2009CondensationFactSheet_112509

7/28/2019 2009CondensationFactSheet_112509

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CONDENSATIONFACTSHEET

METALBUILDINGMANUFACTURERSASSOCIATION1300SumnerAve.,Cleveland,Ohio 441152851 (216)2417333 Fax(216)2410105

Email:[email protected] http://www.mbma.com

11/09


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TheCondensationProcess

Condensationoccurswhenwarmermoist

aircomesincontactwithcoldsurfacessuch

asframingmembers,windowsandother

accessories,orthecolderregionwithinthe

insulationenvelope

(if

moisture

has

penetratedthevaporretarder).Warmair,

havingtheabilitytocontainmoremoisture

thancoldair,losesthatabilitywhenit

comesincontactwithcoolorcoldsurfaces

orregions.Whenthathappens,excessive

moistureintheairisreleasedintheformof

condensation. Inmetalbuildings,thereare

twopossibleconsequencesoftrapped

moistureinwallandroofsystems:(1)

corrosionof

metal

components

and

(2)

degradationofthethermalperformanceof

insulation.

DewPointandRelativeHumidity

Dewpointisthetemperatureatwhich

watervaporinanystaticormovingair

columnwillcondenseintowater. Inother

words,theairissaturatedandcanno

longerholdthemoistureatthis

temperature.Whentheairtemperature

dropsbelow

its

dew

point,

excess

moisture

willbereleasedintheformof

condensation. Condensationproblemsare

mostlikelytooccurinclimateswhere

temperaturesfrequentlydipto35For

colderoveranextendedperiodoftime.

Relativehumidityisapercentage

measurementoftheamountofwatervapor

presentintheairinrelationtotheamount

itiscapableofholdingatthattemperature.

Forexample,

50%

relative

humidity

indicatestheairiscarryingonehalfofthe

maximumamountofmoisturethatitis

capableofcontainingatthegiven

temperature. Thereisarelationship

betweenthedewpointandrelative

humidity. Ahighrelativehumiditymeans

thatthedewpointisnearthecurrentair

temperature. Therefore,arelative

humidityof100%indicatesthatthedew

pointisequaltothecurrenttemperature.

Thisrelationshipbetweendewpointand

relativehumidity

is

given

in

Table

C1.

VisibleandConcealedCondensation

Twothingsmustbepresentfor

condensationtooccur: warmmoistair,and

coolsurfacetemperaturesbelowthedew

point. Thepropercontrolofthesetwo

factorscanminimizecondensation.

Inmetalbuildingsystems,weare

concernedwithtwodifferentareasor

locations:visible

condensation

which

occursonsurfacesbelowdewpoint

temperatures;andconcealedcondensation

whichoccurswhenmoisturehaspassed

intointeriorregionsandthencondenseson

asurfacethatisbelowthedewpoint

temperature.

Signsofvisiblesurfacecondensationare

water,frostoriceonwindows,doors,

frames,ceilings,walls,floor,insulation

vaporretarders,

skylights,

cold

water

pipes

and/orcoolingducts. Toeffectivelycontrol

visiblecondensation,itisnecessaryto

reducethecoldsurfaceareaswhere

condensationmayoccur. Itisalso

importanttominimizetheairmoisture

contentwithinabuildingbytheuseof

properlydesignedventilatingsystems.

Signsofconcealedcondensationinclude

dampspots,stains,moldand/ormildewon

wallsor

ceilings,

delamination

of

laminated

surfaces,bubblesorblistersinasphaltic

surfaces,peelingpaint,anddamp

insulation. Concealedcondensationisthe

mostdifficulttodealwithandcanbethe

mostdamaging. Thistypeofcondensation

maybecontrolledinmetalbuildingsbythe

properuseofvaporretardersandby

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DewPointTemperature(F)1

RelativeHumidity

Design Dry Bulb (Interior) Temperature

32

F

35

F

40

F

45

F

50

F

55

F

60

F

65

F

70

F

75

F

80

F

85

F

90

F

95

F

100

F

100% 32 35 40 45 50 55 60 65 70 75 80 85 90 95 100

90% 30 33 37 42 47 52 57 62 67 72 77 82 87 92 97

80% 27 30 34 39 44 49 54 58 64 68 73 78 83 88 93

70% 24 27 31 36 40 45 50 55 60 64 69 74 79 84 88

60% 20 24 28 32 36 41 46 51 55 60 65 69 74 79 83

50% 16 20 24 28 33 36 41 46 50 55 60 64 69 73 78

40% 12 15 18 23 27 31 35 40 45 49 53 58 62 67 71

30% 8 10 14 16 21 25 29 33 37 42 46 50 54 59 62

20% 6 7 8 9 13 16 20 24 28 31 35 40 43 48 52

10% 4 4 5 5 6 8 9 10 13 17 20 24 27 30 34

1 Chart adapted from ASHRAE Psychometric Chart, 2005ASHRAE Handbook of Fundamentals.

TableC 1:DewPointTemperature(F)1versesRelativeHumidity

minimizingmoisturecontentwithinthe

buildingbyproperventilation. Additional

condensationcontrolcanbeaccomplished

byventingthecavitiesofthewallsandroof.

VaporRetarders

Avaporretarderisusedtoinhibitthe

passageofwarmermoistairintotheinner

regionsof

the

roof

or

wall

system.

The

properselectionandinstallationofthe

vaporretardercanhelpcontrol

condensationproblemsinabuilding. Vapor

retardersareratedbytheamountof

moisturethatcanpassthroughthem. The

lowerthisrating,calledapermrating,the

lessvaportransmissionwilloccurandthe

moreeffectivethevaporretarderwillbe.

Watervaportransmissionrates(perms)are

determinedusingASTME96, StandardTestMethodsforWaterVaporTransmissionofMaterials(ASTM,2005).TypesofVaporRetarders

Therearevarioustypesofvaporretarders

available,such

as:

1. Structuralmembranes,includingrigidsteelsheetsorotherimpermeable

materials. Thelistbelowisnot

exhaustive,norisitanefforttolimitthe

designer,butthesemembranesmay

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includethefollowinginroofand/orwall

construction:

a. Steelformedpanels,properlysealedonedgesandends.

b. Steel,zincalloy,copperoraluminumsheetswithcaulkedandformedstandingseamedgesandends.

c. Vinylsidingappliedtosuitablestructuralsubstrateorsheathing.

d. Tiltupconcretepanels,suitablysealedatendandsidelaps,witha

paintedexterior.

e. Vinyl,metalizedplasticandsimilaroverlays

on

rigid

board

insulation.

f. Foaminsulatedmetalpanels,caulkedandsealedatendsandside

laps.

g. Bituminoussprayortroweloncoatingonconcreteormasonry.

2. Flexiblemembranes,suchasfoils,coatedpapers,orplasticfilms. Usually,

thesemembranesareratedbyperm

of1.0orless,perASTME96,StandardTestMethodsforWaterVaporTransmissionofMaterials(ASTM,2005).Themostcommonapplicationsfor

metalbuildingsaremembraneretarders

laminatedtofiberglassblanket

insulation. Plainwhitevinylwithaperm

ratingof1.0isnotaneffectivevapor

retarder,especiallyinbuildingswitha

highrelativehumidity.

3. Coatingmembranes,whichincludespaints,trowelonbituminouscoatings,

epoxyandurethanefoams.

Laminatedfacingsforfiberglassbattsserve

severalpurposesotherthanappearance.

Theypreventtheblanketfromsagging.

Theypreventmostwatervaporfrom

penetratingthefiberglassbatt. They

provideresistancetoimpactandprovide

reflectivityandemissivitybenefits.Most

facingshavethreeparts:

1. Abasemadeupofnaturalorwhitekraftpaper

or

aluminum

foil.

2. Afiberglassscrimnettingisprovidedforreinforcingandsagprevention.

3. Anexteriorfilmmadeupofpolypropylene,vinyl,metalized

polyester,oraluminumfoilisappliedon

top.

TableC 2listssomeofthemostpopular

typesoffacingsandshowstheirwater

vapor

transmission

rates

(perm

ratings).

TheNorthAmericanInsulation

ManufacturersAssociation(NAIMA)

recommendsthatmetalbuildinginsulation

befacedwithavaporretarderhavinga

permeanceofnotgreaterthan0.10perms.

Besidesthemostpopularfacings,the

FacingDescription Perm Details

WhiteVinyl 1.01.3.003vinyl,economical,butnotan

effective

vapor

retarder

Polypropylenescrimkraft .09.02Whiteormetalizedpolypropylene,fiber

glassreinforcing,11 30lbswhitekraft

Polypropylenescrimpolyester .02 Metalizedpolyester,14lbkraft

Foilscrimkraft .02Aluminumfoil,fiberglassreinforcing,30

lb kraft

Vinylreinforcedpolyester .02 .003vinyl,metalizedpolyester,strong

TableC 2:WaterVaporTransmissionRates

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industryprovidesspecialfacingsthathave

highimpactresistanceforgymnasiums,

facingsforhighUVapplicationsandblack

coloredfacingsforstructureswhereceilings

arenotused.

SealingVapor

Retarders

Carefulattentionmustbepaidtothe

insulationseamsatsideandendlapsto

maintaintheintegrityofthevaporretarder.

Somecommonmethodsusedtosealthe

seamsincluderollingandstaplingtheside

laps,peelandsticktabsatsidelaps,and

sometimesinsulationtape. Theuseof

insulationtapealonetosealtheseamsmay

notbeadvisable. Jobsiteconditionssuch

ashumidity,

dirt,

and

access

to

the

undersideoftheinsulationcanmakethis

difficult.Whereused,tapeshouldbeat

leastthesamequalityasthevapor

retarder,andthetapeshouldbeapproved

bytheinsulationsupplierfortheparticular

product. Inthecaseofamembranetype

retarder,anypuncturesortearsinthe

materialshouldberepairedusingtheself

adhesiverepairtapesuppliedwiththe

insulation.More

information

and

downloadableliteraturecanbefoundon

theNAIMAwebsitevia

http://www.naima.org,orvisitNational

InsulationAssociation(NIA)at

http://www.insulation.org.

Becausethereisagrowingtrendtoadd

insulationtoexistingmetalbuildings,a

propervaporretarderisofcriticalconcern.

Oneofthemostcommonmethodsistoadd

anadditional

layer

of

insulation

to

the

bottomflangeofthepurlinsystem. This

cancreateanairspacewheremoisture

ladenaircanaccumulateiftheintegrityof

thenewvaporretarderclosesttothewarm

insulatingsurfaceofthebuildinghasnot

beenmaintained. Itisimportantthatan

intactvaporretarderisnotleftwithinthe

insulationmass.

Ventilation

Allmetalbuildingsrequiresomelevelof

ventilation,and

more

often

this

ventilation

isbecomingtheresponsibilityofthemetal

buildingcontractor. Alackofventilation

cancreateanuncomfortableworking

conditionthroughelevatedheatlevels,

excessivehumidity,andstaleair. Itcanalso

contributetocondensationproblems.

Ventilationcanbestberepresentedbythe

numberoftimesperhourthebuildingairis

replacedwithoutsideair. Thisisreferredto

asairchangesperhour. Thenumberofair

changesrequired

per

hour

widely

varies

per

application.

Thefollowingexampleillustratesa

calculationforventilationrequirements:

Assumea100feetwidex250feetlongx30

feethighbuildingbeingusedforlight

manufacturing,assemblyandstorage.

1. Determinethetotalvolumeofthe

building.

Volume =100feetwidex250feet

longx30feethigh=750,000ft3

2. Next,determinetherequiredair

flowincubicfeetperminute(CFM)to

provide5airchangesperhour.

AirFlow=750,000ft3x5air

changes/60minutes=62,500CFM

Therefore, 62,500cubicfeetperminuteof

airmustbemovedthroughthisbuildingto

providefive

air

changes

every

hour.

Allowancesmustbemadeforaplaceforair

toenterthebuildingandforairtoexitthe

building,andtheairflowmustbeevenly

distributedthroughoutthebuilding. Typical

methodsofmovingairincludeexhaustand

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http://www.naima.org/http://www.insulation.org/http://www.insulation.org/http://www.naima.org/


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supplyairfans,ridgeventilatorsand

louvers.

ControllingCondensation

TableC 3providesguidanceoncontrolling

condensationproblems

as

listed

in

this

section. Notethatalllistedcontrol

measuresdonotaddressthepossibilityofa

leakdefectthroughtheroofmembrane.

Thisisageneralchecklisttocontrol

condensationthroughpossibleremedial

measures. Itisnotdesignedtoaddress

specificdifficultiesinspecificbuildings.

1. AttheSource Limittheamountofwatervaporwithintheheatedinterior.

a. Provideawelldrainedbasecoursesuchascrushedrockorwashed

gravelundergradelevelslabsto

preventmoisturefrompermeating

intothebuildingthroughtheslab.

b. Provideforadequategroundwaterdrainage.

c. Divertrainandmeltwateraccumulationsawayfromthe

foundation.

d. Ventallmechanicalheatingapparatuswithhydrogenoxygenby

productexhauststotheoutside.

e. Reducesupplementalinteriorhumidification.

f. Beawarethatthepouringofconcretewithinanewlyerected

structurepresentsspecial

considerations.Whilethepractice

shouldbe

held

to

aminimum,

an

individualprojectassessmentmust

bemadetopreventbothvisibleand

concealedcondensation.

2. ByInsulating Aproperlydesignedinsulationsystemeffectivelyraises

surfacetemperatureswithinthe

buildingenvelopeabovethedewpoint

temperature. Thisisaccomplishedby

controllingtheheatlossattheexposed

(exterior)sidesofthosesurfaces.

However,insulationaddedabovean

existingroof

should

be

placed

at

the

existingrooflevelandnotagainstthe

retrofitroofifpossible. Adequately

ventilatedspaceisrequiredbecauseany

trappedwarmairmaycondenseatthe

coldmetalroof.

a. Provideadditionalinsulationinceilingand/orwallcavitiesor

replaceexistingdamporwater

soakedinsulatingmaterialwithin

thesecavities.

b. Installdoubleortripleglazedwindowsorinsulatedstormpanels

withthermalbreakframes.

c. Installinsulateddoors.d. Installinsulationwrap(withexterior

vaporretarder)aroundcoldpipes

and/orcoldairductwork.

e. Insulatefloorslabedgesexposedtoexterior

temperatures

with

arigid

insulationpressuretreatedplywood

combination.

f. Paintcondensationpronesurfaceswithmoistureabsorbingpaints.

3. WithVaporRetarders Avaporretarderisamembraneofpolyethylenefilm,

aluminumfoil,paint,asphalticlaminate

and/orglazed,asphaltsaturated

buildingpaperthatexhibitsa

permeanceoflessthanoneperm. All

jointsandseamsmustbelapped,

sealed,andsecured,inordertoreduce

theamount ofwatervaporthat

combineswiththegivencavityairmass.

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a. Employ(orrepair)avaporretarderatthewarmsidesurfaceofall

insulatingmaterial. Inthecaseof

thefiberglassblanketinsulations,

theretardermaybelaminatedto

theinsulating

fiber.

All

joints

at

crawlspaces,underslabductwork,

atticopenings,ceilingfixtures

and/orotherwall,ceilingandfloor

penetrationsmustbeproperly

sealed.

b. Installavaporretarder groundcoveroverinterior,exposedground

surfaces.

c. Installavaporretarderbetweensub

flooring

and

ground

slab.

d. Installaclearvaporretarderoverskylightopeningsandsealoffto

warmsideinsulationretarder.

e. Installavaporretarderonbothsidesoftheinsulationinbuildingswitha

cooler,controlledatmosphereand

incoldstoragebuildingstoprevent

condensationinside

the

insulation.

4. ThroughVentilation Thedilutionofamoistinteriorairmasswithdrier

outsideairfortheexpresspurposeof

loweringtherelativehumidityoftheair

masscanbeaccomplishedthrough

ventilation. Forretrofitapplications,

ventingaboveexistingroofmaynotbe

necessaryifthesourcebelowis

eliminatedbyadequatemeansofvapor

retarders.The

natural

amounts

of

watervaporfromoutsideairexchange

willdissipategraduallywithoutany

effect.

CondensationProblemMethodsofControl

(SeeSection4.4.5)

Moisture,frostormoldonundersideof

uninsulatedmetalroof.1d,1e,2a,2f,3a,3b,4b1,4b2,4b3

Moistureor

frost

on

skylights.

1d,

1e,

3b,

3d,

4b1,

4b2,

4b3

Moistureorfrostformationsoninteriorvapor

retarder.1d,1e,2a,3b,4b1,4b2,4b3

Moisturedrippingfromceilingfixtures. 2a,4a1,4a4

Moisture,dampnessand/ormildewonfloorareas. 1d,1e,2g,3c,4a3

Moistureand/orfrostonexteriorwindows,doors

andmetalframes.1d,1e,2a,2b,2c,2d,4b1,4b2,4b3

Dampness,stains,mildeworblisteringandpeeling

painton

ceilings.

1d,1e,2a,2f,3a,3b,4a1,4a4,4b1,4b2,4b3

Dampness,stain,mildeworblisteringandpeeling

paintonwalls.1d,1e,2a,2f,3a,3b,4a2,4a4,4b1,4b2,4b3

Moisturedrippingfromcoldwaterpipesorcold

airducts.1d,1e,2d,2f,3b,4b1,4b2,4b3

Soggyordampinsulationinceilingorwalls. 1d,3a,4a1,4a2,4a4

TableC 3:ControllingCondensation

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a. ColdSideVenting Theventingofexteriorcavities(cavitiesatthe

coldsideoftheinsulationenvelope

butcontainedwithinthegeneral

buildingenvelope)ofthebuildings

structuralelements.

One

square

footoffreeventareamustbe

providedforeach300squarefeetof

convectivecavityarea. Ventsshould

beuniformlydistributedtoprovide

thebestoverallairflowandalso

shouldbescreenedandlouveredto

preventinsectsandrainfrom

enteringthecavity.

1) Provideridgeandeaveventsinbuilding

attic

areas.

2) Provideforbaseandeavelineventilationtowallcavities.

3) Installfoundationventstoanycrawlspaceareas.

4) Installexhaustfans.b. WarmSideVenting Theventingof

theinteriorbuildingenvelope.

1)Install

convective

type

venting

apparatus.

2) Installremoteexteriorairchangerswithheatingand

distributingsystemsasrequired.

3) Installexhaustfans. (Note:Borderlineeffectiveness

dependsheavilyuponinfiltration

forairchange.)

HighHumidityConditions

Applicationswheretherelativehumidityin

abuildingisexpectedtobeabove30%

warrantcarefulconsiderationof

condensationissues,andthosewithrelative

humidityabove50%areespeciallyproneto

problemsifproperdesignandinstallation

oftheinsulationretardersystemisnot

carriedout. Someexamplesofthesehigh

humiditysituationsarebuildingsthathouse

swimmingpools,icerinks,wineries,

livestock,highwaymaintenancegarages,

andwasteprocessingfacilities.

Theimportance

of

the

proper

selection,

installationandsealing ofthevapor

retarderismagnifiedinasituationwhere

therelativehumidityishigh. Theperm

ratingofthevaporretardershouldbeless

than0.05forhighhumidityconditions. All

seams,lapsandjointsinthevaporretarder

mustbeproperlysealed. Foldingand

staplingseamsisnotrecommendedfor

laminatedfiberglassinsulation.

Inthe

case

of

aflexible

membrane

type

retarder,anypuncturesortearsinthe

materialshouldberepairedusingtheself

adhesiverepairtapesuppliedwiththe

insulation. Carefulinspectionafterall

tradeshavecompletedtheirworkis

recommendedtoverifytheintegrityofthe

vaporretarder. Insomehighhumidity

applicationswhereaflexiblemembrane

retardercouldbeatriskfordamageand

penetration,astructural

membrane

might

bethemostprudentoption. Another

optionwouldbetheuseofduallayerrigid

boardinsulationssystems. rigidfoam

insulationshouldhaveafoilfacerand

staggeredjoints(min6inches)toprevent

moisturemigration. Insulationjointsshould

bewellsealedandcareshouldbetakento

ensurethatdetailingiswellsealedatrake

edgeofroof/walljunctureandatall

penetrations.

Documents

2009CondensationFactSheet_112509