At the ICC 2018 code hearings in April, two sets of code changes were approved. These changes give builders greater flexibility in meeting the International Residential Code (IRC) and International Energy Conservation Code (IECC) in the areas of unvented attics and buried ducts.
It’s worth noting that these measures received near unanimous support of IRC and IECC Committees and no challenges from the assembly. Once final approval is issued, these changes allow the codes to catch up to building practices that have proven successful in the field and under the DOE's Building America program. This article is the first of a two-part series. An upcoming article will focus on buried ducts.
Championed by Dr. Joe Lstiburek of Building Science Corp in the late 1990s and tested in the Building America program, the concept of moving the insulation off of the attic floor and onto the underside of the roof deck to create an unvented attic first became recognized in the 2006 IRC as an alternative to the traditional roof insulation and ventilation requirements. Moving the thermal barrier to the roof line permitted builders to place the HVAC equipment and ducts inside the conditioned (or indirectly conditioned) space. The heating and cooling loads were reduced and performance increased, providing alternatives to meeting the energy code.
While fiberglass and mineral wool insulation have proven successful in unvented attic applications for years, technically, the language in the IRC did not fully recognize this common practice. In an attempt to remedy this situation, the IRC Committee recently approved a code change proposal that recognizes the use of “air-permeable” insulation-only systems beyond climate zones 2B and 3B (where the practice is currently permitted ) to include climate zones 1, 2, and 3. While other tested means of moisture control exist, the code changes supported by the IRC Committee call for the use of a ‘vapor diffusion port’ in the additional climate zones. The diffusion port acts as an air barrier but not a moisture barrier, so it allows water vapor to work through the air-permeable insulation at the roof’s ridge line and dissipate to the exterior through the port.
The previous lack of approved air-permeable insulation solutions for many unvented attic designs meant that builders had only one choice to insulate attics: spray polyurethane foam (SPF). The new proposal will allow for four more options:
1. wet spray loosefill insulation attached to the roof deck
2. batts pinned to the roof deck or top cord of the truss
3. draped systems where loosefill insulation is installed in netting that is draped from truss top cord to top cord to allow enough insulation to achieve the necessary U-factor for the entire roof assembly
4. box netting insulation system which yields the desired R-value of loosefill uniformly across the roof deck.
Each of these four air-permeable insulation applications are combined with air sealing strategies that put the air barrier plane at the roof sheathing and in contact with the thermal insulation.
Finally, it should be noted that although the ducts are now inside the conditioned space with an unvented attic design, code dictates that the sealing of all duct connections must be done. Best practice calls for at least R-4 insulation on the ducts to ensure the air delivered meets the intended design. Making that air-permeable R-4 insulation on the interior of the ducts will also deliver superior HVAC noise control.
While additional code actions are possible before the 2018 IRC is approved, there is an expectation that air-permeable insulation options will be available in the final version. This will result in more choices for the builder to meet the need for cost-effective conditioned attic systems.