1、44.1CHAPTER 44BUILDING ENVELOPESTerminology . 44.1Governing Principles. 44.2Design Principles. 44.3Quick Design Guide for High-Performance Building Envelopes. 44.8COMMON ENVELOPE ASSEMBLIES . 44.8Roofs. 44.8Walls . 44.9Fenestration 44.10Foundations 44.11Existing and Historic Buildings. 44.11ROPER bu
2、ilding envelope design requires knowledge of thePphysics governing building performance as well as of buildingmaterials and how they are assembled. This chapter provides practi-cal information for designing new building envelopes and retrofits toexisting envelopes, always with the notion that the en
3、velope mustwork well in concert with the buildings surroundings and the HVACsystem. The information can also be useful for those involved withbuilding envelope investigation and analysis.This chapter was developed with the integrated design approachin mind and assumes that the architect, HVAC design
4、er, buildingenvelope designer, and others involved in envelope design and con-struction communicate and understand the interrelationships be-tween the building enclosure and mechanical systems. Integrateddesign requires a clear statement of the owners project requirements(OPR) and design intent, and
5、 is described in greater detail in Chapter58. That chapter may be used as a basis for finding common agree-ment among designers and engineers using the integrated designapproach. The growing use of integrated design in project deliveryhighlights the building envelope as the principal site where arch
6、itec-tural design and mechanical engineering meet.A successful building envelope design requires that the team beknowledgeable about and responsible for the performance require-ments described in this chapter. This chapter does not distinguish theindividual responsibilities of each team member, but
7、rather is in-tended to serve the team as a whole.Buildings are designed and constructed to provide shelter fromthe weather and normally house conditioned, habitable spaces foroccupants. The building envelope is an assembly of componentsand materials that separate the conditioned indoor environment f
8、romthe outdoor environment. The envelope typically includes the foun-dation, walls, windows, doors, and roof. Partitions between inte-rior building zones that have substantially different environmentalconditions (such as a swimming pool compared to an office area) areoften required to function simil
9、arly to building envelopes.Performance requirements for the building envelope include thefollowing (Handegord and Hutcheon 1989; Hendriks and Hens2000; Hutcheon 1963):Control heat flowControl airflow, including airborne contaminantsControl liquid water penetration with rain as the most importantsour
10、ceControl water vapor flowControl light, solar, and other radiationControl noiseControl fireProvide strength and rigidity against outside influences (some-times structural)Be durableBe constructable, maintainable, and repairableBe aesthetically pleasingBe economicalBe sustainableThese performance re
11、quirements and their effects on one anothermust be understood by the project team. Building envelopes shouldbe designed for good overall performance. The first eight listed itemsarise from the envelopes function of separating the conditioned andunconditioned environments. Parties responsible for HVA
12、C andbuilding envelope design must be knowledgeable about how eachsystem affects the performance of the other. Review of the heat, air,and moisture characteristics of the proposed envelope is needed forappropriate design of HVAC systems. The building envelope mustalso be designed with an understandi
13、ng of the interior and exterior en-vironmental design conditions; consequently, the architect or princi-pal designer needs to provide the specific performance requirementsto the HVAC designer, including provisions to achieve minimum air-tightness, interior occupancy criteria, and special-use conside
14、rations.With a building envelope design suited to the operating requirements,the space-conditioning (HVAC) system generally is smaller in capac-ity and may have simpler control and distribution systems, normallyresulting in a system with greater efficiency.This chapter applies information in Chapter
15、s 25 to 27 of the 2013ASHRAE HandbookFundamentals to building envelope design. Itincorporates much of the material from previous versions (until2005) of Chapter 24 in that volume.1. TERMINOLOGYFor definitions related to the physics of heat air and moisturetransport, see the Terminology section in Ch
16、apter 25 of the 2013ASHRAE HandbookFundamentals.An air barrier is a component or set of components in a buildingenvelope that forms a continuous barrier that controls airflow acrossthe envelope or assembly.A building assembly is any part of the building envelope (e.g.,wall, window, roof) that has bo
17、undary conditions at the conditionedspace and the exterior. A building envelope or building enclosure is the overall physi-cal structure that provides separation between conditioned spacesand the outdoor environment or any indoor environment that is sub-stantially different from the conditioned one.
18、A (building) component is any physical element or materialwithin a building assembly.Moisture condensation is the change in phase from vapor to liq-uid water. Condensation occurs typically on materials such as glassor metal that are not porous or hygroscopic and on capillary porousmaterials that are
19、 capillary saturated. Condensation should be distin-guished from phase change between vapor and bound water in cap-illary or open porous materials (see moisture content).Durability is the ability of a building or any of its components toperform its required functions in its service environment over
20、aThe preparation of this chapter is assigned to TC 4.4, Building Materialsand Building Envelope Performance.44.2 2015 ASHRAE HandbookHVAC Applicationsperiod of time without unforeseen cost for maintenance or repair(CSA 1995).Fenestration includes all areas (including the frames) in thebuilding envel
21、ope that let in light. Fenestration includes windows,curtain walls (vision areas), clerestories, skylights, and glazeddoors. Fenestration excludes insulated spandrels and solid doors.Fenestration area is the total area of fenestration measured usingthe rough opening, including the rough opening for
22、doors.Hygrothermal design analysis is a set of calculation proceduresthat uses building design and component physical properties to pre-dict heat, air, and moisture performance of envelopes and assem-blies under design conditions. See Chapters 25 to 27 in the 2013ASHRAE HandbookFundamentals.Infiltra
23、tion is uncontrolled inward air leakage through open,porous materials, cracks, and crevices in any building componentand around windows and doors caused by pressure differences.Exfiltration is uncontrolled outward air leakage through open,porous materials, cracks, and crevices in any building compon
24、entand around windows and doors caused by pressure differences.Wind washing is uncontrolled wind-induced flow of outdoor airin and behind insulation layersAir intrusion is uncontrolled pressure-induced flow of indoorair in and in front of air-permeable insulation layers, caused by windpressures, sta
25、ck effect, or HVAC systems.Convective loop is uncontrolled stack-induced convective flowof cavity air in and around insulation layersThermal insulation is any material specifically designed todecrease heat flow by equivalent conduction through a buildingenvelope or envelope assembly.Moisture content
26、 is the ratio of mass of water to volume of drymaterial in porous and hygroscopic materials, in lb/ft3. Boundwater describes the phase of water bound in hygroscopic materials.Sorption (and desorption) describes the change in phase betweenvapor and bound water.A plenum is a compartment or chamber to
27、which one or moreducts are connected, that forms part of an air distribution system,and is not used for occupancy or storage. A plenum is often underpositive or negative air pressure relative to adjacent spaces.The R-value of a material is the thermal resistance for a giventhickness of that material
28、, as provided by the manufacturer or listedin Table 4 of Chapter 26 in the 2013 ASHRAE HandbookFun-damentals. The system R-value (RS) is the sum of the individualR-values for each material, excluding air films. The total R-value(RT) is the sum of the system R-value and the interior and exteriorair-f
29、ilm resistances (see Chapter 25 in the 2013 ASHRAE Hand-bookFundamentals).A thermal break is a thermally resistive element that decreasesheat conduction through an assembly.A thermal bridge is a thermally conductive element through anotherwise thermally resistive assembly.U-factor or thermal transmi
30、ttance is the rate of heat transferper unit surface of an assembly per unit temperature differencebetween the environments at both sides of the assembly. The clearvalue only considers the surface film resistances and R-values of thematerial layers comprising the assembly. The U-factor is 1/RT. Awhol
31、e-wall or effective U-factor also takes into account thermalbridging, convective loops, wind washing, and indoor air washingeffects (see Chapter 25 in the 2013 ASHRAE HandbookFunda-mentals).A vapor retarder or vapor barrier is any component in abuilding envelope with a low permeance to moisture flow
32、 by dif-fusion.A water-resistive barrier (WRB) is a building envelope com-ponent designed to prevent inward movement of liquid water.2. GOVERNING PRINCIPLESThe building envelope is the key element in managing the envi-ronmental loads on the building. These loads are a function of theclimate and the
33、indoor conditions, such as air temperature, relativehumidity, and air pressure differential. There is a strong interdepen-dence between a building HVAC system and envelope that must beconsidered when designing or modifying a building. This inter-dependence centers on controlling heat flow, airflow (
34、includingcontrol of airborne contaminants), and water and water vapor flow.Design parameters involved are as follows.Design ParametersHeat. The type and amount of insulation to be provided dependson the climate, governing codes, and building use. The insulationshould be continuous, while considering
35、 the limitations of the mate-rials and systems. Discontinuities (or thermal bridges) are the sitesof unwanted heat transfer that reduce energy efficiency which mayresult in premature soiling (e.g., ghosting), surface condensation,and/or mold growth. In heating- and cooling-dominated climates,reduced
36、 thermal performance can affect indoor conditions andincrease HVAC loads. The thermal conductivities and R-values ofinsulating materials allow them to be compared for their effect onheat transfer, though their properties related to air and moisturetransfer vary widely.Air. Some buildings are designe
37、d for natural ventilation whenbuilding use and climate allow, and for mechanical space condition-ing (with ventilation) at other times. During periods of space condi-tioning, the building envelope should show minimal air leakage.This allows better control of (1) HVAC, (2) inflow of airborne con-tami
38、nants in the building, and (3) noise transmission. The HVACsystem can generate pressure differentials across the envelope thatincrease air leakage and may create moisture and thermal problems.It is important to review the interaction of the HVAC system andenvelope at the design stage.Moisture. Build
39、ing envelopes should be designed to shed rain-water, prevent accumulation of moisture in moisture-sensitive mate-rials, and allow draining and drying of water that accumulates. Asecondary means of moisture transport through building envelopesis airflow through openings in the envelopes. Liquid water
40、 and frostcan accumulate on cold materials in a wall assembly along airmovement paths. Vapor diffusion can play a role in wetting andespecially in drying of building materials.Although vapor diffusion control in an envelope assembly isimportant, field experience shows that most moisture problems are
41、associated with bulk water penetration and moisture accumulationcaused by air leakage. Despite the historic and code emphasis onvapor barriers, their effect is often secondary.A beneficial exercise during building envelope design is totrace the continuity of the elements providing thermal, air, andw
42、ater protection to the envelope as assembly details are refined.Continuity of the WRB and the air barrier is essential to their per-formance. Absolute continuity of a vapor barrier is not essential toits performance.Hygrothermal analysis can be used to predict envelope perfor-mance and compare these
43、 results with the requirements. ASHRAEStandard 160 provides guidance for performing a hygrothermaldesign analysis. Inputs for hygrothermal modeling include assem-bly configuration, material properties, initial conditions, and indoorand outdoor climate conditions. Analysis tools generate outputs that
44、may include heat and moisture flux and material moisture content.Because of the number of assumptions and limitations of calcula-tions required to complete an analysis, results should be used forguidance to supplement the designers understanding of envelopeperformance. They should not be considered
45、an absolute predictionof actual hygrothermal performance.Building Envelopes 44.3Other Important Performance CriteriaStrength and Rigidity. The air barrier system must be able towithstand air pressures to which the building will be subjected.These pressures can often be large, and strength is critica
46、l in severe-weather areas such as hurricane zones.Noise. For most occupancies, building envelopes should bedesigned and constructed to reduce noise transfer between condi-tioned and unconditioned spaces. Sound insulation can be particu-larly important near noisy areas such as airports, railways, and
47、highways, especially for occupancies that require indoor quiet (e.g.,hospitals, hotels, residences, theaters).Constructability. During design, how a building envelope willbe built must be considered. If there are practical limitations to howthe envelope elements are physically put together, the desi
48、gn intentwill be lost and problems are likely to develop during construction.Simplicity in design is an effective way to improve the chances ofconstruction in accordance with the design intent. Construction ofthe various components must be sequenced so that all componentscan be correctly assembled,
49、particularly when coordination betweenmultiple trades is required. Investigation of many building failuresdemonstrates that poor sequencing between trades is a commoncontributing factor. Integrating constructability early in projectdesign minimizes the number of failures and helps maximize thepotential to achieve the results described in this chapter. During con-struction review, specific attention should be given to areas of thebuilding where multiple systems are connected and multiple tradesare involved. Use of mockups, design reviews, modeling, and othermethods can enhance constr
