外文翻譯--使用隔汽層防止冷凝

1、<p>　　Use of Vapor Barriers to Prevent Condensation</p><p>　　Whenever insulation is installed in a wall, roof, or slab, its resistance to the flow of heat is so much greater than that of the other eleme

2、nts of the construction that the dew point and resulting condensation may occur within the insulation.</p><p>　　Since water vapor flows from regions of high temperature to regions of low temperature, a simpl

3、e solution to condensation is to stop the flow of water vapor by means of some surface material impervious to moisture---provided this surface is called a vapor barrier. It must always be applied on the warm side.</p&

4、gt;<p>　　Because condensation is generally most severe during the heating season, all vapor barriers should be installed on the interior side of walls and roofs. From a practical standpoint, this means that the va

5、por barrier should be next to and part of the insulation.</p><p>　　One of the best and most economical vapor barriers is aluminum foil. Some insulation come equipped with this foil attached to one surface. H

6、owever, unless reinforced with kraft paper or some other strong material, the foil is easily ripped, torn, or punctured, and so is of little value as a barrier. </p><p>　　Since vapor behaves as a gas, a vapo

7、r barrier, to be effective, must be airtight, or as nearly so as possible. But this is often an impractical requirement. For example, consider a roof with the insulation above the deck and between a vapor barrier and wat

8、erproof roofing. Unless the insulation is of a firm material, the insulation to expand, forming bubbles under the waterproofing. </p><p>　　During the coolness of the night, the bubbles will contract. After a

9、 series of sunny days and cool nights, the bending back and forth of the surface may destroy the roofing. One way to prevent this is to side-vent the roof insulation so the contained air can freely expand and contract. T

10、he side vents must, however, be protected from driving rain.</p><p>　　Vapor barriers can be made of other materials besides aluminum foil. There are aluminum paints, plastic paints, some plastic films, aspha

11、lt paints, rubber-base paints, asphalt, and foil-laminated papers. It must be remembered that water repellent surfaces are not necessarily vapor barriers, that is, airtight.</p><p>　　To evaluate a vapor barr

12、ier, a unit known as the perm is used. It is defined as a vapor-transmission rate of 1 grain of water vapor through 1 square foot of material per hour when the vapor-pressure difference is equal to q inch of mercury (7,0

13、00 grains equal 1 pound). A material having a vapor-transmission rate of 1 perm or less is considered a good vapor barrier. The corresponding unit for permeance of 1-in. thickness is perm-inch.</p><p>　　Resi

14、stance to vapor transmission is the reciprocal of the permeance..</p><p>　　Since vapors flow from the warm side of a wall or roof to the cold side, the exterior surface should be as porous as possible or ven

15、ted and yet offer protection against penetration of rain. This is particularly important with “blown-in” insulation as applied to frame houses, for which a vapor barrier generally cannot be installed. This type of insula

16、tion also involves another principle, which, if ignored, frequently is the cause of peeling of paint and leads to unnecessary repair of rain gutters</p><p>　　“Blown-in” insulation is sprayed into the spaces

17、between the studs of frame construction. The interior surface is generally lath and plaster, or wall-board-----both porous. The exterior is generally wood sheathing, with shingles, clapboards, or stucco. The heat resista

18、nce of the insulation is such that during the winter the location of the dew point falls within the insulation. Theoretically, the resulting condensation should occur within the insulation. This, however, does not occur.

19、 Condensati</p><p>　　The principle involved is this: Whenever the dew point occurs within a material, condensation will not occur until the flow of water vapor encounters the surface of another material of g

20、reater resistance to the flow of water vapor. That is, as long as the air can keep on moving, it will carry the moisture along with it and will not deposit the moisture until it reaches a surface that resists its flow an

21、d is colder than the dew point.</p><p>　　The problem inherent in blown-in insulation can be solved by“cold-side venting.” In applying blown-in insulation, an opening usually is drilled through the exterior w

22、all surface between each pair of studs. These holes should never be scaled, only covered with porous water-repellent material for protection against the weather. Then, whatever water vapor flows through the inside porous

23、 finish can escape to the cold air outside without condensing. With clapboard construction,“toothpick” wedges may </p><p>　　To sum up: vapor barriers, or as much resistance as possible to vapor flow (or air)

24、 should be provided on the warm side of walls and roofs. Openings or porous materials---as little resistance as possible to vapor flow---should be provided on the cold side.</p><p>　　If vapor barriers were p

25、erfect, cold-side venting would not be required. Unfortunately, vapor barriers are not perfect; therefore, cold-side venting is worthwhile insurance against failure of insulation in all cases.</p><p>　　The d

26、iscussions above of winter condensation seem to contradict summer requirements when the warm and cold sides of a construction are the reverse of what they are in winter. In most parts of the United States, however, cooli

27、ng seldom results in maintenance of inside temperatures more than15`F below outside conditions, whereas in winter, inside temperatures ate generally maintained at 60 to 75`F above outside conditions. So in winter, the pr

28、evailing maximum temperature differences are from four </p><p>　　It should be mentioned, however, that in low-temperature work, such as cold storage rooms and low-temperature test cells special conditions ar

29、ise for which it is best to refer to a specialist.</p><p><b>　　使用隔汽層防止冷凝</b></p><p>　　只要在墻、屋蓋、或樓板內(nèi)放置絕熱層，由于它抵抗熱的能力比其它構(gòu)件大得多，在絕熱層內(nèi)可能形成露點(diǎn)和由此產(chǎn)生的冷凝現(xiàn)象。</p><p>　　由于水汽從高溫區(qū)流向低溫區(qū)，解

30、決冷凝的一個(gè)簡(jiǎn)單方法是用某種不透水的表面材料（只要它永遠(yuǎn)在露點(diǎn)以上）阻止水汽的流動(dòng)。這種表面稱為隔汽層。它應(yīng)永遠(yuǎn)裝在暖面。</p><p>　　因?yàn)槔淠F(xiàn)象通常在采暖季節(jié)最為嚴(yán)重，因?yàn)槔淠F(xiàn)象通常在采暖季節(jié)最為嚴(yán)重，因此所有隔汽層都必須設(shè)在墻和屋頂?shù)膬?nèi)側(cè)。從實(shí)際的觀點(diǎn)出 ，這意味著隔汽層應(yīng)緊貼絕熱層并構(gòu)成絕熱層的一部分。</p><p>　　最好最經(jīng)濟(jì)的隔汽層之一是鋁箔。有些絕熱層事先有一

31、面裝有鋁箔。但是除非有牛皮紙或其它結(jié)實(shí)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(méi)有多大價(jià)值.</p><p>　　因?yàn)樗男再|(zhì)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(shí)際要求。例如，一個(gè)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(jiān)固的材料，如泡沫玻璃，否則太陽(yáng)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(shí)，氣泡將收縮。在一連串出太陽(yáng)的白

32、天和涼爽的夜晚之后，表面漲而復(fù)縮將會(huì)破壞屋頂。防止這種現(xiàn)象的一種方法是使屋頂絕熱層有側(cè)邊透氣孔，內(nèi)部的空氣能自由地膨脹和收縮。不過(guò)，側(cè)邊透氣孔必須防止雨水滲入。</p><p>　　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(shuō)，不一定是不透氣的。</p><p>　　為估計(jì)隔汽層的

33、優(yōu)劣，使用了一種叫做perm的單位。其定義為：當(dāng)水汽壓力差等于1英寸水銀柱時(shí)，每小時(shí)通過(guò)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(yōu)質(zhì)隔汽層。滲透1英寸深的相應(yīng)單位為1perm-英寸。</p><p>　　抗水汽傳輸?shù)哪芰κ菨B透能力的倒數(shù)。</p><p>　　因?yàn)樗麖膲蛭蓓數(shù)呐媪飨蚶涿妫獗砻鎽?yīng)盡可能多孔或通風(fēng)，

34、同時(shí)又要防止雨水浸入。對(duì)于構(gòu)架房屋用的“噴吹”絕熱層，這一點(diǎn)尤為重要，這種房屋通常不能設(shè)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(dǎo)致不必要的修理雨水槽，其實(shí)它并不漏。</p><p>　　“噴吹”絕熱層被噴入構(gòu)架結(jié)構(gòu)墻筋之間的空隙。內(nèi)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(yú)鱗板，護(hù)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(diǎn)落在絕熱層之內(nèi)。從理論

35、上講，由此而產(chǎn)生的冷凝也應(yīng)發(fā)生在絕熱層內(nèi)。但事實(shí)并非如此。如發(fā)生冷凝，它并非發(fā)生在絕熱層內(nèi)露點(diǎn)區(qū)內(nèi)，而是在襯板的內(nèi)表面上。</p><p>　　所涉及到的原理是這樣的：每當(dāng)材料內(nèi)部產(chǎn)生露點(diǎn)時(shí)，要等到水汽流接觸到對(duì)水汽流阻力更大的另一種材料的表面時(shí)，才會(huì)產(chǎn)生冷凝。也就是說(shuō)，只要空氣繼續(xù)流動(dòng)。它就攜帶著水汽，直至它接觸到能抵抗其流動(dòng)而又比露點(diǎn)更冷的表面，才使水汽附著下來(lái)。</p><p>　　

36、噴吹絕熱層所固有的這一缺點(diǎn)可用 “冷側(cè)通風(fēng)”法來(lái)解決。在采用噴吹絕熱層時(shí)，通常在外墻表面每對(duì)墻筋之間鉆通一個(gè)孔口。這些孔口決不可封死，只能用多孔防水材料覆蓋以免雨水浸入。于是，無(wú)論什么水汽流到多孔罩面層的內(nèi)側(cè)，都可逃逸到外邊的冷空氣中而不致凝結(jié)。護(hù)壁楔形板結(jié)構(gòu)的牙簽形楔子可楔入每塊板的下端，形成足夠的孔口，供通風(fēng)之用。</p><p>　　總之，隔汽層，即對(duì)水汽流（或空氣）有盡可能抵抗力的材料，應(yīng)放在墻和屋頂?shù)呐?/p>

37、側(cè)，而開(kāi)孔或多孔性材料，即對(duì)水汽流有盡可能少抵抗的材料，應(yīng)放在冷側(cè)。</p><p>　　如隔汽層優(yōu)良，冷側(cè)的通風(fēng)孔就沒(méi)有必要了。遺憾的是，隔汽層總是欠佳；因此，冷側(cè)通風(fēng)孔無(wú)論在什么情況下都是預(yù)防絕熱層失效的可靠保證。</p><p>　　上述有關(guān)冬季結(jié)露現(xiàn)象的討論似乎與夏季的要求相互矛盾，因?yàn)樵谙牧顣r(shí)節(jié)，結(jié)構(gòu)的冷側(cè)和暖側(cè)與冬季正好相反。然而，在美國(guó)大部分地區(qū)，空調(diào)很少使室內(nèi)溫度低于室外溫