Air barrier durability

By Drake A. Wauters AIA, CSI/CDT, SCIP, IES posted 05-17-2017 14:45

  

The design and construction of buildings in the US today often include assumptions, or better yet Owner Project Requirements4, about how well the building will perform over a given time. In the case of energy and comfort performance, the as-built performance characteristics of the exterior building enclosure thermal insulation, radiant heat transfer, and air leakage are completely essential.


While much has been written about how well thermal insulation performs over time on roofs and walls and how well low emissivity insulated glazing performs over time, these systems categorically perform as expected especially since we now widely understand how in-plane wall framing and bridging can impact overall insulation values through thermal bridging. In the case of insulation, we also use aged values for foams now because some loss of insulation values are reasonably well understood. We also understand that if insulation, especially fibrous insulation, is improperly compressed or water-logged in place the thermal values will plummet but other sick building issues will also occur such as mold, rot, water intrusion, and rusting so fixes are likely to be undertaken to address all or most of these issues.


Building enclosure air leakage, on the other hand, is a huge challenge to designers and builders but an even greater challenge to owners and occupants. As norms have changed with regard to building performance and energy usage, air leakage through the building enclosure has been a hot topic for a while. Performance regimens like Passive House5, ASHRAE 189.12, and those of the US Corps of Engineers6 and the US General Services Administration7 have required lower air leakage rates for a long time. However, unlike insulation values in enclosure assemblies or solar heat gain coefficients in glazing, building enclosure air leakage is due to an almost always complicated array of interrelated mechanisms that usually involves many products, many trades, and many diverse conditions throughout the building enclosure. The considerations are also spatial as the ability for trades to access and install the conditions, for installed work to be inspected, and for the conditions to accommodate all aspects movement expected in the building enclosure. To design and build to a required performance level when so many factors are involved requires a concerted and sustained diligence from the owner, architect, and contractor. But it can be done and validated through testing throughout construction and at completion of the building, especially if the building enclosure is commissioned.


So what is the big deal? The big deal is that unlike wall and roof insulation and glazing performance, air leakage rates will not remain intact for very long for most buildings (unless they are biosafety level four facilities or the like) and this will not be evident to the owner and occupants until things are far out of whack and maybe not even then. For air leakage rates to be maintained they need to be planned for, detailed, and bid to be durable and intact for long periods of time. They also need to be constructible and verifiable as actually built correctly while they are still accessible during construction. This means that concerted care needs to be taken when designing, detailing, and specifying each project. And as much care needs to be taken when building.


Aged increases in air leakage rates for building enclosures today (Figure 1) and especially on high-performance buildings that rely so heavily on the building enclosure to perform as promised, has a dramatic and unpredictable impact on the ability of interior air temperatures and humidity levels to be controlled. For reference, these matters are further outlined in the 2013 EPA paper 402-F-13053 “Moisture Control Guidance for Building Design, Construction, and Maintenance.”

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FIGURE 1: AGED AIR BARRIER SEALS PERMIT MORE AIR INFILTRATION AND EXFILTRATION; IMAGE CREDIT: DRAKE WAUTERS


Decades ago engineers assumed heavy air leakage rates through building enclosures. They assumed greater air changes and higher heating and cooling loads to address these low-performance norms. Today buildings use much higher performing designs that necessarily need to assume far better control of air leakage through the building enclosure. The systems do not have excess heating, cooling, or filtration capacity to address unforeseen and escalating air leakage rates. As humidity becomes harder to control due to air leakage increases, sick building issues including condensation and mold can become a problem. Occupant comfort can be sacrificed as cooling is increased in summer months to attempt to handle excess humidity and heated air infiltration. Conversely during winter excessive air leakage can lead to colder spaces, overwhelmed heating systems, and even drier air than designed where humidification is necessary such as on medical, data center, or archival projects.


Though an increasing number of buildings are given a whole building air leakage test, these are still rare, and for larger buildings quite difficult, even when zoned to accommodate multiple more limited volume tests in the design. All the same, once a facility is successfully built and the whole building air leakage rate is validated in some way, why do air leakage rates fall out of sync? User choices can be quite different than planned, for one. Loading bay doors can be left open for longer periods than previously assumed. Interior doors in vestibules or air locks can be chocked open for user convenience. Dampers at locations like the heads of elevator shafts may be left open instead of opened only during life safety functions. Door gaskets are also often an issue as these take a great deal of abuse in service and the alignment of moving door parts at doors can change as fatigue wears in and materials deform, warp, or sag such as near hinges or in regard to door flatness.


However, a more insidious problem are the miles of transitions between materials and assemblies that are concealed within the construction of the building enclosure. These can easily degrade and become more open allowing a great deal more air, humidity, and contaminants to pass through the enclosure without any obvious indications to even the most aware facility professionals. Unlike leaking water, air is not usually visible so air leaks can go unnoticed for years or the life of the building.


Those concealed conditions often involve inattentively planned and/or poorly installed transitions between fenestration and door framing and opaque walls, at active joints especially interstory horizontal joints and vertical expansion joints, at sill joints at foundations or flashing, at dammed ends of flashings, and roof assembly and equipment transitions. They can also include poorly planned and installed transitions between roof air barriers and wall air barriers or seals at penetrations or anchors through air barriers. They are often caused by improper material usage such as self-adhering strips that are incompatible with the barrier and come unbonded or sealants that fail to adhere to the bridged materials. Sealants, tapes, and gaskets can also shrink or crack leading to new openings even if they remain bonded to the materials joined. In many cases, the full travel of active joints and penetration conditions are not accommodated. For instance, a 1/4-inch sealant bead shown between two abutting materials that in reality can be expected to move differentially by over 1-inch, the sealant bead would fail immediately.


There are a great many conditions that can lead to excessive and unplanned air leakage. A best practice is to make sure a seasoned design professional familiar with the mechanics of buildings and the realities of construction peer reviews the building enclosure design drawings and specifications as often as necessary to stay on track. An early review before design development is definitely necessary to help steer away from troublesome design conditions that may be almost impossible to air seal in practice. A review around mid-point and near completion is also necessary to help track and button up design decisions. A good barometer of risk is if the design team does not understand how a condition can be air sealed or how building movement will be addressed there is a good chance construction trades will face the same challenge especially in difficult bidding climates where bidding a great deal of work not indicated would assure the bidder loses the bid. Simply stipulating robust seals without accommodating the space and gap widths in the design to permit their construction and flexure in service even under full design winds and seismic drift can leave the builder with few if any options.


Here are a few numbers for consideration on just how important sustaining low air leakage rates is. The amount of air assumed to be exfiltrating out of buildings and infiltrating into buildings is many times lower today than it was a few decades ago (Figure 2). Those rates are trending downward but according to ASHRAE and the IBPSA-USA3 new construction can readily conform to an air leakage rate of 0.10 CFM75/SF of exterior building enclosure at 75 Pa (0.3 inch of air infiltration pressure or 1.57 psf of air infiltration pressure equating to around 25 mph of wind pressure). The ASHRAE SSPC-90.1 Envelope Subcommittee1 recommends assuming a baseline air leakage rate of 1.80 CFM75/SF of exterior building enclosures at 75 Pa for buildings with uncontrolled air leakage - the way buildings were designed and built decades ago. This 18:1 (1.80 to 0.10) ratio is dramatic. Though organizations like the ABAA (Air Barrier Association of America) recommend a maximum rate of 0.40 CFM75/SF (4 times higher than 0.10) and the US Corp of Engineers requires a maximum rate of 0.25 CFM75/SF (2.5 times higher than 0.10), the ratio between 1.8 CFM75/SF and even 0.25 CFM75/SF is still high at 7.2:1. It should also be noted that Passive House uses an even lower maximum rate of 0.08 CFM75/SF and has long mandated that no more than 0.6 ACH (air changes per hour) may be caused by building enclosure air leakage.


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FIGURE 2: REDUCTIONS IN ACCEPTABLE BUILDING ENCLOSURE AIR LEAKAGE RATES HAVE BEEN ENORMOUS. IMAGE CREDIT: DRAKE WAUTERS


Perhaps a helpful sidebar is that average hourly wind speed in the US is under 10 MPH for most areas so 25 MPH is not the normal condition buildings face thus air leakage would be lower than the theoretical for many days in most regions, until of course increases in air leakage overwhelm the interior conditioning performance margins assumed by the design engineers. On the other hand and less helpful, vapor transmission studies8 have found that even small holes in a vapor retarding membrane can allow vapor diffusion mechanics to equalize humidity levels on both sides of the barrier at a surprisingly rapid rate. Though many air barriers are considered “breathable” the range of vapor permeance can still act to retard vapor transmission and this can be bypassed if uncontrolled openings are present.


One more sidebar is that air permeance rates should not be confused with air leakage rates. Air permeance is the ASTM E2178 tested air passage through materials such as an applied air barrier. Air permeance is typically far lower than assembly or whole building air leakage and though it contributes modestly to air passage it is by no means the major factor for most scenarios. For instance, a typical air permeance rate is 0.004 CFM75/SF, only 4 percent (1:25) of the 0.10 CFM75/SF building air leakage rate previously discussed.


But back to the discussion on leakage rates, using a modest-sized house as an example with an interior volume of 30,000 CF (cubic feet) at perhaps 20’ x 30’ x 50’ the rough difference in air change between 1.8 CFM75/SF and 0.10 CFM75/SF is 8.46 ACH from air leakage versus 0.47 ACH, an approximate 18:1 ratio.

The reason ratios above are so important is that mechanical system conditioning of buildings needs to be right-sized to meet energy use and comfort criteria and to conform to performance requirements in the construction codes. If a building heating, cooling, and humidity control system is designed assuming the low infiltration air change rate per hour that a building enclosure provides when first built but those air leakage rates turn out to increase over time from failures in the building enclosure air leakage effectiveness, it is then only a matter of time before the mechanical design of the facility can no longer keep up with the impacts of higher air changes due to exterior air infiltration and the exfiltration of interior conditioned air. As air leakage increases the impacts on the users and the building will vary depending on how mild or severe the local climate is and how critical the internal controls are for the facility. A basic storage facility in a mild climate might not see much impact at all but an office building in an intensive climate could see a great deal of impact. Some facilities such as hospitals are designed for a higher number of air changes per hour so may not see a dramatic change in energy usage if air leakage leads to a minor uptick in air changes but increased building enclosure air leaks could lead to serious humidity control issues that could require remediation in humidity control and in addressing elevated levels of sick building conditions such as interior condensation and mold growth.


As with air leakage, much can go unseen for some time. For instance, unexpected infiltration of warm moist air can lead to condensation and mold growth within building enclosure assemblies long before they are visually apparent to the occupants or facility manager.


The following figures illustrate a few typical air barrier conditions that often fail and in many instances, are not accessible for inspection or repair without costly invasive efforts (Figures 3-8). It is important to note that the building enclosure includes all components that form the air barrier, not simply the materials specified or labeled as “air barriers”. The US Corps of Engineers have used the pencil test for a long time which includes reviewing building sections, plans, and details and following the path of the air barrier throughout with a pencil, or some virtual means, to help the design team remain mindful of all conditions and transitions that form the building enclosure air barrier.

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FIGURE 3: RAIN SCREEN PANEL WALLS CONCEAL AIR BARRIER SEALS THAT WILL DETERIORATE OVER TIME. IMAGE CREDIT: DRAKE WAUTERS


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FIGURE 4: PRECAST PANEL WALLS CONCEAL AIR BARRIER SEALS THAT WILL DETERIORATE OVER TIME. IMAGE CREDIT: DRAKE WAUTERS


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FIGURE 5: CURTAINWALLS INCLUDE EXPOSED AND CONCEALED AIR BARRIER SEALS THAT WILL DETERIORATE OVER TIME. WET GLAZED DEBOND OR CRACK AND GASKETS SHRINK AND FALL OUT OF PLACE. IMAGE CREDIT: DRAKE WAUTERS


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FIGURE 6: MASONRY RAIN SCREEN WALLS CONCEAL AIR BARRIER SEALS THAT WILL DETERIORATE OVER TIME. IMAGE CREDIT: DRAKE WAUTERS


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FIGURE 7: DOORS INCLUDE FRAME SEALS AND DOOR GASKETS THAT WILL DETERIORATE. GASKETS CAN FAIL QUICKLY. IMAGE CREDIT: DRAKE WAUTERS


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FIGURE 8: AIR BARRIER SEALS AT ROOFTOP CONDITIONS CAN BE OVERLOOKED, INCOMPLETE, OR DETERIORATE OVER TIME AND ARE OFTEN CONCEALED UNDER EQUIPMENT. IMAGE CREDIT: DRAKE WAUTERS


In the past, arguably few building owners or even architects would call for a building enclosure testing firm to revalidate the air leakage performance of the building enclosure as-built until much time has been spent chasing down other sources of performance or deterioration problems. Excessive or unplanned building enclosure air leakage should be among the first few things checked when trouble arises even if that trouble is simply higher utility usage than planned.


Architects are truly in the captain’s chair regarding sound planning, better stakeholder awareness, and vigilance during building occupancy and use. Designing and planning for sustained low building enclosure air leakage rates involves many of the assemblies and materials that architects have the most control over. Clients, users, and engineering consultants rely on the architect’s skill to design and specify buildings that can reasonably be expected to perform as planned. Those who have chosen to walk the path of an architect often say they want to make a difference. Helping to deliver more durable buildings is one key way architects can truly make a big difference as the unseen leakage of air is as essential to the performance of buildings as the more readily seen leakage of water. It is arguably up to the architect to lead the way in helping assure air leakage prevention receives the full attention necessary to meet the challenges of today and tomorrow.


Footnotes

  1. ASHRAE SSPC-90.1 Envelope Subcommittee: As referenced in PNNL-18898 “Infiltration Modeling Guidelines for Commercial Building Energy Analysis”, dated September 2009
  2. ASHRAE 189.1: Referenced compliance with ASHRAE/IESNA Standard 90.1 air leakage rates.
  3. ASHRAE and IBPSA-USA: Air leakage rate design and testing experience for new construction referenced in “Integrating Whole Building Air Leakage Test Data into EnergyPlus Infiltration Models,” August 2016
  4. Owner Project Requirements: As defined by the WBDG Project Management Committee and Commissioning Industry Leaders Council under “Building Commissioning” dated November 2016.
  5. Passive House: PHIUS+ 2015 “Passive Building Standard – North America”
  6. US Corps of Engineers: “U.S. Army Corps of Engineers Air Leakage Test Protocol for Building Envelopes” Version 3, dated May 2012
  7. U.S. General Services Administration: P-100 “Facilities Standards for The Public Buildings Service”
  8. BEST 1 paper “Air Barriers vs. Vapor Barriers” by Roger G. Morse AIA, Stephen M. Lattanzio PE, Paul Haas CIH, Gordon A. Brandon, Brian Crowder
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