Exterior Air Sealing

This online-only article is a supplement to the January/February 2013 print edition of Home Energy Magazine.

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I find inefficiencies in home performance frustrating. Many can be cured by performing simple tasks. The goal of air sealing is to retain Btu within a building, which translates to reduced energy costs, added comfort, and enhanced durability. Most buildings have cold or warm spots; some have ice dams or drafts; and worse, some have mold and mildew problems. All are common signs that a building is not properly sealed against airflows between the conditioned inside and the outdoors.

Air moves from one side of a building assembly to another based on pathways and pressure differences. Up to 25% of heating-and-cooling Btu can escape through unseen cracks and gaps in a building. Replacing these lost Btu, often through the use of HVAC appliances, costs money.

Common air-sealing procedures for an existing building begin with the attic plane. Penetrations are sealed with rigid air barriers, insulation is added, and proper ventilation is established. Generally the contractor is working within the constraints of a finished home interior. He or she is not gutting walls to the shell, as with a new home, so the techniques are restricted to accessible spaces.

There is a work-around for many homes that removes the constraint of a finished interior. That is to work on the outside of the shell rather than the inside. The attic plane is accessible in either case, as is the band joist if the basement is unfinished. In a re-siding job, we often have the opportunity to address all planes of the structure. We can seal the attic, seal and insulate the walls, and seal and insulate the unfinished basement, too.

A National Association of Home Builders (NAHB) economics paper on the durability of various components of a home found that most siding lasts for the life of the home—which, NAHB notes can be 200 years or more, when the home is properly maintained. Despite the functional durability of siding, several million homes per year are re-sided. It is my contention that if the best practices for air sealing were employed during the re-siding of the home, the energy consumption of that home would drop. Retrofitting several million homes per year in this manner would surely have a major impact on home budgets and would probably have a measurable effect on national energy consumption.

Testing This Hypothesis

In order to test this hypothesis, I wrote a prescriptive path to sealing the outside of a building’s shell. I selected a 43-unit townhome project that was taking bids to replace the siding on the entire project. The homeowners’ management committee allowed input on the specifications. The committee also allowed me to measure some pre- and post-testing of airflows. Many case studies have been conducted through the Partnership for Advancing Technology in Housing (PATH), DOE, HUD, and the National Renewable Energy Laboratory (NREL) on restraint of airflows from the inside of the building, but none were found on exterior air sealing. This test project attempted to replicate the research protocols, although no public funding was secured to gather data on every unit. Eight of the units were pre-tested with a blower door for air loss. In one, the homeowner had generally sealed one of the tested units. All the others were in their original condition, which dated from the late 1970s.

Working closely with the homeowners association and educating them on the benefits of exterior air sealing, our rewrite of the bid specifications included lots of air sealing best practices (see Table 1). There was no increased cost to the homeowners association. After the completion of the project, we asked the contractor what his thoughts were. He said that perhaps a thousand dollars of hard costs may have been spent, but the increase in man-hours for diligence was insignificant compared to the good will he gained from the homeowners association for an unprecedented job.

Proper instruction of the men in the field was essential. Each detail on how to apply caulk was carefully explained and monitored—from the contractor to his supervisors, and from them right down to the field installers. Caulk needs to be injected into the seams of the sheathing, not smeared across the face. Air barrier needs to be caulked at the first course, not just taped at the seams. The sheathing needs to be caulked to the foundation to eliminate perimeter leakage. Cutouts at vents and hose bibs all need careful sealing with caulk and expanding foam before any air barrier is applied. Quality control and verification of proper protocols were implemented during the prep and installation stages of the contract. Chuck Juliano of Juliano Roofing and Siding, located in Chester, New Jersey, assisted in the daily site visits and quality control checks.

Post-testing of the units showed a reduction in air infiltration that ranged from 3% (in the home previously sealed by the owner) to 18%. The average improvement in air infiltration was about 15%.

Initial blower door testing was completed to get a baseline of air leakage. All original wood siding and tar paper was removed, revealing major sources of air infiltration into the buildings. These techniques have translated into dramatic savings for the homeowners in the subsequent years. Anecdotal reports of greater comfort, longer cycle time on the furnaces, and stabilized temperatures all make for more satisfied occupants. Getting a great job from the contractor is generally the homeowner’s—or in this case, the association’s—main goal. Having test results proves that the air-sealing job was a success.

Conclusions

In the past, siding has had two functions: to shed water, and to make the house look attractive; it has not been installed to save energy. In this case, we used a foam-backed vinyl siding to replace the wood clapboard; adding R-6 to the insulation value of the wall assembly. Though air sealing was the baseline metric, the wall assembly, formerly R-13, garnered a 50% increase in insulation value. This insulation does a fine job of preventing thermal bridging from interior conditioned space to the exterior. (You can conduct a Manual J load analysis to calculate the insulation improvement for your next project.) There was also a noticeable (though not measured) decrease in cycle time on the HVAC equipment, which should extend its service life as well. In short, this exterior air-sealing practice may be the next quantum leap forward in lowering the operating cost of homes, and in preserving energy resources. The combination of added insulation with best-practice air sealing is a proven winner.

Bill Asdal is a managing member of Asdal Management, LLC, in Chester, New Jersey.

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