ASHRAE FUNDAMENTALS SI CH 36-2017 MOISTURE MANAGEMENT IN BUILDINGS.pdf

1、36.1CHAPTER 36MOISTURE MANAGEMENT IN BUILDINGSEffects of Humidity and Dampness 36.1Elements of Moisture Management . 36.1Envelope and HVAC Interactions 36.2Indoor Wetting and Drying 36.2Vapor Release Related to Building Use. 36.4Indoor/Outdoor Vapor Pressure Difference Analysis 36.6Avoiding Moisture

2、 Problems 36.10Climate-Specific Moisture Management 36.11Moisture Management in Other Handbook Chapters . 36.12HE TERM moisture encompasses the gaseous, liquid, andTsolid states of water and any dissolved contaminants. Examplesof liquid moisture include precipitation, wind-driven rain, construc-tion

3、 moisture, rising damp, and water from incidental pipe drip-pings. Precipitation wets pitched roofs, low-slope roofs, and inclinedfacades, whereas wind-driven rain wets the enclosure as a whole.Buildings, including the envelope, start their service life containingsignificant quantities of constructi

4、on moisture. This is particularlytrue for concrete, aerated concrete, mortar, and plaster. Groundwaterand rain sinks may lead to rising damp, and pipe leakage reflects badworkmanship, lack of maintenance, failing fixtures, or pipe corro-sion (Hens 2016; Mumovic and Santamouris 2009; Trechsel 1994).T

5、his chapter presents data on indoor vapor release and measuredindoor/outdoor vapor pressure or vapor concentration differencesnot included elsewhere in the ASHRAE Handbook, and discussesmoisture sources and sinks that can reduce materials durability, aswell as the negative effects of insufficient or

6、 excessive indoor rela-tive humidity.Gaseous water (vapor) comes from outdoor humidity and frominterior vapor releases. Water vapor in the air, expressed as relativehumidity, governs hygroscopic loading of materials. High relativehumidity values at surfaces favor mold growth and, if reaching100%, ca

7、n lead to surface condensation. When vapor pressure andtemperature gradients in and across assemblies point in the samedirection, interstitial condensation is possible.Chapter 25 contains an in-depth analysis of heat, air, and moistureloads; Chapter 15 discusses surface condensation on windows; andC

8、hapter 16 covers interstitial condensation caused by air leakage.Excessive indoor moisture and humidity interferes with the useand enjoyment of buildings and may shorten their useful life over thelong term. Problems affecting owners and occupants include reducedcomfort, poor indoor air quality, nega

9、tive health effects, damage tothe buildings materials and structural fasteners and wasted energy inHVAC operation. Consequently, moisture management demandsattention from the architect, the builder, the mechanical systemdesigner, and those charged with budgeting, management and main-tenance of the b

10、uilding and its mechanical systems.All buildings experience occasional extremes in relative humidityand moisture. Short-term occurrences of these extremes can gener-ally be accommodated by storage in the building materials, but whenmoisture and humidity accumulate for extended periods in vulnerablem

11、aterials, major problems can and often do occur. Moisture problemsare unfortunately quite common in buildings. It is the responsibilityof those in a position of authority to reasonably reduce the risks asso-ciated with excessive moisture accumulation. Successful manage-ment of moisture and humidity

12、requires understanding the complexand dynamic relationship between the buildings enclosure, its fabric,and the mechanical systems over the entire life of that building. Thisdynamic interaction holds the potential for either an excellent resultover decades, or for frequent, disruptive, and expensive

13、problems.Experience suggests that human behavior can overcome virtually anybuilding technology, so owner and occupant education are importantelements in the successful design and usage of a building.1. EFFECTS OF HUMIDITY AND DAMPNESSMoisture tolerance and appropriate indoor relative humidity levels

14、must be considered requirements for a sustainable built environment:relative humidity affects comfort, indoor air quality, and health, andexcessive wetness can shorten the service life of materials and assem-blies. The preferred relative humidity range for human health andcomfort is between 40 and 6

15、0%, although that interval is often broad-ened from 30 to 70%. High relative humidity degrades thermal com-fort once the operative temperature passes 25 to 27C, making theenvironment feel oppressive. It also facilitates release of volatileorganic compounds (VOCs), especially of formaldehyde, thusdeg

16、rading indoor air quality and triggering olfactory dissatisfaction.Finally, high relative humidity in specific environments (e.g., beds,on surfaces) activates dust mite reproduction and related allergy risks,and can activate mold germination and growth (ASHRAE 2012; IEA-EBC 1990a, 1990b).Very low re

17、lative humidity activates electrostatic discharge andleads to complaints of dry mucous membranes (e.g., nose, lips,throat) and eyes, especially by people wearing contact lenses. On theother hand, some respiratory ailments can be relieved by dry envi-ronments.Excessively low or high relative humidity

18、 creates conditionsfavorable to bacterial and viral infections, allergic rhinitis, andasthma. Chapter 9 gives a more in-depth analysis of the impact ofhumidity on thermal comfort. Chapter 10 discusses the effects of rel-ative humidity on indoor environmental quality, Chapter 11 coversmold, and Chapt

19、er 12 discusses the relationship between relativehumidity and olfactory perception. Additional information is avail-able in Holm (2008).Prolonged and excessive relative humidity and wetness candegrade materials physically, chemically, and biologically. Examplesof physical degradation are frost damag

20、e and salt attack. Chemicaldegradation includes lime/gypsum reaction, carbonization of con-crete, alkali/granulates reaction in concrete, and corrosion of ferrousand nonferrous metals, where moisture determines whether damagewill occur in the presence of corrosive agents (e.g., sulfide, aceticacid).

21、 Wood rot by fungi and bacteria is an example of biological deg-radation. Very dry conditions also can damage wood, causing crack-ing and warping. Fluctuations between extremes of high and lowrelative humidity can induce cracking in hygroscopic materials.2. ELEMENTS OF MOISTURE MANAGEMENTDesigning f

22、or moisture and humidity management includes thechoice of building materials and the layering of the envelope as wellas the design and component selection of the HVAC system. Forinformation on the building envelope, refer to Chapter 15; for detailson building assemblies, see Chapters 25, 26, and 27.

23、The preparation of this chapter is assigned to TC 1.12, Moisture Manage-ment in Buildings.36.2 2017 ASHRAE HandbookFundamentals (SI)The largest contributors of liquid moisture are water from wind-driven rain in building envelopes with insufficient drainage, leaksfrom roof or gutters, and leaks from

24、internal plumbing. Thesesources must be addressed and resolved for the building envelope tosucceed.Once liquid water is addressed, the next factors to consider arevapor pressure and relative humidity. The driving forces responsiblefor water vapor movement within buildings and across the envelopeare

25、air pressure differences that move air and vapor together, andvapor pressure differences that activate vapor diffusion. Tempera-ture-, wind-, and fan-induced air-pressure differentials typicallyoverwhelm diffusion in terms of total vapor flux displaced. How-ever, in hot, humid climates or in buildin

26、gs with high indoor/out-door vapor pressure differences, pure diffusion may neverthelesscause problems. Chapters 16 and 20 contain information on air movement inbuildings. Chapter 13 gives information on intrazone airflow, mul-tizone network airflow, and contaminant transport, included vapor;and Cha

27、pter 24 discusses airflow around buildings. ANSI/ASHRAE Standard 160-2009 provides criteria to evaluate the tran-sient hygrothermal performance of envelopes. This analysis may beused to evaluate moisture tolerance in cases where, besides vapor,liquid water is a primary factor.3. ENVELOPE AND HVAC IN

28、TERACTIONSAs shown by Figure 1, heat, air, and moisture move continuouslythroughout a building, driven by differences in temperature, vaporpressure, and air pressure between the indoor and outdoor environ-ment and by similar differences between adjacent indoor zones.Construction moisture contributes

29、 to the initial wetness of the wholebuilding fabric, including the envelope. Over time, envelope assem-blies exposed to rain and snow may become moist by seeping rainand melting snow or by capillary suction (rising damp) from moistbelow-grade earth, which may dampen walls just above grade.Leaking an

30、d dripping interior pipes can also be responsible forexcessive wetness.The opaque and transparent portions of the building envelopemust be designed so that their thermal transmittance approaches theenergy related economic optimum. Together with the overall build-ing fabric, the envelope should allow

31、 passive solar control, while allassemblies proposed must prevent rain from wetting layers thatmust stay dry. The building details should withhold rainwater run-off and prevent sinking rainwater from wetting basements andfloors on grade. Correct protective measures must exclude risingdamp and promot

32、e effective construction moisture drying; watervapor moving across the envelope assemblies should not result inunacceptable interstitial moisture accumulation. Detailing andworkmanship must minimize air leakage across the envelope as wellas wind washing, indoor air washing, and air looping within en

33、ve-lope assemblies. Correct design, workmanship, and maintenanceshould prevent plumbing leakages or sweating of pipes running inor across walls and floors.The HVAC system must provide a thermally comfortable andhealthy indoor environment. Ventilation is necessary for deliveringfresh air to building

34、occupants. To maintain a set-point temperaturewhen heat losses by transmission, infiltration, and ventilationexceed solar gains through the transparent and opaque envelopeparts and internal gains by lighting, appliances and occupancy, heat-ing is required. If, instead, the gains exceed the losses, c

35、ooling isneeded. In both cases, the temperature difference with outdoors cre-ates a thermal stack effect, which together with wind and fan oper-ation maintains air pressure differentials with the exterior.In heating mode, the indoor vapor concentration is typically leftfree floating, fluctuating wit

36、h the vapor in the ventilation and infil-tration air and the vapor released indoors. The net vapor concentra-tion and the air temperature maintained by the HVAC system thendetermine the relative humidity indoors. Only when the ventilationand infiltrating air is too dry or too humid, when the vapor r

37、eleaseindoors is very high, or when the buildings function requires rela-tive humidity control, must the HVAC system actively intervene tocontrol humidity by removing water vapor from or adding water va-por to the indoor air (dehumidification and humidification). The en-ergy required to humidify or

38、dehumidify is called the latent heatload. In cooling mode, the latent load derives from the supply airtemperature required for cooling, the dew-point temperature of theoutdoor air, and the vapor released indoors. Condensation on the coilremoves some portion of the moisture in the incoming ventilatio

39、n air.4. INDOOR WETTING AND DRYINGHVAC design or a durability assessment of the envelope and thewhole building during design requires knowledge of the expectedindoor humidity conditions. This is essential for making the rightdecisions to prevent mold, surface condensation, problematic inter-stitial

40、condensation, and reduced drying.The function of a building and the conditions outdoors dictatewhether the indoor relative humidity needs control. The vapor bal-ance, which considers the various mechanisms involved in vaporrelease, vapor removal, and vapor storage, is fundamental for thisdecision. O

41、ne of the unknowns in this balance is the vapor releaserelated to building use. A first approach consists of quantifyingthese releases directly. A second approach to get information startsfrom extended indoor/outdoor climate measurements in large sets ofbuildings to deduce a statistically relevant i

42、ndoor-to-outdoor time-averaged difference in vapor pressure or vapor concentration in rela-tion to the outdoor air temperature. Both methods are discussed inthe following.Understanding Vapor BalanceThe air humidity indoors is assumed to be free floating. Five va-por fluxes then contribute to maintai

43、ning the equilibrium in a room:Vapor carried and supplied by infiltrating outdoor air and ventila-tionVapor removed by exfiltrating indoor air and exhaust airVapor released by occupants and their activities, by plants, watersurfaces, and drying fabric partsFig. 1 Dynamic Interaction Between Air, Moi

44、sture, and Materials in HVAC Systems and Building EnvelopeMoisture Management in Buildings 36.3Vapor adsorbed and desorbed by all hygroscopic surfaces presentVapor condensing on indoor surfaces that are colder than the dew-point temperature indoors or evaporating from indoor surfaceswetted by surfac

45、e condensationThese five fluxes load and unload the room air with vapor. Thereis even a sixth flux: vapor inflow and outflow by diffusion throughthe envelope. Its magnitude, however, is insignificant compared tothe five fluxes mentioned, hence ignoring it does not introduce anysignificant error.Assu

46、ming ideal air mixing, the balance per room is expressed by(1)whereGa,in= all airflows entering room, including infiltration from outdoors, ventilation supply air, and infiltration from adjacent rooms, kg/sxv,in= ratio of water vapor to dry air in entering air, kg/kgGa,out= all airflows leaving room

47、, including exfiltration to outdoors, exhaust air, and exfiltration to adjacent rooms, kg/sxv,i= ratio of water vapor to dry air in room, kg/kgj= surface film coefficient for diffusion at each hygroscopic surface, kg/(m2sPa)Aj= area of each hygroscopic surface in room, m2pi= water vapor pressure in

48、room, Papsat,Aj= vapor saturation pressure at different hygroscopic surfaces in room (fabric, furniture, and furnishings), Pasat,Aj= relative humidity at different hygroscopic surfaces (fabric, furniture, and furnishings) in room on a scale from 0 to 1k= surface film coefficient for diffusion at all

49、 room surfaces where vapor condenses or evaporates, kg/(m2sPa)Aj= area of all room surfaces experiencing surface condensation or surface condensate drying, m2psat,Aj= vapor saturation pressure for rooms various condensing and evaporating surfaces, PaGv,P= vapor release in room, kg/sV = air volume of room, m3t= time, sSolving Equation (1) requires knowledge of all in- and exfiltrat-ing airflows between the room and all adjacent rooms and betweenthe room and the outdoors, the ventilation supply airflow, exhaustairflow, water vapor ratio transported by all four airflows, surfacetemp