Energy Efficient Wood Houses Farmington NM
Living With a Tight House
Today's energy-efficient wood-frame houses 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, moisture-caused problems in new houses have skyrocketed. Homeowners complain increasingly of window condensation and mold indoors; of mildew, staining and peeling of exterior coatings; and of rot in windows, doors, trim, siding, sheathing and framing, all within a few years of construction. 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 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 drier air absorbs water vapor inside the house, then carries it outside as it escapes through gaps in the envelope. Reliance on natural leakage of air through random holes in wood-frame houses worked well for centuries. But the rapid evolution of building materials and construction practices created houses whose envelope became progressively tighter and tighter. In the 1920s, for instance, insulation began to be added to walls on an expanding scale. This hindered the movement of air and conduction of heat through a wall. To protect the insulation from being wetted by rain seepage, builders applied rosin paper or building felt over sheathing - at the time, boards on the diagonal - and further reduced airflow through walls. The trend toward building tighter, and therefore colder, walls continued through the 1940s and '50s 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.
The introduction of electric heat; high-efficiency, low-draft furnaces; power-vented, sealed combustion appliances; and heat pumps brought about the demise of the active chimney. As a result, large volumes of moisture-laden air were no longer routinely being expelled from inside a house, and less replacement air from outside was being drawn inside through leaks in the envelope. In the 1970s and '80s, 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 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 wood-rotting fungi to go to work. How 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. After 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 mold-susceptible levels. Entry of liquid water into basements and crawl spaces can be eliminated by installing perimeter drains, sealing cracks and other points of entry, applying waterproofing to the exterior of foundation walls, backfilling with free-draining soil, grading soils so that they slope away from the foundation, and installing gutters and downspouts along eaves. Movement 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 - usually 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 mold and rot because capillary water can travel unimpeded from the soil through the foundation directly into the wood. This can be prevented by inserting a capillary break of metal or plastic between wood and concrete and masonry. Piped Water As we all know, the pipes inside a house occasionally leak. Forceful plumbing leaks are typically discovered and fixed 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 fungus-attractive levels. Condensation dripping from cold-water supply lines onto wood often leads to rot, 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 house to experience three to five 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 mold and wood-rotting 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 release 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 (above about 40 percent). Water condensed from warm, moist indoor air entering into these spaces is deposited as frost or ice, which later wets framing and sheathing to fungus-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 airflow 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 rot.
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 evaporate harmlessly. 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, as well as sheathing and framing. Design features that promote water shedding include steep roof pitches, flashing in roof valleys and at roof and wall intersections, chimney crickets, wide eaves, door and window flashings with built-in drip edges, beveled horizontal trim, and sloped windowsills. 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 mold within walls or permit members to rot. 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 will sometimes be the result. 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 mold, and sometimes with active wood-rotting fungi, when received at the job site. Once sandwiched between the 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. Other Causes of Failure 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 and poor construction practices share the blame.
Although the principles and practices of 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. Inadequate instruction on the proper use of wood in architecture, engineering and construction programs in vocational schools and colleges; curtailment of outreach services provided by state cooperative extension agencies and industry trade associations; and the demise of the apprenticeship system in the United States have left many architects and builders unaware of the new considerations surrounding the proper use of wood in today's tight-walled houses. 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 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 OSB sheathing, 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 rotted quickly. All wood-framed walls will eventually get wet during their lifetime; they must be designed to dry either to the inside or the outside. Even 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 is not told of the importance of back- and end-priming wood siding prior to installation is unlikely to realize intuitively the benefit of this priming. 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. The most important consideration in ensuring the durability of energy-efficient wood-frame houses is to use design features and construction practices that keep wood as dry as possible and promote the drying of wood that does get 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 vapor retarder to the warm side of walls and ceilings;
Using kiln-dried (S-DRY or KD19) framing lumber;
Using caulks, sealants, gaskets and tapes to seal potential air-leakage paths;
Employing 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, 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 of New England, wood-frame houses designed, constructed and maintained in accordance with these principles can last for many centuries. n Stephen Smulski, Ph.D., is president of Wood Science Specialists Inc. in Shutesbury, Mass., a consulting firm that specializes in solving performance problems with wood products.