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1、Use of Vapor Barriers to Prevent CondensationWhenever 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 elements of the construction that the dew point and resulting condensation may occur within the insulation.Since water
2、vapor flows from regions of high temperature to regions of low temperature, a simple 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.Becaus
3、e 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 vapor barrier should be next to and part of the insulation.One of the best and most economical vapor bar
4、riers is aluminum foil. Some insulation come equipped with this foil attached to one surface. However, 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. Since vapor behaves as a gas, a vapor barrie
5、r, 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 waterproof roofing. Unless the insulation is of a firm material, the insulation to expand,
6、forming bubbles under the waterproofing. During the coolness of the night, the bubbles will contract. After a 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
7、freely expand and contract. The side vents must, however, be protected from driving rain.Vapor barriers can be made of other materials besides aluminum foil. There are aluminum paints, plastic paints, some plastic films, asphalt paints, rubber-base paints, asphalt, and foil-laminated papers. It must
8、 be remembered that water repellent surfaces are not necessarily vapor barriers, that is, airtight.To evaluate a vapor barrier, 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 di
9、fference is equal to q inch of mercury (7,000 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.Resistance to vapor transmission is the reciprocal of the permea
10、nce.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 vented 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 ba
11、rrier generally cannot be installed. This type of insulation also involves another principle, which, if ignored, frequently is the cause of peeling of paint and leads to unnecessary repair of rain gutters that do not leak. “Blown-in” insulation is sprayed into the spaces between the studs of frame c
12、onstruction. 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 resistance of the insulation is such that during the winter the location of the dew point falls within the insulation.
13、Theoretically, the resulting condensation should occur within the insulation. This, however, does not occur. Condensation, when it within the insulation, but on the inside surface of the sheathing. The principle involved is this: Whenever the dew point occurs within a material, condensation will not
14、 occur until the flow of water vapor encounters the surface of another material of greater 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 flo
15、w and is colder than the dew point.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 wall surface between each pair of studs. These holes should never be scaled, only covered with porou
16、s water-repellent material for protection against the weather. Then, whatever water vapor flows through the inside porous finish can escape to the cold air outside without condensing. With clapboard construction,“toothpick” wedges may be driven under the lower edge of each clapboard to provide the r
17、equired openings for breathing.To sum up: vapor barriers, or as much resistance as possible to vapor flow (or air) 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.If vapor barriers
18、 were perfect, 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.The discussions above of winter condensation seem to contradict summer requirements when the warm and
19、 cold sides of a construction are the reverse of what they are in winter. In most parts of the United States, however, cooling seldom results in maintenance of inside temperatures more than15F below outside conditions, whereas in winter, inside temperatures ate generally maintained at 60 to 75F abov
20、e outside conditions. So in winter, the prevailing maximum temperature differences are from four to five times what they are in summer. Furthermore, in summer very little cooling is required during the night. Hence, as far as insulation is concerned, summer condensation is so intermittent that it ca
21、n be completely disregarded for the average structure and average occupancy. It should be mentioned, however, that in low-temperature work, such as cold storage rooms and low-temperature test cells special conditions arise for which it is best to refer to a specialist.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
22、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
23、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
24、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
25、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，由此而产生的冷凝也应发生在绝热层内。但事实并非如此。如发生冷凝，它并非发生在绝热层内露点区内，而是在衬板的内表面上。所涉及到的原理是这样的：每当材料内部产生露点时，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露点更冷的表面，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
26、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。护壁楔形板结构的牙签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
