool insulation is rated for this application. 

This article is adapted from content developed by the DOE's Building America Program.