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Building Science Digests
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Digests are papers on specific building science topics written for a broad range of users. Expert advice is given with a focus on establishing an understanding of the theory and translating this theory to practical information.
The construction and operation of buildings consumes over a third of the world’s energy consumption, and 40% of all the mined resources. Striving to make buildings more sustainable, while saving construction and operating costs and improving health and occupant well being is not only possible and practical, it should be the goal of the building industry. Achieving this goal requires an awareness of the problem and the skills to design, specify, construct, and operate buildings in a manner that is often quite different from current standard approaches. This digest will review the challenge of sustainability, discuss methods of assessing green buildings, and recommend a process by which more sustainable buildings can be delivered.
Historical works, notably the Roman Vetruvius’ Ten Books of Architecture, that describe buildings begin with an historical overview. Archaeological and anthropological studies have furthered this understanding. The history of the built form and the building enclosure is more than just a curiosity: understanding the history helps explain many of the buildings types, construction techniques and building materials that we use today. This digest provides a brief overview of the development of the building enclosure and can serve as an entry point into a deeper historically-informed study of buildings and building science.
Predicting the future is very difficult, but examining trends and potential tipping points is useful as an aid to understanding the direction the building industry is headed, and where it might end up. Although some future changes can only be speculated upon, other trends are already occurring and causing changes. Below is a series of changes and possible changes that may influence the building industry and society.
Providing thermal comfort without excess space conditioning costs is one of the primary requirements of buildings. Therefore, thermal control is an important aspect in almost all buildings. Understanding heat transfer and the temperature distribution through building materials and assemblies is also important for assessing energy use, thermal comfort, thermal movements, durability, and the potential for moisture problems.
Heat flow occurs through the building enclosure via opaque enclosure elements, is directly transferred into the building by solar radiation through windows, is carried along with air across the enclosure by unintentional leakage and ventilation, and can be generated within the building by occupants and their activities.
The control of heat flow in buildings requires insulation layers compromised with few thermal bridges, an effective air barrier system, good control of solar radiation, and management of interior heat generation.
Moisture accumulates when the rate of moisture entry into an assembly exceeds the rate of moisture removal. When moisture accumulation exceeds the ability of the assembly materials to store the moisture without significantly degrading performance or long-term service life, moisture problems result.
Moisture is one of the most important agents leading to building enclosure deterioration. Understanding and predicting moisture movement within and through the enclosure is therefore of fundamental importance to predicting and improving building enclosure performance, particularly durability. Since driving rain deposition on walls and roofs is quantitatively the largest single source of moisture for most walls and roofs, it is no surprise that controlling rain penetration is one of the most important parts of a successful moisture control strategy. In fact, failure to control rain is likely the oldest and most common serious building enclosure performance problem. Commentators as long as Vitruvius (70 BC) bemoaned the challenges of controlling rain penetration.
This document will consider rain control from a general to a specific level. The following sections will cover: basic moisture control principles that should be employed in the design of above-grade building enclosures; driving rain as a moisture load on walls; a classification system of the various rain control strategies available for walls; and finally, good design practises for walls. The rain control of roofs will be covered in more detail in another BSD.
The control of air flow is important for several reasons: to control moisture damage, reduce energy losses, and to ensure occupant comfort and health. Airflow across the building enclosure is driven by wind pressures, stack effect, and mechanical air handling equipment like fans and furnaces. A continuous, strong, stiff, durable and air impermeable air barrier system is required between the exterior and conditions space to control airflow driven by these forces.
Air barrier systems should be clearly shown and labelled on all drawings, with continuity demonstrated at all penetrations, transitions, and intersections. In addition, enclosure assemblies and buildings should be vertically and horizontally compartmentalized, may require secondary planes of airtightness (such as those provided by housewraps and sealed rigid sheathing) and may need appropriately air impermeable insulations or insulated sheathing.
That part of any building that physically separates the exterior environment from the interior environment(s) is called the building enclosure or building envelope. Environmental separator is another term used to describe the enclosure, but note that this generic term also applies to separators of two different interior environments. The term building enclosure is preferred to the term building envelope largely because it is considered both more general and more precise. Also note that the building enclosure may contain, but is not the same as, the so-called thermal envelope, a term that is used to refer to the thermal insulation within the enclosure. The enclosure, the loadings it must resist, and its functions are addressed in this digest.
The design of building enclosures to control rain penetration and control rain shedding is typically based on experience and rules of thumb that make use of traditional details. Unlike heat flow, vapor diffusion, air leakage, etc. there is no theory of rain control to aid the designer or analyst of building enclosures. An edited version of this document was published in Journal of Thermal Insulation and Building Envelopes, July 1999, pp. 41-56.
Attics or roofs can be designed and constructed to be either vented or unvented in any hygro-thermal zone (Map 1). The choice of venting or not venting is a design and construction choice not a requirement determined by the physics or by the building code. The model building codes allow both vented and unvented roof assemblies. The applicable physics impacts the design of attic or roof systems as does the applicable building code but neither limit the choice.
Buildings used to be constructed over cellars. Cellars were dank, dark places where coal was stored. People never intended to live in cellars. Now we have things called basements that have pool tables, media centers and play rooms.
Cellars were easy to construct – rubble, stone, bricks and sometimes block. If they got wet or were damp so what? Basements are different. They are not easy to construct if we intend to live in them. They need to be dry, comfortable and keep contaminants out.
Over the last 50 years there has been a notable expansion of living space. The useful conditioned space of building enclosures is expanding to the outer edge of the building skin (Figure 1). Attics, crawlspaces, garages and basements are valuable real estate that are being used to live in or used for storage or places to locate mechanical systems. Basements are viewed by many as cheap space that can easily be incorporated into a home. Keeping basements dry, comfortable and contaminant free is proving to be anything but simple.
Controlling heat flow, airflow, moisture flow and solar and other radiation will control the interactions among the physical elements of the building, its occupants and the environment. Of these four, airflow “merits major consideration mainly because of its influence on heat and moisture flow” (Hutcheon, 1953). Airflow carries moisture that impacts a materials long-term performance (serviceability) and structural integrity (durability). Airflow also affects building behavior in a fire (spread of smoke and other toxic gases, supply of oxygen), indoor air quality (distribution of pollutants and location of microbial reservoirs) and thermal energy use. One of the key strategies in the control of airflow is the use of air barriers.
Controlling rain is the single most important factor in the design and construction of durable buildings and in the control of mold.
Drainage planes are used in the design and construction of building enclosures to control rain.
All exterior claddings pass some rainwater. Siding leaks, brick leaks, stucco leaks, stone leaks, etc. As such, some control of this penetrating rainwater is required. In most walls, this penetrating rainwater is controlled by the drainage plane that directs the penetrating water downwards and outwards.
The function of a vapor barrier is to retard the migration of water vapor. Where it is located in an assembly and its permeability is a function of climate, the characteristics of the materials that comprise the assembly and the interior conditions. Vapor barriers are not typically intended to retard the migration of air. That is the function of air barriers.
Adding outdoor air in hot humid climates causes moisture problems right? Sometimes. It depends on the condition of the house before you start to add outdoor air. Contrary to popular belief, most houses in hot, humid climates are over ventilated due to duct leakage and induced air change from internal air pressure effects due to unbalanced air flow and door closure.
Water comes in four forms: solid, liquid, vapor and adsorbed. All four forms can cause grief to building owners, designers and contractors. When water causes building problems investigating and diagnosing the problem can be challenging because water constantly changes its form inside a building and within its materials. The investigator must hunt down the water thinking like water.
Air flow in buildings is one of the major factors that governs the interaction of the building structure with the mechanical system, climate and occupants. If the air flow at any point within a building or building assembly can be determined or predicted, the temperature and moisture (hygrothermal or pyschometric) conditions can also be determined or predicted. If the hygrothermal conditions of the building or building assembly are known, the performance of materials can also be determined or predicted
Are multifamily buildings one building or a bunch of individual buildings sharing the same structure? Should services and systems be shared or individual? The passions regarding these questions are as strong as those separating Yankee fans and Red Sox fans.
We learn our lessons from disaster. Hurricane Andrew taught us about wind. Hurricanes Charley, Frances and Jeanne taught us about rain. The Red River of the North Basin taught us about floods. Hurricane Katrina had it all: wind, rain and flood.
That we will rebuild, and rebuild in the same place, is not in doubt. This is what we do – for better or worse.
If we are to rebuild and if we are to rebuild in the same place how should we rebuild?
This digest will begin with a brief description of the system and materials, review moisture problems in buildings, and summarize how moisture control should be dealt with in strawbale buildings.
