Rethinking the Perfect Wall (Part 2)

When making enclosure decisions, it’s all about risk management. 

My prior Green Builder magazine article, “Rethinking the Perfect Wall,” might be considered an act of heresy. It dared to suggest that it's time to rethink Dr. Joseph Lstiburek’s “perfect wall,” even though it is broadly embraced by the building science community.

Lstiburek's principles for an environmental separator that optimizes control of moisture and thermal flow are more critical than ever. The problem is that their scope is no longer adequate. So, the last article proposed an alternative “more perfect wall” that includes new risk management imperatives related to natural disasters and adequate workforce. 

Now you’re all caught up. The intent of this second article is to show how the new “more perfect wall” concept can be applied with a simple three-step process to optimize enclosure decisions, using Florida as an example. The entire state is very hot and humid, including climate zones 1A and 2A from the International Energy Conservation Code (IECC). (See Figure 1.) 

Figure 1: Climate Zone Map from IECC 2021.

Step One: Assess Priority Risk Management Issues

The first step in optimizing enclosure decisions is to identify the highest-priority risk management issues. All locations in the Southeast, including Florida, are confronted with significant building science risks related to moisture (vapor and bulk) and thermal control. This is because the area’s much hotter and more humid outdoor air most of the year is being driven through air leakage pathways to the interior, where it is much less hot and humid.

Managing this moisture vapor flow is critical to minimizing condensation when it reaches relatively cold drywall surfaces inside the wall cavity. Wetting is not a good thing. Thus, minimizing air leakage is critical to managing moisture risk. 

In contrast, vapor diffusion imposes much less moisture risk. One research study reports there is 10 times greater vapor flow over an entire cooling season (7 vs. 0.75 quarts of water) through a 4-foot-by-8-foot sheet of drywall with a 1-inch hole, compared to one without the hole. (See Figure 2.) 

Figure 2: Moisture vapor flow in a hot-humid climate from exterior to interior over Spring, Summer, and Fall.

Bulk moisture control risks are also significant in Florida due to the high annual rainfall and severe seasonal storms. Thus, it is critical to ensure effective drainage and flashing details to control bulk moisture that gets behind cladding systems (e.g., roof, walls, openings, foundation and site). And remember, bulk moisture gets behind all cladding systems.

The other key building science risk is excessive thermal bridging. In framed construction without continuous insulation, 20 to 30 percent of the wall will be framing having one-fourth the R-value of typical cavity insulation.

The superhighway for heat transfer through the framing will elevate indoor drywall temperatures during long cooling seasons. This can significantly reduce comfort because mean radiant surface temperatures have a 40 percent greater impact on perceived comfort than ambient air temperature. In addition, utility bills will be higher.

So, it is essential to manage these building science risks in Florida. But it is not nearly enough. Housing in this part of the country is exposed to exponentially increasing natural disaster risks related to hurricanes and sea level rise that require protection from floods, gale wind forces, extreme rainfall, and flying debris. 

I also consider pest damage, especially with subterranean termites, as a natural disaster. This is because pests impose significant financial expenses, property damage, and emotional stress just like other more-traditional natural disasters.

I assume pests are excluded from almost every other list of natural disasters because the damage caused is not covered by home insurance policies. However, every year in the U.S., termites cause billions of dollars in structural damage, and property owners spend over $2 billion to treat them. And no state experiences more termite damage than Florida, with $1 billion of losses each year. 

Another imperative impacting enclosure choice is the lack of an adequate workforce. According to the Bureau of Labor Statistics, the average age of construction workers is more than 42 years old.

This is creating severe challenges in terms of workforce availability, skill and cost with only two new workers in the pipeline to replace every five that retire. And the crisis is well underway, with approximately 723,000 new workers needed each year to combat a housing shortage of more than 1.5 million homes.  

This risk is greatest in states like Florida, which are among the most active in constructing new homes. In 2023, Florida’s nearly 194,000 housing permits were second highest in the country, only exceeded by Texas, with just over 232,000 housing permits. 

Workforce problems significantly increase housing cost while the industry experiences an epic affordability crisis. This is due to significant increases in cycle time that impose higher operating expenses and shrink revenue due to reduced velocity of sales.

Like I said, managing moisture and thermal flow is not nearly enough.

Step Two: Customize Wall Risk Management Issues for Your Location  

After identifying the most critical risk management issues, the next step is to customize the wall assembly risk management matrix (see prior article) for your location. Based on the priority risk assessment issues identified in step one, the matrix has been customized for Florida by eliminating the low-priority issues, earthquake and wildfire resistance, shown in the dashed-line black box in Figure 3. 

Figure 3. Wall assembly risk assessment matrix customized for Florida

Note that concrete block wall construction common in southern Florida is missing as a site-built wall option because it was not considered a high-R wall assembly. But it could be added when installed with loose-fill insulation, foam boards or injection foam. Since workforce issues are national, it is assumed they all apply in Florida. They are shown in Figure 3 without change.

Step Three: Select Optimized Wall Option(s)

The final step is to evaluate one or more preferred “more perfect wall” options that address prevailing building science, disaster resistance, and workforce risks. The wall assembly risk assessment matrix has been customized for Florida in Figure 4. The disaster risks that are not high-priority in Florida (e.g., earthquakes and wildfires) have been removed and the wall options with “low risk” across all risk management issues have been highlighted with a dashed-line black box. 

Figure 4. Wall assembly risk assessment matrix customized for Florida with low-risk options in dashed-line box.

All of these low-risk wall options feature systems-built concrete wall construction: precast insulated concrete walls, precast concrete blocks/panels, and insulated concrete core system. (See images in Figure 5.)

This is the result of concrete’s inherent risk reduction related to building science and disaster resistance issues, and the inherent workforce advantage with systems-built assemblies.

Each of these systems-built wall options also offers quality advantages that add to their value. This includes more-durable construction, superior dimensional accuracy, reduced construction time, and easier integration with Fortified Home certification and related insurance savings. A short description of each of these wall options follows. 

Precast insulated concrete wall systems are commonly used for foundations but are also available for above-grade walls. The panels are made in offsite plants, shipped to the site, and set by the manufacturer in the field. Because the concrete is poured offsite in a controlled environment, it can be cured to 5,000 psi compared to 3,000 psi for poured-in-place concrete. 

The wall system shown uses a ribbed configuration that substantially reduces the amount of concrete needed when compared to poured-in-place concrete walls. The interior bays and ribs are typically lined with R-10 rigid insulation for complete thermal bridging control. The total wall insulation can exceed R-20 by adding insulation between the ribs. Lastly, the ribs are capped with steel channels for installing drywall without any additional framing.  

Insulated concrete panels are similar to insulated concrete blocks but require much less labor, use less concrete, and install much faster. Insulated blocks and panels are assembled onsite, with the foam insulation serving as the forms for pouring concrete in the block or panel voids.

The resulting wall provides complete thermal bridging control with wall insulation that can reach R-24 and higher. These systems include light metal or plastic framing inserts for attaching outside cladding and interior drywall.

Figure 5. Systems-built wall options with low risk for Florida.

An insulated concrete core wall system is being used by a new company. They have a unique automated manufacturing process that uses extensive robotics to produce thin two-inch concrete panels that are assembled in a sandwich with rigid insulation in the middle. The steam curing process results in 8,000 to 10,000 psi concrete providing maximum moisture control. Also unique is the company’s business model operating as the manufacturer, developer, and builder. 

One major advantage of a fully vertically integrated company is that the entire construction cycle time is reduced from the typical eight months for site-built builders to 30 days. In addition to eliminating approximately 80 percent of trades, the industry-leading short cycle time results in substantially lower operational costs and increased revenue from greater velocity of home sales.

As home insurers are rapidly leaving the state and those staying are significantly increasing premiums, this company is able to self-insure its homes and in turn financially benefit from the ongoing insurance revenue. This business choice was made possible by the incredibly low risk of its product. 

Now the hard part begins. Optimizing the wall system selection requires estimating and comparing costs for preferred wall options including any added expenses to manage risks. The wall selection process will be continuously improved with ongoing efforts to measure and track actual costs. This is not an overnight process, but a commitment to embracing a new skill and patience to move up the learning curve. Getting the enclosure right is worth it.

Note that structurally insulated panels also offer a reasonable wall assembly option for Florida that addresses all workforce risks but also entails more rigorous quality control and expenses to manage bulk moisture, flood, and pest risks. If the net total cost is less than the other concrete options, it could be considered a preferred wall assembly.

It’s All About Risk

The example of applying the wall assembly risk assessment matrix is intended to show how it can help quickly identify enclosure options with the least risk for any location. The finding that systems-built concrete wall assemblies in Florida minimize risks associated with building science, disasters and workforce is probably intuitive. Yet, much riskier site-built wood framing and concrete block continue to dominate construction in the state.

A picture tells a thousand words, especially the image below showing a lone home standing in Mexico Beach, Florida in the aftermath of Hurricane Michael in 2018.

In a sea of devastation, this fully intact home—known as The Sand Palace—provides strong empirical evidence that concrete wall options are a “more perfect” wall, providing superior protection from storm surges, gale force winds and flying debris. It uses a concrete pier foundation that raises the home a full story above grade. The walls are constructed with insulated concrete forms. It also helped that a hip-roof over an unvented attic with spray foam at the slope was used to maximize wind resistance. 

Hard Facts

The Sand Palace in Mexico Beach, Florida after Hurricane Michael in 2018, owes its standing—literally—to use of a concrete pier foundation that raises the home a full story above grade. 

It is not mandatory to choose “low-risk” wall options. What should become common practice is identifying the risk management issues and assessing the costs of managing those risks with any preferred wall assembly.

It is also important to recognize that other factors may offset the added cost to manage the higher risks such as supply-chain considerations, retaining hard-earned business relationships with trades, minimizing embodied energy, and preferences for wood assemblies that maximize workability. Then the wall selection decision will be more informed. 

I’m confident that the company that chose to manufacture an advanced concrete insulated core wall assembly was informed by the millions of dollars of added annual revenue made possible by self-insuring a “more perfect” enclosure with minimal risk due to its near-bulletproof disaster resistance.

They also enhanced their future-market-readiness for the state’s rapidly growing homebuyer awareness of exponentially greater disaster risks and resulting buyer preference for homes resistant to them. I assume the great feeling of constructing homes and communities that will stand the test of time is a nice bonus. 

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.

This Housing 2.0 presentation is sponsored by:  Panasonic