Top Ten Air Sealing Opportunities

1I3A1824 web1. Chimney/gas vent chase

Chimneys and gas appliance vent pipes require large clearance to combustible materials like 2×4 framing. Often these chases are open to the attic and surrounding walls or floor joists. Seal them durably with approved materials like sheet metal and fire-rated caulks.

2. Plumbing chase

The plumbing chase is often double framed to accommodate large waste pipes and vent stacks as well as water supply piping. These walls can directly connect the attic to the basement and to sidewalls and tub surrounds. Seal the chase at the top with code approved, durable materials and caulks.

3. Duct chase

Duct chases can run horizontally and vertically connecting many interior walls to attics. Sealing at the attic may not be adequate and may require opening access through interior walls to stop serial leakage. Opening a chase may also present an opportunity to seal the duct seams with mastic to improve overall duct air tightness.

4. Dropped soffits above kitchens and bathtubs

Depending on their construction these areas may connect interior spaces to the attic. An infrared scan can reveal the existence of these leaks easily. Seal the dropped soffits in the attic with durable sheet goods that can withstand the weight of cellulose insulation. Use mechanical fasteners and caulk. Be especially careful around venting connections from a range hood to the outside.

5. Outer wall top plate/strap cavity

Often overlooked in older masonry built homes with lath and plaster walls the strap cavity represents a direct basement to attic connection. It may only be ¾” wide but multiplied by the perimeter of the house it can add up to a large area. One-part expanding polyurethane foam sealants are well suited to sealing this area. In new built homes shrinking lumber used to frame the house can open seams between drywall and framing. This smaller seam can be sealed with caulk.

Steps in air sealing the strap cavity

6. Recessed can lights

Can lights are large leaks often retrofitted into older homes during renovation and modernization. Even if they are IC (insulation contact) rated they can still leak conditioned air to the attic. If possible build a sealed drywall box around the can in the attic. Be sure to size the box to allow clearance to the fixture and to allow it to rise above the level of insulation in the attic. The fixture needs to dissipate heat while operating to prevent a thermal safety switch from shutting off the light and causing a contractor call back.

Steps to air seal a recessed can light

Steps to air seal a recessed can light

7. Hidden knee walls

A hidden knee wall may exist when access to the area behind the knee wall is either limited or non-existent. Air Sealing crews will have to cut the wall to gain access to the area to air seal. This will require extra time and labor and a skilled drywall repairer to patch the access when the job is verified as complete with a blower door test.

8. Cantilever floor

Cantilevered floors may only protrude past the level below by six inches but where ever the plane of the wall is broken an opportunity for air leaks is present. Construction type will dictate the best approach which may involve removing siding to access areas for sealing or dense packing with cellulose to both air seal and insulate.

9. Mud sill connections

Mud sill connections represent an area of a home where dissimilar materials, often concrete foundations and wood wall framing, meet. This area is similar to a top plate leak but is an infiltration site rather than an exfiltration site. Just like a top plate the actual gap may be narrow but multiplied over the length of the perimeter it can represent a large leakage area. Mud Sill connections can be sealed with approved caulks or foam boards and sealants.

10. Porch roofs

A porch roof in an older home may be framed directly to the exterior stud walls without any sheathing between the two components. Because of the large number of framing connections these areas may represent large leakage amounts. Air sealing a porch roof is a challenge that may rely on several methods including cutting access, installing dense pack insulation, or foam products. Occupant interviews and blower door testing in conjunction with infrared imaging will indicate if air sealing a porch roof is a cost effective choice for your client.


Air sealing is best done with a blower door providing the guidance. Run the blower at a low rate of flow and use a smoke puffer to verify and pinpoint air leakage before wasting valuable labor hours chasing leaks that may not exist. A simple process step like this can help you improve efficiency. Practicing proper techniques and learning about different material options will help your crews get it right the first time.

Steve Marchese is a trainer and energy analyst at the Association for Energy Affordability’s Chicago office.

Learn more about AEA Chicago here.

View info about our fall courses on Air Sealing, Combustion Safety Testing, and Infiltration and Duct Leakage at AEA’s Chicago Hands-On Training Lab.

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