1、UseofVaporBarrierstoPreventCondensationWheneverinsulationisinstalledinawall/roof/orslab/itsresistancetotheflowofheatissomuchgreaterthanthatoftheotherelementsoftheconstructionthatthedewpointandresultingcondensationmayoccurwithintheinsulation/Sincewatervaporflowsfromregionsofhightemperaturetoregionsof
2、lowtemperature/asimplesolutiontocondensationistostoptheflowofwatervaporbymeansofsomesurfacematerialimpervioustomoisture-providedthissurfaceiscalledavaporbarrier/Itmustalwaysbeappliedonthewarmside/Becausecondensationisgenerallymostsevereduringtheheatingseason/allvaporbarriersshouldbeinstalledontheint
3、eriorsideofwallsandroofs/Fromapracticalstandpoint/thismeansthatthevaporbarriershouldbenexttoandpartoftheinsulation/Oneofthebestandmosteconomicalvaporbarriersisaluminumfoil/Someinsulationcomeequippedwiththisfoilattachedtoonesurface/However/unlessreinforcedwithkraftpaperorsomeotherstrongmaterial/thefo
4、iliseasilyripped/torn/orpunctured/andsoisoflittlevalueasabarrier/Sincevaporbehavesasagas/avaporbarrier/tobeeffective/mustbeairtight/orasnearlysoaspossible/Butthisisoftenanimpracticalrequirement/Forexample/consideraroofwiththeinsulationabovethedeckandbetweenavaporbarrierandwaterproofroofing/Unlessthe
5、insulationisofafirmmaterial/theinsulationtoexpand/formingbubblesunderthewaterproofing/Duringthecoolnessofthenight/thebubbleswillcontract/Afteraseriesofsunnydaysandcoolnights/thebendingbackandforthofthesurfacemaydestroytheroofing/Onewaytopreventthisistoside-venttheroofinsulationsothecontainedaircanfr
6、eelyexpandandcontract/Thesideventsmust/however/beprotectedfromdrivingrain/Vaporbarrierscanbemadeofothermaterialsbesidesaluminumfoil/Therearealuminumpaints/plasticpaints/someplasticfilms/asphaltpaints/rubber-basepaints/asphalt/andfoil-laminatedpapers/Itmustberememberedthatwaterrepellentsurfacesarenot
7、necessarilyvaporbarriers/thatis/airtight/Toevaluateavaporbarrier/aunitknownasthepermisused/Itisdefinedasavapor-transmissionrateof1grainofwatervaporthrough1squarefootofmaterialperhourwhenthevapor-pressuredifferenceisequaltoqinchofmercury(7/000grainsequal1pound)/Amaterialhavingavapor-transmissionrateo
8、f1permorlessisconsideredagoodvaporbarrier/Thecorrespondingunitforpermeanceof1-in/thicknessisperm-inch/Resistancetovaportransmissionisthereciprocalofthepermeance/Sincevaporsflowfromthewarmsideofawallorrooftothecoldside/theexteriorsurfaceshouldbeasporousaspossibleorventedandyetofferprotectionagainstpe
9、netrationofrain/Thisisparticularlyimportantwith“blown-in”insulationasappliedtoframehouses/forwhichavaporbarriergenerallycannotbeinstalled/Thistypeofinsulationalsoinvolvesanotherprinciple/which/ifignored/frequentlyisthecauseofpeelingofpaintandleadstounnecessaryrepairofrainguttersthatdonotleak/“Blown-
10、in”insulationissprayedintothespacesbetweenthestudsofframeconstruction/Theinteriorsurfaceisgenerallylathandplaster/orwall-board-bothporous/Theexteriorisgenerallywoodsheathing/withshingles/clapboards/orstucco/Theheatresistanceoftheinsulationissuchthatduringthewinterthelocationofthedewpointfallswithint
11、heinsulation/Theoretically/theresultingcondensationshouldoccurwithintheinsulation/This/however/doesnotoccur/Condensation/whenitwithintheinsulation/butontheinsidesurfaceofthesheathing/Theprincipleinvolvedisthis/Wheneverthedewpointoccurswithinamaterial/condensationwillnotoccuruntiltheflowofwatervapore
12、ncountersthesurfaceofanothermaterialofgreaterresistancetotheflowofwatervapor/Thatis/aslongastheaircankeeponmoving/itwillcarrythemoisturealongwithitandwillnotdepositthemoistureuntilitreachesasurfacethatresistsitsflowandiscolderthanthedewpoint/Theprobleminherentinblown-ininsulationcanbesolvedby“cold-s
13、ideventing/”Inapplyingblown-ininsulation/anopeningusuallyisdrilledthroughtheexteriorwallsurfacebetweeneachpairofstuds/Theseholesshouldneverbescaled/onlycoveredwithporouswater-repellentmaterialforprotectionagainsttheweather/Then/whateverwatervaporflowsthroughtheinsideporousfinishcanescapetothecoldair
14、outsidewithoutcondensing/Withclapboardconstruction/“toothpick”wedgesmaybedrivenundertheloweredgeofeachclapboardtoprovidetherequiredopeningsforbreathing/Tosumup/vaporbarriers/orasmuchresistanceaspossibletovaporflow(orair)shouldbeprovidedonthewarmsideofwallsandroofs/Openingsorporousmaterials-aslittler
15、esistanceaspossibletovaporflow-shouldbeprovidedonthecoldside/Ifvaporbarrierswereperfect/cold-sideventingwouldnotberequired/Unfortunately/vaporbarriersarenotperfect；therefore/cold-sideventingisworthwhileinsuranceagainstfailureofinsulationinallcases/Thediscussionsaboveofwintercondensationseemtocontrad
16、ictsummerrequirementswhenthewarmandcoldsidesofaconstructionarethereverseofwhattheyareinwinter/InmostpartsoftheUnitedStates/however/coolingseldomresultsinmaintenanceofinsidetemperaturesmorethan15Fbelowoutsideconditions/whereasinwinter/insidetemperaturesategenerallymaintainedat60to75Faboveoutsidecondi
17、tions/Soinwinter/theprevailingmaximumtemperaturedifferencesarefromfourtofivetimeswhattheyareinsummer/Furthermore/insummerverylittlecoolingisrequiredduringthenight/Hence/asfarasinsulationisconcerned/summercondensationissointermittentthatitcanbecompletelydisregardedfortheaveragestructureandaverageoccu
18、pancy/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***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***以上）阻止水汽的流动。这种表面称为隔汽
19、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
20、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***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。
21、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***落在绝热层之内。从理论上讲，由此而产生的冷凝也应发生在绝热层内。但事实并非如此。如发生冷凝，它并
22、非发生在绝热层内***区内，而是在衬板的内表面上。所涉及到的原理是这样的：每当材料内部产生***时，要等到水汽流接触到对水汽流阻力更大的另一种材料的表面时，才会产生冷凝。也就是说，只要空气继续流动。它就携带着水汽，直至它接触到能抵抗其流动而又比***更冷的表面，才使水汽附着下来。喷吹绝热层所固有的这一缺点可用“冷侧通风”法来解决。在采用喷吹绝热层时，通常在外墙表面每对墙筋之间钻通一个孔口。这些孔口决不可封死，只能用多孔防水材料覆盖以免雨水浸入。于是，无论什么水汽流到多孔罩面层的内侧，都可逃逸到外边的冷空气中而不致凝结。护壁楔形板结构的***形楔子可楔入每块板的下端，形成足够的孔口，供通风之用。总之，隔
23、汽层，即对水汽流（或空气）有尽可能抵抗力的材料，应放在墙和屋顶的暖侧，而开孔或多孔性材料，即对水汽流有尽可能少抵抗的材料，应放在冷侧。如隔汽层优良，冷侧的通风孔就没有必要了。遗憾的是，隔汽层总是欠佳；因此，冷侧通风孔无论在什么情况下都是预防绝热层失效的可靠保证。上述有关冬季结露现象的讨论似乎与夏季的要求相互矛盾，因为在夏令时节，结构的冷侧和暖侧与冬季正好相反。然而，在美国大部分地区，空调很少使室内温度低于室外温度15F以上，而冬天的室内温度通常保持在高于室外温度60至理名言75F之间。因此冬季的最在温差多为夏季的四至五倍。而且，夏季的夜晚几乎不需要空调。因此，对于一般用途的绝热层而言，夏季这种不连续的结露现象，完全可以忽略不计。然而应当指出，对制冷工艺（如冷藏室和低温试验库）中出现的特殊条件，最好请教有关专家。熊洪权0204003
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