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.使用隔汽層防止冷凝只要在墻、屋蓋、或樓板內放置絕熱層，由于它抵抗熱的能力比其它構件大得多，在絕熱層內可能形成露點和由此產生的冷凝現象。由于水汽從高溫區流向低溫區，解決冷凝的一個簡單方法是用某種不透水的表面材料（只要它永遠在露點以上）阻止水汽的流動。這種表面稱為隔汽層。它應永遠裝在暖面。因為冷凝現象通常在采暖季節最為嚴重，因為冷凝現象通常在采暖季節最為嚴重，因此所有隔汽層都必須設在墻和屋頂的內側。從實際的觀點出，這意味著隔汽層應緊貼絕熱層并構成絕熱層的一部分。最好最經濟的隔汽層之一是鋁箔。有些絕熱層事先有一面裝有鋁箔。但是除非有牛皮紙或其它結實材料加固，這種鋁箔很容易被割裂、扯破、或穿孔,所以用作隔汽層沒有多大價值.因為水汽的性質和氣體一樣,隔汽層必須不透氣或盡可能不透氣才能生效.但這往往不切合實際要求。例如，一個屋蓋上的絕熱層位于隔汽層和屋面防水層之間。除非絕熱層是一種堅固的材料，如泡沫玻璃，否則太陽的熱力將使絕熱層中的空氣膨脹，在防水層下形成氣泡。晚上涼爽時，氣泡將收縮。在一連串出太陽的白天和涼爽的夜晚之后，表面漲而復縮將會破壞屋頂。防止這種現象的一種方法是使屋頂絕熱層有側邊透氣孔，內部的空氣能自由地膨脹和收縮。不過，側邊透氣孔必須防止雨水滲入。除了鋁箔，隔汽層還可用別的材料。有鋁涂料，塑料涂料，某些塑料薄膜，瀝青涂料，橡膠類涂料，瀝青，和金屬箔層壓紙板。必須記住，防水表面不一定是隔氣層，也就是說，不一定是不透氣的。為估計隔汽層的優劣，使用了一種叫做perm的單位。其定義為：當水汽壓力差等于1英寸水銀柱時，每小時通過一平方英尺材料為一粒水汽的水汽傳輸率（7，000粒等于是磅）。水汽傳輸率為1perm或者1perm以下的材料就是優質隔汽層。滲透1英寸深的相應單位為1perm-英寸。抗水汽傳輸的能力是滲透能力的倒數。因為水汽從墻或屋頂的暖面流向冷面，外表面應盡可能多孔或通風，同時又要防止雨水浸入。對于構架房屋用的“噴吹”絕熱層，這一點尤為重要，這種房屋通常不能設隔汽層。這種絕熱層還涉及到另一原理，這一原理若被忽略了，常常引起油漆剝落，并導致不必要的修理雨水槽，其實它并不漏。“噴吹”絕熱層被噴入構架結構墻筋之間的空隙。內表面通常是板條和灰泥，或木板都是多孔的。外表面通常是帶魚鱗板，護壁楔形板，或粉飾的木襯板。在冬天由于絕熱層的抗熱性使露點落在絕熱層之內。從理論上講，由此而產生的冷