Dear fellow Energy Wise Enthusiasts,
I hope that the content has value to you either as a builder or consumer. Additionally, I hope that it can help you with your bottom line. I value your input regarding any of the subjects that I address. If there is something that you would like highlighted, I would be happy to research and report the subject. This column will always be a work in progress. You may contact me at email@example.com.
Your Energy Wise Guy…..Willie
WHAT IS BUILDING AIR TIGHTNESS
Building air tightness describes the degree of air leakage into and out of the building’s thermal enclosure which separates conditioned space from the outdoors. Air leakage is the uncontrolled flow through the thermal enclosure due to pressure imbalances caused by wind, stack effect, and mechanical equipment. Air leakage in a building should be minimized; this goal can be effectively and consistently achieved using an air sealing strategy.
Tighter buildings are intended to increase energy efficiency, durability, occupant comfort and indoor air quality. Houses have become considerably tighter over the past couple decades; however, the most recent energy codes mandate even more stringent air sealing and tightness testing requirements.
The building’s air leakage rate is quantified by testing. The most common air tightness test is typically referred to as a “blower door” test. The test equipment consists of a calibrated fan, a panel to seal off the door, and a flow and pressure meter. For residential application, the fan typically depressurizes the house to 50 Pascals at which point the air flow through the fan in recorded.
Potential Concerns with Tighter Houses
– Poor air quality due to “stale” air and indoor contaminants (such as formaldehyde, cleaning agents, and odors) that take longer to dissipate in a tighter house.
– Elevated indoor humidity that could lead to moisture accumulation and damage in the building.
– Increased risk for back-drafting of combustion appliances (caused by exhaust fans creating negative house pressures) that could lead to unsafe levels of carbon monoxide.
Benefits of Tight Houses (Reduced Air Leakage)
– Heating & cooling energy savings
– Reduced potential for moisture movement through the building thermal enclosure
– Improved insulation effectiveness and reduced risk of ice dams
– Improved comfort (reduces drafts and noise)
– Improved indoor air quality (limits contaminants from garages, crawl spaces, attics, and adjacent units)
Example critical Areas for Air Sealing
- Ceiling plane (vented attics)
Penetrations – bath fans, duct boots, electrical
Framed cavities – above kitchen cabinets, soffits, and chases
Bottom plate at deck/slab
Windows & doors
Knee-wall air barriers
Behind tubs & stairs
Framed cavities – within chases and bulkheads
Behind pre-fabricated fireplaces
Around dampers & vents
- Rim Joist Area
Rim board – joist cavity
Sill plate at foundation
Draft stops at garage & knee walls
Above garages, vented crawl spaces, & unconditioned basements
AIR SEALING STRATEGY
Develop a whole-house air sealing plan
Establish a specified house leakage goal to meet or exceed code (e.g., 2.5 ACH50).
Establish the continuous air boundary for the entire house. Avoid installing systems throughout the air boundary (e.g., HVAC systems not in conditioned space).
Prioritize the air sealing locations and efforts. Include all code requirements (See also ENERGY STAR air sealing resources).
Conduct design review meetings with all affected trades and vendors. Include the testing partner if applicable (some may also be air sealing professionals). Evaluate available products, and select methods that are practical to install, cost-effective, and easily inspected for quality assurance. Establish trade partner scopes of work based on mutually agreed upon responsibilities and expectations.
Implement the plan – A thorough air sealing effort is critical for success. Quality inspections should allow for additional “touch-up” air sealing as required. Seal all large holes and focus on high priority areas.
Refine the plan as needed for optimum performance and cost.
Suggested techniques to achieve code compliance
It is easier to air seal while the house is under construction rather than trying to seal after failing a tightness test. Testing prior to completion may identify leakage points that can be easily fixed.
Perform blower door test prior to installing the ceiling-this will allow easier access to air leakage points in the attic if the building does not meet the tightness requirement.
When designing the house, avoid complex architectural designs that may be difficult to seal.
Reduce the number of penetrations to the air barrier (e.g. recessed lights, speakers)
Maintain continuity between air barrier materials
Consider combustion safety, humidity control, and air quality.
What if the house fails to meet the house leakage target?
Search for leaks at the most likely and easily accessible areas, such as the ceiling plane from the attic, with the blower door operating as a diagnostic tool.