Making Air Barriers That Work
Straightforward, but requires careful attention
An air barrier is any material that is impermeable to airflow. This includes drywall, closed-cell foam insulation, and many other materials. (Some, but not all, of these materials are also vapor barriers, blocking diffusion of water vapor.) An effective air barrier reduces air leakage, which offers several benefits: reduced heating and cooling losses, improved thermal comfort, prevention of mold and structural problems due to moisture migration and condensation, and reduced migration of pollutants.
Building an effective air barrier is straightforward, but requires careful attention to detail at all stages of design and construction. An air barrier only works if it is durable and continuous around the space it is designed to enclose. Even a small gap can lead to a lot of air leakage. Incorporating an effective air barrier means identifying appropriate materials that are already a part of, or that will function well with, all six sides of the building assembly. The designer should be able to trace the entire building’s air barrier on paper without lifting pencil from paper. Once that is incorporated...
9 Comments
Integrated Design
When it comes to commissioning and LEED, I would like to see the USGBC take an approach similar to EAc 1 Optimized Energy Performance where the commissioning process all falls under one credit and th number of LEED points would be based on the extent of the commissioning scope. As a commissioning authority, I can say without exception, that most projects are commissioned based on the "status quo". HVAC and Balancing ONLY. This still leaves poorly performing systems and buildings. The real benefits of commissioning will only be realized when we take the "total Cx1. Commissioning (Cx) is the process of verifying and documenting that a building and all of its systems and assemblies are planned, designed, installed, tested, operated, and maintained to meet the owner's project requirements. 2. The process of checking the performance of a building against the owner's goals during design, construction, and occupancy. At a minimum, mechanical and electrical equipment are tested, although much more extensive testing may also be included." approach. For too long, the Cx process has been monopolized by TAB contractors and mechanical engineers with a strong bias toward HVAC only approach. History shows that this approach has failed miserably. To reap the true benefits, the USGBC should get on board with NIBS in promoting the total Cx process which will eventually encompass all areas of the building.
Advanced Envelopes
As LEED Consultants and with LEED v3 2009 coming out we are recommending to our project design teams to just start with and Advanced Envelope. We usually have to give them resources on what that is and then it can go deeper into WUFI studies. We try to choose our battles with training contractors, as Joe used to say Take the human factor out of the equation, i've transferred that to the "knucklehead factor"
Scott,
Good point. We often do the same and use either the ASHRAE Advanced Energy Design Guide or the NBI Core Performance Guide. These give us traction with project teams since LEED recognizes them as prescriptive paths for the energy credits anyway. Do you have other standards you use to get at for advanced envelope items?
Understanding Integrated Design Benefits
No kidding, Al!
I just worked on a project where CAx became such a patting on the back for everyone that the very details of the air barrier and the actual performance of the building envelope design became overlooked. But LEED doesn't address building envelope performance/testing.... right? Just the "same old air balancing" routine (which is still important, don't get me wrong).
The 1st public comment version of the LEED for Healthcare draft included a second option under EAc3 (enhanced commissioning) for Building Envelope Commissioning. I'd suggest you submit this as a suggestion for the rest of the rating systems in the LEED Call for Ideas! http://www.usgbc.org/News/PressReleaseDetails.aspx?ID=4145
Understanding Integrated Design Benefits
Team members on any LEED project should have some understanding of integrated designBuilding or interior design in which different components, such as the building envelope, window placement and glazings, mechanical systems, furniture, and surface finishes are considered as part of building design, especially to achieve goals related to sustainability.. It is impossible to effectively control issues such as humidity without having an effective air barrier. Creating a vapor profile using heat, air & moisture (H.A.M.) analysis can help designers to understand permeance issues. In addition, I like to look at what I call building loading (wind, driving rain index, etc.) to fully understand requirements. Understanding facade materials and how they will react to determined building loads will go a long way in determining air/vapor barrier requirements. It is goo to see more attention being paid to the envelope instead of the same ol "air balancing" is commissioning routine.
For resources section of air barriers and testing
I would just be sure to add some of the articles from Whole Building Design Guide's journal, Journal of Building Enclosure Design, "Characterizing Air Leakage in
Large Buildings: Part I and Part II" from Summer 2007 and Winter 2008 on commissioning of the air barrier.
http://www.wbdg.org/references/jbed.php
Also, maybe something about the standards regarding some the air barrier commissioning through ASHRAE and ASTMVoluntary standards development organization which creates source technical standards for materials, products, systems, and services?
air barriers and vapor barriers
Great point Timothy.
I like the way you distinguish between the two, in terms of function and whether or not the latter is needed, and how the need for a vapor barrier should be determined by way of analysis.
I like to use the term vapor profile instead of vapor barrier, reflecting the need for diffusion analysis. The performance of any building assembly in terms of wetting and drying is based not on the vapor permeability of any one layer, but the individual and then combined vapor permeability of all layers in assemblies. We care about the relative vapor permeability of all layers rather than "assigning" the responsibility for managing wetting and drying to one layer. Sometimes you can do the vapor diffusion analysis by simply qualitatively comparing the relative vapor permeabilities of all layers in an assembly, sometimes it may require a quantitative approach such as WUFI modeling.
Vapor profile is a relatively new term, but I think it was coined at Building Science Corporation, during my tenure there. In any event, we define the term on GreenBuildingAdvisor.com (http://www.greenbuildingadvisor.com/glossary/8#letterv) and the term is used in this great article from BSC: http://www.buildingscience.com/documents/digests/bsd-106-understanding-v....
Air Barriers
It is critical to understand the difference between air barriers and vapor barriers and how each performs in a given climate in order to properly specify them. This discussion deals with air barriers in a general and simplified way. It needs to be emphasized that an effective air barrier is continuous across the building envelop from grade to roof and back to grade and across penetrations. In some instances the air barrier may also need to be a vapor barrier, but not in all cases. A vapor diffusion analysis should be done for the exterior envelop and roof systems to understand the movement of air and vapor for a given condition and climate. Further reading can be done at http://www.airbarrier.org/index_e.php
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