Walls serve multiple functions in a building. They are key elements of the structure, they support the windows and doorways that physically define a space, they are a very important part of the building’s energy efficiency and they dominate the visual aesthetics. It is no wonder that many people come to define their home by the kind of walls it has.
There are many practical measures by which the walls are not as important. They represent only a fraction of the overall cost and construction schedule of a building, and a similar fraction of the total embodied energy. For the many owner-builders who define their projects by the type of walls they are going to use, it can come as a surprise to find out how much time and money is spent on the rest of the building, and what a comparatively small part of the project the walls turn out to be. They are important, to be sure, but no more important than all the other building elements.
Discussing and dissecting wall options is difficult because there are so many possibilities. The multiple functions of structure, environmental separation and aesthetics have multiple options for achieving each function.
Some common wall systems — such as timber frames or stud frames — are only structural elements and do not inherently include a particular insulation strategy or aesthetic finish. Other wall systems — such as straw bale, cob and cordwood — can be a structural system or infill insulation within another structural element and can have many different aesthetic finishes in either case. In the sections of this chapter a range of possibilities are shown, based on combinations of wall structure and wall insulation.
Building Science Basics for Walls
Structurally, a wall transfers loads from the roof and upper floors to the foundation, including gravity loads and the live loads imposed by wind, snow, rain, seismic activity and occupant activity. Thermally, a wall provides insulation from differences in temperature between inside and outside and barriers against air movement from either side. Walls must also deal with moisture loads from either side, including precipitation and differences in humidity between interior and exterior. The wall is also the mounting point for windows and doors, critical seams that will have a large impact on the overall thermal performance of the building regardless of their type, size or style.
Wall structures can be divided into three basic categories:
These systems form a wall in which loads are distributed evenly along the length of the wall. Monolithic walls can be made from materials that are poured or formed into a continuous mass (rammed earth, cob, concrete) or materials that are stacked and bonded such that they form a continuous mass (adobe or compressed earth blocks, CMU). Straw bale walls are a hybrid kind of monolithic wall, in which the bales are stacked like blocks but are not rigid enough to offer the full structural capabilities required, but with a plaster skin bonded to the interior and exterior face the plaster skins become a pair of monolithic walls tied to one another.
These systems form a wall made of individual posts carrying beams on which the loads of the roof/floors are distributed. In timber frames (which use traditional wood-to-wood joinery) or post-and-beam frames (which use metal plates/bracket joinery) the posts and beams are of large dimensions and are designed to be spaced far apart. Lightweight framing systems use a greater number of smaller vertical posts (studs) to support less substantial beams (top plates). Skeletal walls require bracing to handle shear loads (loads imposed in any direction except straight down). Large-dimension frames often use built-in triangulation to provide bracing, and lightweight framing systems often use continuous sheathing.
The proliferation of natural wall systems over the past few decades has created a subcategory of walls, in which skeletal walls are used as a structure in conjunction with monolithic infill systems. In the best-case scenarios, these systems are combined in such a way that the advantages of skeletal systems (quick construction times, the ability to support a roof prior to completing the full wall system, widespread code acceptance) and the shear strength, low embodied energy and thermal properties of the infill system are optimized. In the worst-case scenarios, one or both elements are over-built and redundancies and complications can raise costs, labor time and waste.
In many buildings, hybrids will be created due to design decisions. A monolithic wall with large window openings will rely on skeletal frames of some sort to form and span the large openings. In some buildings with monolithic exterior walls, skeletal interior walls may be an integral structural component.
In residential, low-rise construction all three categories of wall are structurally viable. Choices will have more to do with material selection, thermal properties and code compliance issues than with structural properties.
Wall thermal properties will be determined by the combination of the structural system with an insulation material. For skeletal and hybrid walls there will be two (or more) distinct materials providing structure and insulation. For monolithic walls, it may be the same material or it may also be a combination of materials, depending on the inherent thermal properties of the structural material.
Many builders are obsessed with R-values when it comes to thinking about thermal properties. The R-value measures the resistance of a particular material to the flow of heat energy. R-values are usually given as a “per-inch” figure, or as an overall figure for a particular thickness of insulation, and the higher the number the better the thermal resistance. Insulations can also be rated by a U-value, which measures the ability of a material to conduct heat energy. In the case of U-values, the lower the number the better the insulation. The U-value is the inverse of the R-value; divide either value by 1 to convert one form of measurement to the other. For example, a material with an R-value of 2 will have a U-value of 0.5, reached by dividing 1 by 2.
Understanding how R- and U-values work is key to choosing adequate amounts of insulation for a building. Metric values measure the number of degrees Kelvin required to transfer one watt of energy through one square meter of the material. In imperial measurements, U-factor is a measure of British thermal units (Btu) per hour, per square foot of material, per degree Fahrenheit of temperature difference between the two sides of the material.
From this understanding, two important considerations are made clear. First, diminishing returns are achieved with each successive doubling of the insulating layer. At a certain point, the financial and material costs of adding more insulation do not translate into significant performance improvements. Though highly debatable, in regions with extreme heat and cold, this point is often thought to be around R-40 to R-60 for walls. Second, the measure of R-value and U-value are laboratory figures that are not dynamic and do not represent fluctuating real-world conditions. Standard test practices have a base temperature of 24 °C (+/–3 °C) / 75 °F (+/–5°F), and the temperature differential between the two sides of the test wall must be 22 °C (+/–3 °C ) / 72 °F (+/–5°F). This means that testing is performed at temperatures above 0 °C (32 °F), and at temperature differentials substantially less than the 50 °C (122 °F) often experienced in colder climate zones. This form of testing also concentrates exclusively on the conductive transference of heat, and does not take into account radiant and convective heat flows.
Building science teaches us that thermal resistance is only part of the overall thermal performance of a wall system. Often, builders become obsessed with R-values to the exclusion of other considerations. But R-value alone is not an adequate predictor of the thermal performance of a building enclosure. So what else is going on in a wall system that we should understand?
Airtightness is a major factor. Imagine a home with an excellent R-value and an open window on a cold winter or hot summer day. Whatever effect the insulation might provide is short-circuited by the direct exchange of air between inside and outside through the open window. The comfortable, indoor conditions quickly change due to that air exchange. While a wide-open window may seem like an exaggeration being used to make a point, the truth is that almost all older homes and a shocking number of new homes have an equivalent leakage area (EqLA) the size of a large open window. The EqLA is measured by depressurizing a building using a sealed fan in a doorway (a blower door test) and monitoring the pressure difference between inside and outside. The depressurized building will allow balancing air to make its way inside, and the quantity of incoming air can be translated into the EqLA. Just because you don’t see an open window doesn’t mean your building is not leaking like somebody forgot to close one.
Of all the elements of a building, walls are the most difficult to make airtight. They are bordered on all sides by junctions with floor, foundation, ceiling and other walls, and punctured frequently with openings for doors and windows. Penetrations for electrical and plumbing services riddle many walls. If proper attention is not paid to proper air sealing from the interior and exterior, even the best-insulated walls will not keep a building warm. The “open windows” must first be well and properly closed. Methods for building properly airtight walls exist for all of the materials and systems profiled in this chapter. This book is not a how-to manual intended to guide you with specific advice about air sealing for each type of wall, but there is adequate information in the Resources for Further Research section at the end of this chapter to ensure that the wall is built and detailed properly. Only by building a properly airtight wall will you see the real benefits of the insulative values described for each wall type.
Another form of short-circuit that can affect energy efficiency is thermal bridging. Assuming that a wall is both well insulated and well sealed, heat energy can still be transferred from one side to the other by non-insulating materials that touch the inside and outside surfaces. Skeletal frame members are the main culprits when it comes to thermal bridging. If insulation is placed between framing members, there is significantly less insulation at each stud, sill plate and top plate. Thermal bridges can also occur at window and door openings, where insulation is interrupted. Here, framing members often bridge from inside to outside.
A final thermal consideration is the moisture level within the wall system. Heat will be conducted more easily through a material as the moisture content of that material rises. If the insulation in a wall cavity has elevated levels of humidity or is actually moist or wet, the rated R-value will be subverted by the added conductivity of the water. While rot and mold are key concerns with damp walls, there can be a measurable drop in thermal performance even if there is not enough moisture to create these issues.
All successful wall systems will have four “principle control layers
This element of the building controls the entrance of water into the building enclosure. The exterior cladding of the building is the typical water control layer.
This element controls the flow of air through the building enclosure. The air control layer may be a distinct element in some wall systems, or the role may be performed by a structural and/or thermal component.
This element controls the flow of water vapor through the building enclosure. Water vapor will move from one side of the wall to the other side depending on differential humidity levels. Just as heat always moves from warm to cold, water vapor moves from areas of high concentration to low. The vapor control layer may be a distinct element in some wall systems, or the role may be performed by a structural and/or thermal component.
This element controls the flow of heat energy through the building enclosure.
These layers are a good way to “dissect” a wall assembly and will be used throughout this chapter to identify the elements of various wall systems.
Of the four principle control layers, it is the vapor control layer that is least understood, and worthy of further examination. This layer is a relatively recent addition to wall assemblies. Three factors came together in the past few decades to make it a necessity: increasing airtightness, increasing levels of insulation and greater use of manufactured building materials. In older, less airtight buildings, excessive humidity on either side of the wall was able to move toward equilibrium via direct air exchange through leaks and holes in the building enclosure. Reducing the number of leaks does not reduce nature’s need to balance moisture levels, and water vapor will attempt to diffuse through the building enclosure, molecule by molecule, passing through the building materials themselves. As vapor moves through the enclosure, it often meets a progressively cooler environment, and at some point in its travels it may reach its dew point. At the dew point, water vapor becomes liquid water. This liquid water can accumulate in insulation materials and on framing and sheathing materials, and if it can’t dry out in a reasonable amount of time can lead to rot and mold on wood and degradation of most insulation. Materials that are not permeable to migrating moisture can retard drying and lead to further accumulations of water.
As these moisture problems manifested in modern homes, the initial solution was to build in a vapor barrier on the warm side of the wall (interior in the north, exterior in the south). The plastic sheeting used for this purpose has a very low perm rating, and keeps moisture from diffusing into the enclosure. But notice that the term being used here to describe this layer is vapor control layer, not vapor barrier. A layer can control vapor diffusion without being a complete barrier. Many of the wall systems used by natural builders operate on the principle that an airtight vapor control layer (often in the form of a continuous plaster skin) can greatly reduce the amount of vapor entering the enclosure, and if that enclosure has an equally permeable vapor control layer on both sides and an insulation material that has suitable moisture storage capacity, then the plastic vapor barrier is not necessary. In fact, it is likely to be detrimental.
In practice, the use of plastic vapor barriers in wood frame construction, if done properly, can be an effective vapor control layer. However, so is using a permeable wall strategy. Permeable wall systems bring together low-tech natural builders and leading building scientists in agreement, but they are only slowly being accepted in the broader construction industry.
Wall systems are the most numerous and variable of all the choices a designer will face in creating a sustainable building. The good news is that all of the choices offer excellent potential if well planned and well executed. Weighing the options carefully will ensure that your finished project meets all your cost, energy efficiency, environmental impact and aesthetic goals.
Foundations as walls
Many of the foundation systems described in the previous chapter can also be used as above-grade walls. There can be some advantages to having one continuous material extending from footings to roof, potentially creating less seams and thermal bridges. The foundations that also work as walls nearly all require the addition of an adequate amount of thermal insulation to suit the climate.
Refer to the foundation chapter for detailed consideration of these options:
- Rammed earth tires
- Lightweight CMU
- Aerated autoclaved concrete (AAC)
- Durisol/Faswall insulated concrete forms (ICF)
The resource sections for each of these materials will supply information on using these materials and techniques for walls as well as for foundations. The ratings used in Chapter 1: Foundations will be the same for use in walls.
Whats not included in this Section
There are wall and wall insulation materials that are feasible to use but do not meet our sustainability standards for inclusion in this book. Drawing such lines is always controversial, and there will be green building advocates and practitioners who see this as folly. However, we will present our opinion on these materials and readers who disagree will find ample information in other sources to work with if they choose to pursue these options.
The SIP system uses two thin skins of structural sheathing bound to an insulated core to create a wall (or roof) with excellent structural and insulative properties. In theory, these are ideal building materials as they provide structure and insulation with a single installation, and have reduced thermal bridging and unwanted gaps or voids in the thermal control layer. They are relatively quick to install too.
Unfortunately, most SIPs are made from oriented strand board (OSB) as the structural sheathing and some type of foam as the insulated core. Both materials rely heavily on the petrochemical industry for their component parts. The glues used to bind OSB are typically formaldehyde-based and off-gas for a long time after installation. The reliability of these glues is questionable, especially when exposed to water, and lifespan issues are critical considering these skins provide most of the structural strength of the wall.
Measuring the impacts of the foam insulations used in SIPs will reveal high embodied energy figures and carbon outputs, but these alone are not really reason enough to exclude this category of products. As the foam industry is quick to point out, there is a degree of energy savings from using these products (though the same savings are available with similar amounts of other insulators) that can render the embodied energy less grievous over time. But straight energy analysis, or even life cycle analysis, do not measure the deeper issue as they are typically carried out. Neither of these approaches takes into account the full impact of the petrochemical industry that produce these products. The full “chain of custody” for foam products needs to address the environmentally disastrous processes of this industry (for a fuller discussion of this subject, see the Foam ICF section of Chapter 1: Foundations).
Foam insulations also create vapor-impermeable walls, which are more prone to moisture issues unless continuous mechanical ventilation is provided for the building.
OSB/foam SIPs also represent a problematic disposal issue at the end of their useful life, as neither material is easily recyclable or reusable and the two are bonded together in a way that makes separation difficult and unlikely. The final resting place of a SIP (and all offcuts created during construction) is the landfill.
Not all SIPs are made from OSB and foam, and other options that use the same excellent principle in combination with more environmentally friendly materials have been developed (see Straw Bale SIPs sidebar) and are likely to be developed (see Mycelium foam above).
Foam ICFs are described in the foundation section of this book, but are sometimes used to build continuous foundation and wall systems.
Foam ICFs combine all the suspect characteristics of petrochemical insulation (see Foam ICF section of Chapter 1: Foundations) with high concrete and rebar usage, both of which have high embodied energy and carbon output issues.
This combination of materials will also prove very difficult to disassemble and separate at the end of its life, meaning it’s most likely headed for landfill and not recycling. The whole system is just too dubious in its environmental impacts to recommend in any way, especially when the alternatives that exist are every bit as feasible and practical.