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What's All This Hot Air About Air-Sealing Measures?

Changes in the 2009 IRC Tighten up Thermal Envelopes

Posted on Jul 15 2009 by Lynn Underwood, GBA Advisor

Significant changes in the energy provisions of the 2009 edition of the International Residential Code (IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code.) include air-sealing measures. Section N1102.4, "Air Leakage," has been completely rewritten to include detailed prescriptive air-sealing measures.

The builder has two options to demonstrate compliance. One is commonly known as a blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas., ostensibly be performed by a third party who would issue a report to the builder to pass on to the inspector. The test ensures that air leakage is less than 7 air changes per hour (ACHACH stands for Air Changes per Hour. This is a metric of house air tightness. ACH is often expressed as ACH50, which is the air changes per hour when the house is depressurized to -50 pascals during a blower door test. The term ACHn or NACH refers to "natural" air changes per hour, meaning the rate of air leakage without blower door pressurization or depressurization. While many in the building science community detest this term and its use (because there is no such thing as "normal" or "natural" air leakage; that changes all the time with weather and other conditions), ACHn or NACH is used by many in the residential HVAC industry for their system sizing calculations.) with a pressure of 50 pascals (0.007 psi).

The second option is a visual inspection (by an inspector) of several air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. criteria and insulation. Table N1102.4.2 breaks down what is required for each component:


Component Criteria
Air barrier and thermal barrier Exterior thermal envelope insulation for framed walls is installed in substantial contact and continuous alignment with building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. and air barrier. Breaks or joints in the air barrier are filled or repaired. Air-permeable insulation is not used as a sealing material.
Ceiling/Attic Air barrier in any dropped ceiling or soffit is substantially aligned with insulation, and any gaps are sealed. Attic access (except unvented attic), kneewall door, or drop-down stair is sealed.
Walls Corners and headers are insulated. Junction of foundation and sill plate is sealed.
Windows and doors Space between window and door jambs and framing is sealed.
Rim joists Rim joists are insulated and include an air barrier
Floors (including above garage and cantilevered floors) Insulation is installed to maintain permanent contact with underside of subfloor decking. Air barrier is installed at any exposed edge of floor.
Crawl space walls Insulation is permanently attached to walls. Exposed earth in unvented crawl spaces is covered with Class I vapor retarder, with overlapping joints taped.
Shafts, penetrations Duct shafts, utility penetrations, kneewalls and flue shaft opening to exterior or unconditioned space are sealed.
Narrow cavities Batts in narrow cavities are cut to fit, or narrow cavities are filled by spray sprayed/blown insulation.
Garage separation Air sealing is provided between the garage and conditioned space.
Recessed lighting Recessed light fixtures are airtight, IC rated and sealed to drywall unless fixtures are in conditioned space.
Plumbing and wiring Insulation is placed between outside and pipes. Batt insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. is cut to fit around wiring, and plumbing or sprayed/blown insulation extends behind piping and wiring.
Shower/tub on exterior wall Showers and tubs on exterior walls have insulation and an air barrier separating them from the exterior wall.

Electric/phone box on exterior wall Air barrier extends behind boxes or air-sealed-type boxes are installed.
Common wall Air barrier is installed in common wall between dwelling units (condo or townhomes).
HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. register boots HVAC register boots that penetrate building envelope are sealed to subfloor or drywall.
Fireplace Fireplace walls include air barrier.

In addition to these measures, this section also adds a few extra requirements:

  • New wood-burning fireplaces have gasketed doors and outside combustion air.
  • Windows, skylights, and sliding-glass doors have air infiltration rates of no more than .3 cubic feet per minute (CFM).
  • Recessed lighting installed in the building thermal envelope must be IC rated and sealed to prevent air leakage.

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Wed, 07/15/2009 - 08:48

Thanks, Lynn
by Martin Holladay, GBA Advisor

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This is important news. Thanks for presenting it in a clear, useful format.


2.
Mon, 09/21/2009 - 17:57

Air Barriers
by Jim Baerg

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Thanks for the summary. I'd like to make several comments on the 2009 air sealing requirements. First, 7 ACH50 is an extremely lax requirement and hopefully will be tightened in the near future. I'm assuming this provision's role is to familarize the industry with blower doors. Secondly, we've found that the definition/description of the air barrier in the prescription path needs additional clarification because serious air flow movement often occurs within the insulation cavity. Finally, the effectiveness of the prescriptive path will depend on the quality of inspectors.


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