Article Contents

The Hidden Science of High-Performance Building Assemblies

Printer-friendly versionSend to friend

By Peter Yost and Paula Melton

wallsection.jpg

Directional drying is designed into high-performance buildings, and all three control layers must continuously manage water, air, and heat. Note how the air barrier is primarily accomplished at the interior and how difficult it is to prevent thermal bridging at structural framing if exterior rigid insulation is not used.

Illustration: Steve Baczek Architect

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.

Residential Passive House Building Assembly – Exemplary Details

Res_Assembly.jpg

Illustrations: Steve Baczek Architect

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

Building Science Corporation Assembly – Exemplary Details

Comm_Assembly.jpg

Illustrations: Stephanie Finnegan, Building Science Corporation

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.

Hygrothermal focus

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.

Water barriers

“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.

roof.jpg

A quality builder designed and built an energy-efficient, unvented roof assembly with no drying potential in either direction (asphalt shingles on the exterior and foil-faced polyiso board on the interior are both vapor-impermeable) with airtight can lights. But while the cans themselves were tight, gaps between the cans and the gypsum board ceiling meant substantial air leakage and wintertime wetting of the assembly, with disastrous results.

Photo: Dwight Holmes

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 "Seal, Tape, Gasket: A Sticky Search for Better Materials,” 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 barriers

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.

Continuity Detail – Balcony

balcony_detail.jpg

Balconies are typically accomplished with cantilevered concrete through-beams that are a control layer nightmare. This detail limits structural support and thermal bridging through the continuous air and thermal barriers to as little as four knife-edge connectors.

Image used with permission: “High Performance Enclosures” Building Science Press, Dr. John Straube, 2012

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

Thermal barriers, at least in the opaque portions of assemblies, primarily manage heat loss and heat gain by conduction (see “Choosing Windows: Looking Through the Options,” 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.

Continuity Detail – Residential Window

window_detail.jpg

Continuous barriers or control layers are challenging at windows, particularly with deeper wall assemblies, which can put windows on a different plane from one or more of the barriers. In this detail, the weather-lapped adhered membrane flashing is dragged back to the plane of the window at the head and sill; the sill pan extends up and over the back dam; and the air barrier is achieved at the sill with exterior and interior foam beads against a central backer rod.

Illustration: Steve Baczek Architect

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 “Using WUFI to Prevent Moisture Problems,” 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.”

Draw, But Verify

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.

deck.jpg

In snow country, managing roof load with gutters typically turns into an annual replacement cost. No gutters + backyard deck = a recipe for failed patio doors and the first three courses of lapped siding. Here, the deck board that lines up with the roof eave’s drip line was replaced with a free-draining grate. No splashback, no built-in assembly failures.

Photo: Peter Yost

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

walkway.jpg

During a major renovation of the Frederick, Maryland, town library, the main entrance was moved from the town’s main street to a courtyard near the back of the library. The architect designed a covered walkway completely detached from the original building; it was simpler and faster to build, honored the continuity of the existing wall’s water management, and was less expensive than any attached walkway would have been.

Photo: Peter Yost

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.”

Turning the Corner

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.

Continuing Education

Receive continuing education credit for reading this article. The American Institute of Architects (AIA) has approved this course for 1 HSW Learning Unit. The Green Building Certification Institute (GBCI) has approved this course for 1 CE hour towards the LEED Credential Maintenance Program. The International Living Future Institute (ILFI) has approved this course for 1 CEU.

Learning Objectives

Upon completing this course, participants will be able to:

  1. Define hygrothermal pressure and explain what it requires of water, air, and thermal barriers in high-performance assemblies.
  2. Explain how vapor diffusion is addressed in high-performance assemblies.
  3. Recognize the trend toward field-testing assemblies and the building envelope in commercial buildings.
  4. Summarize how designers and contractors are co-designing installation sequences.

To earn continuing education credit, make sure you are logged into your personal BuildingGreen account, then read this article and pass this quiz. In addition, to receive continuing education credit for ILFI, please add to the discussion forum on this page by providing a thoughtful comment on the article—for example, its effect on your practice and engagement with Living Building Challenge concepts and petals.

Discussion Questions

Use the following questions to inform class discussions or homework assignments.

  1. What has one residential architect done to engage contractors in co-designing the installation sequence of the building envelope? Why is this important? What are some other ways to promote such teamwork?
  2. Recall the three factors needed to move our residential and commercial building assemblies to high performance. How might the factors affect one another?
  3. What is the building-science rationale for placing thermal, air, and water barriers on the exterior of a building?
  4. What does it mean to assess the permeance of each material in a building assembly?
  5. Explore the WUFI design tool. In what ways does WUFI update the outdated Dew-Point method?

Comments (6)

1 structural concerns posted by William (Joe) McNally on 11/08/2012 at 08:16 am

just got this issue in the snail mail today...great to see such large scale details! I have one concern. Supporting the face masonry with a shelf angle is not a good way to go. Much better to rest that masonry directly on the foundation. Shelf angles rust, unless you go for the very expensive option of stainless steel. Paint only delays the problem. And the detail for connecting that angle to the structure is weak. Although not called out, it graphically shows two fasteners, through the rigid insul., connecting to a cast-in weld plate. Depending on how thick that rigid insul. is (and thicker than 2" seems likely), then those fasteners are cantilevering that 2" (or more). I am all for good details from a thermal, air and water barrier point of view, but we can not compromise the strucuture or the durability of that exposed shelf angle. I only use a shelf angle when forced to by my boss or in extenuating circumstances. If an associate brought me those details, I would send him/her back to the drawing board!

2 need 3-d image + how to faste posted by William (Joe) McNally on 11/08/2012 at 08:21 am

more points...I strongly prefer a 3-d image...an isometric works goods...for showing how to seal up a window...there is a lot to do here...sequence and correct overlapping is critical..as is using the correct products

and how does one attach that window? where is the blocking needed for such?..enough belly aching for now:)

3 Need 3-d image + how to faste posted by Steve Baczek on 11/17/2012 at 06:20 am

Being the author of the detail, first, let me say I agree with you. Having drawn literally thousands of residential details there is no "one shot one kill". If it were that simple we wouldn't have plan, elevation, section, and isometric views (at various scales) to describe an assembly or installation. The overall intent here is to stimulate the discussion of installing windows within the plane of the wall. While I agree with you, please understand that the size of the detail in print is restricted in size (scale) and therefore I am subjected to choosing the most revelevant information to illustrate. In comparison this detail was drawn atr 6" (half full size) in the set of construction documents. While 3d views do sometimes illustrate a detail in a better "light", a 3d detail has its informational transfer limits also. If there is a specific attribute of the detail that you would like clarified please let me know and I will ensure the needed clarity. As for the window installation, these windows were properly shimmed at the sill (gravity) and the sides and head were secured with a metal bracket first attached to the window frame, then attached to the rough opening framing, thus providing a secure installation. The resulting space between window unit and rough opening was then properly airsealed. When it comes to window installation details the variances are extremely large, thus creating a need to discriminate information once again. With this particular house, only 1800 sq ft, and using the same windows, I still had 3 variations of window installation details pending different circumstances on this small house. Couple that with the multiple section variances for each window as you move across the sill, and you have an abundance of information to transfer, but only one detail. Again, please let me know if there are any specific aspects that you would like clarifies and I will try to do my best. Thank you for your interest.

4 Radiant Barrier - why no ment posted by Matthew Piner on 08/27/2013 at 03:59 pm

I know it's a controversial topic and laden with manufacturing hype - but why no mention of radiant barriers as a part of a high performance building assembly? Shouldn't color (albedo) of roofing and siding at least be in the conversation? Depending on the climate and for envelope-dominated buildings- rejection of radiant heat gain is as important as the heat loss, air infiltration, control of moisture etc. especially during cooling season. Since radiant barriers need an air space to work - can be part of a rainscreen detail (for west facing walls) or of an air drying convective channel under the roof membrane. I call this a "RaVe Roof®" for radiant-ventilated roof (trademark is pending). It's nothing new, I've just given it a name... Depending on the climate (with significant cooling loads) and whether the attic is insulated at the roof plane level or is being occupied or a part of a cathedral ceiling assembly - this is a valuable detail to consider when the roof plane is sloping (mainly in residential construction). The air space can allow a continuous convective air flow from low to high, allow drying and be that radiant barrier that helps reject heat gain, keep attic temperatures closer to ambient. I have noticed here in Sacramento the insane trend for people to get dark colored roofs as they "look nice" - but the heat gain is crazy! With a RaVe roof system installed, they can have their dark roof and not get the 140 degree attic (although a light colored roof will perform even better, of course!)... Since the insulation has to now work at a much higher delta 'T' and duct work for AC often runs up there, really makes a difference (so does burying duct work in the attic insulation, but I digress)... The cost (price increase to customer) is within a dollar or so per s.f. - or $100/square. Not insignificant, but well recoverable within the life of the roof. Since I am serious about the "branding" of the RaVe Roof® - would like to hear some discussion on this. I have installed several of these (residential construction) as both retrofit (during re-roof) and new construction here in the Sacramento, CA area - with great results. My roofers have been trained on the appropriate detailing and methods for effectiveness, all documented. I can train any roofer with the details - all the materials are available "off the shelf" at most roofing and building suppliers. I think with more heads looking into the details, performance can be enhanced (different kinds of batten systems to reduce thermal bridging, easier methods for air intake and exhaust and moisture barrier, flashing, etc.) - and costs can come down. Have used temperature sensors - before and after - for some empirical data, plus have utility bills - but by not conclusive yet as to cost/benefit over time. Working on getting some independent 3rd party data - looking to work with our local utility, SMUD for some support on it. Would like to know if I can do an article, kick up a little conversation?

Matthew Piner, Architect/Builder/Educator, Sacramento, CA

5 Turn the corner-hydrothermal posted by Walter Currin on 01/21/2014 at 12:51 pm

WUFI does not seem to be on too many architects' desktops just yet, but I suspect it will be on many more in a 5 year window. The bigger barrier seems to be having manufacturer hydrothermal data. I have been seeking data for other information fields for years and it seems manufacturers are so much more likely to share info when it proves their product exemplary. I think it will happen, but significant industry transformation seems to be needed for creative, effective wall construction to take hold. Without richer data we will have to rely on precedent and, probably, continued ample use of "the perfect wall" in commercial / institutional construction.

6 Existing Building Assemblies posted by Ibrahim El-Shair on 03/11/2014 at 03:13 am

In order to optimize the performance of existing building assemblies, we need to: • Assess their current hygrothermal status; • Evaluate and recommend changes to that balance; • Monitor the new balance.

Post new comment

Welcome !
*
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.
  • Glossary terms will be automatically marked with links to their descriptions. If there are certain phrases or sections of text that should be excluded from glossary marking and linking, use the special markup, [no-glossary] ... [/no-glossary]. Additionally, these HTML elements will not be scanned: a, abbr, acronym, code, pre.

More information about formatting options

October 26, 2012