The Perfect Wall and the Building Science Blues

December 01, 2017
Winter 2017
A version of this article appears in the Winter 2017 issue of Home Energy Magazine.
Click here to read more articles about Walls

What is the Perfect Wall? And, why does it give me the building science blues? This article answers both of those questions, and it tells how some of the best building scientists on the continent, with a little help from the Building America program, are working to beat those blues.

In Search of the Perfect Wall

Since DOE hired me in 2011 to lead the Building America program, I’ve been asking myself and many others these burning questions:

Joseph Lstiburek’s The Perfect Wall describes how to build a wall that “keeps the outside out and the inside in.” Lstiburek’s report provides highly regarded guidance, but how many builders actually follow it? If not many (as I suspect), why not?

This question is really important to my program and me. We estimate that the technical savings potential of improving all residential building envelopes in the United States to efficiency levels achieved in DOE Zero Energy Ready Homes is around 3 quadrillion Btu per year. That’s a huge amount of energy, and a lot of money, too. It would be like putting $40 billion back into the pockets of Americans every year. But Building America experts, including Joseph Lstiburek, agree that we can’t achieve this high level of home energy efficiency until we help the industry to solve the building science problems that crop up when building and retrofitting homes requires a lot more insulation and airtightness measures.

The good news is that people around the country are building, even retrofitting, homes to superior levels already. More than 1,000 DOE Zero Energy Ready Homes have been built since the program began and we’re expecting that number to exceed 2,000 in 2017. Building America and DOE Zero Energy Ready Homes have hundreds of real case studies to prove it works. The bad news—and the reason I have the blues— is that the industry is really slow to follow, which brings me back to the Perfect Wall and my burning question.

The Perfect Wall

What is a Perfect Wall? It’s a wall that tightly controls heat flow in and out of a building. You can’t tightly control heat flow in and out of a building without tightly controlling air and moisture flow in and out of the building. It’s applied physics. Apply established laws of physics to buildings and you get the principles—control layers—of the Perfect Wall.

The Four Layers of a Perfect Wall

The Perfect Wall has four control layers. In order of relative importance, they are (1) the rain control layer; (2) the air control layer; (3) the vapor control layer; and (4) the thermal control layer.

In his report, Lstiburek suggests putting all the control layers on the outside of the structure for conventional wood-framed buildings. “Keep the outside out,” he says, “and the inside in.” Protect the structure and all its occupants from Mother Nature. Perfect... See Figure 1.

The Perfect Wall

The Perfect Wall
Figure 1. In concept the perfect wall has the rainwater control layer, the vapor control layer, and the thermal control layer on the exterior of the structure. The claddings function is principally to act as an ultraviolet screen. And architects might consider the aesthetics of the cladding to be important. Source: Lstiburek (2010).

Lstiburek goes on to acknowledge that his Perfect Wall can be expensive. He shows us details of the closest-to-perfect wall he had seen in residential buildings when he wrote the report, with all four control layers on the exterior of the structural frame, including at least half the insulation. The addition of R-10 or more continuous exterior insulation, a drainage plane between the exterior insulation and structural sheathing, and a dual-purpose weather resistive barrier (air-barrier and vapor retarder) together add significant cost to a traditionally framed wood wall. He calls this the Residential (Perfect) Wall. See Figure 2.

The Residential (Perfect) Wall

The Residential (Perfect) Wall
Figure 2. The addition of R-10 or more continuous exterior insulation, a drainage plane between the exterior insulation and structural sheathing, and a dual-purpose weather resistive barrier together add significant cost to a traditionally framed wood wall. Source: Lstiburek (2010).

But many builders and owners prefer not to build with brick or stone veneer cladding, for one reason or another. So how can building designers apply the Perfect Wall principles without regard for cladding type? And without breaking the bank?

An Architect’s Nearly Perfect Wall

My DOE colleague Sam Rashkin is an architect, so he doesn’t think like a scientist or an engineer. Rather than talk about principles and modeled results, Sam prefers to use the following four practical design tricks to achieve a nearly perfect wall, in the spirit of not letting perfection be the enemy of the good:

  1. Put an air gap behind the cladding to drain rain before it has a chance to penetrate the rest of the wall assembly.
  2. Specify a weather-resistive barrier (WRB) that allows the wall assembly to dry to the outside. Sam says that according to Lstiburek, 10–20 perm materials are ideal.
  3. Eliminate thermal bridging by using exterior insulation to keep the dew point (and condensation) away from the wall sheathing material.
  4. Specify a permeable layer (such as latex paint) on the inside of the wall, to allow the interior components of the wall assembly to dry to the inside. Again, Sam says that 10–20 perm materials are ideal.

To check my comprehension, I compared Sam’s four tricks to Lstiburek’s four principles, and I noticed a slight discrepancy. “What about Lstiburek’s air control layer?” I asked Sam. “Don’t we need to keep infiltration of moist air outside of the wall?”

“Yes, of course,” Sam replied. “The WRB acts as an air barrier as well as an air sealer.” He didn’t make that point when he first recited his four architect design tricks. But he’s an architect. What can I say?

Despite his tiny little omission (of a detail that can ruin everything), I believe he’s right that his design tricks will keep architects out of trouble most of the time—if they follow them diligently.

How Close Are We to Achieving the Perfect Wall?

But let’s get back to my first burning question. How many builders today actually follow the Perfect Wall recommendations? I did some research and this is what I found.

First, I wanted to know how common is the use of exterior insulation, one of Lstiburek’s key recommendations. I checked some market data from Home Innovation Research Labs (HIRL), a wholly owned, independent subsidiary of the National Association of Home Builders, and a long-standing member of a Building America Partner team. Table 1 Exterior Insulation shows that a little more than 10% of homes were using exterior insulation in 2013, up from 7% seven years earlier. That’s not very impressive market penetration and a pretty slow trend. Considering the low insulation value and prevalence of insulated sheathing products (Table 1 Primary Wall Sheathing), still less than 15% of homes have any form of exterior insulation. One clear conclusion is that the average builder is not using exterior insulation yet, despite newer codes that prescribe exterior insulation in many climates, the compelling benefits of added thermal performance, and the many exterior insulation products and systems available today.

Table 1. Exterior Insulation and Sheathing Trends

Table 1. Exterior Insulation and Sheathing Trends

The Building Science Blues

Why do I have the building science blues? Simply put, it’s because we know how to build Perfect Walls—super energy-efficient walls that keep the outside out and the inside in, and that last forever—yet hardly any builders are willing to try it. Why? Here’s my hypothesis:

Hypothesis: Design decisions involve numerous and often-conflicting factors. Builders seek cost-optimum solutions that manage their risks, but moisture durability risk—whether a wall "works" in a given climate—is not yet quantifiable. Designers, with or without help from experts, gamble every time they spec a new wall system or a change to their current wall system. The result is that few builders understand the relative risk exposure of any wall they might consider building, and few are willing to bear the learning-curve costs of trying out Perfect Walls.

Why is it so hard to change? Here are several reasons I’ve observed over the years:

  • Moisture risk—at the whole-wall level—isn’t yet quantifiable. So we guess.
  • If you design a wall to resist moisture damage in hot and humid weather, it probably won’t work well in cold weather, and vice versa. This is particularly troublesome when you’re designing buildings that see both types of weather.
  • Building material costs change, making the relative costs of building systems and materials fluctuate, so if you’re looking to optimize costs, be prepared to change systems or materials frequently.
  • Code requirements change every three to six years.
  • Labor shortages change everything.
  • Manufacturers engage in wishful thinking—lots of new materials come on the market, but often with hard-to-believe claims. Sometimes, they exaggerate to make the sale. I can live with that. My mom exaggerates a lot.
  • The properties of some materials vary with temperature and time, so the system you specify today may not deliver the same results at different times of the day or five or ten years from now.
  • Weather is hard to predict, especially over the long term.
  • It’s really expensive to open up walls, and/or measure the moisture inside them for long periods of time. Consequently, we don’t know a lot about how much moisture accumulates inside different walls, in different climates, in different houses, with different occupant activities.
  • Expert advice changes over time, as the experts gradually learn more and more about what’s going on.

Beating the Building Science Blues

One way to beat the building science blues would be to reengineer the way we build. How might we quickly, cheaply, and easily put all the control layers on the outside? One great idea that’s being tested by a Building America team at the University of Minnesota is what they’re calling an Affordable Perfect Wall.

An Affordable Perfect Wall?

The Perfect Wall can theoretically be done cheaply—if builders are willing to completely change their processes and building materials. A current Building America project by the NorthernSTAR team at the University of Minnesota is teaming with Twin Cities Habitat for Humanity, Urban Homeworks, Thrive Home Builders, Building Knowledge, and Huber Engineered Woods to validate the energy, cost, and moisture performance of an innovative new wall system (see Figure 3). Its official name is Monopath, but they recently started calling it the Affordable Perfect Wall. I think that’s because they got tired of listening to me talking about the Perfect Wall.

The Affordable Perfect Wall

The Affordable Perfect Wall
Figure 3. A current Building America project by the NorthernSTAR team at the University of Minnesota is to validate the energy, cost, and moisture performance of an innovative new wall system. Source: Daniel Handeen, Center for Sustainable Building Research, University of Minnesota, 2017.

The Monopath wall system, aka Affordable Perfect Wall is innovative both in technology and in delivery, and it meets all the goals of the Perfect Wall. It is made out of oriented strand board (OSB) panels, and all the control layers are on the outside. The sheathing is the structure. The University of Minnesota team has built some Monopath homes already. Now they’re working to streamline the process and get past the builder learning curve issues. The team is confident this will all come together into a better design, a better construction process, and better performance, all at less cost. So that’s one way to beat the building science blues.

Beating the Blues with More Science and Better Tools

Another way to beat the building science blues is to do the science and develop tools that will help builders better manage the risk of moisture-related building failure when they specify higher insulation levels. Two additional Building America projects are tackling these two challenges.

Much about the science of whole-building performance remains unknown, and very little applied science is done, leading to a great deal of uncertainty about building moisture durability. Based on my own observations, expert opinions, and the limited builder interviews I conducted for this article, this uncertainty leads builders to rely on one or more of the following strategies to manage their risk of moisture-related building failure:

  1. Trusting the judgment of an “expert.”
  2. Doing their own poor man’s research and development.
  3. Most commonly, avoiding or resisting changes (including energy-efficient changes) due to perceived risk.

Proposed Solution: Quantify or qualify the risk of building science failures and make the data or the expert knowledge accessible during the design process.

Two Building America projects are working on this proposed solution now. Read on.

Field Study of Moisture Durability in 20 High-R Walls

In 2016, the HIRL-Building America team announced that it wanted to study moisture performance in walls, and that it needed builders to volunteer (see Figure 4). The team received more than 100 volunteers—far more than the HIRL researchers expected. Builders want to benefit from building science, but most don’t want to do building science—that is, turn their business into a science project. This HIRL study was their chance to learn more about what’s going on inside their walls without having to become real scientists. I think they’re really smart to acknowledge that they’re better at building than they are at science, and that they need help to better manage building science risks, especially the risk of moisture accumulation inside walls.

HIRL Moisture Study Site Map

HIRL Moisture Study Site Map
Figure 4. In 2016, the HIRL-Building America team announced that it wanted to study moisture performance in walls, and that it needed builders to volunteer. The team received more than 100 volunteers. Source: Home Innovation Research Labs, 2017

Another indication that builders are hungry for this information came from a recent HIRL builder survey. The survey found that moisture performance of energy-efficient walls topped the chart of builder concerns related to energy efficiency.

HIRL researchers are well under way with the field moisture study project. They have selected more than 20 different wood walls to study, with willing builder participants in the climates with the most perceived moisture durability risk (3A-7A, 5B, and 4C). The walls selected range from 2 x 4 inches to 2 x 8 inches. They include the three most common types of sheathing (plywood, OSB, and the Huber ZIP system), with and without WRBs; all the common types of insulation, including cavity and continuous exterior insulation up to 2 inches; and four different variations on vapor retardation. For each house in the study, researchers will monitor the moisture levels inside the walls for at least one full year, preferably longer. Their goal is to see what happens.

By collecting and analyzing data on moisture accumulation in many different walls in real houses, HIRL will help us better understand moisture durability risks so we can begin to beat the building science blues. The second part of our proposed solution is to make this building science knowledge more accessible during the design process, which leads us to …

An Expert System for Selecting Moisture-Durable High-R Walls

The 2015 HIRL builder survey also asked how helpful interactive tools would be for builders who need to specify products and wall systems. Of the regional and national builders. 45% picked the highest level (5 points). Extremely Helpful. With builders’ strong interest in such a tool, and with inspiration from a similar tool developed for Canadian builders, Oak Ridge National Laboratory (ORNL) began developing the online Building Science Advisor (BSA) in 2016. The BSA is being built on an expert-guidance framework; it will later be refined based on extensive lab and field measurements (such as the HIRL study), as well as hygrothermal analyses conducted by building scientists.

The BSA has been vetted with U.S. builders and HERS index raters at conferences and trade shows over the past year. ORNL expects to start beta testing late in 2017. With a few simple inputs, users will be able to specify climate, cladding type, wall structure, insulation levels, and other materials parameters. The output screen (Figure 5) then provides data on thermal performance (that is, whole-wall effective thermal resistance) and on moisture performance risk (red-yellow-green indicator). It also provides specific guidance for managing moisture durability risk.

Building scientists may get tired of answering the simple questions. The BSA will help them to triage—when are the answers clear and when should builders seek further expert advice or conduct a hygrothermal analysis before proceeding? We want the BSA to help make building science principles more accessible to anyone who cares about moisture durability. And we want it to provide customized guidance based on these principles to builders who want to choose the best wall system for the climate in the area where they build.

Building Science Advisor Output Screen

Building Science Advisor Output Screen
Figure 5. The output screen provides data on thermal performance (that is, whole-wall effective thermal resistance) and on moisture performance risk (red-yellow-green indicator). Source: Oak Ridge National Laboratory, 2017.


Eric Werling has been the national director of the U.S. DOE’s Building America program since 2011. Prior to DOE, Eric launched the U.S. EPA’s Indoor airPLUS home labeling program, helped develop and implement the successful ENERGY STAR for New Homes program, and led development of popular IAQ and Moisture Control guides for EPA and ASHRAE. Eric is also an amateur musician and plays for 2 bands, including the Building Science Boogie Band.

Note: Nothing in this article is intended to imply that I prefer any particular products or systems over others, and nothing stated represents a product endorsement or guidance from DOE or the Building America program.

Learn More

If you like the sound of the BSA, check out the beta version.

Read more about ORNL research and plans for the BSA.

Oak Ridge National Laboratory (2017). Building America - Building Science Advisor, Oak Ridge National Laboratory under Department of Energy Building America Program. Retrieved August 16, 2017.

It’s not a coincidence that I wrote the “Building Scientist Blues” and have performed it in public.

Lstiburek, Joseph. The Perfect Wall. Westford, MA: Building Science Corporation, July 2010, reprinted from ASHRAE Journal, May 2007, pg. 74, Atlanta, GA.

For an in-depth discussion of the barriers to highly efficient homes.

Download the 2006 International Residential Code for One- and Two-Story Dwellings, Section 406.X.

Use the Canadian Wood Council’s Wall Thermal Design Calculator.

Read about the Monopath Affordable Perfect Wall system.

Learn more about the HIRL moisture study.

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