The Architect’s Challenge – Designing the Best Wall for the Project  

Moisture flow doesn’t care about buildings. Heat flow doesn’t care about buildings. The immutable laws of nature drive both forces. And both could, if not controlled, compromise a building, sicken its occupants, and waste energy and resources. And the professionals who designed and/or constructed that building could learn more about lost reputations, liability, and lawsuits than they ever cared to know.

Wall design is a major key to controlling the forces of nature on a building. While poor thermal control brings discomfort and high operating costs, moisture-related construction defects are among the leading causes of building failures, callbacks, and construction litigation. While exact statistics are hard to come by, some industry experts estimate that up to 90 percent of litigation for construction defects is due to water and moisture problems. ASTM estimates that water-related defects cost Americans more than $9 billion per year.  These billions of dollars are spent each year repairing finish and structural damage due to water- and moisture-related issues. Damage from water leaks not only eats into profits but also can diminish professional reputations, and even lead to structural failures and indoor air quality problems. Damage repair from water infiltration typically requires four to five trade contractors to perform the repairs, which requires significant resources to schedule and correct the damage not to mention inconveniencing the owners.

The challenge for architects is four-fold: to understand the forces of nature, to understand how those forces work on buildings, to learn how to efficiently control those forces on buildings, and to learn how to specify a wall that provides that control.

This article is about the importance of moisture and thermal controls in a wall design, and how the architect can design and specify the right wall system for the project. We’ll also focus on several products that help achieve those performance goals, including drainable building wraps and three forms of insulation.

Architects Must Take the Lead

While wall design might seem to be the purview of others, from engineers to builders, there is no doubt in the mind of Achilles Karagiozis, a nationally regarded building science expert, exactly who should take the lead to make buildings more durable, safer, and energy and resource efficient.

“The architects’ job is expanding,” Karagiozis says, citing sustainability, extreme energy efficiency, and durability as the architect’s concerns. “No one else has more of that responsibility. Everyone is looking to architects.”

But architects must learn more about building science—the study of the effects of natural forces on structures.

“Building science,” Karagiozis adds, “is key to durable design.”

“It’s lack of education,” agrees Ian Daniels, who holds a master’s degree in architecture and is a technical expert for a company that designs and manufacturers moisture management systems. According to Daniels and others, it would behoove architects to learn how the laws of nature move moisture and temperature into and out of a building.

The Building as a System

Understanding wall design means understanding nature. Here are the immutable laws:

• Moisture moves from wet to dry.
• Energy transfers from hot to cold.
• Pressure moves from high to low.
• Gravity pulls matter down.

Keeping a building dry and conditioned inside is not just one problem to be solved. Success means to balance all factors. If moisture does intrude, as it surely will, the building must be designed so it can dry out. In other words, buildings must be designed for failure.

“Architects need to understand that if they are to be designing energy-efficient structures,” Karagiozis says, “their walls are becoming more and more sensitive to moisture, more and more and more sensitive to air flow.”

A building is, simply put, an environmental separator. Whereas primitive structures once separated occupants and their possessions from lightning strikes, downpours, mountain lions, bears, locust, and other threats, we now demand much more of our building envelopes. In order to function as a superior environmental separator today, the following attributes of a structure must be achieved:

• Control of heat flow
• Control of airflow
• Control of water vapor flow
• Control of rain
• Control of groundwater
• Control of light and solar radiation
• Control of noise and vibrations
• Control of contaminants, environmental hazards, and odors
• Control of insects, rodents, and vermin
• Control of fire
• Provide strength and rigidity
• Be durable
• Be aesthetically pleasing
• Be economical

Control Layers

For a structure to be durable, safe, and healthy, it must be protected by four control layers. Water exists in all three phases (solid, liquid, and gas). These following control layers are deployed to control water (in all phases) and thermal movement:

• Water control layer
• Air control layer
• Vapor control layer
• Thermal control layer

While many configurations are possible in the ever-changing world of material innovation and budgets, the optimum configuration for the control layers is likely to be as shown in Figure 1.

While current building science indicates that control layers on the outside of the structure bring the best moisture and thermal bridging protections, cavity insulation should not be overlooked. According to Karagiozis, the stud cavity is “valuable real estate” that can be insulated to further the building program’s energy efficiency goals.

The most important factor to consider when specifying external control layers is their continuity; they should be continuous around the entire perimeter of the building enclosure, including foundation, walls, and ceiling.

This emphasis on moisture and air control layers has shifted since the beginning of the green building movement, which followed the oil embargo of the 1970s, when most importance was put on thermal control. However, as buildings have become tighter and tighter, the science has shown that controlling the water, air, and vapor going into and out of a structure is actually more impactful for achieving a sustainably-minded architect’s goal—designing a durable building. Only when the forces of moisture intrusion are handled, and controlled ventilation is provided, does the energy efficiency of a building matter. To complicate matters further, each construction material stores water—this specific property is called sorption. Some materials have high affinity to store water (wood), while others like extruded polystyrene (XPS) have very little when placed in the same relative humidity environment.

We’ll take a look first at the role that water-resistant barriers play in the specification of a superior wall. After that, we’ll look at three types of insulation—extruded polystyrene, mineral wood, and fiberglass—and the applications in which each of them contributes to a superior wall design.

 

Keys to Moisture Management: Understanding Water-Resistant Barriers

Exterior cladding systems are generally classified in two categories: barrier systems and rainscreen systems. Barrier systems are intended to prevent exterior liquid water from penetrating into a wall system. However, over time some components, like sealants, in a barrier system inevitably break down and create a leak.

Rainscreen cladding materials like stone, brick, and stucco actually store water and are designed with weep holes to let water escape precisely because they are not intended to be watertight. Therefore, it is inevitable that all cladding systems leak and it’s not a matter of if, but when. Most of the time, these weep holes allow the moist air in the air cavity between the cladding and the water-resistive barrier to ventilate to the outdoors. This mode of drying is an essential feature that an architect can include in their design for more durable wall performance.

Clients expect their buildings to not leak inside and to be durable, long lasting, healthy, and comfortable to occupy. Water infiltration can lead to mold and indoor air quality problems that can require expensive mitigation and cause disruption to the occupants.

Behind the cladding, the weather-resistant barrier (WRB) works to stop the rainwater from getting into the wall system and allows it to drain, via the natural force of gravity, down and out of the wall system. But if the WRB and cladding are touching, the wall will not drain because of tension between the two surfaces. Under the influence of that force, the water sticks and stays and the structure suffers.

To mitigate that, innovative sheet WRB products effectively eliminate excess moisture and thus the damaging effects of mold and rot. Because of their design, these products remove at least 100 times more bulk water from a wall versus standard housewraps. This is achieved through the gap created by 1.5-millimeter spacers bonded to a high-performance housewrap. This gap design provides a true drainage space between the sheathing and cladding material.

Further, the innovative configuration of these spacers allows drainage of the plane in any direction—horizontal, vertical, slanted—which helps eliminate negative consequences from the common irregularities of field installation. In other words, if the drainable WRB is installed upside down or sideways, no harm, no foul. While furring strips have traditionally been employed to separate the cladding from the WRB to allow space to ease the hydrostatic pressure and to allow drainage to occur via gravity, this method of providing drainage by the very design of the WRB saves time and money over the older methods.

Liquid Water vs. Vapor

To understand how weather-resistant barriers work to impede water intrusion, it’s important to distinguish between liquid water and vapor.

Moisture vapor, composed of water droplets in a vaporous form, is very small and invisible. Water molecules, in liquid form, have a stronger attraction to one another, making movement through space different than that of gases. WRBs are water resistant because the pores (spaces) in the sheet material are too small for liquid water to pass, but with pores that are large enough to allow moisture as a vapor to pass through. Thus they are said to “breathe.” WRBs need to be breathable to allow interior moisture to escape and to allow for drying if the building assembly gets wet. The advantage of a wrap product over building paper is that it is more breathable, more durable, and the seams can be easily taped to create a continuous water and air barrier system.

Building codes require WRBs under all cladding systems to prevent water penetration into building assemblies. The type of WRB is not specified in the codes. WRBs can be as simple as building paper or as be as multifaceted as a high-performance drainage wrap.

The function of a WRB or building wrap is to:

1. Create a weather barrier behind exterior cladding to protect the sheathing and aid in reducing water intrusion into the wall cavities.
2. Promote fast and efficient drainage of water out of the assembly.
3. Provide a vapor-permeable membrane that allows moisture trapped in sheathing to escape.
4. Provide a vapor-impermeable membrane that stops moisture ingress to sheathing.
5. Be an energy-efficient air barrier to stop air infiltration and exfiltration through walls.

Building Standards Require Water Tight Performance

Codes and building standards have progressively gotten stricter as a result of the increase in water-related damage in homes.

The 2009 & 2012 International Residential Codes (IRC) state:

The building code is very clear in its requirements to not only use a weather-resistive barrier but also to provide a drainage path for water to drain from the wall assembly.

Water management is also a significant component of green building programs because longer lasting, more durable homes and buildings equates to lower greenhouse gas emissions because less resources and energy are used to repair and replace them. “The greenest building you can build is one you don’t have to rebuild,” says building science consultant Steve Easley.

A Word about Air Barriers

Air barriers are used to control air infiltration and air exfiltration. Air barriers save energy and reduce the potential for moisture problems. Air leakage in buildings pushes out conditioned air requiring additional energy to reheat or cool the air. A typical home has more than a half-mile of cracks and gaps. Air barriers help save energy by reducing the buildings air exchange rate. Properly installed wraps can save between 15 and 20 percent on space conditioning costs.

Air barriers are also an important element in managing moisture. Air contains moisture, and when moisture vapor comes in contact with cold building materials it can be absorbed by those materials and even condense. This increases the moisture content of building materials, or local condensation, and can lead to mold, corrosion of steel structure or accessories and fungal decay in cellulotic materials. Air barriers reduce the flow of moisture-laden air in and out of structures thus reducing the potential for moisture accumulation.

Vapor Barriers and Retarders

It’s important not to confuse building wraps with vapor barriers/retarders. Vapor barriers and vapor retarders, are required by codes in the colder climates zones. They are predominantly installed on the warm side of the insulation. The original intent was to prevent moisture-laden air from entering conditioned living spaces from diffusing through the wall finishes and insulation and then condensing in wall cavities on cold surfaces.

The reality is that under normal circumstances, there is not much moisture flow by diffusion. Since vapor barriers and retarders are typically not sealed, and are installed with many gaps and voids, they are often not very effective at preventing moisture vapor from entering wall cavities.

The primary mover of moisture into wall cavities is air exfiltration/infiltration. Many building science experts believe that vapor barriers cause more problems than they solve because they reduce drying potential to the inside and trap moisture. Unlike vapor retarders, some WRBs are more vapor permeable, so that moisture can readily diffuse through the wrap and allow wet building assemblies to dry.

AC38 and How WRBs are Tested

The International Code Council (ICC) has acceptance criteria for WRB properties titled ICC-ES AC38. This acceptance criteria is designed to give the industry a list of tests that best evaluate the necessary performance factors required of a building wrap. These tests are helpful in comparing products.

The required tests as per AC38 are:

• Water Resistance—AATCC Test Method 127: Hydrostatic Pressure Test
• Water Vapor Transmission—ASTM E96 Desiccant Method
• Ability to impede airflow—ASTM E2178 Air Permeance of Building Materials
• Durability and tear resistance—ASTM D5034 Breaking Strength and Elongation of Textile Fabric
• Cold weather flexibility—During this test a specimen of housewrap is conditioned to 32°F (0°C) then bent over a 1/16-inch-diameter mandrel, and to pass it must not crack
• Flammability and smoke developed—ASTM E84 Surface Burning Characteristics of Building Materials

• And there is one more test that is optional:
• Drainage Efficiency—The optional test for this is ASTM E2273. During this test a wall assembly is created with the WRB over the sheathing behind the cladding. Water is applied to the assembly with the amount of drainage-over-time recorded. AC38 minimum is 90 percent drainage, with some WRBs achieving up to 96 percent.

To specify a WRB that has been tested to the highest levels, make sure that it also passes the testing criteria for ASTM E2273, for its drainability capacity, which is currently voluntary but that could be adopted into future codes.

Final Assurance: Taping the WRB

To be fully functional, the seams of the WRB must be sealed. There are two issues at play here. If the WRB is woven, the cut ends can fray and thus be vulnerable to gaps when sealing is attempted. For best results, specify a non-woven WRB. Next, the tape used should be compatible with the product, thus ideally from the same manufacturer. A newer generation of tapes with extremely strong sticking power are double sided, which eliminates the critical need for a typical 6-inch overlap.

Of course, proper installation and standardized flashing details are key to ensure that the specification of high-quality building materials was not for naught.

With the moisture issue clarified for specification purposes, we’ll now focus on how the wall is insulated.

The Right Insulation for the Application

Though control of water, air, and vapor are necessary for a safe and durable wall system, the insulation layer brings big benefits noticed by owners and occupants: comfort and savings, with the latter including both operating costs and planetary resources associated with energy use. Some insulation also provides acoustic benefits.

The type of insulation used in the wall design is highly specific to the application. It must meet the codes and be compatible with all other components in the assembly, from the framing or structural supports to WRB to hardware and more. A century ago, we designed and built with a handful of materials and asked far less of our buildings. Today there could be a matrix of multiple dozens or even hundreds of combinations.

“It’s a bewildering mix of product and system choices to get into,” acknowledges Herbert Slone, the chief architect and senior manager of commercial building systems for a global building products company.

The best strategy, Slone says, is to determine the performance goals for a wall system, then specify the system as “a system” to achieve those goals, and from that flows the products that will make up that system. A basic understanding of insulation properties helps make sense of proper wall design.

Let’s look at three distinctly different types of insulation—extruded polystyrene, mineral wool, and fiberglass—and their basic properties.

Extruded Polystyrene (XPS) 
Extruded Polystyrene (XPS) insulation, a type of rigid foam insulation, is recommended for, among other things, exterior above grade walls, and foundation walls, applied either on the exterior or interior face. It typically has an R-value of 5 for a 1-inch thickness.

XPS contains hundreds of millions of microscopic closed cells filled with a captive, low-conductivity blowing agent to provide its legendary thermal control. Perhaps the most notable quality of XPS is that it is virtually impervious to moisture, thus preventing moisture absorption and loss of R-value. Non-structural XPS weighs quite a bit less than plywood, OSB or other structural non-insulation materials (which typically have much lower R-values) and is easy to install. Because it’s rigid, it can be scored and snapped, or cut or sawed, with common tools. It never sags or settles.

In some framing applications where allowed by codes, XPS can be used as an insulating sheathing, with OSB-reinforced corners, in place of full OSB sheathing. XPS is also commonly used as continuous insulation sheathing, either over an exterior gypsum board sheathing or applied directly to the steel studs, in independently braced steel-stud framing systems. This means more R-value and reduced energy loss in stud framing systems.

Wall System Fire Performance
To meet energy efficiency standards, commercial buildings often incorporate foam plastic insulation and air/water resistive barriers in the building envelope. All foam plastic insulation is combustible, including XPS, expanded polystyrene (EPS), polyisocyanurate (ISO), and spray polyurethane (SPF). Many WRBs are also combustible. Commercial buildings, because of their area, height, proximity to property lines, and/or the nature of their use, are often required to be constructed in whole or in part of noncombustible materials. Noncombustible construction “Types” are defined in Section 602 of the International Building Code (IBC). Types I and II are defined as essentially all building elements consisting of noncombustible materials. Types III and IV are defined as the exterior walls being constructed of noncombustible materials. Type V is wholly combustible construction.

Limiting Fire Spread, NFPA 285
The IBC requires the exterior walls of many commercial buildings to be constructed of noncombustible materials, as is the case in Types I, II, III, and IV construction. The ASHRAE 90.1 energy standard for commercial buildings prescribes the use of continuous insulation (ci) over steel framing to minimize energy-inefficient thermal bridging. It also requires air-/water-resistive barriers to minimize air leakage. As explained earlier, continuous insulation is often combustible foam plastic insulation, and WRBs are often combustible. To address the dual requirements of noncombustible walls that are at the same time required to contain combustibles, with some exceptions, the IBC requires wall assemblies that are required to be Type I, II, III, or IV construction, be tested to comply with the acceptance criteria of National Fire Protection Association (NFPA) 285.

To pass the NFPA 285 test, a wall system must demonstrate limited fire spread vertically and horizontally away from the area of fire exposure. The IBC imposes two additional criteria for exterior wall assemblies:

• Potential heat: The potential heat of foam plastic in walls, expressed in Btu per square foot, is limited to the amount that has been successfully tested in the required NFPA 285 full-scale wall test.
• Ignition: Exterior walls shall not exhibit sustained flaming when tested in accordance with NFPA 268. Walls that are protected on the outside with a minimum of 1-inch thick masonry, concrete or a minimum of 7/8-inch thick stucco, are not required to be tested for ignition.

Mineral Wool—Fire Ratings and More
In applications where fire resistance is required and is a primary concern to save lives, mineral wool is the insulation of choice. For instance, it is used extensively in One World Trade Center. (See case study sidebar.)

It makes sense that mineral wool has good fire performance; it’s basically made of rocks.

“Think about rock, you don’t think of it as something that will melt and go away,” says Angie Ogino, technical manager for a company that makes mineral wool.

Ogino explains how mineral wool was made initially of naturally occurring rocks. But today, some sustainability minded companies use up to 70 percent slag, which is a byproduct of the steel industry, what’s left over after the iron ore is extracted. Those are melted at 2,600˚F and essentially taken back to their original molten state. The next step mimics the making of cotton candy, where the molten rock is introduced into an airstream, creating strands of fiber.

Mineral wool’s extraordinary resistance to fire makes it an integral part of the fire safety system for commercial buildings, including tall buildings. The mineral wool fiber is used within the spandrel area and joint created where a non-rated exterior curtain wall intersects with a rated floor assembly. Compression-fit mineral wool within the joint allows for the movement that is unavoidable in tall buildings, and creates a barrier to keep the fire compartmentalized and confined to the room of origin for up to two and three hours, preventing it from spreading from floor to floor.

Mineral wool can also be used as continuous insulation in cavity wall and rainscreen applications. Especially in systems that include an open-joint rainscreen application where UV-resistant insulations are required.

Because of its temperature and fire-resistant characteristics, mineral wool is also used in such industrial applications as high-temperature ovens and sound enclosures for industrial rooms. It is not affected by ultraviolet exposure, which can degrade other materials.

For residential use, some multifamily structures need a firewall and some UL-rated assemblies call out mineral wool.

While the fire-resistant qualities of mineral wool are widely appreciated, there are also thermal and acoustic benefits. Sound control in particular, Ogino notes, “is becoming an ever-increasing requirement for sustainable buildings.” In terms of aesthetics, mineral wool’s natural dark color provides camouflaging in open-joint facades.
 
The choice of insulation is application specific. When all is calculated, the fire performance of mineral wool insulation is most important.

“That’s my number one priority,” Ogino says of her work, “keeping people safe.”

Fiberglass Insulation
Perhaps the most flexible insulation, both in application and installation, fiberglass in batts or loosefill can provide R-values from about 10 to almost 100 and comes unfaced or faced with either a kraft or foil vapor retarder. Batts can be manufactured in thicknesses from 3½ inches to 14 inches.

Fiberglass insulation can be used in a wide range of exterior wall and roof/ceiling applications. The product can be installed in wood or metal framing cavities, installed between furring strips, or pinned to surfaces.

Because of its range of R-values and thicknesses available, it can meet many thermal specifications that exclude other materials. As an example, some products provide excellent thermal performance in the limited space of cathedral ceilings. Additionally, fiberglass insulation enhances interior noise control by improving the Sound Transmission Class (STC) of walls and floor/ceiling assemblies.

While some insulation can compress in the wall cavity and leave thermal voids, the best-quality fiberglass materials on the market are in fact dimensionally stable and will not slump within the wall cavity. Due to its inorganic fibers, it will not rot or mildew and is noncorrosive to steel, copper, and aluminum.

The Installer’s Point of View
Architects carry the mantle of creating healthy buildings more than most others in the built environment industry, and that concern likely extends to the health of laborers and crafts workers. New generations of ecologically sensitive fiberglass insulation produce less dust than other fiberglass insulation products, and some are Greenguard certified and verified to be formaldehyde free. Insulation manufacturers continue to find new ways to be environmentally friendly. At least one brand has a minimum of 50 percent total recycled content.

Kraft and standard foil facings on fiberglass insulation exceed the maximum flame spread limits prescribed by the building code. Those facings must not be used in Types I and II construction, and must not be left exposed. Facings are available that have a flame spread rating less than 25 and that can be used in those situations.

New and Developing Analysis Tools

In addition to understanding the forces of nature and how to control and channel them, architects in the United States face a challenge that architects in other countries do not: geographic diversity. With geographic and seasonal climates ranging from cold and dry to hot and humid, and mixtures of those, the rules of wall design change. While the architects of long ago calculated dew point—a measure of atmospheric moisture—with a calculator and pencil, computer capacity has allowed building scientists to create newer tools that take into account the entire expected atmospheric conditions of a geography for the entire year, and help match that with products and a system to achieve optimal desired performance.

Karagiozis helped design the sophisticated WUFI (the name stands for Wärme und Feuchte Instationär, or “Transient Heat and Moisture Transport”) software, which he says is now an industry standard that takes the guessing out of whole-building material and system specification. According to the Oak Ridge National Laboratory, where Karagiozis helped introduce the software and trains architects to use it, WUFI “is a menu-driven PC program which allows realistic calculation of the transient coupled one-dimensional heat and moisture transport in multilayer building components exposed to natural weather. It is based on the newest findings regarding vapor diffusion and liquid transport in building materials and has been validated by detailed comparison with measurements obtained in the laboratory and on outdoor testing fields.”

Further, software can now help architects design wall systems for desired service life. For instance, an architect designing a one-story strip mall that will be refurbished every 10 or 15 years does not need to specify a 500-year wall.

“He or she does not need to invest valuable resources for that period of time,” Karagiozis says. Rather, the architect can now, aided by software, design the building for the anticipated service life, whether it is 15 years for a strip mall or indeed 500 years for an art museum.

“I think this is the power era we are entering,” he says. “This is the era of breakthroughs that we are in today.”

Looking to the Future: A Systems Approach

To review the basic concepts:

• The forces of nature work on a building and must be controlled.
• Liquid control is on the outside, not the inside of a wall system.
• Thermal control’s optimum is outside to shut down thermal bridging, but architects can also take advantage of valuable space by insulating inside the stud cavity.

With thousands of materials on the market, compatibility and complete system performance are becoming ever-increasing concerns. Some building product manufacturers have teamed up with other manufacturers and compiled and tested complete systems for various applications, climates, performance, and anticipated service life. The architect would be well-served to take advantage of those complete, tested assemblies when specifying walls systems.
 
But the savvy architect will strive to understand building science: the forces of nature, and the building materials and systems that control those forces.

“I really think architects need to own this space as they own this space in other countries,” says Karagiozis, the building systems expert.

“Architects,” he believes, “are society’s heroes of the future.”

CASE STUDIES

 

 

 

Endnotes

https://bloomington.in.gov/green-building-benefits

http://bta.seattleschools.org/assets/Uploads/documents/Building_Green_Enhances.pdf

http://www.researchgate.net/publication/260016614_Energy_saving_in_ceramic_tile_kilns_
Cooling_gas_heat_recovery

http://www.madehow.com/Volume-1/Ceramic-Tile.html

http://www.exponent.com/construction_materials/

https://www.cornellengineers.com.au/a-holistic-approach-to-building-sustainability/

http://www.designsforcreation.com.au/the-25-principles-of-baubiologie.html

http://www.usgbc.org/articles/classic-timeless-and-sustainable-cities-jiaming-investment-looks-towards-future

Chris Sullivan is an author and principal of C.C. Sullivan (www.ccsullivan.com), a marketing agency focused on architecture, construction, and building products.