1 Hw t Avid Misture Damage t Walls frm Cndensatin Backgrund Infrmatin Exterir building walls are barriers between the atmspheric cnditins n the utside and the atmspheric cnditins n the inside. Certainly the temperatures n the inside will be different than that n the utside, but when we talk abut atmspheric cnditins, we must als include the humidity, which is the water vapr present in the atmsphere. The prper design f an exterir wall must take in t accunt the subtle differences in atmspheric cnditins, alng with the walls ability t d its main jb f resisting wind, rain, snw, sleet, hail and the sun. There are cnsensus standards available (ASTM, ANSI, etc.) that by examining varius expsures in the labratry, can measure the ability f an exterir wall t perfrm the jb f prtecting the inside f a building frm the weather cnditins n the utside. These test standards may even help t prvide an answer t the questin n hw lng an exterir wall will be able perfrm its main functin. Hwever, methds that are available t measure the effects that the unseen atmspheric cnditins (temperature and humidity) can have n the perfrmance f an exterir wall, generally use extensive calculatins in additin t lab data. A ptentially damaging result f the differences in temperature and humidity is cndensatin that may ccur within an exterir wall. The nly readily available and practical methd f determining this pssibility is by means f cmputer prgrams that calculate dew pints in walls. The calculatins fllw methds given in the Handbk f Fundamentals published by ASHRAE (American Sciety f Heating Refrigeratin and Air-cnditining Engineers). Althugh these prgrams are nt perfect and leave a lt t be desired, they d prvide sme infrmatin t the design prfessinal t avid lng term damage. Water Vapr Migratin There is always sme amunt f water vapr in the atmsphere, and this amunt f water vapr will typically be different n the inside f a building frm that n the utside. As is always the case, nature will try t equalize this imbalance, which means that the water vapr will try t migrate frm a lcatin f higher cncentratins t a lcatin f lwer cncentratins. This results in a water vapr drive acrss the bundary between the tw lcatins. The usual water vapr migratin directin is frm higher temperatures twards lwer temperatures, s in clder nrthern climates, where buildings are heated
2 mre than cled, the primary vapr drive is frm the interir twards the exterir (Figure A). Cld Climate - Figure A Fr warm suthern climates, buildings are air-cnditined mre than heated, s the primary vapr drive acrss the wall is frm the exterir twards the interir (Figure B). Ht/Humid Climate - Figure B During summertime in nrthern climates, the interir t exterir vapr drive is minimized, and winter in the suthern climates minimizes the exterir t interir vapr drive. Lking at a map f the United States, we ve defined three areas in which the water vapr drive is predminant. First, the nrthern r cld climate is established as the area f the cuntry nrth f the 4000 heating degree day line (Fr explanatin f Heating Degree Days, see Definitin f Terms). The secnd line is the brder f the ht/humid climate, which is defined in the ASHRAE Handbk, and this runs frm the Texas Gulf Cast acrss the sutheast t the
3 Nrth Carlina Atlantic cast. The area between these tw climate extremes is cnsidered a mderate climate where the vapr drive is essentially equal in bth directins. NrthernCld Climate - Vapr Drive Interir t Exterir Mderate Climate - Vapr Drive Equal Bth Directins Ht/Humid Climate - Vapr Drive Exterir t Interir Definitin f Terms The definitins belw are simplified descriptins f the terms used in the prgram s that the results generated by the cmputer prgram can be easily understd and interpreted. Saturated Vapr Pressure This is a measure f the ttal amunt f water vapr that the atmsphere can hld at a given temperature. The higher the temperature, the mre water vapr the atmsphere can hld, and cnversely at lwer temperatures, the atmsphere can hld less amunts f water vapr. Partial Vapr Pressure The actual amunt f water vapr that is in the atmsphere at a given time, and it is usually given in terms f percentage f the saturated vapr pressure. In weather terms, it is called the relative humidity. S when the weather reprt states the Relative humidity is 80% it means that the amunt f water vapr in the atmsphere is 80% f what the atmsphere culd actually hld.
4 Dew Pint The pint at which the partial vapr pressure equals the saturated vapr pressure and cndensatin ccurs. In weather terms, the relative humidity is 100%, and there is precipitatin falling frm the sky. Heating Degree Days (HDD) This is a indicatin f what perid f time a building must be heated t maintain a temperature f 65 degrees Fahrenheit. As an example, take a building that nly has t be heated fr tw days, and the temperature ne day is 35 degrees Fahrenheit and 25 degrees Fahrenheit the secnd. The ttal is 70 heating degree days (35 degrees t 65 degrees is 30 degrees and 25 degrees t 65 degrees is 40 degrees, s fr the tw days f heating, yu have 30 plus 40 fr the 70 HDD). Thus, as ne travels nrth, the number f heating degree days increases (e.g. Nashville, TN is 3696 HDD and Chicag, IL is 6127 HDD). What s Happening in a Wall? As nted earlier, the cnditins n the interir f a building are typically different frm the exterir cnditins, and this means that there are gradients acrss the wall frm the interir t the exterir. One gradient acrss the wall is the temperature gradient as shwn in Figure C belw: Figure C Interir 72 F Exterir 32 F Temperature Gradient Batt Insulatin The greatest temperature drp acrss the wall is at the wall cmpnents that exhibit gd insulating prperties. In the example abve, the greatest temperature drps are at the batt insulatin and the Expanded Plystyrene (EPS) bard in the Exterir Insulatin and Finish System (EIFS). Remving the batt insulatin frm the wall cavity, the temperature gradient wuld be similar t that shwn in Figure D.
5 Figure D Interir 72 F Exterir 32 F Temperature Gradient N Batt Insulatin Alng with the temperature gradients acrss the wall, there are als the vapr pressure gradients fr bth the saturated and partial pressures. Since the vapr pressures are directly related t the temperature, these gradients will reflect the changes in temperature acrss the wall. Figure E shws hw these gradients may appear acrss the wall assembly. Figure E Interir 72 F Exterir 32 F Temperature Saturated Vapr Pressure Partial Vapr Pressure Nte that in Figure E, the partial vapr pressure gradient never meets the saturated vapr pressure gradient, s there is n dew pint r cndensatin ccurring in this example.
6 Dew Pint Lcatins Cnditins can exist in any gegraphical lcatin that are favrable fr the frmatin f cndensatin in a wall assembly. Assuming that under certain interir and exterir cnditins that the prbability fr cndensatin is high, then the mst likely lcatin f the dew pint within the wall becmes very imprtant. As is always the case, sme cmpnents that make up the wall assembly are mre resistant t misture than ther, s if the dew pint falls within a mre resistant cmpnent, then pssible damage t the wall will be minimized r eliminated. Examples f tw different cnditins can be seen in Figures F and G. Figure F Dew Pint Temperature Saturated Vapr Pressure Partial Vapr Pressure The partial vapr pressure equals the saturated vapr pressure in the expanded plystyrene insulatin bard in Figure F. This lcatin fr the dew pint is ideal as the fam plastic insulatin bard is nt affected by misture, and there is virtually n pprtunity fr lng term misture accumulatin. If the exterir and interir design cnditins allw fr the dew pint t ccur in the EPS bard, then this wall assembly shuld remain free frm misture damage. Figure G examines anther lcatin fr the ccurrence f the dew pint and the pssible lng term cnsideratins fr misture damage:
7 Figure G Dew Pint Temperature Saturated Vapr Pressure Partial Vapr Pressure Here the partial vapr pressure equals the saturated vapr pressure in the batt insulatin. Prlnged expsure t the cnditins that result in a dew pint in this lcatin will lead t accumulatin f misture. T much accumulatin fr t lng a duratin culd eventually damage the surrunding wall cmpnents. This is nt a gd lcatin fr the dew pint t ccur. Keep in mind that as exterir and interir cnditins change, the dew pint within the wall can mve. Cnditins that may cause the dew pint t fall in the batt insulatin may ccur very infrequently, and if that is the case, the likelihd f misture accumulatin and pssibility f damage is reduced. Cmputer Wall Analysis There are several cmputer prgrams available t analyze a wall assembly t determine the lcatin f the dew pint (if there is a dew pint), and in sme cases, the amunt f cndensatin ver a perid f time. Typically these prgrams evaluate the temperature drp acrss each f the cmpnents f the wall assembly based n the R-value and thickness f the cmpnent. Included in the data base f building materials in the cmputer prgram is the ability f each f the cmpnents t allw water vapr t pass thrugh it. This ability fr water vapr transmissin is given a value as measured by a cnsensus standard and generally has units f US perms, permeance, permeability, water vapr transmissin rate, etc. Based n these values, the partial vapr pressure n each side f a wall cmpnent can be calculated, s alng with the temperature, the partial vapr pressure is knwn at the interface f each wall cmpnent.
8 Infrmatin that must be entered int these cmputer prgrams include a list f all the cmpnents and the thickness f each cmpnent that make up the wall assembly frm the exterir t the interir. Als needed by the prgrams are the interir and exterir design temperatures and humidities t which the wall assembly will be expsed. These temperature and humidity values can be average cnditins, as fund in natinally published weather vlumes, r extreme values as may be encuntered in a wrst case scenari. Belw is an example f the results f a cmputer wall analysis. The cmpnents and thickness f each are listed, alng with the exterir and interir cnditins used in the analysis, the three gradients acrss the wall are defined, and the lcatin f the dew pint with an estimate f the vlume f cndensatin expected. The infrmatin cntained in here will enable the design prfessinal t assess the ptential lng term effects, and take the necessary precautins t reduce r eliminate any pssible damage. Typical Residence - Lake City, MN N Vapr Retarder EXTERIOR Big Lake, MN C (-11.0 F) 90.0% RH INTERIOR 21.0 C (71 F) 35.0 % RH DESCRIPTION RSI R Exterir air film Parex Lamina, 3.2 mm Plystyrene expanded T1, 25.0 mm Plywd exterir, 13.0 mm Fibreglas batt, 89 mm Gypsum panel generic, 13.0 mm Interir air film Ttal Thermal Resistance There is cndensatin in the given assembly at this lcatin. The cndensatin rate is 1.168E-03 g/m2/sec. r 1.009E-01 litres/m2/day. The heat lss rate is Watt/m2. The dewpint temperature is -1.7 degrees Celsius. Scale 1 = 10 Legend Temperature Vapr pressure fr cntinuity f flw Saturated vapr pressure Nte: Abve prgram was develped in Canada, which uses metric units and Celsius temperature.
9 Vapr Retarders and Prper Lcatin A building cmpnent that is resistant t the migratin f water vapr is frequently called a vapr barrier, but the mre apprpriate term wuld be vapr retarder as there are very few building materials that d nt allw any transmissin f water vapr. Vapr retarders are frequently emplyed in wall cnstructin, especially in the nrthern climates, t reduce the amunt f water vapr that can enter the wall assembly. By reducing the amunt f water vapr in the wall, the partial vapr pressure in the wall is far less than it wuld nrmally be, and thus the likelihd f the partial pressure equaling the saturated pressure and frming cndensatin is all but eliminated. Thus vapr retarders serve a very useful purpse in wall cnstructin, but they must be placed in the prper lcatin in the assembly. Placement f a vapr retarder in the wrng place in a wall assembly, by a well-intentined design prfessinal, can actually create mre damage t the wall than if it had been mitted. A cmmn ccurrence f an imprperly lcated vapr retarder is the use f a vinyl wallpaper inside f a building in a ht, humid climate like Flrida. In this case, the vinyl wallpaper, acting as a vapr retarder, is n the inside f the wall. The predminate vapr flw thrugh a wall in Flrida is frm the utside twards the interir (ht utside t air-cnditined inside), s the water vapr travels thrugh the wall until it meets the vinyl wallpaper and can travel n further. Misture starts t build-up behind the vinyl until the cncentratin is high enugh t frm cndensatin, and, withut a means t escape, there will be damage t the gypsum wallbard and pssible frmatin f mld and mildew. Figure H demnstrates this prblem. Vinyl Wallpaper Vapr Retarder Ht/Humid Climate - Figure H
10 Design Optins In the event that a cmputer analysis f a wall assembly indicates a pssible cndensatin prblem within the system based n the variables entered int the prgram, there are ptinal curses f actin that the design prfessinal can utilize t crrect any deficiencies. One r mre f these design cnsideratins may be incrprated in the wall cnstructin, and sme f these methds are bvius, and thers may seem a bit bscure. A. The mst frequently used methd f preventing cndensatin is the installatin f a vapr retarder n the warm side f the wall. This is an effective means f slving the prblem, but as discussed abve, the lcatin f the vapr retarder is critical, and the vapr retarder must be able t functin prperly (i.e. must have a lw water vapr transmissin rate, and must nt be breached by penetratins r breaks). Althugh this is the first chice, there are instances where this slutin cannt be used effectively, s ther ptins have t be reviewed. B. If the analysis indicates that mst f the time the dew pint is ging t be lcated in ne f the wall cmpnents that is unaffected by misture, r accumulatin cannt ccur, then it may be pssible t mit the vapr retarder. C. A technique used in clder climates is t add mre insulatin t the exterir f the wall. This has the effect f mving a dew pint twards the exterir f the wall and perhaps int a cmpnent that des nt allw misture t accumulate and cause damage. There may be sme restrictins n hw much insulatin can be added t the exterir, and it des take a cnsiderable amunt f insulatin t mve the dew pint a shrt distance. D. Cnsider the cnditins that were used in the cmputer analysis, and if a wrst case scenari was chsen, then perhaps the perid f time that cndensatin is likely t ccur is very shrt. Damage t mst wall cmpnents ccurs nly when misture is allwed t accumulate and cntact is prlnged. Given an adequate time fr drying, then intermittent perids f cndensatin may nt pse a prblem. E. Allwing the wall t drain by means f weeps is ne methd f preventing misture accumulatin, especially in cavity wall designs. Venting the wall t allw evapratin is emplyed in curtain wall systems. These ptins are typically nt dne in an EIFS clad cnstructin. F. Finally, the remval f all insulatin in the wall cavity may have t be cnsidered. Dew pints ccur mst cmmnly where the temperature drp is the greatest, and this happens acrss materials that have gd insulating prperties. Remval f the cavity insulatin will alter the temperature gradient acrss the wall and effectively mve the dew pint t a lcatin that
11 may be mre acceptable. The dwn side t this apprach is the reductin f the verall R-value f the wall system. Parex ffers the wall analysis service t design prfessinals s that they can btain basic data n water vapr migratin thrugh the wall assembly being designed using reasnable techniques t avid ptential lng term cndensatin prblems. If yu have any questins regarding this service r the resulting analysis, please call us at
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