Walls of the future
By Greg Hildebrand Petr Vegh and Brian Burton
A historical perspective on the building envelope
By Greg Hildebrand Petr Vegh and Brian Burton
In today’s world we have come to accept that the interaction and
performance of building envelopes – a term that describes the perimeter
walls and roofing systems that separate the indoor and outdoor
environments – is a very complex topic from a scientific, psychological
and cultural perspective.
In today’s world we have come to accept that the interaction and performance of building envelopes – a term that describes the perimeter walls and roofing systems that separate the indoor and outdoor environments – is a very complex topic from a scientific, psychological and cultural perspective. The term “envelope” itself has become common in describing enclosure systems because over the past four decades the components have tended to become lighter and lighter, as well as much thinner. It is generally acknowledged today that glazing and other fenestration elements are vital components of these building systems.
|In Europe, people are reluctant to demolish structures. This retrofitted post-war highrise is an example of the future of the building envelope as we move toward replaceable facades. PHOTO BY Graeme Stewart, E.R.A. Architects|
It is also recognized that, in addition to becoming more and more complex, buildings are tremendously important to humans. They always have been and always will be.
The study of the building envelope has been primarily focused on performance characteristics related to 11 key functions that were described by one of Canada’s best-known building scientists, Neil Hutchinson, in 1963. They are:
- Control heat flow
- Control air flow
- Control water vapour flow
- Control rain penetration
- Control light, solar and other radiation
- Control noise
- Control fire
- Provide strength and rigidity
- Be durable
- Be esthetically pleasing
- Be economical
For this particular article we researched a number of sources related to historical information about the functions of the building envelope, and will present some thoughts about buildings and building envelopes. As you will see, our expectations of the enclosure system have gradually increased over the last 50 years.
Putting aside for the moment the fact that engineers and architects are always attempting to build taller or more distinctive buildings, historical documents show that the physical protection of the occupants and their personal property is the most important design consideration overall.
An unending search for better security
The importance of security is clearly apparent in the huge investment of resources in buildings that were designed primarily to provide protection of the occupants and, at the same time, protection of essentials and valuables. These assets include a long list of items necessary for survival, and objects and materials with real or perceived value for trading and/or acquisition of goods.
In the early development phase of human history, the ease of construction and mobility of a shelter were also considered to be very important factors, a consideration we rarely take into account in today’s world. This is partly because when humans survived as hunters and gatherers they were constantly on the move. It is somewhat surprising that the efficient use of existing resources does not appear to have been an important consideration in many civilizations, and buildings were not typically viewed as an investment the way they are today until the beginning of the 20th century. For most of history it appears that providing comfort to the occupants was a secondary consideration. In some cases, the tools to provide comfort may not have been available or practical. Rather than comfort, buildings were designed to provide basic environmental separation: protection from inclement weather of all sorts, from precipitation in all its forms, and from temperature variations, wind, particulate matter, unwanted solar radiation, and unwanted intrusion.
As these issues became an essential part of most designs, such other issues as comfort and control of the indoor environment started to receive more attention.
We have gradually come to expect that enclosure systems will provide natural ventilation, daylighting, access and/or egress and a view of the outside environment. But this was not always the case. This is one area where glass and fenestration components come into play and in which their roles are very significant.
Building envelopes and enclosures came to provide a sense of dimension. They took on an esthetic role, providing decoration and ornamentation. In some very early historical examples the buildings, rather than providing security and comfort, were more likely a symbol of personal, religious and/or political power. Buildings served as a symbolic representation that demonstrated the relationship between leaders and citizens. Early examples would be the palace at Knossos (Crete, 1700 B.C.) and the pyramids of Egypt (2400 B.C.).
In effect, these enclosures served to protect and celebrate the power of the community, the church, the state, the powers that were, or, in some cases, individuals. This can be seen quite clearly in monolithic structures, not all of which were buildings (think of the Great Wall of China, or the pyramids), and later the construction of temples, cathedrals and castles.
So many other factors entered into building envelope design in addition to protection from the elements. There are numerous cases in history where buildings were actually works of art rather than structures designed for human occupancy.
Classifications of building enclosures
Over the long history of our habitation of buildings there have been two major classifications of building enclosures. The first is usually constructed of a solid mass of blocks or linear elements made mostly from locally available materials.
| Photo by Scott McAlpine|
The second describes buildings that incorporate active assemblies and such components as adjustable devices for daylighting, ventilation, thermal comfort and security, which have the capability to actively affect the inside environment.
Most buildings in the past were of the first kind: constructed of inexpensive, readily available and easily transportable materials that were present in the local environment. Two interesting examples are the Inuit igloo and the native tent made from animal skins, which served as a transportable environmental envelope. Here in Canada, that meant timber, clay, natural stone, wattle-and-daub (walls fabricated from clay, straw, mud and just about any other material), thatched roofing, and pebbledash (walls roughly textured by pebbles or stone fragments embedded in mud or clay).
Another excellent local example is the use of Dundas shale in the City of Toronto and surrounding areas. For almost 100 years beginning in the 1830s, Lake Ontario between the Credit River and Burlington Bay supported an entire industry of so-called “stonehookers.” These were boats that harvested stone from the bottom of the lake which was then extensively used in foundations and walls. There are still examples of buildings where this stone was used for foundations, walls and landscaping.
The use of locally available material in areas with a high population density often led to over-exploitation of resources. The use of timber for construction in Britain is a good example. The local supply of inexpensive timber was quickly exhausted and a wide range of other locally available materials began to be employed out of necessity. Canada fared somewhat better with the population spread out and our large supply of timber. However, replanting did eventually become necessary and over time we were forced to travel greater distances to obtain an adequate supply. The same phenomenon occurred in other countries with different materials as a result of population growth and building booms.
We have seen building envelopes fabricated from an amazing range of materials.
Animal skins including yak, camel, goat, buffalo, bison, ox, bear and beaver skins have been used to shelter people since antiquity and are the traditional walls of choice for yurts and teepees. Skins were also frequently used to fabricate primitive windows. Hides were softened by soaking the animal skin in a lime mixture, after which the hair was removed with a knife and oil was worked into the skin to improve its water-resistance. The finished hide was then stretched over a wooden frame to dry. Other local materials employed in envelope construction included bamboo, eucalyptus, banana fibre, cane, canvas, cork, driftwood, dung, flax, grass, hemp, jute, palm fronds, pumice blocks, ramie, reeds, seagrass, sisal, straw, turf and grass sod.
The future of the building envelope
Throughout history, the facade has served as the face of a building. The facade often reflects the importance and wealth of the owner. Today, modern buildings are often constructed using especially expensive materials to demonstrate wealth and importance. These buildings employ high-quality natural stone, glass, stainless steel, copper, and zinc. Architecture using these materials appears to exude an aura of richness or prestige. For these buildings, the cost of the envelope in relation to the total cost of the building has increased to a substantial degree. The Sony Center in Berlin, where the facade structure is primarily constructed of stainless steel elements and glass, is a good example. The cost for the facade equals the cost for the basic concrete load bearing structure. However, this is not always the case with opulent facades. Through innovative architecture and engineering, it has been demonstrated that an esthetic goal can be achieved with lightweight envelope components, such as those clad in a window wall, or an exterior insulating finish system. While the durability of these systems is yet to be determined, it is clear they can be economically employed to provide a functional and appealing building envelope.
Our expectations of the functional facade have expanded to a certain degree in recent times. The facade that was traditionally designed to provide support by withstanding any structural loading, and to control moisture, heat and air in the enclosure, is now expected to protect the occupant from atmospheric pollution, smoke, odour, mould growth, blast impact, natural hazards and loud noise – an overall much higher standard of physical security. Because modern envelopes are constructed of lightweight layers, each with a specific function, it must accommodate differential movement created by moisture, temperature variations and structural movement. Some of these functions seem contradictory, especially with glass facades where technology is required to unite thermal insulation, condensation resistance and solar control. Current systems employ a myriad of materials and systems to achieve these goals. Most of the common systems are static in nature and include, for example, IGUs with spectrally selective low-emissivity coatings and various gas fills to improve solar control and thermal insulating performance. At present, it is common to employ different systems on the various elevations to optimize these performance characteristics; for instance, solar control film on the west and south only. There is a new class of affordable switchable glazing systems that are now coming on to the market. Although they are not likely to be employed in the new construction highrise market for a few years, they will become more prevalent in the commercial, institutional and highrise residential markets.
Another example of conflicting expectations in building envelope construction is the call for a vapour-permeable drainage layer to prevent passage of liquid moisture but allow the passage of vapour-phase moisture. Many of the new membranes that can perform this task are recent developments and also serve as an integral part of the air barrier.
In view of our greater awareness of our depleted resources, more attention is now paid to maintainability and the ability to replace or refurbish components as well as their constructability. Accordingly, sustainability has become a central issue and not only includes designing buildings constructed of long-term durable materials, but demands scrutiny relating to the choice of building envelope materials with respect to their carbon footprints, long-term supply, renewability, recyclability and disposal. A common example of the issue would be the use of a window wall incorporating sealed insulating glass units versus a solid masonry or dimensioned-stone-clad wall. From a sustainability perspective, the masonry wall will have a much longer useful service life than the IGU, perhaps an order of magnitude in some cases. However, depending on the type of stone, limited availability or the costs associated with procurement or use of it (i.e., placing and anchoring) may make the shorter-lived IGU system appear to be a better choice from a sustainability perspective. In this case, when adding in the operating costs associated with both systems over their useful service lives, the stone wall becomes a more sustainable system over the long term. Long term is often the issue, given the conflicting goals of the owner and the builder regarding the importance of short-term cost. This is a problem because the builder is not usually the end user of the building.
In some modern building envelopes we see systems that serve to distribute services throughout the building. The future of the building envelope will also include changes in building shape and orientation as we are no longer limited to linear construction. In fact, computer-integrated design and manufacturing have made practically impossible building shapes and facade designs a reality.
Demographics will also affect the construction of the building envelope. With the population of seven billion expected to double over the next decade we can expect to see both modern and developing nations faced with growing population densities. Design and construction of buildings will almost certainly be affected.
The same is true for environmental factors as we begin to build in areas where environmental conditions may be more severe. It has also been suggested that building design and construction will be affected by climate change, requiring modification to both new and existing buildings.
Cultural and psychological factors may assume a greater role in future building envelope design driven by changes in the workplace environment.
Quality control issues
In the future, it is likely we will see modifications to our overall approach to quality assurance in the construction of the building envelope because at the current time over 70 per cent of all construction defects related to the building envelope are due to water penetration or accumulation. Our National Building Code requires the envelope to be weathertight. The waterproofing or water-shedding characteristics of a building’s envelope are vital for ensuring long-term structural integrity, along with reducing water penetration and accumulation that can result in interior damages and foster mould or other biological growth.
Unlike the robust, solid, masonry walls we have traditionally used, modern building envelopes typically consist of five lightweight critical barriers that serve different essential functions: primary water-shedding surface, waterproofing barrier, thermal barrier, air barrier and vapour retarder.
Unfortunately, nearly half of all building enclosures fail prematurely at some point because the design violates basic building science fundamentals. Builders often fail to identify the proper location and construction sequencing to ensure adequate detailing and constructability. They fail to recognize that the location of the air and vapour retarders within the wall assembly will vary according to climate and occupancy. The waterproofing barrier is compromised in over 90 per cent of all buildings, usually at penetrations through the wall system. And, often, designs submitted cannot be constructed in the manner shown in the construction documentation.
The near future will bring us a wide variety of possibilities and opportunities. Taken as a whole, the construction and maintenance of modern building facades will also present a formidable challenge for the professionals who deal with these components of modern buildings.
About the authors
Greg Hildebrand, C.E.T., M.Sc. (Eng) is head of Exp’s Façade Engineering Group, Building Engineering Team. He is a member of a number of ASTM and CSA standards-setting committees.
Petr Vegh, PhD, P.Eng. is head of Exp’s Structural Group and a member of the Executive Council of the International
Association for Shell and Spatial Structures (IASS).
Brian Burton is a research and development specialist for Exp, a certified CGSB/ICPI construction inspector and a columnist for Glass Canada magazine.