Evolving buildings yield evolving design principles

June 17, 2019

by Ted Winslow

All images courtesy CertainTeed Insulation[1]
All images courtesy CertainTeed Insulation

Construction professionals seem to find ways to add more layers and new materials to building envelopes. While these additional layers have helped reduce energy losses, they have not managed to keep water out of the walls. For example, the use of continuous insulation (ci) on exterior walls and the airtightening of building envelopes have only increased the need for materials with elevated drying potential (e.g. fiberglass insulation) because when (not if) moisture infiltrates the assemblies, it needs to escape. This is where resilient design comes into play.

Buildings must be durable and treated individually based on their climate zone (CZ), region, and the location—they must be robust. However, to do so, architects and specifiers must first understand how systems behave in different markets and how designs can be optimized to make the wall assemblies truly robust. Managing moisture would be the key to successfully constructing the sustainable buildings of the future.

In several building science circles, the concept of a ‘perfect wall’ is often discussed. It has a lot of sound principles, such as ‘keeping the outside out and the inside in’ and designing the wall assembly to not just manage, but also control rain, air, vapor, and heat. However, even walls deemed ‘ultimate’ or ‘perfect’ fail due to the multitude of unforeseen events, such as faulty installations and poorly communicated design details. Due to this uncertainty, the ultimate perfect wall is only flawless until it fails. What will happen then? Can the wall withstand failure?

A robust wall is able to anticipate the areas where a wall system or component may not succeed. It includes layers that are designed to reduce the impact of failure. Alternatively, a robust wall could be considered the perfect failure, as it is designed to fail. It embraces the fact that building professionals cannot always predict complete success. However, the building team can anticipate at-risk areas and potential failures. By minimizing risks and anticipating the perfect failure, construction professionals can create truly robust building assemblies.


Expanded view of vapor permeance of an assembly and the components therein.[2]
Expanded view of vapor permeance of an assembly and the components therein.

Let us take a step back and first understand the evolution of building construction. How has the industry shifted in the robustness of today’s building assemblies? How have building methods changed, and how are they impacting things like insulation?

The evolution of framing techniques has largely been driven by the need to build more efficiently. Platform framing became the norm as it was much more efficient to construct than balloon framing. Traditional roof rafters have been shifted to roof trusses as they can be built on the roof, ground, or even prefabricated offsite—making room for more options and versatility as well as expediting the construction. Even framing is changing with high-performing, lightweight joists and open-web trusses. Industry professionals are also seeing more and more elements being borrowed from prefabricated construction, which, on its own, is rapidly growing and changing how buildings are put together.

All of these changes come at a cost, especially as the industry is heading toward higher R-values and airtightness levels. There are even more challenging details to address and increasing opportunities for air to leak through the walls. Having said that, the primary goal still resonates within these structures—building professionals do want to develop healthy, durable, and energy-efficient structures. To do this, the project team must understand how the building works and where to look for key problems.

It is important to keep in mind contractors are no longer just responsible for one role or function—even trades are evolving. For example, the insulation contractor is now required to be an energy expert for the entire building envelope, the most important piece of the entire building. The contractor must be equipped with the tools and knowledge to decrease air infiltration and increase building comfort. He/she must understand where to air seal, how to inspect work for quality, and potential problem areas and how to prevent them from happening.

Builders are balancing and juggling multiple management strategies to address the needs—especially the management of heat, air, and moisture—of their projects. No single product or system can be used to address all of these. At a practical level, building project teams strive to achieve a balance between costs and performance, and sometimes one is compromised for the other. Trade-offs are made to reduce project costs. This can, in turn, negatively impact the building’s ability to control the flow of heat, air, and moisture through the building envelope.

Enter the HAM principle—heat, air, and moisture flows must be balanced to ensure optimum performance for any building.

Heat flow

The different climate zones of the United States.[3]
The different climate zones of the United States.

When design professionals manage heat, they are trying to keep what is in inside and what is out outside, at least to the best of their abilities. Heat will always try to move from a warm area to a cold zone. The rate of transfer can be slowed by adding insulation between those areas.

When it comes to developing strategies for managing heat flow, the author’s preference is to divide the wall into two sectors: the cavity and the exterior. There are many reasons for this, but the two primary ones for the author are:

The entire project team, including the architects and contractors, plays a huge role in building the ultimate perfect wall in all phases of a project. From the tradesperson installing the materials onsite to the specifiers designing the assembly, each of these roles need to be considered when designing crucial components and details for wall and roof assemblies.

The shift to tomorrow’s building code requirements.[4]
The shift to tomorrow’s building code requirements.

Air permeable materials (e.g. fiberglass, mineral wool, and cellulose) and products like spray foam, radiant barriers, and even hybrid systems combining multiple types of insulation and sealing techniques are employed within the cavity. Vapor impermeable insulations (e.g. expanded or extruded polystyrene [EPS or XPS], polyisocyanurate [ISO], etc.) or vapor permeable materials such as fiberglass or stone wool are utilized on the exterior wall.

The challenge becomes when to choose certain insulations over others and the factors that come into play when making those decisions. For instance:

When managing heat flow, the goal is to limit heat transfer to improve comfort and protect the building from moisture damage to improve the wall’s durability.


Building codes—particularly standards such as the International Energy Conservation Code (IECC) are focusing on reducing things like air changes per hour (ach) by evaluating them at certain pressure levels (typically 50 Pa [1 psf]) to simulate how air will leak in and out of a building. Tools like infrared (IR) and blower doors can be employed to pinpoint the location of leaks.

Just like heat, there are both interior and exterior strategies to address airflow. Exterior air barrier strategies employ products like fluid-applied membranes, water-resistive barriers (WRBs), and insulation boards. Combining these systems with tapes and joint sealants is a great way to address the leakage paths on the exterior side of the wall.

Interior air barrier strategies include a variety of solutions. SPF can be used to address challenging details (e.g. rim joists) or with hybrid applications in more cost-conscience jobs. It can also be utilized as a full-cavity solution or in conditioned attic assemblies with mechanical equipment. Airtight drywalls, window foam sealants, and caulk and seal packages can also be suitable to address a variety of details from the inside. Newer, more-advanced approaches include the installation of a smart vapor retarder as a continuous air barrier with tapes and sealants. Alternatively, the building team could utilize a similar system integrated with batt insulation that can be simply taped to tackle air, moisture, and heat in a single application.

 What makes it ‘smart’?

A balance is needed when managing heat, air, and moisture flow.[5]
A balance is needed when managing heat, air, and moisture flow.

A smart vapor retarder has the ability to react to changes in relative humidity (RH) by altering its physical structure. During winter, when RH is low, smart vapor retarders have the ability to provide resistance to vapor penetration from the interior. However, when RH rises to 60 percent or above, its permeance also increases, thus allowing the water vapor to pass through, facilitating the drying of wet building systems.

When it comes to air infiltration and exfiltration, “air out always equals air in,” so if air can be either stopped from coming in and/or going out, the building team can be fairly confident in the success of the system as a whole. One key advantage of also including an interior air barrier system is they are far better protected from hazards (e.g. rips, tears, getting blown-off, ultraviolet [UV] degradations, etc.) that could diminish performance. Another detail to consider is climate, as it also dictates the amount of airtightness a building would need. For example, a home in the northern half of the United States will generally need an ach 50 of three or less; whereas, the same home in the South would require an ach 50 of five or less. This
is because buildings in the northern climates are susceptible to losing more energy due to the larger temperature differentials between the inside and the outside, especially in the winter months, so losing less air from the building allows it to more easily maintain internal air temperatures, thereby saving energy.

Another interesting nuance is exterior systems involve exterior trades (framers, siders, etc.) and interior strategies that generally involve insulators, drywallers, and other interior tradespersons. This raises the question of who is responsible for the occupants’ comfort, indoor air quality (IAQ), and energy efficiency? Regardless of whether the responsibility lies on the inside, outside, or both, the project team must make sure that the above-mentioned strategies are utilized. Otherwise, hot air would be let out instead of keeping it under control.

Moisture flow

Building codes are focusing on heat and air flows. This leads to an increased risk for moisture flow because the drying potential is reduced.[6]
Building codes are focusing on heat and air flows. This leads to an increased risk for moisture flow because the drying potential is reduced.

How are building professionals trying to manage moisture? Similar to heat and airflow, the goal is to always keep what is in inside and what is out outside. Of course, all of this changes once moisture gets inside the wall. The focus then is to provide the moisture with a pathway to escape the assembly and ‘breathe,’ so it can dry out.

Every component of the building assembly has a different vapor permeance and falls into one of the following four categories:

Quite often, construction practitioners focus on the permeance of just a few materials, but it is critical to take a much more holistic approach and understand how the entire assembly behaves from a moisture management standpoint. It should inspire building professionals to consider solutions that are robust in managing moisture. For example, smart vapor retarders are able to adapt their permeability based on the moisture trapped within a wall cavity—providing it has the means to dry out and minimize risks.

Good design and practice involve controlling the wetting of building assemblies from both the exterior and interior, and each climate zone requires a different approach.

Back to robustness

Continuous insulations (ci) with a higher permeance are more favorable as they dry toward the exterior and interior.[7]
Continuous insulations (ci) with a higher permeance are more favorable as they dry toward the exterior and interior.

Heat and air flows are relatively static in how they are managed. For example, a hole can be sealed (hopefully during the construction phase itself) and additional insulation can be installed either in the wall’s interior or exterior. However, moisture is a bit different. No wall or roof is completely impermeable to moisture. A robust wall is one with the ability to actively and passively manage moisture while accounting for the fact moisture management changes with the time of day, season, and life of a building.

Additionally, building solutions need to evolve with codes that are driving the increased levels of insulation and airtightness. In the past, R-15 or even R-13 were considered acceptable thermal insulation targets for exterior walls, but today’s walls mostly require either increasing the wall thickness toward the inside (e.g. 2×6) to achieve R-20 and more, which is a code-driven requirement, or adding exterior insulation to the outside of the wall (up to R-10). Airtightness requirements are also more stringent than before. This is great from an energy efficiency perspective, but it increases the risk of moisture condensation in a wall with less drying potential. Surprisingly, there has been limited change in the building and energy codes (e.g. the International Codes [I-codes]) to account for these moisture-management challenges. It should be clarified this does not mean there are no areas where moisture management strategies are suggested (or implied) in some building codes. The issue is there are not enough deliberate references to construction practices to prevent moisture issues from occurring altogether. For example, Section 1404.3, “Vapor retarders,” in the 2018 International Building Code (IBC) suggests “approved designs for accepted engineering practices for Hygrothermal analysis” could be utilized. However, this “code speak” is vague and does not clearly identify acceptable assemblies that would enhance a building’s performance directly. The author anticipates focus will tilt toward areas like durability and resiliency during the next few code cycles as moisture-related problems arise and ideally there will be a shift in a direction that will provide content in an easy-to-understand format for all parties involved.

Ci with R-13 in cavity can still be at risk if rain infiltrates the wall assembly.[8]
Ci with R-13 in cavity can still be at risk if rain infiltrates the wall assembly.

The challenge with moisture management led insulation product manufacturers to develop criteria to evaluate the durability and resiliency of a wall and to assess the robustness of an assembly. To be robust to moisture, the wall assembly needed to perform without moisture issues all year long. Based on research and analysis from dedicated building scientists throughout North America, the team was able to conduct a moisture analysis across various regions built on the following three fundamental performance benchmarks:

Polyethylene (PE) has high risk of summer condensation as assembly dries toward the interior of the building envelope.[9]
Polyethylene (PE) has high risk of summer condensation as assembly dries toward the interior of the building envelope.

Here is what the team discovered when looking at various regions throughout the United States.

Marine (CZ 3C and 4C) – San Francisco and Seattle, and cold humid (CZ 5A and 6A) – Boston and Minneapolis

Hot humid (CZ 2A and 3A) – Houston, Tampa, Atlanta, Oklahoma City, and warm humid (CZ 4A) – Nashville and Philadelphia

Making these learnings universal – available tools and resources

Cities with cold winters could still be at risk even though they pass Criteria 1 and 2.[10]
Cities with cold winters could still be at risk even though they pass Criteria 1 and 2.

The great news is the industry is at a point where access to reliable knowledge and information focused on the science and performance of buildings is readily available. Manufacturer websites have a plethora of information. BuildingScience.com is another great resource.

Trade associations like the North American Insulation Manufacturers Association (NAIMA) are also developing excellent resources to utilize and leverage like the Canadian Wood Council’s (CWC’s) simulator tool that allows design professionals to evaluate various assemblies across Canada against several performance criteria.

Recently, the US Department of Energy (DOE) and Building America have been developing a building science advisor tool to evaluate the performance of various wall assemblies across the country. The intent of these tools is to provide the industry with a better gauge to measure how buildings will perform.

Embrace failure to learn the path forward

Kraft batts have moderate risk of summer condensation because the wall assembly dries toward interior.[11]
Kraft batts have moderate risk of summer condensation because the wall assembly dries toward interior.

Everyone fails. Many thought leaders have been quoted about being fearless in their pursuit of accepting failure, but one quote from Woody Allen has always stood out to the author: “If you are not failing every now and again it is a sign that you are not doing anything very innovative.”

As codes move forward, building professionals cannot remain complacent and static in their approach to construction. They need to adapt, change, evolve, and be dynamic. If the same thing is done constantly despite environmental changes, construction teams cannot anticipate different results, or it may yield different results, just not the expected ones.

The industry needs to keep moving forward and try to understand the local considerations of product, material, and system selections as well as the behaviors they exhibit to make durable and resilient buildings. The structures must be robust by maintaining a greater potential to dry than wet.

It is advisable to consider more resilient and adaptive product choices such as smart vapor retarders and hybrid systems in certain climate zones to optimize their performance. Building professionals need to not just anticipate failures, but also understand how structures fail. Systems designed to recover when exposed to failure must be employed to make the building envelopes truly robust.

Ted Winslow is the brand product manager of building science, systems, and technical marketing for CertainTeed Insulation. He serves the company as a technical resource on topics ranging from code reviews to sustainability programs, and oversees development of CertainTeed insulation systems. Winslow holds a bachelor of science degree in mechanical engineering from Temple University. He can be reached at ted.winslow@saint-gobain.com[12].

  1. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2019/06/Opener1.jpg
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