1、2008 ASHRAE 167ABSTRACT In 1996, ASHRAE formed a new Standard ProjectCommittee, ASHRAE 160P, to develop a standard for moisturecontrol in buildings. The draft standard“Criteria for Mois-ture Control Design Analysis in Buildings”intends toprovide performance-based procedures for moisture designanalys
2、is for buildings. The standard sets criteria for moisturedesign loads, moisture analysis methods, and building perfor-mance and applies to the above-grade portions of all types ofbuildings. It can be used for design analysis of the above-gradeportion of the building envelope or help guide specificat
3、ionsfor HVAC equipment and controls. Eventually it should formthe basis for moisture design rules based on a uniform set ofdesign assumptions and loads. This paper describes the ratio-nale behind this standard, its current outline, and its potentialuses.INTRODUCTIONIn April 1996, the American Societ
4、y of Heating, Refrig-erating, and Air-Conditioning Engineers (ASHRAE) createda new Standard Project Committee 160P. The initial title of theStandard Project Committee (SPC) was “Prevention of Mois-ture Damage,” and the SPC was asked to “define the role thatmoisture plays in the degradation of buildi
5、ng materials,components systems and furnishings.” However, the SPCmembers felt that the original scope was too broad and that itthreatened to parallel efforts within other standards organiza-tions, in particular ASTM. The SPC recognized an emergingneed for criteria for moisture design analysis and t
6、hereforeredefined its purpose and scope and changed the title to“Design Criteria for Moisture Control in Buildings.” Inresponse to comments from public review (ASHRAE 2006),the title was recently changed once more, to “Criteria forMoisture Control Design Analysis in Buildings.”The need for criteria
7、for moisture design has becomemore urgent with a rapid improvement in the capabilities ofcomputer-based moisture analysis tools that predict move-ment and accumulation of moisture in building componentsand materials. Although analytical tools are becoming moresophisticated and accurate, relatively l
8、ittle attention has beenpaid to appropriate inputs and boundary conditions. Whenusing these tools to analyze a failure of an existing buildingthis poses no problem, because only the data for conditionsduring the period before the failure are needed. Obtainingaccurate and sufficient data in such a fo
9、rensic application isoften difficult enough. However, the choice of appropriateinput values is even more uncertain when the analysis is usedfor design purposes before the building has been built andbefore actual loads can be measured.Another reason for creating this standard is that currentrecommend
10、ations and rules for moisture control are not basedon a set of consistent underlying assumptions. This has attimes led to largely pointless discussions about the need forvarious design features, because the need often depends onwhat indoor or outdoor conditions are assumed. For instance,a computer a
11、nalysis by Tsongas et al. (1995) of moisture accu-mulation in a wood frame wall in Madison, Wisconsin,showed that the need for including a vapor retarder in thedesign completely depended on the selection of indoor humid-ity. Thus the choice of input values for a design analysis is crit-ical. Whether
12、 a design analysis will show acceptable orunacceptable performance of a particular design largelydepends on the magnitude of the design loads operating on thebuilding. Although it is difficult to imagine structural designASHRAE Standard 160PCriteria forMoisture Control Design Analysis in BuildingsAn
13、ton TenWoldeMember ASHRAEAnton TenWolde is a research physicist in the Forest Products Laboratory at USDA Forest Service, Madison, WI.NY-08-0222008, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 114, Part 1.
14、 For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.168 ASHRAE Transactionsdecisions without knowledge of appropriate structural designloads, until now we have in fact made moisture c
15、ontrol deci-sions without defining “moisture design loads.”TenWolde (2001) showed how the use of a moisturedesign standard such as 160P might have alerted manufac-tured-home builders to the potential of widespread decay ofplywood sheathing that occurred in the mid-1980s in a groupof manufactured hom
16、es in the Midwest (Merrill and TenWolde1989). The article (TenWolde 2001) also shows how use of thestandard could have led to the solution and prevention of prob-lems that occurred, and could have circumvented a lot of thedisagreements, and perhaps litigation, that took place follow-ing the discover
17、y of the building failures.In summary, the standard is intended to bring moisturedesign out of the realm of purely prescriptive measures andturn building moisture design into a performance-based proce-dure, with the potential for greater flexibility and a better abil-ity to incorporate new designs a
18、nd building materials. Inaddition to uniformity of design assumptions, the standardalso seeks to make the moisture design analysis proceduremore transparent by requiring documentation of the assump-tions, material properties used, and other choices made for theanalysis. The standard is attempting to
19、 strike a balancebetween simplicity of use and the actual complexities of thebuilding environment, building performance, and design. Thishas led to many simplifications and sometimes parallelcompliance pathways for the designer. The SPC is aware thatother technically supportable choices could have b
20、een made inthe standard. This is why it is important that the standard isfollowing the consensus procedure required by ANSI.Finally, the SPC also found that sufficient informationwas not always available to provide a solid technical basis forthe criteria. In those cases, the criteria are based on ou
21、r bestprofessional judgment, with the expectation that they will bechanged if technical information becomes available support-ing different criteria or procedures. Since ASHRAE has long been a leader in providing tech-nical guidance for moisture control in buildings, we felt that aconsensus standard
22、 procedure through ASHRAE provided themost logical vehicle for these needed moisture design criteria.PURPOSE AND SCOPEThe purpose statement of ASHRAE Standard 160P readsas follows: Given the role that moisture plays in the degrada-tion of building envelope materials, components, systems andfurnishin
23、gs, the purpose of this standard is to specify perfor-mance-based design methods for predicting, preventing, miti-gating or reducing moisture damage depending on climate,construction type and system operation. These methodsinclude (a) criteria for selecting analytic procedures, (b)design input value
24、s, and (c) criteria for evaluation and use ofoutputs.Thus the standard specifies the minimum attributes foranalytical procedures, depending on the building design andother parameters. For instance, if the construction is a brickwall, the analytical procedure should be able to handle waterabsorption
25、and redistribution in the brick. In contrast, in theunlikely event that the construction does not contain hygro-scopic materials and is airtight, a simple vapor diffusion anal-ysis may be sufficient. The standard also defines design inputvalues, or “design moisture loads,” primarily by prescribingde
26、fault values in case the designer does not have actual designspecifications. This includes interior as well as exterior (suchas rain, humidity) loads. Finally, the standard describes howthe results from the analysis should be interpreted. It providescriteria to determine if the building component is
27、 likely toperform satisfactory or not.Standard 160P is intended to apply to new buildings, addi-tions, or retrofit and renovation of existing buildings. Itincludes all types of buildings, building components, andmaterials. An important limitation in the scope is that the stan-dard will not address e
28、xtraordinary loads due to gross waterleakage from sources such as rain water, ground water, flood-ing, plumbing leaks, or ice dams. The SPC believes thatmassive intrusion of rain water should be prevented by properdetailing and flashings, and intrusion of ground water byproper site grading and drain
29、age. However, the committee alsorealized that occasional intrusion of a small amount of water,especially around doors and windows, is probably inevitable,even in a building with proper detailing and flashing. Theseloads may still be significant, and the standard thereforeincludes small rain penetrat
30、ion loads that the building will beexpected to be able to accommodate.The standard is most easily used for the design of theabove-grade portion of the building envelope. Because noanalytical procedures are yet available for reasonably accuratemoisture analysis of below-grade building elements, it is
31、 notyet feasible to apply the standard to such building elements.Below-grade moisture loads are also far more variable andcomplicated, and quantitative data are very scarce. The perfor-mance of the building design is evaluated under design loadsusing the performance criteria in the standard. If the
32、perfor-mance is unacceptable, the design should be changed andreevaluated. Another viable option is to use the standard todesign the HVAC equipment and controls. The indoor designloads are manipulated by varying the design of HVAC equip-ment and controls, and the design analysis is used to evaluatet
33、he building performance in response to this manipulation. In the following section, an outline of the content of thedraft standard is given. Some of this information may changeduring the second public review and finalization of the stan-dard. Some of the following also reflects the opinion of theaut
34、hor, which may or may not be shared by all SPC members.DESIGN LOADSIdeally a design analysis involves determining the prob-ability of failure, treating all design parameters and loads asstochastic variables (Geving 1997). However, sufficient dataare usually not available to make a full statistical t
35、reatmentpractical. Instead, a moisture design protocol will have to beASHRAE Transactions 169based on a combination of statistical data and professionaljudgment where only limited data exist. Another judgmentinvolves the choice of an acceptable probability of the occur-rence of damage. Although impo
36、sing very stringent criteriafor structural design is common because of safety concerns,moisture damage usually occurs over a long period of time andusually has less catastrophic, although sometimes costly,consequences. An international consensus has emerged that a10% likelihood of failure is an appr
37、opriate level in buildingmoisture design analysis, and this standard has adopted thisapproach. To the extent possible, the loads prescribed in thestandard are based on this 10% exceedance approach.Design Initial Moisture ConditionsSome building materials, such as concrete, wet-spraycellulose insulat
38、ion, and wood, may contain large amounts ofwater at the time of building enclosure. This moisture is oftencalled construction moisture. A design analysis shouldaccount for this initial moisture load by assuming high initialmoisture contents for those materials, unless specific planshave been include
39、d in the construction cycle to dissipate thismoisture or to prevent this moisture from accumulating in thematerials through proper storage and protection from rain andflooding during construction. If such measures are included inthe design and construction plans, the initial conditions to beused, ac
40、cording to Standard 160P, are the equilibrium mois-ture content (EMC) of each material at 80% relative humidity(rh). This moisture content is named the EMC80 in the stan-dard. This level was chosen because the standard defines thisas the highest possible moisture level that does not lead tomold grow
41、th (see Performance Criteria section), therebyallowing for a reasonably high, but not destructively highmoisture content. As discussed in the section on PerformanceCriteria, there is no guarantee that no mold will be present atlower moisture contents, but the SPC needed to strike a reason-able balan
42、ce between accuracy and simplicity. The designinitial moisture content of concrete is EMC90 (EMC at 90%rh) if specific care is taken to limit initial moisture conditions.If no such measures are planned, the design moisture contentsmust be doubled (i.e., 2 EMC90 for concrete and 2 EMC80for all other
43、materials). The factor of 2 was chosen somewhatarbitrarily, but no published quantitative data are available onconstruction moisture, actual moisture contents are likely tobe extremely variable, and the SPC felt strongly that someform of accountability for construction moisture and dryoutprocedures
44、should be included.Internal LoadsIn a moisture analysis for building envelope design, thechoice of indoor environmental conditions is extremelyimportant, especially for buildings in cold climates. This stan-dard opts for a design indoor climate definition that is based onengineering principles and r
45、eflects the influence of ventilationand air-conditioning equipment and controls that may or maynot be part of the building design. In buildings where indoorhumidity and temperature are explicitly controlled, the build-ing envelope performance should be evaluated with theintended indoor design condit
46、ions. In residential buildings,indoor humidity is rarely explicitly controlled, and defaultdesign assumptions are needed for these buildings. In general,the standard encourages designers to use their own designparameter values if they are known and part of the design. Ifthey are unknown or not inclu
47、ded in the design, the standardprovides default values for those loads and parameters.Internal moisture design conditions include temperature,humidity, and air pressures. In case design values are availablefrom HVAC or other design specifications, they should beused. If design values are not availab
48、le, the standard prescribesdefault design values.Design Indoor Temperature. If the operating indoortemperature is specified, or the design temperature isprescribed by code or law, that temperature should be used fora design moisture analysis. If the indoor temperature is notspecified, the default in
49、door design temperature is 21.1C(70F) during heating and 23.9C (75F) for cooling, if air-conditioning equipment is included in the design. Heating isassumed to take place when the 24-h running average fallsbelow 18.3C (65F), and cooling when the running averagerises above 21.1C (70F), if air-conditioning equipment isincluded in the design. When the 24-h running averageoutdoor temperature falls between 18.3C (65F) and 21.1C(70F), the default indoor temperature is assumed to “float” at2.8C (5F) above the 24-h running average outdoor temper-ature. If no air-conditioning eq
