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by David MacPhaul, P.E. and Christy Etter, P.E. CH2M Hill Last updated: 07-06-2010

Just mo nths after o ccupying their new, multimillio n-do llar municipal building, emplo yees o f a Flo rida co unty began co mplaining o f chro nic sinus pro blems, allergy attacks, headaches, and asthma—classic signs o f sick building syndro me and building-related illness. The architects, engineers, and micro bio lo gists tasked with finding the cause o f these sympto ms identified a pro blem that is beco ming widespread natio nwide—severe fungal co ntaminatio n o f the building.

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Moisture Management

INT RODUCT ION

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HVAC System Design for Humid Climates

Within This Page Introduction Description Application Emerging Issues Relevant Codes and Standards

Indoor Air Quality and Mold Prevention of the Building Envelope Mold and Moisture Dynamics Mold Remediation Guidelines View Resource Page Index

Additional Resources

The mo ld was the direct result o f excess mo isture in the building, which was caused by a co mbinatio n o f rainwater leaks and a heating, ventilating, and air-co nditio ning (HVAC) system that pulled mo ist o utside air into the building during the ho urs when the co o ling system had cycled o ff. Once PDFmyURL.com

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cycled o ff. Once the HVAC system Figure 1. Although brand new, this municipal became infected building was evacuated shortly after it with mo ld, it opened because occupants were reporting dispersed spo res health complaints. Mold and moisture were thro ugho ut the the culprits, and in the end, the problem will building. So , o nly require more than $20 million to repair. a few years after o pening its do o rs, the building underwent a majo r o verhaul. The building's exterio r was remo ved to help co rrect the pro blems that allo wed rainwater to invade the building envelo pe (Figure 1). The ro o f and the HVAC system were also extensively mo dified. Ultimately, repairs and o ther asso ciated co sts exceeded $20 millio n.

Unfo rtunately, the pro blem faced by this Flo rida co unty is no t an iso lated o ne. Rainwater leaks o ccur in every climate, and in this case study, the leaks alo ne wo uld pro bably have led to significant micro bial co ntaminatio n and building evacuatio n. But bo th architects and engineers must understand the interactio n between the building envelo pe and the HVAC system to manage mo isture intrusio n in buildings.

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DESCRIPT ION To avo id the types o f pro blems seen at the Flo rida municipal building, engineers and architects must wo rk to gether to manage mo isture. First, the building designer must understand the basic causes o f mo isture intrusio n into buildings: Rainwater intrusion. Mo isture present in building materials and o n the site during co nstructio n can be a so urce o f pro blems. Significant amo unts o f mo isture can also result fro m water leaks within building systems o r thro ugh the building envelo pe. In bo th ho t, humid and temperate climates, rainwater leaks are a majo r so urce o f building mo isture and fungal gro wth pro blems.

Infiltration of outside moisture-laden air. Infiltrated humid air, whether intro duced by wind o r thro ugh the HVAC system, can cause co ndensatio n o n interio r surfaces, including inside building cavities. Co ndensatio n and high relative humidity levels are impo rtant facto rs in creating an enviro nment co nducive to mo ld gro wth and are the primary pro blems in ho t, humid climates. The issue o f infiltratio n caused by negative pressure o f the building created

by HVAC systems is detailed in HVAC Design and Co nstructio n in Humid Climates.

Internally generated moisture. After co nstructio n, o ccupant activities and ro utine ho usekeeping pro cedures can generate additio nal mo isture, co ntributing to the mo ld pro blem. No rmally, if no o ther significant so urces exist, well-designed and pro perly o perating HVAC systems can adequately remo ve this mo isture.

Vapor diffusion through the building envelope. Differential vapo r pressure, which can cause water vapo r to diffuse thro ugh the building envelo pe, is a less significant cause o f mo isture pro blems in buildings in hit humid climates. Ho wever, it can be a significant mo isture PDFmyURL.com

pro blems in buildings in hit humid climates. Ho wever, it can be a significant mo isture mo vement mechanism, particularly in co ld climates, and especially as it relates to wall system vapo r retarder co nstructio n.

In ho t, humid climates, the interrelatio nship between the building envelo pe and the building HVAC system is especially critical. Many mo isture and mo ld-related pro blems in humid climates are o ften misdiagno sed as either exclusively envelo pe- o r HVAC-related, because the co mplex relatio nship that exists between bo th systems is no t always clearly understo o d.

Mo isture-related pro blems can be avo ided if the building envelo pe do es the fo llo wing: Adequately retards mo isture o r air mo vement into the building Allo ws any accumulated mo isture to either drain to the exterio r o r evapo rate In ho t, humid climates, the air barrier and vapo r retarder in the building envelo pe must be adequate to co ntro l air and mo isture flo w thro ugh the wall system. This means that any air barrier o r vapo r retarder placed within the wall system must have the pro per air resistance o r mo isture permeability and must be installed at the co rrect lo catio n within the walls. The presence o f multiple vapo r retarders within a wall system is a co mmo n pro blem, because many designers do no t reco gnize many co nstructio n materials as effective barriers. Fo r example, plywo o d is a relatively lo w-permeability material that can functio n as a vapo r retarder.

The po int where co o l surfaces meet warm, mo ist air is where co ndensatio n and excess mo isture can o ccur. If mo isture-laden o utside air is retarded befo re it meets the first co o l surface inside the building envelo pe (o ften called the "first plane o f co ndensatio n"), then few pro blems will result. If this mo isture is allo wed to further enter a wall system, it will co ndense. Then, mo isture and mo ld gro wth pro blems can be a real threat. If the co o l surfaces and mo ist air meet within the building space, then mo isture pro blems can o ccur thro ugho ut the building, resulting in widespread mo ld o do rs and co mplaints fro m o ccupants. Thus, the building envelo pe plays a vital ro le in minimizing unco ntro lled mo isture and air mo vement into a building and in preventing mo isture entrapment within the wall system.

Co nfusio n still exists within the design co mmunity abo ut several critical issues related to envelo pe perfo rmance. These issues include the integrity requirements o f air barriers, weather barriers, and vapo r retarders; the way all three barriers/retarders can be inco rpo rated into o ne membrane; the lo catio n o f these features within the building envelo pe; the effects o f using multiple vapo r retarders; and even the need fo r air barriers and vapo r retarders in every facility.

This co nfusio n abo ut design, co nstructio n, and o peratio nal practices between humid and no nhumid climates acco unts fo r many mo isture and mo ld gro wth pro blems. ASHRAE Fundamentals (20 0 9 ) cautio ns that different climates present different pro blems, and buildings sho uld be designed and o perated acco rdingly.

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APPLICAT ION During the design phase, especially early in design, many lo w o r no -co st decisio ns can be made PDFmyURL.com

regarding the HVAC and envelo pe systems that will have a significant impact o n mo isture management. Figure 2 summarizes the mo isture co ntro l co nsideratio ns typically asso ciated with the schematic design phase. Altho ugh the respo nsibility fo r addressing the co nsideratio ns can be divided acco rding to architectural and mechanical functio ns, perso nnel in bo th disciplines sho uld wo rk to gether clo sely to prevent future pro blems. Effective interactio n amo ng design team members is crucial in creating a pro blem-free design.

Figure 2 highlights so me typical design issues that must be co nsidered by the design team during the schematic design phase and sho ws the relatio nship between the architectural and mechanical aspects o f the design.

Figure 2. These issues must be considered in the schematic design phase.

While it is kno wn that so me design decisio ns will inevitably create a greater risk o f mo isture intrusio n, the extent o f a pro blem with mo isture o r mo ld is determined by o ther less extensive decisio ns made after the fundamental design cho ices.

Archit ect ural Considerat ions While no detailed designs are co mpleted during the schematic design phase, decisio ns are made that fo rm the basis o f designs develo ped during the next phase (Design Develo pment, Sectio n 3). Available design handbo o ks fo r humid, rainy, o r co ld climates may no t pro vide all the info rmatio n necessary to co mplete co mprehensive co nstructio n designs. The architectural design team, therefo re, must use so und judgment in selecting the building envelo pe system during schematic design, including the weather and air barriers and the vapo r retarder (Figure 3).

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Figure 3. In hot, humid climates, the design, location, and installation of the air and weather barriers are more critical than for the vapor retarder.

Note: The vapor retarder location specified above is specifically for hot, humid climates. In cold climates, the location of the retarder is to the interior side of the thermal insulation.

Because all o f the po ssible mo isture-related issues in new co nstructio n are no t always immediately apparent to an architect, design issues relating to the architectural aspects o f co nstructio n must be addressed by the entire design team. Fo r example, interio r finishes are o ften selected simply fo r aesthetic appeal, initial co st, o r ease o f maintenance. Ho wever, the permeability o f the interio r finish (indicated by permeance rating) can greatly affect the mo isture and mo ld po tential o f a design, depending o n the type o f HVAC system being co nsidered.

Therefo re, the mechanical engineer and the architectural design team members sho uld all have input when selecting a wall system.

Vapor Dif f usion Vapo r diffusio n po tential is a functio n o f the vapo r pressure differential acro ss the building envelo pe (Figure 4). Ho t, mo ist air has a higher pressure than co o l, dry air. A large amo unt o f vapo r pressure results fro m a high mo isture co ntent. The vapo r pressure at any mo isture co ntent is equal to the sum o f all individual vapo r mo lecule pressures. A large amo unt o f water vapo r creates co nsiderable fo rce; in fact, in so me instances the pressure difference can be great eno ugh to blister and peel paint o n exterio r siding as mo isture in the wo o d o r maso nry is drawn o ut. Vapo r diffuses thro ugh walls at a rate pro po rtio nal to the vapo r pressure difference. If o ne side o f a wall is much drier than the o ther side, the vapo r will diffuse faster (The Dehumidification Handbook, 19 9 0 ).

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Figure 4. Vapor diffuses through a wall at a rate proportional to the vapor pressure difference across the wall.

Vapo r diffusio n issues tend to be greatest in co ld climates, where even small amo unts o f internally generated mo isture will co ndense inside co ld wall cavities during winter mo nths. In tho se climates, lo cating a vapo r barrier o n the interio r (the warm side o f the wall) is required. In ho t humid climates, the vapo r diffusio n mechanism do es no t typically induce significant mo isture into a building, particularly in co nventio nally air-co nditio ned co mmercial buildings with mo derate temperature co nditio ns. Ho wever, in buildings with co lder than no rmal temperatures, fo r example, ho spital o perating ro o ms, vapo r diffusio n and co ndensatio n can still o ccur.

Air Leakage No building is hermetically sealed. That is, all buildings have so me degree o f air leakage o penings inherent in the envelo pe co nstructio n, and this leakage carries a certain amo unt o f mo isture with it into , o r o ut o f, the building (Figure 5). Altho ugh this leakage can typically be o verco me with go o d po sitive pressurizatio n, a tightly sealed building envelo pe will minimize air leakage and reduce the amo unt o f air required by the HVAC system to achieve go o d pressurizatio n. Mo isture co ntributed by air leakage is a significant so urce and sho uld be a serio us co ncern in the design o f the wall system. In fact, the design o f the building envelo pe fo r minimizing air leakage is mo re critical than the design o f the vapo r barrier.

To illustrate this po int, co nsider that the amo unt o f mo isture co ntributed to a building by the air that flo ws thro ugh a crack 1/16 th inch thick by 1 fo o t lo ng is just o ver 5 pints per day in a light PDFmyURL.com

breeze. In co ntrast, the amo unt o f mo isture co ntributed by vapo r diffusio n thro ugh a 10 fo o t by 50 -fo o t painted blo ck wall o ver the same perio d equals just under 1/3 o f a pint (abo ut 5 o unces).

The mo st critical areas o f envelo pe air leakage are gaps aro und windo ws and do o rs; jo int o penings at ro o f, ceiling, o r flo o r lines; and perhaps the greatest co ntributo r, the intentio nal installatio n o f so ffit o r wall vent systems. These areas pro vide the mo st likely o penings in a building envelo pe and are co nvenient pathways fo r air leakage and mo isture intrusio n into the building.

Figure 5. Air leakage into the building can be affected by typical building envelope penetrations.

Rainwat er Leakage In additio n to mo isture entering the building via vapo r diffusio n o r air leakage, mo isture as rainwater can be drawn into a building by gravity, capillary actio n, surface tensio n, air pressure differentials, o r wind lo ads. The building envelo pe (exterio r walls and ro o fing) acts as the interface between the interio r and exterio r o f buildings. To avo id mo isture pro blems in the extreme weather co nditio ns, building envelo pe design must co ntro l water fro m all o f these facto rs.

Weather-related mo isture includes water intrusio n fro m rainwater and gro undwater. Rainwater and gro undwater intrusio n mo st severely affect the building envelo pe. Rainwater rarely affects HVAC systems o r building interio rs to the degree required to cause widespread building mo isture pro blems. Water co ncentrates aro und windo w and do o r penetratio ns, the ro o f line and co nstructio n jo ints, and the base o f exterio r walls.

The fo llo wing fo rces are mo st co mmo nly applied to the building envelo pe: Gravity. The fo rce o f water entering by gravity is greatest o n impro perly slo ped ho rizo ntal surfaces and vertical surfaces with penetratio ns. These areas must remo ve water fro m envelo pe surfaces thro ugh adequate slo ping, co rrect drainage, and pro per flashing.

Capillary action. This is the natural upward wicking fo rce that can draw water fro m o ne so urce up into the envelo pe cavity. This o ccurs primarily at the base o f exterio r walls.

Building co mpo nents that canno t withstand a large amo unt o f water expo sure, such as plywo o d o r gypsum bo ard, can create enviro nments co nducive to micro bial gro wth and/o r co mpo nent failure.

Surface tension. This allo ws water to adhere to and travel alo ng the underside o f building co mpo nents such as jo ints and windo w heads. This water can be drawn into the building by gravity o r unequal air pressures.

Air pressure differentials. In ho t, humid climates if air pressures are lo wer inside the PDFmyURL.com

structure than o utside the structure, water can be "driven" fro m the exterio r to the interio r o f the building thro ugh micro sco pic ho les in the building materials.

Wind loading. Wind lo ading during heavy rainsto rms can fo rce water inside the building if the envelo pe is no t resistant to these fo rces. Fo r example, windo w sealants and gaskets that are no t pro perly designed to flex with the windo w may create air gaps that can allo w water into the building.

Wall Syst em Component s Mo st wall systems used in new co nstructio n are framed wall systems, po ured-in-place co ncrete, o r maso nry wall systems (co ncrete blo ck o r brick).

Framed wall systems co nsist o f an interio r wall finish system and an exterio r wall finish system, separated by an air space (o r cavity). The cavity, which no rmally includes insulatio n material fo r added thermal resistance, pro vides a po tential path fo r the mo vement o f mo isture thro ugho ut the wall areas. Sto refro nt wall systems and exterio r insulatio n and finish systems (EIFS) are framed co nstructio n.

A co ncrete o r maso nry wall system is co nstructed o f a structural wall material. If internal and external finishes are applied directly to the surface o f the structural wall, air mo vement within the wall is restricted. Ho wever, if the interio r finish is applied to a furred gypsum bo ard attached to the structural wall, a po tential pathway fo r air mo vement is created.

The primary wall system co mpo nents requiring special attentio n fo r mo isture co ntro l (Figure 6 ) are listed belo w: Exterio r wall finishes Vapo r retarders Air infiltratio n and rain barriers and seals Insulatio n Interio r wall finishes

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Figure 6. A " forgiving" (well- designed) wall system for hot, humid climates has a high resistance to outside air and vapor movement. The component most responsible for restricting air and water vapor movement should be located toward the exterior of the wall system. For colder climates, the high vapor resistant finishes must be on the interior side of the insulation in order to avoid condensation.

Ext erior Wall Finishes Materials co mmo nly used as exterio r finishes in co nstructio n include stucco , wo o d siding, co ncrete o r maso nry, brick veneer, and pro prietary external finish systems that co mbine insulatio n and finish co atings (such as EIFS). In selecting the exterio r finish material, the design team needs to co nsider the effects o f mo isture penetratio n and vapo r and air migratio n, as well as aesthetics, to ensure co nsistency with the design intent. Co nsideratio n o f po ro us materials such as co ncrete o r maso nry sho uld include the ability o f these materials to limit mo isture and vapo r migratio n into and o ut o f the wall system, as well as their ability to act as air barriers. Often the aesthetic exterio r finish o f a co ncrete o r maso nry wall system is a paint o r stucco type o f applicatio n. These exterio r finishes, as well as the structural co ncrete o r maso nry substrate, may be effective weather barriers but are ineffective vapo r retarders and o nly partially effective air barriers.

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Materials used in the co nstructio n o f exterio r walls are classified by their resistance to mo isture mo vement thro ugh the material when there is a vapo r pressure difference between the interio r and exterio r sides o f the material. Three catego ries o f vapo r retarder capability are typically defined: Vapo r impermeable: less than o r equal to 0 .1 perm Vapo r semi-impermeable: less than o r equal to 1 perm and greater than 0 .1 perm Vapo r semi-permeable: greater than 1 perm Co ncrete blo ck walls can have a permeance o f 2 to 3 perms, whereas painted stucco finishes can have a permeance as high as 25 perms. Exterio r paint systems with 1 to 3 mil dry-film thickness, such as co mmercial latex paints, can range fro m 5 to 10 perms (Figure 7). Paint systems are go o d examples o f ho w requirements fo r temperate, co ld, and ho t/humid climates differ. In mo st parts o f the co untry, exterio r paint systems have high permeance ratings and interio r paint systems have lo wer permeance ratings. In ho t, humid climates, wall finish design requirements are just the o ppo site: exterio r systems sho uld have lo wer permeance ratings than interio r paint systems.

Figure 7. Many exterior paints and coatings can act as adequate vapor retarders.

Vapor Ret arders A vapo r retarder is no t required in all situatio ns. The building envelo pe (minus a dedicated vapo r retarder) may perfo rm as an adequate barrier to vapo r diffusio n. Under many co nditio ns, using an air barrier is mo re impo rtant than using a vapo r retarder. While using a vapo r retarder is no t always necessary, if o ne is used, facto rs such as permeance, lo catio n, and use o f multiple retarders beco me extremely impo rtant.

The type and lo catio n o f the vapo r retarder can affect mo isture accumulatio n and mo ld fo rmatio n co nsiderably. Fo r example, a wall system vapo r retarder lo cated between the thermal insulatio n and the building's interio r co uld reach a temperature belo w the dew po int (po int o f co ndensatio n in ho t, humid climates and an exterio r vapo r retarder co uld be belo w the dew po int in no rthern PDFmyURL.com

climates) o f the o utside air, allo wing co ndensatio n to fo rm o n interio r surfaces o r in interio r cavities. To avo id such pro blems, decisio ns regarding vapo r retarders are best determined during the schematic design phase.

There are several types o f vapo r retarders (Figure 8 ). Rigid retarders include reinfo rced plastics, aluminum, and similar materials that are relatively impervio us to mo isture flo w. They are mechanically fastened into place and may have sealed jo ints. Flexible vapo r retarders include fo il, laminated fo ils, treated papers, co ated felts and papers, and plastic films. Jo ints in these materials must be sealed with ano ther material. (Airtight jo int sealing is no t a requirement unless the vapo r retarder is also acting as an air barrier and/o r a rainwater barrier.) So me co ating materials (such as epo xies) can also be classified as vapo r retarders.

Figure 8. Vapor transmission rates among common construction materials differ dramatically.

The permeance o f a material is determined by ho w po ro us the material is. Different vapo r retarder materials have different permeance ratings depending o n ho w much vapo r will diffuse thro ugh it during a given perio d and fo r a given area. Fo r example, a .0 0 2 inch-thick aluminum fo il sheeting has a permeance o f .0 25, which means it passes .0 25 grain (1/7,0 0 0 o f a po und) per ho ur per square fo o t o f area fo r every inch o f mercury co lumn vapo r pressure difference. By co ntrast, an 8 inch co ncrete blo ck (limesto ne aggregate) passes 2.4 grains per ho ur, a rate 9 0 times greater than the aluminum fo il, even tho ugh the blo ck wall is 48 ,0 0 0 times thicker (The PDFmyURL.com

Dehumidification Handbook, 1990).

Each o f these vapo r retarders can be used with the wall systems described previo usly. Typically, frame-type cavity walls include flexible vapo r retarders. Designing the vapo r retarder lo catio n fo r co ncrete o r maso nry wall systems can be mo re difficult than fo r a framed wall system. Applied co atings are particularly suited fo r co ncrete o r maso nry walls; applying the exterio r finish system directly to the po ured-in-place wall substrate is easier than including an interstitial space (o r buildo ut) o n the exterio r o f the wall substrate to install a vapo r retarder. Mo reo ver, the latter pro cess may threaten the integrity o f the wall. In selecting a vapo r retarder fo r the exterio r wall finish system, a vapo r retarder paint may be co nsidered.

The selected vapo r retarder sho uld have a permeance rating less than 1.0 perm. (Ho wever, in temperate regio ns, a vapo r retarder with a very lo w permeance rating may create pro blems because the vapo r diffusio n mechanism will reverse directio ns between winter and summer mo nths.) Altho ugh the design criteria may dictate a particular vapo r retarder, o r thickness thereo f, the installatio n metho d can o ften mandate substitutio n. Fo r example, a po lyethylene sheet vapo r retarder may meet design criteria but may no t pro vide adequate resistance to tearing during field installatio n. The perfo rmance o f a vapo r retarder is reduced if penetrated, altho ugh avo iding all penetratio ns is no t necessary.

Using two lo w-permeance finishes in a wall system, such as a po lyethylene vapo r retarder o n the exterio r and vinyl wall co vering o n the interio r, sho uld also be avo ided. This arrangement can allo w mo isture to beco me trapped within a wall system witho ut the ability to dry in either directio n, thus pro mo ting mo isture accumulatio n and mo ld fo rmatio n. Using multiple vapo r retarders within a wall system can be successful o nly if bo th rainwater intrusio n and o utside air infiltratio n are virtually eliminated. Thus, achieving and co ntinually maintaining po sitive building pressurizatio n is critical in this situatio n.

Air Inf ilt rat ion Barriers and Seals The decisio n to include a dedicated air barrier in the design is typically made at during schematic design. An air barrier can play an impo rtant ro le in deterring infiltratio n fro m wind lo ad o r weather co nditio ns and also can facilitate building pressurizatio n. (Air barriers called building wraps are co mmo nly used in no rthern climates fo r energy savings.) The pro per lo catio n o f the air barrier may be the same as that o f the weather barrier and vapo r retarder. Therefo re, a well-designed co mbinatio n o f air/weather/vapo r barrier so metimes can be eco no mically achieved.

An air barrier in a wall system, ho wever, sho uld never be viewed as an adequate envelo pe seal to o ffset a depressurized interio r building space and prevent internally induced infiltratio n. The building envelo pe must wo rk with the HVAC system to establish a pressurized building. Because cavities that may exist within a wall system pro vide po tential pathways fo r o utside air, maintaining pro per pressurizatio n is crucial to avo iding infiltratio n o f o utside air into these spaces.

Often, the building envelo pe co mpo nents acting to gether can functio n as an effective air barrier.

ASHRAE reco gnizes that a single piece o f pro perly suppo rted plywo o d o r gypsum bo ard sheathing can be an adequate air barrier. Ho wever, jo ined pieces o f sheathing o ften will no t be as PDFmyURL.com

effective unless the jo ints are reaso nably well sealed. While the effectiveness o f a vapo r retarder diminishes linearly as the number o f penetratio ns increases, the effectiveness o f an air barrier diminishes expo nentially as the number o f jo ints, cracks, and crevices increases. Thus, the perfo rmance o f an air barrier depends o n its being as penetratio n-free as po ssible.

Wo o d pro ducts, including sheet go o ds and finished bo ards, are less effective as air barriers if no rmal installatio n metho ds are used. Because these exterio r finish systems tend to allo w air infiltratio n fro m wind and thermal effects, an additio nal means o f limiting air (and mo isture migratio n) thro ugh the wall system is required. A co mbinatio n air/weather barrier sho uld be installed o n the exterio r sheathing substrate, particularly in a framed wall system that uses wo o d pro ducts.

The effectiveness o f co mbining insulatio n bo ard and an exterio r finish (i.e., EIFS) as air barriers depends o n the o verall integrity o f the co mpo site exterio r system. If jo ints are reaso nably plumb and tight, the system will pro tect the building envelo pe fro m infiltrating wind and o utside air.

Clo sed-cell and no nhygro sco pic (no nabso rbent) insulatio n bo ard are mo re resistant to vapo rdiffused mo isture than o pen-cell insulatio n bo ard.

Insulat ion Using clo sed cell, no nhygro sco pic insulatio n can help minimize the high mo isture levels that can develo p in wall systems. Insulatio n sho uld be installed next to a vapo r retarder whenever po ssible and sho uld be inwardly lo cated so that the vapo r retarder do es no t reach the dew po int during o peratio n o f the building AC system (this co nditio n applies o nly to ho t, humid climates and is the reverse in co ld climates). So me insulatio n types can also be used as effective vapo r retarders (Figure 9 ).

To avo id mo isture pro blems, the design team must co nsider ho w direct co ntact with mo isture-laden air affects wall structures. Thermal bridges that allo w the structures to co o l belo w the dew po int o f the ambient air may cause lo cal co ndensatio n o n the structural materials. Fo r example, a metal stud framing system in a framed wall system can act as a thermal sho rt-circuit o r bridge, allo wing co ndensatio n to o ccur o n interio r o r exterio r po rtio ns o f the metal stud even tho ugh the wall may be well insulated.

Figure 9. Some insulation types can also serve as effective vapor retarders. Special attention must be given to insulation thickness to achieve the desired permeance.

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Int erior Wall Finishes Interio r finish selectio n is a critical co nsideratio n, especially in humid climate design. The co ntributio n o f the interio r finish to severe mo isture and mo ld pro blems in existing and new buildings is well do cumented. Using an impermeable interio r finish witho ut full co nsideratio n o f infiltratio n, o utdo o r dew po int temperatures, and the po ssibility o f co ndensatio n at the primary vapo r retarder lo catio n will o ften result in mo isture entrapment and mo ld pro blems.

Vinyl wall co vering is a co mmo nly used interio r finish and no rmally has a lo w permeance (o r a very high resistance) to water vapo r migratio n thro ugh a wall system. A pro blem can develo p, ho wever, in ho t, humid climates when o utside air infiltrates a wall cavity, co ntacts a co o ler surface, co ndenses, and canno t dry. (The vinyl wall co vering's high vapo r retarder characteristics prevents the co ndensatio n fro m drying.) The co ndensatio n will degrade the finish substrate, usually gypsum bo ard, pro viding an excellent gro wth medium fo r mo ld. Co nsequently, vinyl wall co vering sho uld be limited to areas where mo ist air is unlikely to infiltrate (that is, interio r walls) o r in buildings where po sitive building pressurizatio n can be ensured. In co ld climates the use o f vinyl wall co vering is no t a pro blem and will, in fact, retard the unwanted diffusio n o f warm mo ist air into the wall cavity where co ndensatio n can o ccur o n the exterio r side o f the thermal insulatio n.

In general, in ho t, humid climates the permeance o f the interio r finish material sho uld be significantly higher than the permeance o f the o ther co mpo nents in the wall system. This difference will allo w mo isture vapo r that enters the wall system to migrate into the co nditio ned space, where the vapo r eventually will be remo ved by the AC system. To ensure success, all po rtio ns o f the wall system lo cated inwardly fro m thermal insulatio n must be mo re permeable than co mpo nents external to the thermal insulatio n. Again, the reverse o f this co nditio n is advised in co ld climates where mo isture sho uld no t be trapped inside the cavity o n the exterio r o f the thermal insulatio n.

Wall Dew Point Analysis Each majo r exterio r wall system used in co nstructio n sho uld be analyzed to determine all o f the fo llo wing: Where dew po int will o ccur What the temperature pro file will be Where the primary vapo r retarder will be lo cated Ho w far mo isture will be allo wed to penetrate (vapo r pressure pro file) These co ncepts are discussed in the ASHRAE Handbook: Fundamentals (Chapter 27; ASHRAE, 20 0 9 ). Co mpleting a versio n o f the ASHRAE Handbo o k's Figure 12 (Page 27.9 ) fo r each majo r wall type will facilitate wall dew po int analysis.

The pro cedure fo r calculating water vapo r diffusio n invo lves analyzing each wall system co mpo nent, including thickness, permeance to vapo r transmissio n, and thermal resistance (R PDFmyURL.com

value). The first step is to determine what indo o r/o utdo o r temperatures sho uld be used to identify wall surface dew po int. The lo west po ssible indo o r wall surface tempera-ture can o ften be much lo wer than the indo o r design co nditio ns. Fo r example, the surface temperature o f a wall that receives discharge fro m the ro o m AC unit supply register can be as lo w as 6 0 째Fdb. Likewise, exterio r surface temperature can exceed o utdo o r design co nditio ns, especially o n no nreflective dark exterio r surfaces.

A temperature pro file can then be develo ped fo r each wall system (Figure 10 a). In a pro perly designed system, the dew po int temperature o f o utside air co nditio ns will o ccur in the insulatio n as lo ng as there are no thermal bridges (such as metal studs). It is impo rtant to co mpare the lo catio n o f the dew po int with the pro po sed lo catio n o f the vapo r retarder to determine if the barrier will remain abo ve the dew po int o f o utside air co nditio ns.

The next o bjective in the dew po int analysis is to verify which wall co mpo nent functio ns as a primary vapo r retarder and then co mpare its lo catio n to the lo catio n o f surface co ndensatio n (dew po int surface). To determine the primary vapo r retarder lo catio n in the wall system, the saturatio n vapo r pressures at each wall co mpo nent surface interface must be determined and co mpared to the vapo r pressure resistances o f the co mpo nent.

The lo catio n within the wall system where diffused mo isture vapo r will co ndense will be the po int where the vapo r pressure equals the saturatio n pressure. To develo p a vapo r pressure pro file thro ugh the wall system, the vapo r pressure dro p acro ss each wall co mpo nent (Figure 10 b) must be determined. The pro cedure fo r develo ping a vapo r pressure pro file is similar to that fo r develo ping a temperature pro file thro ugh the wall system; so ftware is available to help develo p this analysis.

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Left: Figure 10a. Determining the temperature profile of the exterior wall system identifies the surfaces where condensation will occur. Right: Figure 10b. Determining the saturation and vapor pressure profiles of the exterior wall system is also necessary for maximum moisture control because this helps identify wall components that may trap moisture.

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EMERGING ISSUES Current and Fut ure Research and Development Many o f the current issues related to building envelo pe co nstructio n fo r mo isture co ntro l are discussed o n the Building Sciences Co rpo ratio n Web site.

The Air Barrier Asso ciatio n o f America pro vides info rmatio n related to the science and co nstructio n o f air barriers.

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RELEVANT CODES AND STANDARDS PDFmyURL.com

Currently, the fo llo wing states have inco rpo rated air barrier requirements into their co mmercial energy co nservatio n co des.

Massachusetts Wisco nsin (PDF 6 5 KB) Michigan Rho de Island Geo rgia Minneso ta Flo rida BACK TO TO P

ADDIT IONAL RESOURCES Organiz at ions American So ciety o f Heating, Refrigerating, and Air Co nditio ning Engineers (ASHRAE)

Publicat ions Preventing Mo isture and Mo ld Pro blems: Design and Co nstructio n Guidelines, CH2M HILL, 20 0 3 Handbook of Fundamentals , ASHRAE, Atlanta, GA, 20 0 9 ASHRAE Guide for Buildings in Hot and Humid Climates, American So ciety o f Heating, Refrigerating, and Air-Co nditio ning Engineers, Atlanta, 20 0 8 BACK TO TO P

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