Metaphor for Progress: The Wood Framed Wall
by: Paul Kando
Log walls (1) constructed of tree trunks go back to ancient times. Even a modest structure required the felling of many trees. Construction involved heavy labor but few tools. Trees could be felled and logs shaped by an axe. But for the gaps between logs, log walls were solid wood. The gaps had to be sealed - with mud, pitch or whatever was available. Still, log walls were prone to leak as both the logs and the sealing material shrank while drying. The insulation value of a log wall is less than R-1 per inch of thickness, allowing for the gaps between the logs.
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Post and beam construction (2), dominant from 1630 to 1830, still used whole tree trunks, but only for the structural framing. Precise joinery connected posts, beams and braces, requiring considerable skill and more sophisticated tools. The walls were sheathed with one or more layer of sawn boards, often laid up diagonally, to provide additional bracing. Insulation was minimal; there was no wall cavity to speak of. An inch thick wooden board has an R value of about one. Mud and horse-hair plaster were sometimes used to seal the walls from the winds.
Balloon framing (3) came into vogue after 1830 and remained the dominant way to build wooden structures until the late 1930s. It introduced standard dimensional framing lumber -- 2" thick sawn planks 4,6,8,10 and 12 inches wide, instead of whole logs -- a huge advance in material utilization. Long wall studs reached from the foundation to the top of the second floor. Floor joists were attached to these vertical studs, in effect suspending the intermediate floor within two story high walls. Balloon framed walls were sometimes insulated with improvised materials, like sawdust or crumpled newspaper, or were left uninsulated - a cause of large heat losses due to the chimney effect within the tall wall cavity.
Platform framing (4) - from the late 1930s onward -- represented an important step toward standardization, improved productivity and materials use. Walls could be constructed on the floor deck, then tilted up into place. A standard 8' ceiling height permitted the use of standard-length, 8' long studs and standard-sized (4'x 8') sheathing materials -- plywood, oriented strand board (OSB), gypsum wallboard -- with minimum cutting and waste. (For custom designs other standard dimensions are also available.)
Passivhaus (PH) construction introduced information technology - the use of computers and sophisticated software -- to the building arts for the first time. Building materials, components, appliances are all system-optimized using computer-simulation in advance of any construction. The aim is to meet a set of predetermined performance parameters using superinsulation, avoidance of thermal bridges across the conditioned building envelope, air-tight construction, mechanical ventilation to control air supply and remove excess moisture, and the use of high efficiency windows, doors, heating systems and appliances. PH does not prescribe building components or methods. Instead it introduces affordable comfort conditions as a primary goal to be met. The three PH wall systems shown all use standard platform-style 2x4 framing on the interior, sheathed with air-tightly taped OSB as a tight air/vapor barrier. Inside this air-sealed layer, conventional utilities can be installed, along with insulation. Outside, additional framing is added to accommodate extra insulation required by the local climate. Various methods are possible. Of those shown, (5) employs I-joists as studs, (6) uses Larsen trusses, and (7) affixes rigid foam insulating panels. In all three the "outer wall" is reminiscent of balloon framing, but uses engineered materials, rather than dimensional lumber. These form a spacious cavity that reaches from the foundation to the top of the top floor. Variants (5) and (6) can even replace the conventional wooden outer wall sheathing with a super-strong engineered building fabric that is weatherproof from the outside, but vapor-permeable from the inside out. This keeps the wall structure dry, shedding its moisture in both directions from the air-tight OSB layer.
I focused here on the evolution of wooden walls. Need I emphasize that the whole house is a system in which the heated envelope must be surrounded by an air-tight cocoon of superinsulation? In energy-efficient structures, walls (incorporating highly efficient windows and doors), floors and ceilings that form the boundary of the heated space are seamlessly sealed together. Mechanical ventilation is introduced to ensure the removal of stale air and excess moisture, while preserving most of the heat contained in the outgoing stale air. The latest state of the art ventilating systems reclaim as much as 94% of this heat, which conventional ventilation would waste.
The story of the evolving wooden wall is also a metaphor for beneficial change, based on science, rational analysis, continual learning, and sharing of information. Note the increasingly frugal use of materials, the gradual replacement of hard labor with skills and knowledge, the use of increasingly advanced tools. Note also the incorporation of new knowledge (particularly system knowledge), and improved energy efficiency. Most importantly, note the introduction of comfort as a fundamental value (rather than some minimum dictated by market tolerance and demand for builder-profitability) and the key role of the computer in achieving pre-set comfort and performance goals.
They all add up to system optimization, purposely addressing not only the occupants' needs, but also those of the climate, the economy, and the pursuit of happiness. That's true human progress.