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.使用隔汽層防止冷凝只要在墻、屋蓋、或樓板內放置絕熱層，由于它抵

22、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

23、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