b'The Effects of Water Vapor in Buildings By Bob LaCosse, CAE Ventilation Rule The air inside an occupied building is aA general rule ofbe to provide thumb would mixture of dry air and water vapor.Thea minimumone top side ventevery offor water vaporbeof"steam" at1,000ofarea orThe canthoughtassquare feetroofless. a very low pressure, and it acts like a gas.toproofarenotto sideventsdesignedonly Its pressure forces itmove to areas that toprovide relief to moisture sealed into the have a lower vapor pressure.This steamroof system during application, butto also cannotbeseenexceptwhensaturatedrelieve moisture that may penetrate the conditions occur, and then it appears asvapor retarder. fog. be Great care shouldtaken when fabricating Thequantityof water vapor in the airand installing these relief vents to ensure depends upon the rate that the moisture isBob IACosse, CAEthat they don\'t disturb the vapor retarder released within the building and the relativeMRCA Technical Advisorand its function. the building. tightness of co1d underside of the roofing membrane.Some buildings may never experience high Ifofairfree toThis is done by the use ofvaporinternal moisture loads and, therefore, do large quantitiesoutdoorarearetarder enterthebuilding,therelativelydrieronwarm side ofnotequire a vapor thethe insulation.r retarder for protection. outside air will dilute the moisture in theKeep in mind, however,great amounts that building. Theair inAs long as the water vapor remains a gas,ofof result is relatively drierwater are released during the curing the building.This drier air will have ano problems can occur, and it will remainconcreteandplasterandwater-based lower vapor pressureand the air-watera gas as long as it cannot reach a surfacepaints.Ifrelease happens to occur this vapor will have a lower dew point.where the temperature is below the dewduringcoldweatherandadequate point ofair-water vapor mixture.ventilation isprovided in the thenotbuilding, Condensationthis moisture may be driven into the roof Ifis purposely sealed so thatVapor Retarderssystem, causing considerable damage. the building the outside airexcluded from the building,A solid, unbroken vapor retarder over the is the water vapor content of the inside airroof deckofWhileunder somecircumstancesitis is the best solution for control will in\\:rease and finally reach a point wherewater vapor inside a building. Itassumed,possible that the vapor retarder may be is condensation will occur on cool surfaceshowever, that the insulation on top of theomitted,thisshouldonly be done with suchastheoutsidewalls,windows,vapor retarder is dry when it is installedextreme caution and after careful study. undersideofaroofdeckand/ortheover the vapor retarder, and that water isneed them Where some buildings may not underside ofnotvaporandinitially, subsequent usage or change of roofing membranes.trapped between theretarder the roof membrane.occupancy may alter the humidity inside Condensation of liquid water from waterthe building.Also,the vapor pressure vapor occurs whenevertemperature ofIfcannot be assumed, some meanscouldsignificantlychange,resultingin thethis damagetheinsulationeventual a surface is below the dew point ofair must be provided for venting the trappedtoroofand the roof watermixture inmoisture out of the assembly.Ifnotfailure. vaporthe building.it is exVaporretardersaremorecommonly vented, the trapped moisture~and Practically every structural roof deck isduring hot weather, causing blisters and requiredthe northern climates than in the\' porous to water vapor transfer, and it isbubblesform. Ittothatin tois importantnote southern regions of possible for condensation to occur on thea vapor retarder handled improperly canthe United States, where bottomroofing membranessomecreate problems instead ofdownward pressure can be expected and ofattimesolving them. the roofas a vapor or another.Therefore, the condensationmembrane itself serves may occur as liquid waterfrost dependingA vapormeansretarder. orretarder provides a positive uponthetemperatureoutsideoftheof protection from condensation becauseWithregardtooutsidewalls,moisture building.ofinternal moisture loads.It will not highcondensation wille place inof provide protection against moisture sealedtak any type wall, whether itinsulated or Initinto the roofing system during constructionisnot, provided either case,will eventually saturate thethe conditions areThese conditions roofinsulationandreduceitsthermalor which enters the roof assembly as aright. efficiency or run back into the building,resulta water leak.A venting system isare a high inside relative humidity, a low ofoutside temperature and a construction such staining the ceiling or dripping onto therequired to do this.that the materials nearest to the inside of floor.Initin the many cases,will stayfelts,thearetoThe asbuilt-upthento ice andVentingbeatroofwallpermeablewater vapor. inroofing,returnmayprovidedtheedges,water vapor filters through to the colder causeproblemsinthecompleteroofortheis very large, top ventingthe ifrooforouterit will condense. assembly.use of a relief vent may be needed.Thesections where exact number andofwill locationthe vents The only way to prevent this problem isvaryaccordingtothedensityofthe to prevent the water vaporinsulationandtheconfigurationof the from reaching the building. 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