Designing for environmental control compels professionals to address the building enclosure (BE) system as a whole while recognizing that both the choice of materials and the design details affect the environmental performance of the BE system. Achieving the right harmony between materials, design and system performance depends on integrating two extremes in conceptual thinking: qualitative assessment based on experience in use and quantitative evaluation based on results of testing and analysis.
On the qualitative side of the environmental control is the knowledge of what makes a building enclosure function plus a general understanding of how suitable the materials are for a given use. However, qualitative decisions can appear somewhat arbitrary, so attempts are made to support them with a quantitative analysis. Additional effort is directed at developing national evaluation criteria (as they are equivalent to the national standards) towards being based less on the properties of the materials and more on field performance.
While many of today's achievements in building science demonstrate successful understanding of both the scientific principles and the art of construction, there is no formal procedure underlying the process of designing for environmental control. Experience suggests a three-pronged strategy for ensuring environmentally sound building enclosures
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strive for the continuity of barriers
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incorporate a second line of defense
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ensure that the design is buildable
Continuity of barriers
Discontinuity can significantly reduce the efficiency of the environmental barrier. Indeed, in the extreme case, air barrier discontinuity may result in a failure of the building enclosure system. As previously discussed, air transport represents a critical factor in environmental control. It underscores virtually all facets of environmental control as it moves both heat and moisture through the building enclosure.
While most designers and specifiers consider air barrier systems carefully, some aspects of air barrier performance, such as details of joints and differential movements of construction, may pass unnoticed. Likewise, details involving steel columns, roof/wall junctions, or brick ties in masonry walls are often overlooked. Not fully appreciated is also installation of different air barrier systems, such as trowel applied, torch applied, adhesive back or mechanically fastened materials.
Differential movements in the structure also affect the continuity of air barriers. These movements develop after construction because of thermal expansion or contraction of the building elements, deflection of beams or mortar shrinkage. In addition, air barrier materials differ in crack-bridging ability. (Rigid paging materials usually do not offer protection from cracks developing in masonry walls. However, reinforced flexible membranes with adequate thickness may perform well.) While some structural movements can easily be predicted, yet in many cases, designers must rely on experience and judgment to anticipate the impact of differential movements on the specific design.
Most designers and specifiers realize the need for continuity of the air barrier, but they often accept discontinuities in thermal insulation without realizing their effect on energy performance of the enclosure. Consider a prefabricated concrete panel where the joint between the panels causes a break in the insulation, creating what is termed a thermal bridge. A simple model of heat transfer, the ASHRAE parallel heat flow model, assumes that heat flows only perpendicular to the wall surface. Using this model, a 60 mm gap in the insulation formed between two 3 m panels (creating a thermal bridge that occupies 2% of the surface area) results in average thermal resistance of the wall which is 8 percent lower than the thermal resistance in the cross-section through the insulation.
This model, however, fails to account for lateral heat transfer adjacent to the thermal bridge. So correcting for the multidirectional flow of heat results in thermal resistance which is 28 percent lower than the thermal resistance in the cross-section through the insulation.
This example, though extreme, is far from being the worst case of reduction in field performance caused by the discontinuity of insulation barrier. Much higher relative reductions of thermal performance have been measured, for example, in walls where spaces remained unfilled following pneumatic application of loose-fill mineral fiber insulation. In one case, wood frame wall with pneumatically filled mineral fiber insulation and 0.5% of the area unfilled, the effective thermal resistance of wall was reduced by 10% (ASTM STP 789 p.341). An even greater reduction of performance resulted when convective movements were generated by installation defect in the space adjacent to wood frame members. When outdoor temperature reached minus 35 oC, unfilled 6% area in the wall cavity caused 36 % reduction of R-value for the frame wall (J. of Building Physics, April 1993).
In light of these findings, how can the designer calculate thermal resistance of the actual design? Firstly, one must use thermal characteristics that relate to performance of these materials under field conditions, for instance, by accounting for the reduction in thermal resistance caused by material aging and weathering. Secondly, one combines parallel and in-series models described by the ASHRAE Handbook of Fundamentals.
The parallel model of heat flow assumes the shortest heat flow path, without considering lateral heat flow. The series-parallel model estimates thermal resistance by assuming a perfect equalization of temperature in each layer interface. The actual thermal resistance is always between these models. Thermal resistance measured on several wood frame walls fall within 10% of the mean value of the results obtained from each model (1:1 ratio). Europeans, on the other hand, prefer for masonry walls use of 2:1 ratio (parabolic rule), since this mixing rule gives thermal resistance closer to wall assemblies, typical of Europe, in which one path of heat flow shows significantly higher conduction than the other.
For instance, an assembly consisting of an insulated steel stud frame placed between drywall and 100 mm (4 in) of concrete was tested at a mean temperature of 24 oC and thermal resistance of the assembly was determined as 1.32 (m2K)/W. Calculating thermal resistance of this wall with the ASHRAE parallel heat flow model alone gives thermal resistance of 2.36 (m2 K)/W. Using the parabolic rule, however, yields 1.29 (m2 K)/W, a good agreement with the measured thermal resistance.
Redundant design (or second line of defense)
Through experience, philosophy for good environmental design incorporates designing a second line of defense. Experience shows that in construction, sooner or later, something goes wrong. It rains during construction time, a roof leaks some time later. Alternatively, water enters for other reasons, for instance, roof drainage does not lead water away from the building but directs it right to the basement. For these and many other reasons some moisture finds its way into the enclosure. So, as the second line of defense, walls are constructed to permit draining and drying of any excess moisture. But how long would the drying take and what effect would the moisture have on other materials? As there are no quantitative answers, designers must look to logic and experience.
The second line of defense forces designers and builders to prepare for the issues they can't predict, such as material changes or the workmanship issues that may escape notice. Typical unnoticed leakage paths occur when air traveling from inside to outside passes over more than one building component, such as passing through unfinished paging of masonry walls behind radiator cabinets, or through holes used to pass wires to the suspended ceiling or corrugated roof decks. This list also includes large masonry partition walls that stretch above the roof of an adjacent section, openings cut for electrical heating, ventilation and plumbing services, cladding materials that are not drawn tight to metal studs and furred partition walls connecting with a suspended ceiling.
Practitioners observed that using external insulating sheathing has a number of advantages. The sheathing increases temperature in the wall cavity thereby reducing the potential for condensation in the wall cavity. It also helps achieve continuity in thermal insulation of the basement walls and roofs. If, however, the external sheathing acts as both air and vapor barrier material, placing it on the cold side may create risk of moisture accumulation if a defect develops during the service period.
Well, how could a designer apply the principle of the second line of defense when considering an external insulating sheathing as the air barrier material for wood frame wall? The designer has four options:
1) The designer may use impermeable foam in the exterior insulation and fiber insulation in the cavity but increasing thermal resistance of external sheathing to a level high enough to reduce the risk of moisture accumulation if a defect develops during the service period.
(2) The designer chooses an impermeable foam and fibrous insulation but requires improved drying ability inwards. The wetting potential is not reduced but the designer ensures that for a given climate and use the wall will dry inward in sufficiently short time.
(3) The designer chooses impermeable foam but chooses a different thermal insulation for the
cavity. By selecting a thermal insulation that is resistant to air and moisture flows (e.g., spray polyurethane foam) the potential for moisture accumulation is reduced.
(4) The designer uses the fibrous thermal insulation in the wall cavity but chooses an external insulating sheathing that is impermeable for air but sufficiently permeable for thermally driven water vapor. In this case the drying potential is increased.
Whatever the choice, its basis is always the building science principles and designer's experience together.
Buildability
Like the second line of defense, buildability relates more to judgment and knowledge than to mathematical analysis. Buildability reflects whether the specific design can be assembled by various trades without compromising the functional requirements during construction.
Contrary to a frequent misconception, buildability is more related to a good design than to superior workmanship because, as experience indicates, only good design can combine all the environmental factors while presenting an easy construction pattern. For the most part, it is the designer who attends to the aspects of buildability such as material installation under different weather conditions, level of skill required for installation, and construction tolerances. Often buildability problems arise when different professions are involved, for instance the window /wall interface is not of interest to the window manufacturer nor to the wall designer.
There is a need for careful review (troubleshooting) of shop drawings of the assembled system as a whole and the architectural details in particular. The significance of such review cannot be overstated. Furthermore, the designer must review the specification terminology used on the architectural details to ensure that everything is understood by those who rely on the drawings. The designer may even choose to have a specific section addressing the joints and junctions of various sub-assemblies within the overall building enclosure system. Of particular concern are such areas as wall/roof and wall/window interfaces.
It is clear that the design of building enclosures for environmental control requires a number of iterations. Each material must be examined with regard to its compatibility and interaction with the adjacent materials and components. Each modification of the performance requirements or change in the material selection must be followed by a review of architectural details. These issues highlight the importance of review (troubleshooting) of drawings of the assembled system and on the design details. To maintain a high standard of quality and clarity, some specifiers prefer to have one section of the specification addressing the joints and junctions of various sub-assemblies within the overall building enclosure system. In this respect the concepts of buildability and second line of defense complement each other.
Yet, so often the key to the good, long-term performance of the system depends on the quality of the architectural details. Jean-Claude Perrault stated in a seminar on construction details for air tightness: "A good or bad detail will often make the difference between good and bad installation. Building designers should bear in mind that those who actually build the buildings usually have no design background. They should not be forced to guess the designer's intention, or to play the role of designer, but should only be expected to build carefully, as detailed. That is why a detail must be precise, easy to understand and predictable."
Some building scientists have suggested that construction details should be drawn one-half full size to permit easier review and to force designers to achieve a greater clarity of thought. But whether the size of the detail drawings will improve the flow of information from designer to construction site is, perhaps, a moot point. For in the final analysis, it is the consolidation of quantitative evaluation and logical experience which makes the difference between a building enclosure that works and one that merely frames an indoor space.