Durability of Energy-Efficient Wood-Frame Houses
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Durability of Energy-Efficient Wood-Frame Houses
Smulski, Stephen1
ABSTRACT
The energy-efficient wood-frame houses of the 1980s and 1990s are the most expensive, the most comfortable, and very
likely the least durable residential structures ever built in the United States. This is due largely to the rapid evolution of
building materials and construction practices in the 20th century—particularly since the global energy crisis of the 1970s—
that resulted in the creation of houses whose walls became progressively tighter and tighter. Tight walls markedly improve
energy efficiency and occupant comfort. Those that get wet, however, tend to stay wet longer, and in some cases,
accumulate enough moisture to cause mildew, extractive staining, and peeling of exterior coatings, and rot in windows,
doors, trim, siding, sheathing, and framing, all within a few years of construction. Because tight walls hinder the escape of
moisture released inside a house, excessive window condensation and mildew can appear in living spaces. The switch
from leaky, warm walls forgiving of getting wet to tight, cold, less forgiving walls is but one reason for the lesser
durability of some new houses. Declining technology transfer, dubious designs, poor construction practices, and new
wood products that have not performed as promised share the blame.
INTRODUCTION
The energy-efficient wood-frame houses of the 1980s and 1990s are the most expensive, the most comfortable, and very
likely the least durable residential structures ever built in the United States. Over the past 20 years, the occurrence of
moisture-caused problems in new houses has skyrocketed. Homeowners complain increasingly of window condensation;
of mildew and mold indoors; of mildew, extractive staining, and peeling of exterior coatings; and of rot in windows,
doors, trim, siding, sheathing, and framing, all within a few years of construction.
TIGHTER WALLS, COLDER WALLS
Historically, and perhaps unwittingly, the architects, builders, and occupants of wood-frame houses relied upon natural
infiltration and exfiltration of air as a means of both controlling indoor relative humidity and keeping walls dry.
Differences in temperature, vapor pressure, and air pressure across a house’s envelope provide the driving force for
moving air through random leaks in its walls, foundation, and attic. As a result, warm, moist indoor air is constantly
flushed out and replaced with cooler (and usually drier) outdoor air. Once inside, the cooler air is heated and its relative
humidity lowered. This newly warmed and now drier air absorbs water vapor inside the house, then carries it outside as it
escapes through gaps in the envelope.
Reliance upon natural leakage of air through random holes in wood-frame houses worked well for centuries. However, the
rapid evolution of building materials and construction practices in the 20th century occasioned the creation of houses
whose envelope became progressively tighter and tighter. In the 1920s, for example, insulation began to be added to walls
on an expanding scale. Two things changed as a consequence. First, insulation posed a physical barrier that hindered
movement of air and conduction of heat through a wall. Second, the insulation often lowered the temperature of the
interior side of the sheathing (at the time, boards installed on the diagonal) to below the dew point during cold weather.
Mildew and mold sometimes appeared on the back of the sheathing as a result of condensation from warm, moist indoor
air that leaked through or around the insulation. Sills and the bottoms of studs sometimes rotted as well. To improve the
thermal performance of insulation and to protect it from being wetted by rain seepage, builders applied rosin paper or
building felt over sheathing, further reducing airflow through walls. The trend towards building tighter, and therefore
colder, walls continued through the 1940s and 1950s as builders switched from lath and plaster to gypsum wallboard on
the inside and from lumber sheathing to insulation board or plywood on the outside.
1
President, Wood Science Specialists Inc., 453 Wendell Road, Shutesbury, Massachusetts 01072, U.S.A.The introduction in the 1960s and beyond of electric heat; high-efficiency, low-draft furnaces; power-vented, sealed
combustion appliances; and heat pumps brought about the demise of the customary active chimney. Elimination of the
active chimney meant that large volumes of moisture-laden air were no longer routinely being expelled from inside a
house. As a result, less replacement air was needed and therefore less outdoor air was being drawn inside through leaks in
the envelope. In the 1970s and 1980s, widespread adoption of insulating windows and doors, continuous vapor retarders,
and air infiltration barriers, coupled with extensive use of caulks, sealants, gaskets, and tapes, further reduced the amount
of air and heat flowing through a house’s envelope.
The upshot is that the walls of older houses (defined here as those built before the global energy crisis of the 1970s) tend
to be leakier, warmer, and more forgiving of getting wet, while those of newer houses tend to be tighter, colder, and less
forgiving. When the walls of an older house get wet, they are soon dried sufficiently by air and heat from inside the house
flowing through them. In contrast, the passage of air and heat through the tight walls of newer houses is drastically
reduced. As a consequence, tight walls that get wet tend to stay wet longer, and in some cases, accumulate enough
moisture for mildew, mold, and decay fungi to go to work.
HOW ENERGY-EFFICIENT HOUSES GET WET
Energy-efficient houses can be wetted by ground water, piped water, condensation, and precipitation. In some cases, water
naturally contained in green framing members at the time of construction is at fault.
Ground water
Soil surrounding a house’s foundation and floor slab always contains water in both liquid and vapor form. Liquid water
can seep into basements and crawl spaces through shrinkage and settlement cracks, joints, utility cutouts, and other
penetrations in foundation walls and floors. It can also be drawn by capillary suction from soil into the micropores inside
concrete and masonry. Water vapor in soil can diffuse through foundation walls and floors. Upon reaching an exposed
surface, the water evaporates, sometimes leaving behind a telltale crystalline efflorescence. Evaporating water increases
the relative humidity inside a basement or crawl space, which in turn, can raise the moisture content of sills, girders, joists,
and subflooring to mildew- and mold-susceptible levels.
Entry of liquid water into basements and crawl spaces can be eliminated by installing perimeter drains, by sealing cracks
and other points of entry, by applying waterproofing to the exterior of foundation walls, by backfilling with free-draining
soil, by grading soils so that they slope away from the foundation, and by installing gutters and downspouts along eaves.
Transport of water into a basement or crawl space via capillary suction or vapor diffusion through walls is controlled by
applying dampproofing to a foundation’s exterior. Capillary suction and vapor diffusion through the floor is prevented by
installing a vapor retarder (most commonly polyethylene sheeting) under the slab or over exposed soil in a crawl space.
The circle of dampness that sometimes shows up around the base of a foundation owes to capillary movement of water
from the soil through the footing and up into the wall. Known as rising damp, this can be stopped by placing polyethylene
over the footing before walls are cast or built. Sills, girders, and other framing members in direct contact with concrete
and masonry are prone to mildew, mold, and decay because capillary water can travel unimpeded from the soil through the
foundation directly into the wood. Wetting of wood by capillary water is prevented by inserting a capillary break of metal
or plastic between wood and concrete and masonry.
Piped water
The labyrinth of pipes inside a house that provides water and sometimes, heat, occasionally leaks. Forceful plumbing leaks
are typically discovered and corrected immediately. Although wood may be temporarily saturated as a result, it dries out
before fungi can get established. Slow, persistent leaks hidden inside walls and floors, however, can go undetected for
long periods, and can elevate the moisture content of wood to fungi-attractive levels. Condensation dripping from cold
water supply lines onto wood often leads to decay, especially in damp basements and crawl spaces. Prevention entails
insulating cold water pipes.
Condensation
The benefit of tight walls in a wood-frame house is a marked increase in energy-efficiency and occupant comfort. The
downside is that tight walls substantially lower the rate of natural ventilation. While it is not uncommon for an older houseto experience 3 to 5 (or more) air changes per hour, the replacement rate in an energy-efficient house may be as low as 0.5 to 1. As a result, water vapor generated by occupants’ activities and released from other interior sources lingers longer indoors and is carried into walls, floors, ceilings, attics, basements, and crawl spaces on convection currents of air flowing through joints between materials; penetrations in walls, floors, and ceilings; and other hidden air leakage paths. When warm, moist air entering into these spaces is cooled below the dew point, the excess moisture (the amount above 100 percent relative humidity) is deposited as condensation on framing, sheathing, and other cold surfaces, creating conditions favorable to mildew, mold, and decay fungi. Water vapor diffusing into these spaces can do the same. Significant sources of indoor moisture usually include soil moisture migrating through foundations and floor slabs; showering and bathing; humidifiers; clothes dryers that are not vented to the outside; indoor storage of firewood; and backdrafting of gas-fired appliances. Seemingly minor sources such as occupant respiration and perspiration; cooking; dishwashing; floor mopping; plants; aquariums; and seasonal desorption from building materials, when considered in total, can make a sizable contribution. In northern climates, condensation within walls, floors, ceilings, attics, basements, and crawl spaces can occur during both the heating and cooling seasons. Condensation on the inside of windows and within walls, floors, ceilings, and attics happens mostly during the heating season, and usually because of excessively high indoor relative humidity. Water condensed from warm, moist indoor air entering into these spaces is deposited as frost or ice which later wets framing and sheathing to fungi-favorable levels when it melts. In summer, water vapor held in hot, humid outdoor air entering into basements and crawl spaces can condense on cooler framing and subflooring, creating conditions irresistible to fungi. In southern climates, condensation is a problem primarily during the cooling season. Here, the culprit is hot, humid outdoor air that seeps into walls, floors, ceilings, attics, basements, and crawl spaces cooled to below the dew point by air conditioning. Framing and subflooring in crawl spaces under air-conditioned rooms is especially at peril. Condensation-caused problems can be kept in check by reducing indoor relative humidity in the following ways: eliminate potential sources of moisture; seal potential air leakage paths; place vapor retarders against the warm side of walls, ceilings, and floors; vent clothes dryers, heating appliances, and kitchen range and bath exhaust fans directly to the outside; provide the needed ceiling insulation and roof and attic ventilation; and dehumidify. Precipitation Siding, trim, windows, doors, and other exterior wood products are routinely exposed to dew, rain, and melted snow and ice. Because these products are almost always finished with a film-forming paint or solid color stain that repels liquid water, the underlying wood is generally wetted only superficially. However, liquid water can be driven by wind or drawn by capillary suction into uncoated wood inside joints and overlaps to wet products internally from the ends or back. Absorption of water into end grain surfaces is especially rapid. Because water enters wood as a liquid, but leaves it as a vapor, exterior wood products get wet much faster than they dry. Once wet, exterior wood products on energy-efficient houses take even longer to dry because of the reduced air flow through tight walls and their generally lower temperature. As a consequence, some tight walls accumulate enough moisture to cause mildew, extractive staining, and coatings failure or rot in exterior wood products, sheathing, and framing. A film-forming coating on the surface of exterior wood retards the escape of water vapor outward into the air. The housewrap and sheathing behind exterior wood slows its movement inward, while the vapor retarder deeper in the wall essentially stops it altogether. Rather than drying out exterior wood completely, heat from the sun drives some of the water to the back of the product during the day. The process is reversed at night when warmth from inside the house pushes the water back to the product’s face. That is why a blister under a coating, when pricked, is often found to be filled with water early in the morning, but empty in the late afternoon. The sun can drive water vapor through the housewrap, where it is absorbed by the sheathing and transferred to the framing. Through repeated wettings, enough water eventually accumulates in exterior wood to raise its moisture content to the point where the coating mildews, stains, blisters, cracks, and peels, or the wood rots. Plywood, OSB, and dimension lumber behind exterior wood can temporarily store only so much water before they too become susceptible to decay. Precipitation-caused problems can be avoided in new construction by installing siding over furring strips applied over sheathing and housewrap. This creates a vented air space behind the siding into which water driven by the sun to the back of the siding can harmlessly evaporate. Lap siding already in place can be retrofitted with thin plastic shims called siding
wedges that are inserted into the overlap between courses. The gap opened by the wedges reduces the potential for water
to be sucked into the overlap by capillarity, and allows air to reach and dry out the back of the siding, and sheathing and
framing as well.
Design features that promote water-shedding include steep roof pitches, flashing in roof valleys and at roof/wall
intersections, chimney crickets, wide eaves, door and window flashings with built-in drip edges, beveled horizontal trim,
and sloped window sills. Gutters and downspouts prevent water running off of a roof from cascading directly down walls,
or splashing back onto walls from a lower roof, deck, or the ground. The ends and back of siding, trim, windows, doors,
and other exterior wood products should be finished with a water repellent. The exposed surfaces of these products should
be finished with opaque film-forming coatings that combine high water repellency with high permeability to water vapor.
Exterior wood should be at least 8 inches off the ground to reduce wetting by splashback.
Green wood
Energy-efficient houses are sometimes framed with green timbers or green dimension lumber. Normally, water in the
wood is slowly and harmlessly released to the air inside the house or to surrounding materials as members dry, eventually
escaping to the outside. Under certain circumstances, however, water in green wood can give rise to window
condensation, facilitate growth of mildew and mold within walls, or permit members to decay.
The total water given off by a green timber frame can count in the hundreds of gallons, with the greatest release occurring
during the first heating season. Severe window condensation that damages sashes, sills, and walls sometimes results.
Solutions include using partially air-dried timbers and dehumidifying a house during the first heating season. Because
green dimension lumber (S-GRN in the grade stamp) is wet when solid-piled and banded, and does not dry during
shipment and storage, it is often infected with live mildew and mold, and sometimes, with active decay fungi, when
received at the job site. Once sandwiched between sheathing and vapor retarder, the drying of infected lumber is hindered,
allowing fungi to flourish inside walls. This can be avoided by using kiln-dried lumber (S-DRY or KD19) whose moisture
content is too low to support fungal growth.
THE LESSENED DURABILITY OF ENERGY-EFFICIENT HOUSES
The switch from leaky, warm, forgiving walls to tight, cold, less forgiving walls is not the only reason for the lesser
durability of some new houses. Declining technology transfer, dubious designs, poor construction practices, and new
wood products that have not performed as promised share the blame.
Declining technology transfer
The durability of energy-efficient wood-frame houses arises from the proficiency of their design and the quality of their
construction. Although the principles and practices for designing and constructing durable wood-frame houses are well
established and thoroughly documented, this information is either not reaching some practitioners or is being ignored. The
decline in technology transfer probably traces to retrenchment of the traditional routes by which this knowledge has been
communicated. Because of the emphasis placed on concrete and steel, students in architecture, engineering, and
construction programs in vocational schools, colleges, and universities often receive little or no instruction on the proper
use of wood in residential construction. Curtailment of outreach services provided by state cooperative extension agencies
and industry trade associations in the past decade has made the acquisition of information somewhat more difficult. The
demise of the apprenticeship system in the United States has largely denied novice builders the opportunity to learn from
experienced masters. The confluence of these events has left many architects and builders unaware of the new
considerations surrounding the proper use of wood in today’s tight-walled houses.
Dubious designs
Dubious designs that undermine the durability of energy-efficient houses flow directly from the decline in technology
transfer. The architect who is unaware of the importance of designing a wood-frame house to shed water cannot knowingly
incorporate the proper details into his or her plans. Hence, the recent prevalence of questionable architectural trends such
as the narrowing of eaves and a reluctance to use gutters. Devolution of the roofed porch into the open deck and the
popularity of complicated roofs with multiple intersecting planes, valleys, and dormers promote the wetting of walls by
splashing. Designs that sandwich wood between materials of low permeability can trap water and encourage mildew,
mold, and rot. This inherently risky approach was used in some houses in the 1990s when exterior insulation and finish
systems (EIFS) were introduced into residential construction. With EIFS, rigid foam panels are glued to plywood or OSBsheathing, then skim-coated with synthetic stucco. Touted as being impervious to water when correctly applied, many
EIFS installations leaked around doors and windows. The foam prevented wet sheathing and framing from drying to the
outside, while the vapor retarder blocked drying to the inside. Sheathing and framing quickly rotted. All wood-framed
walls will eventually get wet during their lifetime; they must be designed to dry either to the inside or the outside.
Poor construction practices
Good house designs can be defeated by poor construction practices that allow excessive moisture to build up inside or
permit water to intrude from outside. Most poor construction practices stem from declining technology transfer. The
builder who has not been told of the importance of back- and end priming wood siding prior to installation is unlikely to
realize intuitively the benefit of doing so. Labor statistics reveal that builders’ ranks swell during economic upturns, then
shrivel when booms go bust. The effect is that much of the workforce building houses today is transient and lacking in
some of the training, skills, knowledge, and experience necessary to construct durable houses. Some builders continue to
commit such basic errors as venting clothes dryers and bath and kitchen exhaust fans into attics, basements, and crawl
spaces; blocking soffit vents with insulation; and omitting flashing and caulking around doors and windows.
New products, new problems
Even the best design and construction practices can fall victim to wood products that are not inherently durable. A few
wood products introduced in the last two decades have not performed satisfactorily in the field over time.
The 1980s saw the introduction of an ill-conceived OSB lap siding. Consisting of a narrow strip of OSB with an uncoated
back, primed edges, and a resin-impregnated paper overlay on its face, this product was fated to fail. Dew, rain, and
melted snow and ice are readily drawn by capillary suction into its edges, ends, and back, then trapped inside by the
overlay. Made from aspen and the sapwood of softwoods that possess no natural decay resistance, OSB lap siding
frequently rotted within a few years of installation.
Hundreds of millions of dollars have been spent replacing failed products and fueling litigation spawned by the OSB lap
siding debacle. The wood products industry suffered a black eye and many builders burned by this blunder switched to
non-wood alternatives.
More recently, finger-jointed and edge-glued solid wood siding and trim are proving to be problematic. The number of
complaints of telegraphed joints and open joints in these products is climbing. Despite being manufactured for exterior
use, many of these products are made with Type II water-resistant adhesives rather than waterproof adhesives. Apparently,
manufacturers are relying on the film-forming coating applied to these products to protect the glueline from getting wet.
The Achilles’ heel of this approach is that differential shrinkage and swelling of the wood on either side of a joint can
rupture the coating, allowing water to have access to the adhesive, thereby degrading it. Because much of the wood used is
sapwood, rot sometimes follows. These products would benefit from waterproof adhesives, grain matching, and a higher
moisture content at the time of gluing.
ENSURING THE DURABILITY OF ENERGY-EFFICIENT WOOD-FRAME HOUSES
The most important consideration in ensuring the durability of energy-efficient wood-frame houses is to utilize design
features and construction practices that keep wood as dry as possible and promote drying if the wood gets wet. Experience
has shown that the moisture-caused problems plaguing new houses can be largely avoided by:
• Employing design features and construction practices that promote water-shedding;
• Installing wood siding on furring strips;
• Back- and end priming exterior wood products;
• Finishing exterior wood products with opaque, film-forming coatings of high permeability;
• Designing wood-frame walls to dry either to the inside or the outside;
• Applying a continuous vapor retarder to the warm side of walls and ceilings;
• Utilizing kiln-dried framing lumber;
• Using caulks, sealants, gaskets, and tapes to seal potential air leakage paths;
• Using design features and construction practices that limit migration of soil moisture into basements and crawl
spaces;• Dehumidifying basements and crawl spaces during the cooling season;
• Venting clothes dryers, and bath and kitchen range exhaust fans directly outdoors;
• Keeping indoor relative humidity below 40 percent during the heating season;
• Augmenting low rates of natural air exchange with mechanical ventilation;
• Providing adequate roof and attic ventilation.
As evidenced by Norwegian stave churches, Asian pagodas, and the colonial meeting houses and covered bridges of New
England, wood-frame houses designed, constructed, and maintained in accordance with these principles can last for
centuries.
BIBLIOGRAPHY
Air Vent Inc. 1987. Principles of Attic Ventilation. Air Vent Inc. Peoria Heights, Illinois.
American Society of Heating, Refrigerating and Air-Conditioning Engineers. 1985. Moisture in building construction.
Pages 21.1-21.20 in ASHRAE Handbook Fundamentals. ASHRAE. Atlanta, Georgia.
Anderson, L. 1972. Condensation Problems: Their Prevention and Solution. USDA Forest Service Research Paper FPL
132. Forest Products Laboratory. Madison, Wisconsin.
Angell, W. and W. Olson. 1988. Moisture Sources Associated with Potential Damage in Cold Climate Housing. CD-FS-
3405. Cold Climate Housing Information Center. St. Paul, Minnesota.
Canadian House Builders’ Association. 1989. Canadian House Builders’ Association Builders’ Manual. CHBA. Ottawa,
Ontario, Canada.
Hutcheson, N. and G. Handegord. 1983. Building Science for a Cold Climate. John Wiley & Sons. New York, New York.
Kubler, H. 1982. Effect of Wood on Humidity in Homes. Forest Products Journal. 32(6):47-50.
Lstiburek, J. 1987. How Insulation Can Peel Your Paint. New England Builder. June:27-31.
Lstiburek, J. and J. Carmody. 1993. Moisture Control Handbook. Van Nostrand Reinhold. New York, New York.
National Association of Home Builders. 1978. Basement Water Leakage…Causes, Prevention, and Correction. NAHB.
Washington, D. C.
Oatman, L. and C. Lane. 1988. Mold and Mildew in the House. NR-FO-3397. Cold Climate Housing Information Center.
St. Paul, Minnesota.
Olin, H., J. Schmidt and W. Lewis. 1990. Moisture control. Pages 104-1-104-24 in Construction Principles, Materials &
Methods. Van Nostrand Reinhold. New York, New York.
Peterson, R. and L. Hendricks. 1988. Humidity and Condensation Control in Cold Climate Housing. CD-FO-3567. Cold
Climate Housing Information Center. St. Paul, Minnesota.
Peterson, R. and L. Hendricks. 1988. Ceiling Airtightness and the Role of Air Barriers and Vapor Retarders. CD-FO-
3568. Cold Climate Housing Information Center. St. Paul, Minnesota.
Peterson, R. and L. Hendricks. 1988. Exterior Wall Airtightness and the Role of Air Barriers and Vapor Retarders. CD-
FO-3569. Cold Climate Housing Information Center. St. Paul, Minnesota.
Quarles, S. 1989. Factors influencing the moisture conditions in crawl spaces. Forest Products Journal. 39(10):71-75.Rose, W. and A. TenWolde. 1994. Issues in crawl space design and construction—A symposium summary. ASHRAE Transactions. 100(1). Scheffer, T. and A. Verrall. 1973. Principles for Protecting Wood Buildings from Decay. USDA Forest Service Research Paper FPL 190. Forest Products Laboratory. Madison, Wisconsin. Sherwood, G. 1985. Condensation Potential in High Thermal Performance Walls Hot, Humid Summer Climate. USDA Forest Service Research Paper FPL 455. Forest Products Laboratory. Madison, Wisconsin. Sherwood, G. and A. Tenwolde. 1982. Moisture movement and control in light-frame structures. Forest Products Journal. 32(10):69-73. Smulski, S. 1994. Energy conservation backlash: mildew, mold, decay and insects in wood-frame construction. Pages A123-A132 in Proceedings of the Joint Conference of the Energy Efficient Building Association and the North East Sustainable Energy Association. EEBA. Wausau, Wisconsin. Smulski, S. 1994. Water in the walls: three case studies. Journal of Light Construction. 12(11):58-60. Smulski, S. 1997. Premature Failure of Paints and Solid Color Stains on Energy-Efficient Homes. Paint & Coatings Industry. 13(4):68, 70. Smulski, S. 1997. Controlling Indoor Moisture Sources in Wood-Frame Houses. Wood Design Focus. 8(4):19-24. Smulski, S. 1998. Preventing Wood Degradation. Pages 3.1-3.44 in Timber Construction for Architects and Builders. E. Goldstein. McGraw-Hill. New York, New York. Tenwolde, A. 1994. Indoor humidity and the building envelope. Pages 37-41 in Bugs, Mold and Rot II: Proceedings of Workshop on Control of Humidity for Health, Artifacts, and Buildings. National Institute of Building Sciences. Washington, D. C. Verrall, A. 1966. Building Decay Associated with Rain Seepage. USDA Technical Bulletin 1356. Southern Forest experiment Station. New Orleans, Louisiana. Verrall, A. and T. Amburgey. 1980. Prevention and Control of Decay in Homes. USDA Forest Service. US Government Printing Office. Washington, D. C. Zabel, R. and J. Morrell. 1992. Wood Microbiology Decay and Its Prevention. Academic Press, Inc., New York, New York.
Table 1. Indoor moisture sources.a _________________________________________________________________________________________________ source estimated release (pints) occupant-generated aquariums variable evaporative loss bathing tub (excluding towels and spillage) 0.12 shower (excluding towels and spillage) 0.52 per 5 minutes clothes washing (lid closed, standpipe discharge) virtually 0 per load clothes drying dryer vented outside virtually 0 per load dryer vented inside or indoor line drying 4.7 to 6.2 per load (more if gas dryer) combustion unvented kerosene space heater 7.6 per gallon of kerosene burned cooking breakfast (four persons) 0.35 (plus 0.58 if gas range) lunch (four persons) 0.53 (plus 0.68 if gas range) dinner (four persons) 1.22 (plus 1.58 if gas range) simmer in 6-inch pan at 203 §F for 10 minutes
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