A few papers and books are used as references for principles of building enclosure design. We will discuss two of them, namely work of Adelson and Rice (1991) and Hutcheon (1963), We start with the first one as they are discussed by Bomberg at al. (2016)[1]:

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A. Objectives

     A1. Provide continuity of functions

     A2. Provide redundancy of design (second line of defence)

     A3. Integrate interactive effects

 

B. Constrains

 

     B1. Consider separate lives of components or assemblies

     B2. Consider flow of fluids and energy from high to low potentials

     B3. Consider moisture-originated deterioration mechanisms

 

C. Balance

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    C1. Keep balance between continuity and separation

    C2. Assess heat, air and moisture flows and their effects

    C3. Use economic considerations for interactive effects

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In designing a building we need to consider both objectives and constrains and try to establish a balance between them. In the traditional masonry construction all functions were achieved by a composite of masonry and plasters. Emergence of framed and layered structures initiated a process, accentuated by codes and standards, to link each material to its main function in the assembly. This approach, however, confuses people and makes them forget that any system always performs as an entity. We will, therefore, examine how design principles can be applied to determine potential design weakness already in the design stage.

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Objective A1: Continuity of functions (continuity of performance attributes)

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We need to achieve continuity of all environmental functions (sound, heat, air and moisture transfer, fire and smoke protection etc.). A good tool to visualize this principle is a funnel. In a narrow part of a small funnel water runs faster than in the wide one, but when we increase the size of a funnel and fill it with a high water height, water runs amazingly fast.  Similarly, a thermal bridge in a well-insulated wall has much higher impact on wall thermal performance than the same thermal bridge in a poorly insulated wall; or a hole with same size has much higher impact on air tightness if the wall is very tight.

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Objective A2: Second line of defense (redundancy).

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Since buildings are erected in uncertain weather conditions with different materials and may encounter different deficiencies during design or workmanship stages, Adelson and Rice (1991) introduced a principle called “creative pessimism” that we re-named to "the second line of defense". This concept follows recognition that uncertainty is caused by variability of materials, workmanship, and weather. One, therefore, requires applying two different measures of control. The need for the second line of control is most visible in moisture management of cladding systems.

As the saying goes the perfect design exists only on paper, in the real world sooner or later something goes wrong. The failures of “face seal” approach in moisture management of cladding (stucco or EIFS) highlighted the risk of neglecting possibility of failures. Sealants were the main measure to control water entry, so the water resistive barrier was added as the second line of defense.

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Objective A3: Integrate interactive effects

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This principle applies when a final effect can be achieved by a different combination of environmental factors, e.g., temperature of the indoor air depends on thermal mass, thermal insulation, air infiltration, air ventilation, fraction and orientation of widows, outdoor weather etc. Changing one of these factors may affect others and modify the final effect. People who calculate effect of adding thermal insulation on energy assuming that it has constant correlation make a mistake. This principle tells us that any change in the interacting factors must be evaluated in context of the whole building in both technical and economical manner.  

When trying to fulfill these three objectives we encounter the following three constraints:

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Constrain B1. Consider separate lives of components or assemblies

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Materials have different thermal and moisture expansion coefficients, exhibit differential durability and they may even exhibit lack of chemical compatibility each with another. This may be a problem if moisture is accumulating in the interface. Consider a joint between the exterior plaster and rough opening of a window. Typically, the fresh Portland cement plaster is applied directly to the rough opening frame. Yet, as all cement-based materials, it will shrink away from the window opening frame developing a small crack. This crack will draw water inwards (from the wall surface) and water comes in contact with wood that is a moisture sensitive material. Historically, when plaster was lime-cement, it allowed good drying from the surface. Today, the plaster contains hydrophobic (water repelling) agents they slow both the rate of water entry as well as the rate of water drying. This type of failure has been frequently seen in warm and humid climates

So, if you want to use modern acrylic finishing plaster you must respect the principle of the separate lives and place a gasket (or sealant on backer rod) between the plaster and the rough opening of the window.

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Constrain B2: High to low (follow the gradients)

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This principle relates to energy and mass flows: heat, air, water, vapor or electric current all flow from high, to low potential being it temperature, pressure, or substance concentration. This law also applies to materials that have been enriched during manufacturing e.g. oriented strand board panel will reverse to wood fibers. In the latter case, one talks about durability of materials under effects of environmental factors where the rate of damage depends on the severity of exposure.

Examples of high to low principle are: shedding of rain water flowing under action of gravity, need for a drop edge under windows and need for overlap of water resistive barriers.

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Constrain B3. Consider moisture-originated deterioration mechanisms

 

Moisture has not been a consideration in the traditional, massive masonry walls that had large capacity to absorb and store water. This constrain has been added because today most materials, even masonry walls lack moisture storage capability and have to be considered as damage prone.

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Balance between objectives and constrains

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We need to achieve a balance with changes in the outdoor environment and near constant indoor environment, we need to achieve balance between the various materials in the assembly to avoid distortions and deformations and similarly a balance between different components of the building. A good example of design with balance in mind is plywood with oriented strands going in two different directions. Another example is traditional three coat stucco, where starting from the substrate each layer has higher water vapor permeability and lower mechanical stiffness to avoid warping of the stucco under wetting or drying conditions.

 

Balance C1. Keep balance between continuity and separation

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Often the continuity of function can be achieved by an adequately designed discontinuity; e.g., by using an overlap of roofing tiles or use of the flashing to compensate for the effect of "separate lives". Other examples may include movement joints e.g. when two adjacent panels can expand and contract but the joint maintains the connection or reviles in a plaster where a thin section will crack but the crack will not be visible because the shape of the profile hides the crack and the undelaying material (substrate) will provide the continuity of function.

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Balance C2. Use risk assessment for flows and their effects

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A good example of risk assessment is the requirement in ASHRAE standard 160 that assumes 1 percent of rain load to have passed the first layer of defence and one must calculate if the specified wall system in the given climatic conditions has an adequate capability to dry this moisture within one year.

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Balance C3. Use economic considerations for interactive effects

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This requirement has been added because one often selects one parameter out of many when dealing with an interactive situation e.g., increasing thermal insulation or air tightness without consideration what effect it has on other factors. This approach often results in a large gap between design intentions and practice of construction. An  example is the requirement of the nominal thermal resistance of the opaque part of the wall in high rise buildings Rsi 4 while the average thermal performance including windows and air infiltration divided by the wall area makes an effective R-value index about 0.7 (Cianfrone et al 2016).

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The second source of design principles can be derived from a slightly modified list of factors proposed by Hutcheon (1963) for the performance analysis (Table 2). We will compare their application with thesis of Olie (1996)

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Table 2 Principal performance requirements for walls

1.         Control of heat flow;

2.         Control of rain penetration

3.         Control of air flow;

4.         Control of water vapour flow;

5.         Control of light, solar and other radiation;

6.         Control of noise;

7.         Control of fire;

8.         Control of insects, vermin, mold

9.         Provide strength and rigidity;

10.       Be durable;

11.       Be aesthetically pleasing;

12.       Be economical.