Affordable Air-sealing Upgrades Provide High-yield Returns

Zip System, air sealing, building science

The largest energy consumer in most U.S. structures is heating, cooling and ventilation. The heating and cooling load is determined based on air infiltration, insulation size, and window and door openings. Per Allison A. Bailes III, Ph.D., in his book, A House Needs to Breathe
… Or Does It?: An Introduction to Building Science
, the building enclosure is the boundary between conditioned space and the various types of unconditioned spaces surrounding the house. The design and installation of building-enclosure components determine the structure’s energy efficiency.

The cheapest and easiest way to reduce the HVAC load is to ensure the building is airtight. Therefore, a renovation contractor should complete air sealing when doing a major remodel of a home. Air sealing and insulation can decrease the cost of the HVAC system and operational costs, which is a win-win for building owners. Improvements in air sealing and insulation help reduce energy costs year-round. Most occupants associate these improvements primarily with colder temperatures; however, heat and energy transfer through structures all year long.

If you are retrofitting the siding and windows on a home, a liquid-applied membrane is a good way to air seal and add the needed water-resistant barrier at the same time.
PHOTO: Matt Hoots

Although making a building tight is one of the easiest ways to make a remodel more energy efficient, it also tends to be the most overlooked. Building tight, either in new construction or retrofit projects, prevents energy loss. It also helps control pollutants from entering the structure from the exterior. Ventilation is required when building tight. However, this should be achieved through mechanical ventilation that can be filtered and conditioned. In a leaky building, you cannot control the air, which could affect the health of the occupants.


Building science professionals use ACH50 to measure how leaky a building is. It stands for air exchanges per hour at 50 pascals pressure differential. Lower numbers mean there’s less air leakage through the air-control layers, which are 3D systems of materials designed, constructed and/or acting to control airflow across a building enclosure or between a conditioned space and an unconditioned space.

Existing buildings are much more difficult to make airtight because you may not be able to reach all the control layers. On most existing homes, the major control layers are water control, air control, vapor control and thermal control. If you have brick or siding still attached to an existing structure, you may have to do the air sealing from the inside. If the sheathing is not air-sealed (and most isn’t), the drywall becomes part of the air-control layer. It is better if you can control the air before the drywall. Drywall tends to get cut, and these holes make houses leakier. If the control layer is at the sheathing, it will stay intact longer because sheathing stays in place longer.

Traditionally, air sealing from the inside was executed at holes where wires come through and at gaps in framing, like bottom plates and between studs, with a caulk gun and a can of spray foam. If drywall is not going to be removed, then sealing around the outlet boxes, trim, etc., is the best option. Next best is sealing at the framing from the inside. Caulking once everything is finished does help a little; however, it is not as effective.

A fine mist of sealant can be released on the inside of a home while the house is under pressure. It can find and seal most of the cracks and gaps in the building. PHOTO: Aeroseal

A more effective approach is a product that releases a fine mist of sealant while the house is under pressure. It can find and seal most of the cracks and gaps in the building. The technician can monitor the ACH50 as the product is finding air gaps. Once the target ACH50 is reached, the machine is turned off and a report shows the results of the process. The report assists HVAC designers to factor in real data to their load calculations.

This fine-mist air sealant is designed to work on existing and new homes. Although it provides a good measured air barrier, it does not provide a water-resistant one.

If brick or siding can be removed from the exterior of an existing building, an air- control layer at the outside of the sheathing provides a good air barrier and a water-resistant one in the same application. There are many different ways to achieve this, such as liquid-applied membrane or oriented strand board (OSB) with a water-resistant barrier.

The key for any water-resistant barrier is to keep moisture out while allowing moisture or vapor that is in the exterior wall assembly to dry out. Most renovation contractors simple apply or reapply house wrap before installing new siding on the house. Although this does meet code, it is not
the best material to deal with air leakage. Liquid-applied membranes act as a water- resistant barrier but also help air seal at the same time. A liquid membrane is applied in two steps: First, seal all the joints and transitions; next, roll or spray the membrane onto the plywood or OSB. If you are retrofitting the siding and windows on a home, a liquid-applied membrane is a good way to air seal and add the needed water-resistant barrier at the same time.

About the Author

Matt Hoots
Matt Hoots is a registered licensed contractor and green builder. As owner of SawHorse Inc. Design + Build in Atlanta, Hoots specializes in building science.

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