b'YOUR TOOLKIT FOR BUILDING EXCELENCEsurface temperatures at the eave are below freezing andbe at the same absolute humidity as the upper chords. temperatures further up the roof are above freezing. In thisHowever, if covered with insulation and in direct contact situation, water running down the roof will form a buildupwith the finish ceiling, the lower chords will be measurably of ice (i.e., an ice dam) at the eave. As water pools behindwarmer than the upper chords throughout the late fall and this dam, the ice dam grows leading to a larger pool ofwinter months. This means that the relative humidity of the water. This can result in leaks through the roof into areasspace surrounding the lower chords will be significantly where the water can cause wood decay and mold growth.lower than the relative humidity of the air in contact with Alternatively, a decrease in temperature also freezes thethe upper chords. And it is the relative humidity (not the water behind the dam, and as it expands, it forces apartabsolute humidity) that largely dictates the equilibrium building components above the eave. Ice dams form inmoisture content (EMC) of wood. In fact, between 30F and buildings with poor attic ventilation or where a measurable80F, and between 20% and 60% relative humidity, wood quantity of heat is escaping into the space under the roof.EMC can be approximated within 0.2% by the equation:Ice dams can be largely prevented by (1) ensuring that theEMC (%, d.b.) = 0.16 x (%RH) + 1.3%entire underside of the roof surface is properly ventilated so that the temperature above and below the roof surface areDuring cold winter months, a difference in air relative humidity similar, and (2) installing ceiling insulation and foaming/ may occur between top and bottom chords of 60%, which for sealing around ceiling fixtures and ducts to prevent buildingmost conditions translates into an EMC difference around 10%. heat from reaching the underside of the roof. Alternatively, a building owner can remove roof snow shortly after aThe impact of truss uplift increases as the distance between snowfall, de-ice eave ice with calcium chloride, install heatingtruss bearing points increases. The most significant truss tapes on the eave, and/or blow outside air into the attic. uplift I have encountered was in a hog finishing building. In cold climates, vapor retarders are installed on theDue to significant uplift, interior partitions dropped out inside of thermal envelopes to prevent building moistureof the U-shaped plastic extrusions that held them in place from diffusing into exterior wall and roof cavities duringat the ceiling. In smaller structures, problems associated colder weather. When vapor retarders are incompletewith truss uplift are mostly cosmetic (e.g., nail pops, orhavebeencompromised,moisturediffusesintoopen seams in drywall) and can be eliminated through exterior cavities and condenses on exterior insulationproper attachment of drywall and interior walls to trusses.and sheathing or cladding. During prolonged periods ofBuildings in cold climates are subjected to freeze-thaw subfreezing temperatures, ice crystals will form and grow,cycles that can deteriorate concrete, masonry products, and remaining until outdoor temperatures move back abovemortars. This deterioration is significantly reduced with freezing at which time the melt water will trickle intoproper air entrainment and surface sealing to limit intrusion other areas where it can enhance decay and mold growth. of moisture. Be aware that ordering properly air-entrained Truss uplift occurs when the moisture content (MC) ofconcrete will not result in improved freeze-thaw resistance the trusss bottom chord is driven to a lower level than itsunless the concrete is properly placed. Figure 8 shows a top chord.A decrease in moisture content shortens theconcrete surface one year after placement. Although this bottom chord, and as with any beam (and a truss is a veryconcrete had a very low water/cement ratio and was ordered narrow and deep beam), when you shorten the bottom ofwith 6% air entrainment, it was over-worked after placement the beam, the beam arches upward. Truss uplift is largely(i.e., excessively confined to trusses in attic spaces that: (1) are well ventilatedpower-floated with outside air, (2) are isolated from a heated buildingandpower-interior with a properly installed vapor retarder that preventstroweled) which moisture from entering the attic space, and (3) has insulationremoved surface covering the bottom chords. In well ventilated attics inair entrainment.cooler climates, the moisture content of top chords typicallyFIGURE 8. reaches its maximum in December and its minimum inOVER-FINISHED AIR-ENTRAINED April. In the upper Midwest, the maximum MC rangesCONCRETE between 14.5% and 16% db, and the minimum betweenAFTER ONE 12% and 13% db (USDA Forest Service, FPL-RN-0268YEAR OF WINTER Equilibrium Moisture Content of Wood in OutdoorEXPOSURE.Locations in the United States and Worldwide by William T. Simpson). When isolated from the indoor environment via a vapor retarder, lower chords in the attic space will continued on page 18 FRAME BUILDER - JAN2023 / 17'