36 February 2002|ASHRAE JournalMoisture BalanceMoisture accumulates in the buildingenvelope when the rate of moisture entryinto an assembly exceeds the rate ofmoisture removal. When moisture accu-mulation exceeds the ability of the as-sembly materials to store the moisturewithout significantly degrading perfor-mance or long-term service life, moistureproblems result. The moisture storagecapacity of a material depends on time,temperature, and material properties.This moisture storage capacity is sig-nificant in determining performance.Consider three examples: a wood framewall, a steel stud wall and a masonry wall.In an exterior wood frame wall with awood-based sheathing, the wood cansafely store moisture until the moisturecontent by weight exceeds 16% (the “sur-face mold limit for wood”). The equilib-rium moisture content of wood exposedto a relative humidity of 80% is 16. Inmost climates, most wood materials cometo equilibrium at around 5% to 6% mois-ture content by weight. The differencebetween the surface mold limit and thetypical average condition in an exteriorwood frame wall is approximately 10%moisture content by weight. In otherAbout the AuthorWWWWWBy Joseph Lstiburek, Ph.D., P.Eng., Member ASHRAEhen designing a building’s envelope and its interaction with themechanical system, temperature, humidity, rain, and the interiorclimate often are ignored. The focus for the building may be more onaesthetics and cost than on performance.Joseph Lstiburek, Ph.D., P.Eng., is a principalwith Building Science Corporation, Westford, Mass.The concept of limit states (limitingconditions) plays a key role in buildingdurability. In structural engineering,loads and load resistance are consideredand limiting states, such as deflection,are specified.A similar approach is applied to mois-ture engineering. Rain, temperature, hu-midity and the interior climate are con-sidered environmental loads with princi-pal limiting conditions such as rot, de-cay, mold and corrosion. A damage func-tion (damage process) analysis is then usedto determine whether a limiting condition,such as mold growth, is achieved.Moisture engineering uses an iterativeand interdisciplinary systems approachto develop performance metrics to meetmoisture-related objectives.Environmental LoadsHygro-thermal regions, rain exposurezones and interior climate classes areenvironmental loads used in applyingmoisture engineering to building enve-lopes and mechanical systems. Figure 1shows hygro-thermal regions and Figure2 shows rain exposure zones for NorthAmerica. Table 1 describes interior cli-mate classes.words, the moisture storage capacity orhygric buffer capacity of most exteriorwood frame walls with wood-basedsheathings is approximately 10%. Ifmoisture accumulates beyond about 16%by weight, wood surfaces are likely todevelop mold.In the average home approximately4,000 to 5,000 lbs (1814 to 2267 kg) ofwood are in the exterior walls. This yieldsa hygric buffer capacity of approximately400 to 500 lbs (181 to 226 kg) of wateror approximately 45 to 50 gallons (170to 189 L). From a performance perspec-tive, the average home can easily accom-modate 45 to 50 gallons (170 to 189 L)of water via hygric redistribution. Mostwater leaks are not a problem because ofthis large capacity to store water.IssalCdetaredoMerutarepmeT•dellortnocnUerusserPropaV•• dellortnocnUerusserPriAIIssalCdellortnoCerutarepmeT•• detaredoMerusserPropaV• detaredoMerusserPriAIIIssalCdellortnoCerutarepmeT•• dellortnoCerusserPropaV• dellortnoCerusserPriATable 1: Interior climate classes.Copyright 2002, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).Reprinted by permission from ASHRAE Journal, February 2002. This article may not be copied nor distributed in eitherpaper or digital form without ASHRAE’s permission.ASHRAE Journal|February 2002 37MoistureMatters are considerably different when the exterior wallsare constructed with steel studs and gypsum sheathing. Steelstuds have no water storage capacity. Gypsum sheathing canstore approximately 1% moisture content by weight beforemold colonization occurs. Constructing the average home withsteel studs and gypsum sheathing yields a hygric buffer capac-ity of 5 gallons (19 L). In this type of assembly, even the small-est leak can lead to problems.In contrast, consider a similar sized home built with ma-sonry exterior walls and masonry cladding. That constructionyields a hygric buffer capacity of approximately 500 gallons(1892 L).Hygric Buffer Capacity for 2,000 ft2 (186 m2) HomehtiwemarFleetSgnihtaehSmuspyG)L91(snollag5.xorppAhtiwemarFdooWgnihtaehSdooW)L981(snollag05.xorppAllaWyrnosaM )L2981(snollag005.xorppAThe quantity of accumulated moisture in assemblies is af-fected by the energy flow through the assemblies. In general,more thermal insulation increases the dwell time ofmoisture in the assembly. Dwell time — or drying time —should be as short as possible to avoid moisture problems.Constructing highly insulated steel frame assemblies withgypsum sheathing is one of the significant challenges ofmoisture engineering. This assembly combines two perilouscharacteristics: low hygric buffer capacity (low safety mar-gin) with slow drying times. So even small amounts of mois-ture will cause problems.Moisture ControlVarious strategies can be implemented to minimize the riskof moisture damage. The strategies fall into the following threegroups:1. Control of moisture entry,2. Control of moisture accumulation, and3. Removal of moisture.These are best used in combination. Strategies effective inthe control of moisture entry, however, often are not effective ifbuilding assemblies start out wet. In fact, these strategies canbe detrimental. A technique that is effective at preventingmoisture from entering an assembly is also likely to be effec-tive at preventing moisture from leaving an assembly. Con-versely, a technique effective at removing moisture also mayallow moisture to enter. Balance between entry and removal iskey in many assemblies.The most significant wetting mechanisms are liquid flowand capillary suction. Groundwater and rain are the moisturesources. Controlling groundwater entry below grade and rainentry above grade has long preoccupied builders and design-ers. Air transport and vapor diffusion are less obvious contri-butions to the wetting of building assemblies. All of thesemechanisms are capable of leading to moisture-related build-ing problems.All