Build a Better Home THE ENGINEERED WOOD ASSOCIATION APA WALLS AVOIDING MOISTURE ACCUMULATION IN WALLS Walls are an integral part of a structure’s weather-resistive system. Details in wall design and construction are important in preventing damaging moisture build-up, whether the moisture originates from outside or inside the building. The Build a Better Home program from APA – The Engineered Wood Association is designed to provide builders and homeowners with the construction guidelines they need to protect their homes against damaging moisture infiltration. Key elements in the building envelope are the roof, walls, and foundation. This publication outlines the two primary sources of moisture in wood wall construction and methods of preventing its accumulation. FIGURE 1 CROSS-SECTION OF WINDOW SHOWING INTEGRATION OF STRUCTURE’S WEATHER-RESISTIVE SYSTEM IN A WALL WITH WOOD SIDING Window sill with flange Pan flashing (or felt sill strip) continuation Wood structural panel sheathing Caulking/sealant with backer rod Drip edge Pan flashing (or felt sill strip) Weather-resistive barrier – lap over top of metal head flashing Wood siding Metal head flashing Sloped top and drip-edged head trim Sealant Sealant Window flange Weather-resistive barrier Wood structural panel sheathing Wood siding
Transcript
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THE ENGINEEREDWOOD ASSOCIATION
APA
WALLS
AVOIDING MOISTURE ACCUMULATION IN WALLS
Walls are an integral part of a structure’s weather-resistive system. Details in wall design andconstruction are important in preventing damaging moisture build-up, whether the moistureoriginates from outside or inside the building.
The Build a Better Home program from APA – The Engineered Wood Association is designed toprovide builders and homeowners with the construction guidelines they need to protecttheir homes against damaging moisture infiltration. Key elements in the building envelopeare the roof, walls, and foundation.
This publication outlines the two primary sources of moisture in wood wall construction andmethods of preventing its accumulation.
FIGURE 1
CROSS-SECTION OF WINDOW SHOWING INTEGRATION OF STRUCTURE’S WEATHER-RESISTIVE SYSTEM IN A WALL WITH WOOD SIDING
Window sill with flange
Pan flashing (or felt sill strip)continuation
Wood structural panel sheathing
Caulking/sealant with backer rod
Drip edge
Pan flashing (or felt sill strip)
Weather-resistive barrier –lap over top of metal head flashing
Wood siding
Metal head flashing
Sloped top and drip-edged head trim
Sealant
Sealant
Window flange
Weather-resistive barrier
Wood structural panel sheathing
Wood siding
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Water can accumulate in walls from twosources: water leaks, and vapor laden air that penetrates the wall to producecondensation. Water from leaks presentsthe greatest threat of water accumula-tion in walls. Since water can leakdirectly into the wall, it can quicklyaccumulate to levels that will degradethe wood components as well as otherproducts in the wall. Moisture vaporfrom air penetration and vapor diffusionare important, but represent muchsmaller amounts of water accumulation.
HOW WATER LEAKS INTOWOOD WALL CONSTRUCTION
Water leaking through the envelope of astructure is the largest contributor tobuilding damage. Leaks are caused by anumber of factors, including:
■ Improper or missing flashing■ Improper installation of weather-resistive barriers■ Poorly designed or executed wallintersections and penetrations
Wood structures have the ability toabsorb, distribute and dissipate smallamounts of water, especially from inter-mittent sources. The problems arisewhen there are design or constructionerrors that allow water into wall cavitiesat a rate that exceeds the structure’sability to absorb and eliminate the water.Wood construction will perform indefi-nitely but is subject to failure if exposedto prolonged wetting where the woodmoisture content exceeds 19 percent.
The control of water leaks into wallsinvolves proper design, constructionand maintenance. Design features suchas roof overhangs can provide moistureprotection. Proper construction incorpo-rates products like flashing, weather-resistant barriers, and caulks with thestructural and architectural componentsin such a way that water is deflected ordrained down and away from the wall.Proper maintenance of caulks and paintis necessary for long-term moistureperformance of walls.
PREVENTING LEAKS WITH FLASHING
Flashing is used to deflect water andthus prevent leaks around wall inter-sections, window and door openings,and penetrations. Flashing can be made from galvanized steel, copper,aluminum, lead, vinyl or cut strips ofweather-resistant barrier materials. Atsmall wall penetrations, such as exhaustvents, custom flashing is used in lieu of conventional flashing because of the irregular shapes.
Flashing directs water flow down andaway from the interior of the structureto the outside of the wall covering. Inevery example shown here, the weather-resistant barrier laps over the top edgeof the flashing. In such a manner, theflashing is part of a whole weather-resistive system that is continuouslyredirecting water flow down and awayfrom the interior of the structure.
Figures 1 through 14 illustrate examplesof typical flashing details for wood-framed walls with various exterior finishes.
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FIGURE 3
FLASHING WINDOW WHEN USING BUILDING PAPER
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WEATHER-RESISTIVE BARRIERS
Long-term durability of wood-framedwalls relies in part on protecting mois-ture sensitive wood sheathing with aprotective weather- or water-resistivebarrier (i.e., building paper), or anapproved alternate, such as “housewrap” that also sheds water. In thispublication, all references to weather-/water-resistive barriers and house wrap will bereferred to as weather-resistive barriers.
Weather-resistive barrier materials providea line of defense for the building enve-lope against the intrusion of water. Thematerials and labor costs associated withthese products provide inexpensiveprotection for structural components andreduce the risk of moisture accumulationand damage to moisture sensitive materi-
doors, plumbing hose bibs, electricalboxes, wall-mounted air conditionersand vents for appliances; and at junctures with horizontal surfaces such as exterior decks and cantileveredbalconies, and sloping roof-to-wallsurfaces. It is especially important inthese cases to insure that water leaks are not directed down the sheathingonto structural elements below, or into the wall cavity.
Figures 1 through 14 illustrate how theweather-resistive barrier materials andflashing work together to channel anywater running down the inside face ofthe barrier over the flashing and outsideof the envelope.
als. The barriers must be installed prop-erly, however, to prevent water leaks intothe wall cavity. Proper installation atcorners and intersections and incorpora-tion of flashing are particularly important.
The basic principle behind weather-resistive barrier materials is to provide acontinuous drainage plane that shedsmoisture down and away from the planeof the structural wall surface. This isaccomplished by overlapping successivelayers of weather-resistive barrier behindthe exterior finish and over the struc-tural sheathing. In conjunction withproperly applied flashing, weather-resistive barriers direct leakage awayfrom the wood structural panel sheath-ing. This principle also extends to wallpenetrations, such as windows and
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FIGURE 4
SILL FLASHING AT SLIDING GLASS DOOR
FIGURE 5
PROPER INSTALLATION OF Z-FLASHING IN A SINGLE WALL SYSTEM WITH APA RATED SIDING
1/8" gap to prevent wicking
Exteriorwall stud
Lower course of weather-resistive barrier
Lower panel
Z-flashing
Upper course of weather-resistive barrier overlaps Z-flashing
Deck flashing
Caulk/sealant orsolder betweenpan flash and deck flashing
Sill or pan flashing counter-flashes deck flashing and/or weather-resistive barrier protecting wall below
Caulk/sealant betweenframe and flashing
Seal all penetrationsthrough pan
Weather-resistivebarrier
Sliding glass door frame
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FIGURE 6
CROSS SECTION OF WINDOW SHOWING INTEGRATION OF STRUCTURE’S WEATHER-RESISTIVE SYSTEM IN A WALL WITH BRICK VENEER
Weather-resistive barrier
Air gap
Woodstructuralpanel
Through-wallflashing (installedbefore window)
Sealant
Wood structuralpanel sheathing
Weather-resistive barrier
Air gap
Through-wallflashing
Steel lintel
Sealant
Weepholes
1" min.
Weepholes
FIGURE 7
CROSS SECTION OF WINDOW SHOWING INTEGRATION OFSTRUCTURE’S WEATHER-RESISTIVE SYSTEM IN A WALL WITHPORTLAND CEMENT STUCCO EXTERIOR WALL COVERING
Flashing over drip cap
Two layers of weather-resistive barrier
Stucco
Sealant
Flashing under sill
Stucco
Two layers ofweather-resistive barrier
Metal lath
Metal lath
Wood structural panel
Wood structural panel
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FIGURE 8
FLASHING AND WEATHER-RESISTIVEBARRIER INSTALLATION AT BRICK LEDGE
Weather-resistive barrier
Brick tie
Wood structuralpanel
Weepholes
Air gap
Treated lumber
FIGURE 9
FLASHING INSTALLATION AT TERMINATION OF PORTLAND CEMENT STUCCO EXTERIOR WALL COVERING
Where wood contacts stucco, cover with two layers of weather-resistive barrier
Flashing
Grade
Note:Metal lath and weather-resistive barrier must extend down over flashing
Wood structural panel
Metal lath
Treated lumber
Wall Intersections and PenetrationsUnique construction detailing situations call for special attention, including:
■ Deck to wall intersections■ Wall to roof intersections■ Gutter to roof or wall intersections■ Skylight installation
Figures 10 and 11 illustrate typical wall intersection details.Figure 12 shows a very common wall penetration detail. Figure 13 is an example of the detailing required at the intersection of an outside deck and an exterior wall. Figure 14 shows the flashing and use of building paper around a door opening.
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FIGURE 12
INTEGRATION OF STRUCTURE’S WEATHER-RESISTIVE SYSTEM AT A TYPICAL WALL PENETRATION
Weather-resistive barrieroverlaps flashing
Weather-resistive barrier under flashing
Nail first piece of flashing at top
Set second piece of flashing in caulking
2" overhang
3"
6"
Nail second flashing piece at top
Sheathing not shown for clarity
FIGURE 11
FLASHING INSTALLATION AT PORTLAND CEMENT STUCCO-TO-ROOF INTERSECTION
Base flashing
Roofing androof deck
Counter flashing
Metal lath Wood structuralpanel
Two layers of weather-resistive barrier
Note: Metal lath and weather-resistive barrier must extend down over flashing
FIGURE 10
FLASHING INSTALLATION AT BRICK VENEER-TO-ROOF INTERSECTION
Weather-resistive barrier
Through-wall flashing
Weepholes
Counter flashing
Base flashing
Air gap (1" typical)
Weather-resistive barrier
Roofing and roof deck
Brick tie
Wood structural panel
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FIGURE 15
CAULKED JOINT GEOMETRY
Backer rod
Hour-glass caulk profile
Fillet caulk profile
FIGURE 13
WEATHER-RESISTIVE SYSTEM AT AN EXTERIOR DECK
Metal flashing
Use staggeredbolts or screws
Sealant
Weather-resistive barrier
Use bearing plates, washers, or prefab spacers to allowfor drainage
Wood structural panel
Treated lumber
FIGURE 14
PROPER INSTALLATION OF BUILDING PAPER – SHOWN AROUND A DOOR OPENING
6" min. vertical laps of building paper
Horizontal laps of building paper (2" min.) 4"-6" recommended
3rd course of building paper
Sealant betweenflashing andbuilding paper
Pan flashing
Install successive layers of building paper“shingle-lap” layers starting at bottom and proceed to top of wall
2nd course of building paper
1st course of building paper
Flashing
Flashing underbuilding paper. Seal to sheathing.
CAULKING AS AWATERPROOFING COMPONENT
Elastomeric exterior sealants, known ascaulks, are a popular component of thewaterproofing system used in modernstructures. Used to seal the cracksbetween individual elements of thebuilding’s exterior finish, caulks helpkeep wind and water from penetratingthe skin of the structure. Caulks arenever perfect, even when carefullyinstalled. However, caulks can be usedas a secondary or tertiary part of theweather-protective system.
Caulks are not permanent! They havea limited lifetime and must be replacedon a periodic basis. As a result, acaulked joint cannot be the sole form of waterproofing at a given location.Intelligent building design, the use ofback-up methods of waterproofing suchas building paper or “house wrap,” and
proper placement of flashing shouldalways be used in conjunction withcaulked joints.
A caulked joint is ineffective if it is notproperly applied to a clean surface. Awell-caulked joint should maximize thesurface area between the caulk and theapplication surface. It should also have asmaller cross section between the con-tact surface than it has at the matingsurface. Figure 15 illustrates this. Asmaller cross section in the middle ofthe caulked joint allows differentialmovement within the caulked joint,alleviating concentration at the matingsurface between the caulk and the sur-face to which it is applied. While thecaulk is flexible, the joint between thecaulk and the mating surface is not. Ifstress is concentrated at the joint, it willcause premature failure of the joint.
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MOISTURE FROMCONDENSATION
Condensation of vapor is a source ofmoisture intrusion. Condensationoccurs if there is a significant drop in the air’s temperature as it passesthrough an insulated wall such that the air temperature falls below the dewpoint. The dew point is the temperatureat which moisture vapor in the air con-denses. If it happens to be within thewall cavity, the building materials absorbthis moisture, and thus the moisturecontent of the building materialsincreases. The moist air can enter fromthe inside or the outside, depending on the vapor pressure differential acrossthe wall. In a hot, moist climate with air-conditioned buildings, there couldbe infiltration from the outside to theinside. In cold, dry climates, the insideair leaking out could cause the problem.
Air Infiltration in Wood Wall ConstructionCondensation in wall systems may becaused by air infiltration. Even relativelysmall differential pressures across agiven wall can cause a large volume ofmoisture-laden air to leak into or out of a structure, thereby increasing therisk of condensation within the wall.
buildup can cause moisture damage tothe structure and degrade the livingconditions therein.
An air infiltration barrier such as housewrap retards the flow of moisture-ladenair into the wall cavity. Because it doesnot matter where the airflow is stopped,the air barrier can be placed on theinside or outside surface of the wall. In a cold climate that requires a warm-sidevapor retarder, the vapor retarder mayact as the air barrier as well, if properlyapplied and sealed.
Air Infiltration BarriersDifferential air pressures existing acrossthe wall cause air infiltration. This differ-ential air pressure can be caused by anunbalanced ventilation system, the stackeffect caused by hot air rising within thestructure, the use of unvented heatingappliances, or wind. The actual differen-tial pressure does not have to be verylarge to cause a significant amount of airleakage in one direction or another. Ifthe moisture-laden airflow persists for asignificant length of time, the moisture
FIGURE 16
PROPER AIR BARRIER INSTALLATION DETAILSTWO-STORY WALL SHOWN (not to scale)
6"-12" overlap at corner and vertical joints Upper roll overlaps
bottom by 6"
2"-3" overlap at sill plate/foundation
Cut air barrier andwrap around sill andsides of window opening
6"-12" overlap at allhorizontal joints.
Tape all tears and holes
Attach per manufacturer’srecommendations
Tape all joints withair-barrier tape.
Wood structural panel
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Because the major model building codesallow air infiltration barriers to be usedin lieu of building paper for most appli-cations, the use of these products is onthe rise. To get the full benefits of an airbarrier, it must be sealed as describedbelow to ensure that it is airtight.
Air infiltration barriers are available inrolls up to 9 ft wide, allowing the builderto wrap the barrier all the way around the house during construction. This isthe origin of the term “house wrap.” The large size speeds up installation andminimizes the number of seam seals.When the wrap is used as an air barrier,all of the splits, seams, penetrations anddamaged areas must be repaired using aspecial adhesive-backed seam tape. Inthis respect, an air barrier differs from avapor retarder.
Figures 16 and 17 show general installa-tion techniques for proper application ofan air barrier.
Vapor Transmission in Wood Wall ConstructionVapor transmission is the molecularpassage of water through the compo-nents of a building. A differential watervapor pressure across the wall causesthis movement. In cold weather, vaporfrom the interior of the structure canpermeate through the interior wall finishand condense on cooler framing andsheathing surfaces in the wall cavity ifthere are surfaces colder than the dewpoint temperture. To prevent this, aneffective interior (‘warm side’) vaporretarder, installed beneath the interiorwall finish, is recommended by codesfor most cold climate regions.
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The “warm side” vapor retarder in exte-rior walls may be omitted in regionswith moderate temperatures, such asthe southern and southeastern UnitedStates. In warm, humid regions close tothe Gulf of Mexico, and in Hawaii andthe Caribbean regions, where air condi-tioning is prevalent, the vapor retardershould be installed on the exterior sideof the wall, behind the sheathing. Thiswill prevent humid air from penetratinginto the wall cavity and causingincreased condensation on the cooler interior wall surface.
Selection and Installation of Vapor RetarderWhen the warm side is determined tobe the inside wall, the vapor retardercan be a kraft paper or foil/kraft paperfacing on the wall insulation. The effec-tiveness of this vapor retarder dependson how carefully the insulation isinstalled. The most effective installationtechnique is to cut the insulation battlength slightly oversize so it can befriction-fit to avoid gaps at the top andbottom wall plates. Also, the installationtabs of the insulation facing should belapped and stapled onto the nailingsurface of the studs, instead of the sidesof the studs, to “seal” the insulationfacing against air and moisture leakage,and to minimize gaps between theinsulation and studs.
Alternatively, an effective continuousvapor retarder can be installed by usinga separate layer of 4-mil polyethylenesheeting, stapled over the interior sideof the wall framing. In this case, unfaced
FIGURE 17
AIR INSTALLATION DETAILS USING HEADER WRAP
Header wrap
Header wrap
House wrap
Seam tape
Woodstructural panel
Woodstructural panel
insulation without an integral vaporretarder facing may be used, and frictionfit to fill the stud cavities without gaps.
While polyethylene sheeting makes avery good vapor retarder, it is relativelydifficult to install. In most cases, the useof polyethylene is not necessary, even invery cold regions. Ordinary interior latexpaint applied over drywall can providesufficient vapor retardant properties.
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INSTALLING RAIN-SCREEN WALLS
The entire exterior finish, weather-proofing, and flashing system in woodconstruction relies on gravity to keepbulk water out of the building envelope.Wind-driven rain can compromise thesesafeguards because the water is hittingthe wall from a different angle. If wind-driven rain is an infrequent occurrence,the forgiving nature of wood construc-tion can often account for the occa-sional influx of water into the buildingsystem. The moisture will be removedthrough capillary suction and the entirebuilding frame will dry.
In areas where wind-driven rain is frequent, the amount of water driveninto the wall system could be moredamaging. In these cases, double-wallconstruction – also known as a rain-screen wall – is often used. Double-wallconstruction creates an air spacebetween the exterior finish system andthe weather-resistive system. This sepa-ration is made with the use of pressure-treated lumber spacers that are installedvertically and carefully detailed aroundopenings and penetrations to allowdrainage of any water that makes itthrough the exterior finish. This space –3/4 to 1 inch – is open at the bottom topromote drainage and closed at the topto allow the air space to equalize withthe exterior air pressure. The opening atthe bottom has a pest screen. This sys-tem is often used with an interior airbarrier to allow the air pressure in theinterior of the wall to equalize with thatin the air space behind the cladding.This will eliminate the driving force thatcauses water to leak into the wall.
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FIGURE 18
RAIN-SCREEN WALL DETAILS
Weather-resistive barrier
Wood studs
Drywall
Wind
Furring/airspace
Siding
Vapor retarder(if appropriate)
Wood structural panel sheathing
Wind
Wind
Wind
Siding
Furring/airspace Weather-resistive barrier
Wood structural panel sheathing
Vapor retarder (if appropriate)
Drywall
Wood studs
Insulation baffel
Treated lumber
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APA’s services go far beyond qualitytesting and inspection. Research andpromotion programs play importantroles in developing and improving paneland engineered wood systems, and inhelping users and specifiers betterunderstand and apply products.
For additional information on woodconstruction systems, contact:
APA – The Engineered Wood Association,P.O. Box 11700, Tacoma, Washington98411-0700.
More Information OnlineVisit APA’s web site at apawood.org formore information on engineered woodproducts, wood design and construc-tion, and technical issues and answers.
Online publication ordering is alsoavailable through the web site.
THE ENGINEEREDWOOD ASSOCIATION
APA
ADDITIONAL INFORMATION
About APA – The Engineered Wood AssociationAPA – The Engineered Wood Association is a nonprofit trade association whosemember mills produce approximately70 percent of the structural wood panel products manufactured in North America.
The Association’s trademark appearsonly on products manufactured bymember mills and is the manufacturer’sassurance that the product conforms tothe standard shown on the trademark.That standard may be an APA perfor-mance standard, the Voluntary ProductStandard PS 1-95 for Construction and Industrial Plywood, or VoluntaryProduct Standard PS 2-92, PerformanceStandards for Wood-Based Structural-Use Panels. Panel quality of all APA trademarked products is subject toverification through an APA audit.
We have field representatives in most major U.S. cities and in Canada
who can help answer questions involving APA trademarked products. For additional
assistance in specifying engineered wood products, contact us:
APA – THE ENGINEERED WOOD ASSOCIATION
HEADQUARTERS7011 So. 19th St. ■ P.O. Box 11700Tacoma, Washington 98411-0700
(International Offices: Bournemouth, United Kingdom;
Mexico City, Mexico; Tokyo, Japan.)
The recommendations in this publicationare based on a review of credible sources.APA – The Engineered Wood Associationmaintains a continuing program of labora-tory testing, product research and compre-hensive field experience on engineeredwood products. However, because theAssociation has no control over quality ofworkmanship, the conditions under whichengineered wood products are used, or thequality of other associated products used inconstruction or their interaction, it cannotaccept responsibility for performance ordesigns as actually constructed. Becauserequirements vary geographically, consultyour local architect, engineer or designprofessional to assure compliance withcode, construction, and performancerequirements.
