Rethinking the Perfect Wall

The new environmental separator goes beyond internal performance. It’s also a safeguard against natural disasters. 

This is Sam Rashkin’s latest in a series of articles based on his second book, “Housing 2.0: A Disruption Survival Guide.” It is intended as a roadmap for high-performance builders to become the most successful in the industry.

Widely renowned building scientist Dr. Joseph Lstiburek introduced us to the “perfect wall” as an effective environmental separator for keeping the outside out and the inside in.

The resulting moisture risk management has become an imperative as the latest International Energy Conservation Codes (IECCs) and certification programs (e.g., ENERGY STAR Certified Home and Zero Energy Ready Home [ZERH]) are effectively transforming U.S. housing to high-performance enclosures.

Consider how much better insulated and air-sealed new homes are compared to a few decades ago. This means framed walls today experience significantly less air flow and thermal flow with colder surfaces inside their cavities. Walls today have significantly less drying potential and more wetting potential that create greater moisture risk.

A "perfect wall" in a building, such as with this project from Rauser Design, requires many variables, including local climate and penetrations to how material selections may interact.

The secret sauce for Lstiburek’s “perfect wall” is to provide adequate continuous insulation outboard the structure to provide an effective thermal break and reduce surface temperatures inside the framed cavity to mitigate condensation risk. A common variation of the “perfect wall” is shown below in Figure 1.

Figure 1: 2021 International Energy Conservation Code compliant assembly for Climate Zone 3B Source: U.S. DOE Building America Solution Center

Unfortunately, controlling rain, air, vapor and heat flow is no longer enough. We can no longer ignore exponentially greater disaster risks and construction industry workforce challenges. They have a profound impact on what qualifies as a perfect wall.

Disaster Risks

Managing risk is about predicting the future accurately. The key is knowing how to distinguish between a soft trend (e.g., latest color preference) from a hard trend (e.g., increasing global human population). It is about knowing how to recognize certainty.

One clear hard trend is that the planet is getting warmer. According to NASA, the 10 most recent years are the warmest years on record, including 2023 as the hottest. 

When predicting the future, I also care about science. As the planet gets warmer, temperature gradients increase, and ocean temperatures rise. Then physics kicks in. Following the second law of thermodynamics we experience greater “more to less” driving forces (e.g., pressure, heat, and moisture flow) that, in turn, increase the frequency and magnitude of disaster events.

According to NOAA data, the frequency between the most extreme billion-dollar disasters has increased more than four times since the 1980s. This has led to six times more billion-dollar severe storms in the past two decades than the prior two decades. 

Over one five-week period, five 1,000-year rain events struck the U.S. We’re not in Kansas anymore. It’s time to recalibrate what is a 1,000-year event.

Along with greater frequency comes greater magnitude driven by greater atmospheric temperature gradients and moisture absorption. This results in many-fold increases in storm size, wind speed, ocean water surge, polar ice melting, droughts, and heat storms.

For example, hurricanes a few hundred miles in diameter used to be considered extreme. Now we are experiencing storms exceeding 1,300 miles in diameter. One foot of sea rise is forecasted for the next 30 years, which is more than three times greater than that experienced in the past 100 years. Similarly, we’re seeing devastating wildfires that destroy whole cities, and rainstorms that dump a year’s worth of rain in just a couple of days. 

Most significantly, we can’t afford the cost of this exponentially increasing disaster risk. There is $6.6 trillion of estimated market value for the 43 percent of U.S. homes in counties with high natural disaster risk.

In September, hurricanes Helene and Milton hit the gulf coast within two weeks of each other, causing an expected $100 billion of wind and flood damages. In response to this new reality, insurance companies are pulling out of high-risk states (e.g., California, Florida) and significantly raising costs.

And this risk is not lost on the public. In one survey, 75 to 80 percent of respondents would hesitate to buy a home in areas with increased flooding, intensity of natural disasters, extreme temperatures, and rising sea levels. 

Aftermath of Hurricane Milton in September 2024. CREDIT: Flickr/Calmuziclover

Workforce Risks

The construction industry is facing a huge workforce crisis that is frequently in the news. The key challenges are availability, skill and productivity. 

Regarding workforce availability, in one recent survey, 94 percent of construction firms reported they have a hard time finding workers to hire. Fifty-four percent report project delays due to this shortage. 

In addition to added costs and the burden of securing workers, the increased cycle time imposes significant expense for additional overhead (e.g., rent, salaries, equipment, vehicles, construction loans, insurance) and reduced sales velocity (lost profit margins).

What is most disturbing about the workforce shortage is there is no end in sight. According to the Bureau of Labor Statistics, the average construction worker’s age is 42.5, with the Home Builders Institute reporting there are only two workers in the construction workforce pipeline to replace every five workers that age out or retire. The result is a labor shortage Tsunami looming ahead with approximately 723,000 new workers needed each year to combat a 1.5-million-home shortage.

In addition to a critical workforce shortage, there is a substantial skill deficit among those workers that can be found. According to one survey, inadequate skills is one of the main reasons labor shortages are so severe, since most job candidates are not qualified to work in the industry.

This problem is compounded in high-performance housing that require much greater skills for quality installed moisture control, insulation, air sealing, air barriers, HVAC equipment and ductwork, whole-house ventilation and smart home systems.

The final workforce issue, productivity, is a huge problem that also contributes to the housing affordability crisis. This is because home building productivity is actually going backward, with an approximately 50 percent decline since 1968 while every other industry has become more productive. The outcome is an average cycle time that has increased to 8.6 months from permit to finish construction.

Analysis by McKinsey and Company suggests the U.S. construction industry is suffering from a $200 billion labor productivity gap that could be closed by adopting 21st-century manufacturing methods. 

The bottom line is there is a new imperative for the “perfect wall” to resist exponentially growing disaster and workforce risks. 

The New “Perfect Wall”

The original “perfect wall” was a great revelation when introduced in 2010. It’s time to expand the scope of what qualifies as a “perfect wall” beyond building science to also include disaster and workforce risks.

The risk assessment matrix in Figure 2 below is a first attempt to integrate all these risk factors into a single tool. This is achieved using a “low,” “medium” and “high” assessment for each risk factor across a diverse array of wall assemblies. Importantly, wall assemblies are split into site-built and systems-built (e.g., offsite construction) to account for workforce risks. 

It is important to recognize that a “medium” or “high” risk assessment for any risk factor of concern does not mean a wall assembly should not be used. It just means it is critical to manage that risk. For example, if a wall assembly is considered “high risk” relative to bulk moisture, I’m confident tried-and-true moisture control strategies with adequate quality control can manage the risk (e.g., weather resistant barriers, flashing details and capillary breaks). 

What “medium” or “high” risk assessments do mean is that the extra costs to manage the greater risk must be considered for a fair comparison with other lower-risk wall assemblies. This is especially important for new innovations providing low-risk alternatives to 150-year-old traditional framing assemblies. Too often innovative wall systems are handicapped with simple bid cost comparisons with conventional wall framing that ignore these costs.

This is an especially difficult handicap when coupled with normal builder resistance to change and lack of substantial learning curve cost savings that only come with use. Thus, this tool is an attempt to begin the process of fully integrating all the costs of risk management along with other benefits when choosing wall assemblies (e.g., reduced cycle time, reduced rework, reduced waste, superior user experiences). 

Higher Risks, Greater Gains

The strawman wall assembly risk assessment matrix in Figure 2 suggests there are much higher risks for wood framing compared to concrete wall assemblies and for site-built compared to system-built wall assemblies. No surprises. 

Wood framing dominates home construction because it is abundant, easy to work with, lightweight, and renewable. However, objectively it is a poor wall assembly material because it:

• Lacks dimensional stability
• Burns
• Rots due to moisture
• Gets moldy with moisture
• Creeps under sustained loads
• Doesn’t withstand impact 
• Attracts destructive pests 
• Warps based on temperature and humidity
• Expands and contracts based on relative humidity
• Has different strength properties depending on orientation
• Requires job site protection
• Tends to be labor intensive

Also obvious is that system-built walls substantially reduce workforce risks compared to site-built walls. 

Where a systems-built assembly that doesn’t use concrete is preferred, structural insulated panels (SIPs) offer an option for a more perfect wall. They enhance impact resistance, improve wildfire resistance when paired with an SIP’s unvented attic, and address workforce risk factors. However, they still must manage high risks related to bulk moisture as well as flood resistance where it applies.

Remember that this assessment is just an initial attempt to compare wall options relative to a much more comprehensive set of risk factors than the original “perfect wall.” 

This is just a first step. Assembling diverse stakeholders and experts to collaborate on a consensus risk assessment for wall assemblies would help provide more robust guidance. The key point is that we need to be better informed choosing wall assemblies that are more disaster resistant and workforce ready at a time when housing is confronting a massive affordability crisis.

Figure 2: Wall assembly risk assessment matrix for IECC climate zones 5 through 8.

This Housing 2.0 presentation is sponsored by:  Jinko Solar,  Panasonic, Schneider Electric and LG HVAC