The Hidden Science of High-Performance Building Assemblies
By Peter Yost and Paula Melton
For children, the delicate, fern-like patterns left by Jack Frost on the inside of a windowpane can be magical. But for the owners of a brand new commercial building in the Midwestern U.S., they were more like a nightmare.
“There were 71 punched window openings, all of which had condensation on the inside that turned to frost in the winter,” explained a consulting architect who was hired to diagnose the problem and suggest a solution. “They tried lowering the interior relative humidity to get the condensation to stop. That didn’t work.”
Interior condensation on glazing often indicates a problem with the window itself, but here the culprit was “a poor detail on the architect’s part,” explained the consultant. “They were trying to do this unique architectural feature where they had a steel element above the window that protruded to the inside.” Unfortunately, the steel was a thermal bridge, so no matter how well insulated and airtight the walls and roof might have been, those 71 “unique architectural features” spent the winter relentlessly chilling the rest of the building envelope. In the short term, a problem like this would likely cause major comfort issues and strain the mechanical system; because the condensation was not only on the glass but also on the window frame and the surrounding drywall, the thermal bridges also threatened the durability of the building materials. “The only solution was taking out all of the windows, cutting the steel that was the thermal bridge, and installing some additional insulation,” the consultant said. Fortunately, no lawsuit was filed. But with more attention to detail, the architecture firm responsible for this building design could have saved itself a great deal of time, expense, and embarrassment.
High-performance buildings begin with a very complex big picture, integrating site-responsive orientation, climate-responsive form, hefty R-values, efficient mechanical systems, healthy indoor air, and glazing that effectively balances daylight and heat gain. But there’s more. In a reversal of the usual rule, the whole building can be less rather than more than the sum of these parts if attention isn’t also paid to hundreds of hidden components that we don’t talk about much. Things like corner joints. Window flashing. Hundreds of beads of sealant and runs of tape. Poorly designed, specified, or installed details in these areas can bring the proudest solar-powered building owners to their knees with moisture and mold problems, façades falling to pieces, and drafty interiors that send tenants packing—and even suing.
Assemblies Put It All Together
The building enclosure manages everything that might get into and out of a building: water, wind, light, sound, air, pests, and people. Assemblies are the foundations, above-grade walls, and roofs that make up the enclosure, and how they’re put together makes all the difference to how well the building performs—and for how long.
High performance, says Z Smith, AIA, director of sustainability and building performance at New Orleans-based Eskew+Dumez+Ripple, is all about “using your 100-year glasses” and in the process “thinking in two and three dimensions” about thermal bridges, penetrations, and opportunities for air and water leakage—something he says architects weren’t that attuned to 20 years ago.
This goes for residential projects as well, adds Steve Baczek, R.A., a residential architect who has worked on Passive House projects, deep energy retrofits, and high-performance production homes. Baczek says he’s seen high-end projects that didn’t give a thought to performance: “If the homeowner gave me the house, I couldn’t afford to operate it monthly,” he said. “As an architect, it’s your responsibility to not only do the drawings but to ensure that the building is a responsible effort. What I’m designing is going to be here for 100-plus years.”
High-performance buildings aren’t just those that offer superior energy performance: we also need them to provide a durable, safe, comfortable, and healthy space in which to live and work. Meticulously designed, specified, and installed assemblies are an integral part of building performance—and because some of the tiniest assembly details can affect all these functions, achieving project goals will require cooperative input and accountability from architects, engineers, building scientists, and contractors.
It’s the Heat and the Humidity
For most buildings and most climates, moisture management is key to efficiency, durability, and resilience. This is true because heat flow is inextricably linked to moisture flow.
In today’s energy-efficient, multilayered building assemblies, durability and resilience rely heavily on moisture management to control mold, rot, corrosion, and even many pests that thrive in humid conditions, such as termites, carpenter ants, and dust mites.
We use the term hygrothermal to characterize the inexorable relationship between heat and moisture; the unrelenting nature of hygrothermal pressures requires rigorous continuity of water, air, and thermal barriers in all our assemblies: you should be able to trace the barrier with your finger from footing to ridge or parapet of a building without lifting it off the cross-section even once.
Note that the challenges to continuity for all these barriers happen at the same key places—penetrations, assembly transitions, and assembly margins—and that proper installation is critical.
“Lack of water control causes much more damage and attracts lawyers much faster” than other types of barrier problems, says John Straube, Ph.D., P.Eng., of Building Science Corporation. Water in liquid form moves about in two ways: as bulk water driven by gravity and wind, and as capillary water driven by the porous nature of so many building materials, such as concrete, brick, wood, and paper.
What we think of as the hidden or concealed continuous water barrier—weather-resistive sheet goods, flashing tapes, and sealants—should actually be the second line of defense, managing only the leftover bulk water that claddings can’t handle (see ",” EBN Sept. 2012).
Continuous capillary breaks are achieved in one of two ways: free-draining spaces and non-porous sheet goods or membranes between porous building materials.
Air infiltration and exfiltration make up 25%–40% of total heat loss in a building in a cold climate and 10%–15% of total heat gain in a hot climate. Air barriers manage not only this but also the moisture that leaking air inevitably carries with it.
“Air infiltration is the leading cause of mold, and mold is the leading cause of lawyers” is an oft-repeated chestnut among Z Smith’s colleagues. “Wall details are more than the sectional drawings,” he told EBN. “The air-leakage load of the envelope is dominating all the thermal things we are thinking of doing.” Even in his hot-humid climate, where he says dehumidification is the largest operational load, vapor drive is way down the list as a concern, completely overshadowed “by the one little crack near the window.”
Air barrier materials are pretty easy to come by—gypsum board, concrete, housewraps, some spray-foam insulation, and oriented-strand board (OSB) all meet the definition of an air barrier—but achieving continuity in air barrier assemblies requires careful integration. In the end, it’s not just each material’s air-leakage rating that counts but the performance of the entire building, as measured by a blower-door test.
The dedicated, continuous air barrier in any assembly can be on the exterior, on the interior, or interstitial (in the assembly cavity). The key is connecting the air barrier in the field of the wall or roof to details at penetrations (such as windows) and transitions (such as the eaves and the top of the foundation wall).
Thermal barriers, at least in the opaque portions of assemblies, primarily manage heat loss and heat gain by conduction (see “,” EBN Feb. 2011, for details on how glazing can manage radiant heat transfer). We most often think of thermal barriers as the bulk insulation we install in assembly cavities. But assembly cavities are not necessarily the best place to locate thermal barriers; they are simply the most convenient and least expensive place. In fact, insulating assembly cavities introduces a sometimes very steep temperature gradient across the assembly, risking condensation and the attendant mold, rot, and corrosion.
Leading building scientist Joseph Lstiburek, Ph.D., P.Eng., of Building Science Corporation identifies the “perfect assembly” as one with all thermal insulation (along with continuous air and water barriers) on the exterior, effectively pulling the interstices into conditioned space and avoiding many moisture-related risks.
Managing Moisture in the Air
Water, air, heat … wait, where’s the vapor barrier?
If you’re like many building professionals, you’re probably wondering whether you need one. (And if not, why does the building inspector insist that you do?) The way we deal with vapor diffusion in high-performance assemblies is different from how we deal with bulk water, air, and heat flows:
When we need one, we probably want a retarder, not a barrier.
A dedicated vapor retarder does not have to be continuous in order to work well.
We can do more harm than good by trying to block vapor diffusion into our assemblies— because that vapor retarder also keeps moisture from getting out of our assemblies.
All building materials—not just the ones we call retarders or barriers—affect vapor movement to some degree; serious moisture problems can occur if the permeability of each and every assembly component is not accounted for in assembly design.
In fact, as mentioned above, the best defense against water vapor in interstitial cavities is often a continuous air barrier. Although codes can be slow to catch up, the design world is moving away from dedicated vapor retarders and toward the concept of a vapor profile, assessing the permeance of each material in a building assembly and ensuring that any water leaks or condensation can diffuse readily through at least one side of the assembly. For more on these advancements, see “,” EBN, Nov. 2012.
Rubber, Meet Road
The physics behind high performance are constant, but material offerings are changing rapidly—and when it comes to delivering high performance, building assembly designs have generally fallen into a gray area—meaning opportunities for better performance have literally fallen through the cracks. Details like corner joints, window flashing, and all those beads of sealant were typically left to the contractor to work out based on manufacturers’ instructions.
But that too is changing, says Z Smith. Increasingly, “we are telling a contractor exactly how to do his job,” he said. “We certainly show things in 3-D now, and we are showing the sequencing. It almost starts to look like an IKEA construction diagram. We are taking manufacturers’ ideas and applying them to our own details.” Because of construction management and integrated project delivery, “we work together; we co-design that installation sequence.”
This new, more collaborative way of doing things invites other opportunities as well, says Luke Leung, P.E., director of sustainable engineering at SOM. “The world as a whole is moving more toward the actual performance of the wall,” looking at “how that wall is actually performing in the actual condition,” he says. This is not just true for light construction anymore: he’s seen blower-door testing on 30- and 40-story buildings and has worked on contracts for the U.S. General Services Administration and the U.S. Army Corps of Engineers that required it.
Testing sometimes doesn’t go as well as expected, in which case things could potentially get less collegial rather quickly. “Depending on where [the leaks] are, it could be as simple as people forgetting to seal up a certain part of the building,” Leung explained. “Those are easy to fix, but if you have a wholesale problem, it requires a much deeper discussion on how to remediate that.” Still, the prospect of testing tends to keep everyone on their toes. “It’s a different attitude if you know the building will get tested in the future. You’re going to be a little bit more careful because you know Big Brother is watching,” Leung jokes.
Smith is seeing the same trend toward more field-testing of assemblies and the building envelope in the commercial building world. There used to be an assumption that commercial buildings were inherently airtight, he says, and commercial architects used to view residential building as “kind of simple, something craftsmen threw together.” But in fact, he claims, not only do conventional commercial buildings (and particularly curtainwalls) turn out to be pretty leaky, but also “I think the people trying to build net-zero and Passive House are pushing the boundary in a way that is rebounding to commercial [projects].” At the same time, the lessons of high-performance commercial buildings are being applied even in production homes. “It’s like a badminton game between the commercial and residential worlds—and the net keeps being raised.”
On the residential side, Steve Baczek has hosted a “trade day” on his projects to help with follow-through from design through construction. The team met with all the trades and asked, “What do you think of these details?” and received valuable insights, he said. This practice also “gets them to buy into the project. If you offer insight, you’re going to feel emotionally tied to the project.”
Teamwork Is Key
The lives of buildings and building professionals used to be a lot simpler, Baczek points out. “Even 50 to 75 years ago, all residential wall systems were the same, and performance expectations were the same,” he says. Designs were less complex, there were not many material choices, and interior environments came with fewer health concerns and wider comfort tolerances. “You didn’t care that the house was an energy pig. Now people care.”
But caring has consequences. “Ten years ago, there was the code,” Baczek said, and now we have a whole buffet of building and testing standards and product certifications to choose from. Not only has the number of ways of doing things “right” proliferated, but there are also “new materials [that] come online almost weekly” that may look similar to the materials we’re used to working with but can have fundamentally different hygrothermal properties. Staying on top of it all isn’t easy: “As an industry, I see us wanting to turn the corner, thinking we should turn the corner, talking about turning the corner. But I don’t think we’ve turned the corner yet.”
Today’s high-performance building assemblies demand a lot more from everyone on the project team and require building assemblies that purposefully manage all the flows on and through the building. “Things have gotten too complicated,” Baczek concludes. “You cannot approach any of these projects as anything less than a good team.”
To move our residential and commercial building assemblies to high performance, a lot is needed.
• Clients have to see the value.
• Architects, specifiers, and builders need to get the details right.
• Manufacturers need to provide the full slate of hygrothermal properties for how their products perform in terms of all heat and moisture transfer.
We will turn the corner when clients demand durability and energy efficiency right along with curb appeal; when building practitioners use products only from manufacturers supplying all hygrothermal data and indicating how their products integrate within assembly systems; and when codes, standards, and programs fully recognize sustained performance right along with more conventional metrics for our better and our best buildings.