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UseofVaporBarrierstoPreventCondensationWheneverinsulationisinstalledinawall,roof,orslab,itsresistancetotheflowofheatissomuchgreaterthanthatoftheotherelementsoftheconstructionthatthedewpointandresultingcondensationmayoccurwithintheinsulation.Sincewatervaporflowsfromregionsofhightemperaturetoregionsoflowtemperature,asimplesolutiontocondensationistostoptheflowofwatervaporbymeansofsomesurfacematerialimpervioustomoisture-providedthissurfaceiscalledavaporbarrier.Itmustalwaysbeappliedonthewarmside.Becausecondensationisgenerallymostsevereduringtheheatingseason,allvaporbarriersshouldbeinstalledontheinteriorsideofwallsandroofs.Fromapracticalstandpoint,thismeansthatthevaporbarriershouldbenexttoandpartoftheinsulation.Oneofthebestandmosteconomicalvaporbarriersisaluminumfoil.Someinsulationcomeequippedwiththisfoilattachedtoonesurface.However,unlessreinforcedwithkraftpaperorsomeotherstrongmaterial,thefoiliseasilyripped,torn,orpunctured,andsoisoflittlevalueasabarrier.Sincevaporbehavesasagas,avaporbarrier,tobeeffective,mustbeairtight,orasnearlysoaspossible.Butthisisoftenanimpracticalrequirement.Forexample,consideraroofwiththeinsulationabovethedeckandbetweenavaporbarrierandwaterproofroofing.Unlesstheinsulationisofafirmmaterial,theinsulationtoexpand,formingbubblesunderthewaterproofing.Duringthecoolnessofthenight,thebubbleswillcontract.Afteraseriesofsunnydaysandcoolnights,thebendingbackandforthofthesurfacemaydestroytheroofing.Onewaytopreventthisistoside-venttheroofinsulationsothecontainedaircanfreelyexpandandcontract.Thesideventsmust,however,beprotectedfromdrivingrain.Vaporbarrierscanbemadeofothermaterialsbesidesaluminumfoil.Therearealuminumpaints,plasticpaints,someplasticfilms,asphaltpaints,rubber-basepaints,asphalt,andfoil-laminatedpapers.Itmustberememberedthatwaterrepellentsurfacesarenotnecessarilyvaporbarriers,thatis,airtight.Toevaluateavaporbarrier,aunitknownasthepermisused.Itisdefinedasavapor-transmissionrateof1grainofwatervaporthrough1squarefootofmaterialperhourwhenthevapor-pressuredifferenceisequaltoqinchofmercury(7,000grainsequal1pound).Amaterialhavingavapor-transmissionrateof1permorlessisconsideredagoodvaporbarrier.Thecorrespondingunitforpermeanceof1-in.thicknessisperm-inch.Resistancetovaportransmissionisthereciprocalofthepermeance.Sincevaporsflowfromthewarmsideofawallorrooftothecoldside,theexteriorsurfaceshouldbeasporousaspossibleorventedandyetofferprotectionagainstpenetrationofrain.Thisisparticularlyimportantwith“blown-in”insulationasappliedtoframehouses,forwhichavaporbarriergenerallycannotbeinstalled.Thistypeofinsulationalsoinvolvesanotherprinciple,which,ifignored,frequentlyisthecauseofpeelingofpaintandleadstounnecessaryrepairofrainguttersthatdonotleak.“Blown-in”insulationissprayedintothespacesbetweenthestudsofframeconstruction.Theinteriorsurfaceisgenerallylathandplaster,orwall-board-bothporous.Theexteriorisgenerallywoodsheathing,withshingles,clapboards,orstucco.Theheatresistanceoftheinsulationissuchthatduringthewinterthelocationofthedewpointfallswithintheinsulation.Theoretically,theresultingcondensationshouldoccurwithintheinsulation.This,however,doesnotoccur.Condensation,whenitwithintheinsulation,butontheinsidesurfaceofthesheathing.Theprincipleinvolvedisthis:Wheneverthedewpointoccurswithinamaterial,condensationwillnotoccuruntiltheflowofwatervaporencountersthesurfaceofanothermaterialofgreaterresistancetotheflowofwatervapor.Thatis,aslongastheaircankeeponmoving,itwillcarrythemoisturealongwithitandwillnotdepositthemoistureuntilitreachesasurfacethatresistsitsflowandiscolderthanthedewpoint.Theprobleminherentinblown-ininsulationcanbesolvedby“cold-sideventing.”Inapplyingblown-ininsulation,anopeningusuallyisdrilledthroughtheexteriorwallsurfacebetweeneachpairofstuds.Theseholesshouldneverbescaled,onlycoveredwithporouswater-repellentmaterialforprotectionagainsttheweather.Then,whateverwatervaporflowsthroughtheinsideporousfinishcanescapetothecoldairoutsidewithoutcondensing.Withclapboardconstruction,“toothpick”wedgesmaybedrivenundertheloweredgeofeachclapboardtoprovidetherequiredopeningsforbreathing.Tosumup:vaporbarriers,orasmuchresistanceaspossibletovaporflow(orair)shouldbeprovidedonthewarmsideofwallsandroofs.Openingsorporousmaterials-aslittleresistanceaspossibletovaporflow-shouldbeprovidedonthecoldside.Ifvaporbarrierswereperfect,cold-sideventingwouldnotberequired.Unfortunately,vaporbarriersarenotperfect;therefore,cold-sideventingisworthwhileinsuranceagainstfailureofinsulationinallcases.Thediscussionsaboveofwintercondensationseemtocontradictsummerrequirementswhenthewarmandcoldsidesofaconstructionarethereverseofwhattheyareinwinter.InmostpartsoftheUnitedStates,however,coolingseldomresultsinmaintenanceofinsidetemperaturesmorethan15Fbelowoutsideconditions,whereasinwinter,insidetemperaturesategenerallymaintainedat60to75Faboveoutsideconditions.Soinwinter,theprevailingmaximumtemperaturedifferencesarefromfourtofivetimeswhattheyareinsummer.Furthermore,insummerverylittlecoolingisrequiredduringthenight.Hence,asfarasinsulationisconcerned,summercondensationissointermittentthatitcanbecompletelydisregardedfortheaveragestructureandaverageoccupancy.Itshouldbementioned,however,thatinlow-temperaturework,suchascoldstorageroomsandlow-temperaturetestcellsspecialconditionsariseforwhichitisbesttorefertoaspecialist.使用隔汽层防止冷凝只要在墙、屋盖、或楼板内放置绝热层,由于它抵抗热的能力比其它构件大得多,在绝热层内可能形成露点和由此产生的冷凝现象。由于水汽从高温区流向低温区,解决冷凝的一个简单方法是用某种不透水的表面材料(只要它永远在露点以上)阻止水汽的流动。这种表面称为隔汽层。它应永远装在暖面。因为冷凝现象通常在采暖季节最为严重,因为冷凝现象通常在采暖季节最为严重,因此所有隔汽层都必须设在墙和屋顶的内侧。从实际的观点出,这意味着隔汽层应紧贴绝热层并构成绝热层的一部分。最好最经济的隔汽层之一是铝箔。有些绝热层事先有一面装有铝箔。但是除非有牛皮纸或其它结实材料加固,这种铝箔很容易被割裂、扯破、或穿孔,所以用作隔汽层没有多大价值.因为水汽的性质和气体一样,隔汽层必须不透气或尽可能不透气才能生效.但这往往不切合实际要求。例如,一个屋盖上的绝热层位于隔汽层和屋面防水层之间。除非绝热层是一种坚固的材料,如泡沫玻璃,否则太阳的热力将使绝热层中的空气膨胀,在防水层下形成气泡。晚上凉爽时,气泡将收缩。在一连串出太阳的白天和凉爽的夜晚之后,表面涨而复缩将会破坏屋顶。防止这种现象的一种方法是使屋顶绝热层有侧边透气孔,内部的空气能自由地膨胀和收缩。不过,侧边透气孔必须防止雨水渗入。除了铝箔,隔汽层还可用别的材料。有铝涂料,塑料涂料,某些塑料薄膜,沥青涂料,橡胶类涂料,沥青,和金属箔层压纸板。必须记住,防水表面不一定是隔气层,也就是说,不一定是不透气的。为估计隔汽层的优劣,使用了一种叫做perm的单位。其定义为:当水汽压力差等于1英寸水银柱时,每小时通过一平方英尺材料为一粒水汽的水汽传输率(7,000粒等于是磅)。水汽传输率为1perm或者1perm以下的材料就是优质隔汽层。渗透1英寸深的相应单位为1perm-英寸。抗水汽传输的能力是渗透能力的倒数。因为水汽从墙或屋顶的暖面流向冷面,外表面应尽可能多孔或通风,同时又要防止雨水浸入。对于构架房屋用的“喷吹”绝热层,这一点尤为重要,这种房屋通常不能设隔汽层。这种绝热层还涉及到另一原理,这一原理若被忽略了,常常引起油漆剥落,并导致不必要的修理雨水槽,其实它并不漏。“喷吹”绝热层被喷入构架结构墙筋之间的空隙。内表面通常是板条和灰泥,或木板都是多孔的。外表面通常是带鱼鳞板,护壁楔形板,或粉饰的木衬板。在冬天由于绝热层的抗热性使露点落在绝热层之内。从理论上讲,由此而产生的冷

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