1 General principles of the building envelope
In cooperation with NKUA
1.1 Functions of the building envelope
According to R. Rush [RUS86] a building consists of four parts:
§ Envelope
§ Structure
§ Mechanical systems
§ Interior (furnishings)
Figure 6: Schematic representation of the building systems [OCW08]
1: Envelope 2: structure 3: mechanical systems 4: interior
According to this categorization [RUS86], "The envelope has to respond both to natural forces (rain, snow, wind and sun) and human values (safety, security, health and task success). The envelope provides protection by enclosure and by balancing internal and external environmental forces. To achieve protection it allows for careful control of penetrations. A symbol of the envelope might be a large bubble that would keep the weather out and the interior climate in."
According to Straube [STR98], the envelope can be described in terms of performance and function. Thus, the envelope experiences a variety of loads, including, but not limited to, structural loads, both static and dynamic and also air, heat and moisture loads. The enclosure must support structural loads and copes with environmental loads, both in the short-term and in the long-term. The enclosure is also often used to carry and distribute services within the building. In addition, the envelope (primarily the wall) has several aesthetic attributes that can be summarized as finishes. Among the four parts of the building, the elements of the envelope and their assembly contribute thus to a great extent to the overall building performance. The main expectable non-structural functions of the building envelope can be summarized in the following aspects:
§ Energy conservation: Resist thermal transfer through radiation, convection and conduction.
§ Air: Resist excessive air infiltration and ensure sufficient ventilation.
§ Sound: Attenuate sound transmission.
§ Water: Resist water penetration and enable humidity discharge.
Other aspects should also be taken into consideration such as:
§ Transfer lateral loads to building frame and support own weight (if the wall is not part of the main building structure),
§ Condensation (resist condensation on interior surfaces under service conditions),
§ Movement aspects (differential movement caused by moisture, seasonal or diurnal temperature variations, and structural movement),
§ Fire safety and security aspects (occupants protection from outside threats)
Finally, constructability and aesthetic aspects should be taken into account [RUS86].
1.2 General principles of an efficient building envelope
In general, the building envelope influences the heat exchange between the building and its environment, and regulates the penetration of solar energy into the buildings as well as humidity exchange. Energy losses through the building’s exterior walls, floors, roof, windows and doors account for 10 to 25 % of the energy used by most buildings, depending on the outdoor conditions and construction of the building elements [BAL94]. Energy losses are governed by two main parameters:
§ Temperature difference between the indoor and outdoor environment
§ The ability of the envelope to resist heating transfer due to conduction, convection, infiltration and absorption of solar radiation.
1.3 Points of attention
§ Opaque elements: Insulation of the external opaque elements or the use of additional insulation to the facades and the roof is one of the simplest and also very efficient measures to consider. It is possible to insulate a wall so well that the heat loss will be practically eliminated (for example: well-insulated offices are often self-heating during much of the year). Technological improvements, mainly in the field of the transparent insulation, offer additional alternatives. Proper location of insulation may increase the thermal inertia of the building and buildings with condensation problems on surfaces will experience a reduction or even an elimination of this kind of problem.
§ Transparent elements: Windows and other types of transparent areas in the building envelope determine the penetration of solar radiation to the building, daylight entrance, natural ventilation rates and heat flow to or from the ambient environment. They improve visual contact with the outdoor environment of the building and influence the acoustics of the building. Actions to improve or replace transparent components should combine all criteria in order to enhance penetration of solar radiation during the heating period and minimize it during the cooling period, decrease heat flow through the opening, improve day lighting, and permit a more efficient airflow during the summer season.
Figure 7: Tacoma Union Station [WBD08]
§ Infiltration: Infiltration of external cold air into the buildings can be responsible for a high part of the heating load. Air filtrates into or out of the building through cracks, openings, gaps around windows and doors, etc. In particular, air leaks through the building enclosure can take one of several forms: 1. Orifice flow that occurs when the air movement encounters across a linear pathway such as in the crack between a window rough opening and its frame, 2. Diffuse flow that happens when materials are used in the envelope, which are ineffective in controlling air infiltration and exfiltration due to many cracks or their high permeability to air, such as fibrous insulation or uncoated concrete block, and 3. Channel flow is probably the most common and serious of all types of air leaks and is shown in Fig. 2. The air entry point and exit point are distant from each other, giving the air enough time to cool below its dew point and deposit moisture in the building enclosure [WAG07].
Figure 8: Orifice and diffuse flow (left picture) and channel flow (right picture [WBD08]
§ Airtightness: This is a general descriptive term for the leakage characteristics of a building. This issue has to be considered carefully in cases of replacement of openings as well as when any civil works are carried out to retrofit a building. Air tightness is important because it impacts building energy use and the transport of contaminants between indoor air and outdoor air. From an energy standpoint alone it is almost always desirable to increase air tightness, but then indoor air quality may suffer in certain cases. In many countries infiltration (as natural ventilation) is the dominant source of outdoor air. Providing appropriate indoor air quality at minimal energy costs is a complex optimization process that includes, but may not be dominated, by air tightness concerns. A high degree of air tightness will provide insufficient air through infiltration and thus necessitates a designed ventilation system. Furthermore, if the building is completely sealed then moisture migration is prevented; this can be a serious problem especially for historical buildings. Thus, without some infiltration, condensation problems could occur throughout the building. Air tightness is usually estimated with blower door tests. “Blower door” is the popular name for a device that is capable of pressurizing or depressurizing a building and measuring the resultant airflow and pressure. The name comes from the fact that in the common utilization of the technology there is a fan (i.e. blower) mounted in a door; the generic term is “fan pressurization” and the basic technique involves measuring the steady-state flow through the fan necessary to maintain a steady pressure across the building envelope. The steady pressure usually acquired is 50 Pa [SHE04].
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1Figure 9: Blower Door [CIE08]
§ Moisture problems: Sometimes an important effect of filtration through the building envelope can be the condensation of moisture from the exfiltrating air (this usually takes place in northern climates) and from infiltrating hot humid air (this usually takes place in southern climates). Moisture may be responsible for mold growth, decay and corrosion, which in turn cause health problems and premature building deterioration. Therefore moisture problems are especially important for historical buildings. Air pressure differences between the indoor and outdoor environment can transport significant amounts of water vapour through air leaks in the building envelope. This water vapour can condense within the envelope in a concentrated manner, wherever those air leaks may occur [WAQ07]. In order to avoid this kind of problems in northern climates a vapor barrier is added facing in. On the other hand, in southern climates, the insulation is installed with the vapor barrier facing out. In order to control the flow of water, vapour retarders are used, i.e. elements that are intended to control or otherwise limit the flow of water in its vapor form (diffusive vapor flow, or vapor "drive") across an exterior wall system or assembly.
1.4 Rules of thumb
§ Adding insulation to the exterior walls and especially to the façade elements can be almost always achieved when façades get renovated for other (non energy-optimization) reasons. Insulation material can be added either to the inside or the outside surface or by filling the cavities within the wall structure. Use of additional insulation decreases the rate of heat loss, may improve the air tightness of the wall and reduce condensation problems on surfaces, while it generally increases indoor thermal comfort. Possible impact on storage thermal mass accessibility - and thus building inertia - should nevertheless be considered.
§ Opening windows is the basis of natural ventilation design since they allow a controllable flow of air, which can satisfy the needs to carry away surplus heat in summer and provide a pollution free environment.
§ Balanced mechanical ventilation systems require low infiltration rates. The rate of air entering the building must equal the air leaving the building. If the rate of the exhaust air is higher than the rate of air introduced into the building, then unwanted infiltration occurs.
§ The weather stripping of windows and doors also aims at decreasing infiltration and can be particularly effective measures.
§ Creating an atrium or a light well to provide natural light when required for the new use in a manner that assures the preservation of the structural system as well as character-defining interior spaces, features, and finishes is recommended. On the other hand damaging the structural system or individual features, radically changing, damaging, or destroying character-defining interior spaces, features, or finishes in order to create an atrium or a light well should be avoided [NPS08].
§ Vestibules may be very useful since they create a secondary air space at a doorway to reduce air infiltration occurring while the primary door is open. If a vestibule is in place, it would be nice to retain it. If not, adding a vestibule, either on the exterior or interior, should be carefully considered to determine the possible visual impact on the character of the building. The energy savings would be comparatively small compared to construction costs. Adding a vestibule should be considered in very cold climates, or where door use is very high, but in either case, the additional question of visual intrusion must be resolved before it is added. For most cases with historic buildings, adding a vestibule is not recommended (NPS08].
1.5 Practical examples
§ Natural ventilation schemes: Small offices can receive adequate ventilation by a combination of low and high level openings on one side of the buildings. Larger office buildings and open plan offices will normally require windows on two opposite sides, so that air moves through the office from one side to another, by normal pressure differences across the building.
§ Insulation materials used to fill cavity walls can be foam, mineral wool or polystyrene beads or granules. Attention should be paid to ensure that the wall materials are compatible with the type of the considered insulation, that interaction will not occur and that there will not be any damage either of the wall structure or the insulation.
§ Replacement of old frames may drastically reduce infiltration rates. For buildings located in areas with enhanced traffic, the use of vestibules may reduce airflow through exterior doors. The same effect is achieved using revolving doors.
1.6 Related chapters
§ Part I Chapter 2: Penetrations and additions
§ Part I Chapter 3: Exterior Insulation
§ Part II Chapter 2 : Thermal mass
1.7 References
[RUS86] Rush R. (Editor) The Building Systems Integration Handbook, Wiley, New York, 1986
[STR98] Straube, J.F. and Burnett, E.F.P., 'Vents, Ventilation and Masonry Veneer Wall Systems', Proc. of the Eighth Canadian Masonry Symposium, Jasper, Alta., Canada, May 31-June 3, 1998, pp. 194-207.
[BAL94] Balaras C. “Energy conservation in office buildings”, M. Santamouris and D.N. Asimakopoulos, (Editors), SAVE Program, European Commission, Directorate General for Energy, CIENE Editions, Athens, 1994.
[SHE04] Sherman M.H. and Chan R. “Building Airtightness: Research and Practice”. Lawrence Berkeley National Laboratory Report No LBNL-53356
[WAQ07] Wagdy A. “Air Barrier Systems in Buildings”. http://www.wbdg.org/resources/airbarriers.php?r=env_introduction. Last accessed 28-8-08.
[NPS08] Standards for Rehabilitation and Guidelines for Rehabilitating Historic Buildings. http://www.nps.gov/history/hps/tps/standguide/rehab/rehab_strucsystems.htm. Last accessed: 28-8-08
[WBD08] http://wbdg.org/design/historic_pres.php. Last accessed: 28-08-2008
[OCW08] http://ocw.mit.edu/OcwWeb/web/home/home/index.htm
[CIE08] Central Institution Energy Efficiency Education, University of Athens, Athens, Greece.