Water penetration

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I N BU I L D ING ENV E LOP E SBY CHRISTINE BEALL, NCARB, CCS

Water penetrationthrough abuilding enclosure dependson the simultaneousoccur-

renceof threethings: The presence of water. An opening through which water

canenter. A physical force to move the water.Water can be present as rain, melting

snow, and soil moisture. Buildings haveopenings for egress, daylight, ventilation,and utilities, as well as unintentional gaps,

voids, cracks, and joints. Several forcescancause water penetration through theseopenings (Figure 1):

Gravity Air currents Capillary suction Surface tension Kinetic energy (momentum) Air pressure Hydrostatic pressureGravityJoints andopenings in

horizontal surfaces are particularly vulner-

able to water penetrationcausedby gravi-ty.

Air CurrentsRainoccurring in theabsence of wind (or onthe leeward side ofa building) strikes only the horizontal andsloping surfaces of a building, while the vertical surfacesstay dry.Rain driven by wind, though, iscarried at an angle by theaircurrents and strikes the walls as well. Wind patterns createswirling currents at the tops and corners of buildings, increasingthe wetted areaat these locations (Figure2). Thetops and edgesof a building may be subjected to 20 to 30 times as much rainwetting as the center portion ofa facade. Thismeansthat para-pet wallsare at a higher risk ofwind-drivenrain penetration thanany other element of a building.

Figure 1: Forces causing water penetration through building enclosures.Capillary SuctionSmall capillary passages (less than

1/8) in and between exteriorbuilding materials can create a suc-tion force whichdraws in water. The smaller the passage, thegreater the force and the higher the capillary potential. A mater-

ial with fine capillary passageswill attract and retain more waterthan onewith larger capillary passages.

Surface TensionWaterclings to a surface by surfacetension. Water that is flowing down the face of a vertical surfacewill turn and follow the profile of the surface flowing horizontal-

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Figure 2: Wind incrementswetting at building top and corners.ly ontosoffits and into cracks, joints and openings.

Kinetic Energy (Momentum)The momentum ofblowing rainwillcarry it directly through open windows, doorsand joints. If the openings are small, orif the trajectory oftherain is such that it cannot enter directly, water can splash intoopeningsrain drops can splashfrom a sill throughan openwindow or attic vent or splash into a joint.

Air PressureIn addition to the force of wind drivingrain against a surface and blowing it into openings, wind alsocreates air pressuredifferentials acrossthe building envelope.The total air pressure difference between the outsideand theinside of a wall is produced by the combined effects of wind,stack effect, and mechanical ventilation. Wind pressure hasthemost effect on rain penetration. The exact pressure differential atany given pointon a wall or roofwill vary depending on thelocation with respect to the wind direction and the shape of the building. Pressuredifferences on a wall can forcewater throughevenmicroscopicopenings in a building envelopethrough a sortof pumping action, and wind pressure can evendrivewateruphill.

Hydrostatic PressureBothstanding water and groundwater can exerthydrostatic pressure against a building envelope.

Ground waterin the soil exerts hydrostatic pressure againsta basement waterproofing membrane. The deeperthe struc-ture, the greater the pressure exerted. Ponded water exertsgravitational pressure on a roofmembrane, but also exertshydrostatic pressure on the vertical flashings and joints.Continuous hydrostatic pressure places stringent require-

ments on waterproofing membranes and will cause failure at

even small flaws or discontinuities.PREVENTING WATER PROBLEMS

It is not possibleto eliminate rain, snow, and groundwater or the forces that move them, and it is impossible or impractical to eliminate all of the intentional and inadver-

tent openingsand penetrations which occur in a buildingenclosure. It is possible, though, to mitigate the factorswhichcontributeto waterpenetration by applying threebasic moisture protection strategies.1. Limit water penetration into a building or building

enclosure with:- Barriers such as membranes and joint sealants.- Diversionssuch as sloping surfaces and gutters.- Screens such as projections and baffles.

2. Prevent wateraccumulation by providing:- Drainage.- Drying/evaporation.- Ventilation.

3. Neutralize the physical forces that transport water with- Capillary breaks.- Drips.- Protected openings.- Rain screens.

Limiting Water PenetrationIt is difficult to eliminatewaterpenetration but relatively

simple to limit it. Continuous roofing and waterproofing mem-branes form barriers to prevent waterpenetration throughcracksand openingsand into porous surfaces. The effectivenessofthese barriers, however, depends on membrane continuity, theintegrity of seams and joints, and properly detailed flashing atperimeters and penetrations. Elastomeric sealants prevent waterpenetration through large joints, but installation must be nearperfect to form a complete barrier.

Waterpenetration can be limited by diverting the flowofwateras it crosses the building enclosure. Sloped surfaces, forexample, areeffective in draining waterquickly from a roof, cop-ing, orsill so that it has less opportunity to penetrate through cracksor imperfections. Considering the inaccuracy of the con-struction process and the anticipated tolerancesin erecting andassembling building components, most flat surfacesactuallyslope in onedirection oranother and the unintended slope caneasily be in a direction whichcollects rather than sheds water.

Waterpenetration can also be limited by reducing theamount ofwater that strikes building surfaces and joints. Over-hangsand projections can protect walls and openingsfrom rain;louvers can shield an attic vent; baffles or batten strips can block

joints; and gutters withdownspouts can direct wateraway fromfoundation walls.

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Figure 3: Shieldsand labyrinths keep water out of joints.PreventingWater Accumulation

Water that penetrates a building enclosure may not necessari-

ly cause damageif it can be drained ordried quickly. Flashingand weepholes areused inmany different kinds ofbuilding sys-temsto collect and drainwater that has penetratedthe exteriorsurface. Porous materials can be protected from excessive surfaceabsorption with coatings or water repellent treatments, and dry-ing canbe expedited by allowing air circulationbehind thecladding.

Neutralizing ForcesTo resist water penetrationcaused by

gravity flow, particular attention must bepaid to horizontal surfaces. Wallcaps orcopings shouldbe sloped toshed water,and the joints sealed with a high-

performance sealant. Roofs should slope todrains, scuppers, or overhangs. Windowsillsand other projections should besloped to the outside so that water will runoff. A minimum 15-degree slope is gener-

ally recommended for sills.To prevent rainpenetration causedby

the kinetic energy or momentum of therain, roof overhangs and projectionssuchas balconies and porches providemuchthe same protection as an umbrella, inter-

cepting the rain drops before they strikethe wall surface. Similarly, a shield in frontof an opening will stopthe rain. A shieldcan be incorporated in the joints betweencomponents of the cladding suchas a bat-ten stripon vertical woodsiding, or the

joint itself can be shaped orbaffled toavoid a straight-through opening (Figure3).It is not necessary for these devices to besealedtightly to prevent the momentumof the rain from carrying it through theopening, so small imperfections are of lit-

tle consequence.To prevent the penetration of water

caused by surface tension, the surfacepro-file should be designed tobreak the surface

tension and force waterto drip off. Since watercannot flowupward without some force beingapplied, a sharp change in direction to another

vertical surfacewill cause the waterto collectuntil its weight is sufficient that gravity over-

comes the surfacetension, and the waterdripsdownward. Figure 4shows several examplesofdrip profiles or drips that can be incorporated

in masonry, precast, EIFS, and stucco systems.Capillary suction can be eliminated by

interrupting the passageswith a sheet materialsuch as metal flashing ora waterproofingmembrane. Capillary suction can also be elimi-

nated by introducing an airgap into the mois-ture path (Figure5). Air gaps, drainage cavities,and gravel beds are effective capillary barriers

commonly used in drainage type wall systems such as masonrycavity walls, underfloorslabs, and adjacent to basement walls.Airgaps in vertical walls or joints can also serve as drainagechannelsto direct penetrated waterback to the outside.

To prevent the penetration of rain blown against a buildingby aircurrents, openings in the enclosure must be shielded in amanner similarto that discussed under kinetic energy forces.

Roof overhangsprotect the tops of walls, balcony and porch

Figure 4: Drips breaksurface tension.



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projections shield door openings, and awning-type windowsallow naturalventilation while shielding the opening from blow-ing rain. Similarly, a shieldincorporated in the joints betweencomponents of the cladding in theform of baffles or splineswill

Figure 5: Capillary breaks prevent suction.

Figure 6: Hydrostatic pressure relief.

prevent rain from blowing into the opening.

Equalizing the pressure differentialson two opposite sides ofa wall stops rain penetration caused by air pressure. Pressureequalization requires an airchamberbetween the inside and out-

side face of the enclosure that can be sealedagainst airleakage at the sides and at theinside plane. Pressure equalization principleswere first developed for use in the curtainwall industry, but have been adapted with

varying degrees of successto other cladding

systems.

Wind pressure can also drive waterthrough openings and up a vertical surface.Where wind pressures cannot be equalized, aback dam or flashing overlap must be of suffi-

cient height to contain this water, dependingon the expected wind speed and pressure.

Hydrostatic pressure leaks can be prevent-

ed by a waterproofbarriermembrane andbelow-grade drainage. The hydrostatic pres-

sure can be relieved with a permeabledrainage mat orgravel layeradjacent to base-ment walls and belowslabs(Figure 6).BARRIERS, DRAINS, AND RAINSCREENS

The design conceptswhichincorporateoneor more of these three basic moistureprotection strategiesare barriersystems,drainage systems, and rain screen systems.Barriersystems attempt to limitthe number ofopenings in a building envelope. Drainagesystems limit the amount of waterthat canaccumulate by controlling its flow. Rainscreens neutralizesome of the primary forceswhichmove water through the enclosure.

Barrier SystemsBarriersystems are also sometimes

referred to as face seal or primesealmethods ofmoisture protection.Barriersystems are commonly used inroofing, below-grade waterproofing,EIFS, architectural precast cladding,and metal building systems(Figure 7).Barriersystems rely exclusively on anairand water seal at the exteriorweathering surface. All exteriorjointsand openings must be perfectlysealed orwater will enterand betrapped inside the wall. To expectsuch perfection is unrealistic becausea building that relies on joint sealants

and surface treatments asitsfirst andonly lineof defensewill eventuallyleak.

Barriersystems are unforgiving ofeven minorerrors in design or instal-



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Figure8: Drainagetype systems.

Figure 7: Rainbarrier type systems.lation. Exterior seals are difficult toachieve and maintain becauseof expansion and contraction and long-term exposure to ultra-

violet radiation. Interstitial condensation from interior moisturesources can also be a problem since barrier systemsdo not incor-porate any method of venting or drainage. Barrier systems are the least expensivemethodof providing initial weatherresis-tance, but they require frequent maintenanceof sealant joints toassure satisfactory performance.

Drainage SystemsDrainage systems incorporate a cavity or mem-

brane behindthe exterior weathering surface tocollectmoisturewhich may penetrate theouterplane. The cavity is then drained of penetratedwater by flashing and/or weeps (Figure 8). Drainagesystems can be usedin masonry, precast concrete,stucco, metal roofs andmetal windows. Drainagesystems provideredundant protection where theexterior skinstops most of the incident rain butdoes not haveto function as a perfect barrier. Theinternal collection and drainage system provides asecond lineof defense and prevents water frompenetrating to the interior. Drainage systemsaregenerally moreexpensive than barrier systems, butwhenproperly designed and constructed, theyrequireless maintenance, have a longerexpectedservice life, and provide a higher level of protectionagainst water penetration.

Drainage systems are more forgiving than barri-

er systems but can still experiencewaterpenetra-tion problems ifthe flashing andweeps arenotadequately designed and installed.Rain Screen Systems

Rainscreen systems create a pressure-equalizedcavity behind the cladding and must include a struc-

turally supported air barrier, a sealed and compartmented airchamber, adequate drains, and external vents. Wind pressure is transmitted to the cavity through drain and vent holes, while anair barrierand sealed compartments confine the cavity pressureand prevent airfrom moving to the building interior or aroundcorners(Figure 9).

Figure9: Rain screen system concept.



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As a rule of thumb, intentional openingsin the outer wallshould be ten times the area ofthe unintentional openingsin theair barrier. The force producingrain penetration and the quanti-ty of water penetrating the exterior surface isthus reduced toabout one tenth of what it wouldhavebeen. Theair chambermay also serveas a drainage cavity forcollecting penetratedwater and directing it back to theoutside.

Rainscreens are high performance wall systems. They pro-vide multiple layers ofprotection by limiting water penetration,collecting and draining penetrated water, and moving the prima-ry air seal fromthe weathering surfaceof the wall to a protectedlocation within the wall. Rainscreen systems are more expensivethanbarrier and drainage wallsystems, but they alsohave thelowest maintenance andthe highest safety factor in protectingagainst water infiltration.

Combining Protective StrategiesIn reality, many enclosure systems use a combination of

methods to resist waterpenetration. A building might consist ofbarrier membrane waterproofing below grade witha drainagemedium provided as a capillary break to hydrostatic pressure; a sloping roofto shed snow and rain; a pressure-equalized alu-minum curtain wallsystem; and a masonry cavity wall with flash-ing and weeps for drainage. A combination of protectivestrategies canbe selectedbasedon the type ofbuilding and itsoccupancy requirements, the severity of macro- and micro-

environmental exposure, and appropriateness for the typeofbuilding system under consideration.

**This article is based on material from Ms. BeallsbookThermal andMoistureProtection Manual(McGrawHill, 1999),reprinted here with permission.

REFERENCESAllen, Edward,ArchitecturalDetailing: Function, Constructability,

Aesthetics, New York, John Wiley & Sons, 1993.Lstiburek, Joseph and John Carmody, Moisture Control Handbook,

New York: Van Nostrand Reinhold, 1993.Quirouette, R.L. Rain Penetration Control, The Construction

Specifier, (November1994), pp. 48-56.

American Architectural Manufacturers Association,AluminumCurtain Wall Design Guide Manual, Palatine, Illinois, AAMA,1979.

Beall, Christine,Masonry Design andDetailing,4th ed., NewYork:McGraw-Hill, 1997.

ChristineBeallisaconsultingarchitectfromLakeMcQueeney,Texas,whospecializesinproblemsofarchitecturaldetailing,specifications,claddingsystems,andmoistureprotection.Shehaswrittenextensivelyonavarietyofdesignandconstruction topics,andhaspublishedseveralbooksincludingtheThermalandMoistureProtectionManualonwhichthisarticleisbased.ChristineisamemberofASTM,TheMasonrySociety,andtheAssociationforPreservationTechnology.ShehasbeenaspeakeratnumerousRCIfunctions,includingtheMarch2000RCIconventioninReno.Visitherwebsiteatwww.christinebeall.com.

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