Practical Guide

PRACTICAL GUIDE

UNDERSTANDING WHAT HUMIDITY DOES AND WHY By Kenneth M. Elovitz, P.E. Member ASHRAE

P

eople sometimes attribute ef fects to “humidity” without understanding the underlying physics. For example, we have all

experienced “hot, humid” summer weather. Yet the outdoor air relative humidity on a “hot, humid” summer day (95°F db/78°F wb [35°C db/26°C wb]) is less than 50%. By contrast, the outdoor air relative humidity on a “cold, dry” winter day is typically around 80%. This article examines the difference between relative humidity, specific humidity, and vapor pressure. It goes on to explore how those measures influence phenomena loosely attributed to “humidity.”

Measures of Humidity

Different measures of humidity quantify different physical properties of the mixture of water vapor (moisture) and air. Understanding how moist air behaves requires understanding those measures of humidity. Relative humidity is the ratio of the amount of water vapor in the air to the amount of water vapor air can hold at that temperature. At 100% relative humidity, the dry bulb, wet bulb, and dew point temperatures are equal. At 100% relative humidity, the air is saturated, which means it cannot hold any more moisture. Raising the temperature without changing the amount of moisture in the air reduces the relative humidity. The relative humidity goes down because warmer air can hold more moisture than colder air. For example, a comfort cooling system might be designed to maintain 75°F (24°C)/55% RH at design load using 56°F (13°C) coil leaving air temperature. The system might have enough sensible capacity to cool the room to 70°F (21°C) at less than design load, or the system might be oversized. The 84

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coil leaving air temperature does not change, so the available dehumidification capacity does not change. The resulting room relative humidity at 70°F (21°C) will be 65%, possibly generating complaints that the relative humidity is too high. While room conditions should be analyzed in accordance with ASHRAE Standard 55-1992, Thermal Environmental Conditions for Human Occupancy to evaluate comfort, if relative humidity itself is the problem, one practical solution might be to operate the system at the design temperature setpoint of 75°F (24°C). Achieving moderately low humidity at low room temperatures may require using a reheat system. Achieving low relative humidity at low temperatures usually requires specialized systems like desiccant dehumidification. Specific humidity is the amount of moisture in the air per unit mass of air. It is usually expressed as grains of water per pound of dry air (gr/lb) or pounds of water per pound of dry air (lbw/lbda, kgw/kgda). Specific humidity is proportional to the enthalpy or total energy content of the moist air mixture. Specific humidity changes when moisture is added or removed. Changing temperature does not change specific humidity unless the air is cooled below the dew point. Dew point is the temperature where moisture begins to condense out of the air. When air is cooled to its dew point, it reaches 100% relative humidity or saturation. Cooling the air any further causes water vapor in the air to change to the liquid phase. Liquid water molecules accumulate, droplets form, and moisture condenses out of the air. At the new conditions, the air contains less moisture, has lower specific humidity, and has a lower dew point temperature, but it is still at 100% relative humidity. Raising the temperature of air at its dew point reduces its relative humidity but does not change its water vapor content (specific humidity) so does not change its dew point. Vapor pressure is the pressure exerted by free molecules at the surface of a solid or liquid. Consider water in a closed vessel at 75°F (24°C). Water will evaporate until the partial pressure of the water in the vessel reaches 0.44 in. Hg (1.49 kPa), which April 1999

Museums & Renovation

Figure 1a: Temperature and moisture gradient in a wall (condensation).

Figure 1b: Temperature and moisture gradient in a wall (no condensation).

is the vapor pressure of water at 75°F (24°C). For a given substance, vapor pressure is a function of temperature. As temperature increases, vapor pressure increases. When the vapor pressure reaches atmospheric pressure (29.92 in. Hg [100 kPa]), the liquid boils. For water at sea level, this condition occurs at 212°F (100°C). At 5,000 ft (1524 m) above sea level, atmospheric pressure is only 24.89 in. Hg (84 kPa). That is why water boils at 202°F (94°C) in Denver. Vapor pressure is a measure of the affinity of a substance for itself. If a substance has low affinity for itself, it evaporates readily even at low temperature. The substance will have a high vapor pressure. For most HVAC processes, the vapor pressure of interest is for water in contact with itself. However, water in contact with other substances (e.g., wood, paper, salt) also has a vapor pressure. The vapor pressure of water in contact with those other substances may be different from the vapor pressure of water in contact with itself.

cific humidity, or vapor pressure. Engineers must identify the operative parameter before they can design HVAC/R systems that avoid or mitigate the effects of moisture in the air.

Effects of Humidity

Understanding how moisture affects materials and processes requires understanding whether those effects are a function of relative humidity, specific humidity, or vapor pressure. Much of the literature on effects of humidity covers a narrow temperature range. Those studies likely used relative humidity because it is easy to measure. At constant temperature, relative humidity varies directly with moisture content — the lower the moisture content, the lower the relative humidity. Since the studies were conducted over a narrow temperature range, the data lend little insight into whether the operative factor is relative humidity, speApril 1999

Condensation

Condensation is strictly a function of relative humidity. When air cools to a temperature below its dew point, moisture condenses out of the air. It is not necessary to cool the entire air mass to get condensation. Condensation occurs on the coldest surface in a room. A cold window might cool nearby air below its dew point and cause condensation while the rest of the room remains at normal temperature. Condensation causes a variety of problems. Condensation is a housekeeping problem if moisture puddles on the floor or if droplets stain the materials they contact. Condensation can damage wood, paper, and fabric, and it accelerates rusting of steel. It can also hurt products like frozen foods in a supermarket. No one wants to buy the package of ice cream coated with frost. Moreover, for water vapor in the air to form frost on the package of ice cream, it must give up its heat of vaporization (approximately 1000 Btu/lb [2326 kJ/kg]) and its heat of fusion (approximately 144 Btu/lb [335 kJ/ kg]). It gives up some of that heat to the air and some of it to the ice cream. The ice cream warms up a bit and can even begin to soften or melt if the freezer is not cold enough. HVAC/R designs generally try to avoid condensation in the conditioned space. For cooling applications, they accomplish that goal with dehumidifying coils that remove moisture from the supply air before it enters the conditioned space. Most comfort cooling systems are designed to control temperature, so they control ASHRAE Journal

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Practical Guide INDOOR TEMPERATURE INDOOR MOISTURE

70°F (21.1°C) 22.4 GR/LB

OUTDOOR TEMPERATURE OUTDOOR MOISTURE

R-VALUE

CUMULATIVE R-VALUE

FRACTION OF TEMP. DIFF.

SFCE. TEMP. °F (°C)

PERMS

Inside air film

0.68

0.68

0.091

62.7 (17.1)

nil

Paint

nil

0.68

0.091

62.7 (17.1)

5.0

½ inch Wallboard

0.45

1.13

0.151

57.9 (14.4)

3½ inch Air space

1.01

2.14

0.286

47.1 (8.4)

¾ inch Polystyrene

3.75

5.89

0.786

½ inch Plywood

0.62

6.51

0.869

Clapboards

0.81

7.32

0.977

Outside air film

0.17

7.49

1.000

ITEM

Notes: (1) Perms are grains/hr per sq ft per in. Hg Pressure difference (2) Reps are 1/perms

–10°F (–23.3°C) 1.3 GR/LB

CUMULATIVE REPS

MOISTURE DIFFERENCE

SURFACE GR/LB

DEW POINT °F (°C)

0.00

0.000

22.4

28 (–2)

0.20

0.20

0.074

20.8

27 (–3)

37.5

0.03

0.23

0.084

20.6

26 (–3)

34.3

0.03

0.26

0.094

20.4

26 (–3)

7.1 (–13.8)

1.6

0.63

0.88

0.325

15.5

20 (–6)

0.5 (–17.5)

0.7

1.43

2.31

0.852

4.4

–4 (–20)

–8.2 (–22.3)

2.5

0.40

2.71

1.000

1.3

–27 (–33)

–10.0 (–23.3)

nil

2.71

1.000

1.3

–27 (–33)

REPS

(3) SFCE. Temp. is on outside face of surface. (4) SFCE. Temp. = Indoor Temp. – (Frac. of Temp. Diff. x Total Demp Diff.)

Table 1: Stud wall dew point analysis.

relative humidity and the risk of condensation only indirectly. However, matching both the sensible (temperature) and latent (dehumidification) capacities to the cooling loads is part of a successful design. Excessive winter humidification risks condensation on cold window and wall surfaces. Excess humidification is humidity above what the building envelope was designed to accommodate. Besides condensation, excess humidification can cause problems like peeling paint, either inside or outside.

Moisture in the Building Structure

Condensation problems are not limited to the occupied space. Condensation inside walls can be a serious problem. Any conditioned building has a temperature gradient between indoors and outdoors. The temperature difference across each element of the wall structure is proportional to the insulating value of that element. Buildings also have a moisture gradient between indoors and outdoors. The moisture difference across each element of the structure is proportional to the vapor diffusion resistance of the element. Figure 1 illustrates the temperature and vapor pressure gradients in a wood stud wall and shows how insulation placement affects performance.1 While this example is for a modern house, the analysis applies to any structure, including historic buildings. The house had urea formaldehyde foam insulation that had shrunk away from the studs, leaving large areas effectively uninsulated. Moisture from the humidified house condensed on the back side of the sheathing, ruining it. The owner wanted to install insulated sheathing for energy conservation and to avoid another condensation problem. Table 1 is a dew point calculation for Figure 1a. Like the temperature gradient, the moisture gradient is proportional to the resistance of each element in the wall. Where the temperature gradient is expressed in degrees, the moisture gradient is expressed in vapor pressure (in. Hg or kPa) or specific humidity (grains/lb, lbw/lbda, or kgw/kgda). Since vapor permeance data are commonly tabulated in grains in the I-P system of units, it is 86

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easier to work in grains/lb than lb/lb. Although the units are different, the principle is similar to the more familiar temperature gradient calculation: Quantity R-Value Btu/h Permeance (perms) gr/h

Area ft2 (m2) ft2 (m2)

Driving Force °F (°C) in. Hg (kPa)

In Figure 1a, with the insulation inside the exterior sheathing, the surface of the sheathing falls below the dewpoint and damaging condensation can occur. The following calculation shows the basis for that conclusion: R-value of all components up to plywood: 5.89 Total R-value of assembly: 7.49 Temperature on inside surface of plywood: Inside R-value Temp. Ratio

Temperature Difference

70°F – 5.89 × [70°F – (–10°F)] = 7.1°F (–13.8°C) 7.49 Vapor diffusion resistance of components up to plywood: 0.88 Total vapor diffusion resistance of assembly: 2.71 Dew point calculation for surface of plywood Inside Moisture

Rep Ratio

Moisture Difference

22.4 gr/lb – 0.88 × (22.4 – 1.3) gr/lb 2.71 = 15.5 gr/lb dew point = 20°F (–6°C) Since the temperature on the plywood is lower than the dew point, moisture can condense. Figure 1b shows that installing the insulation outside the sheathing keeps the sheathing above the local dewpoint, avoiding conApril 1999

Museums & Renovation densation. Note that these conditions result in part from the fact that plywood sheathing is a moderately effective vapor retarder.2 In hot, humid climates, the indoor temperature and dewpoint are below the outdoor temperature and dewpoint much of the year. In those situations, the vapor retarder is usually installed outside the insulation.3 If a wall is not designed for the anticipated indoor/outdoor moisture gradient, or if the indoor humidity is higher than the building design contemplated, moisture can condense inside the wall. That moisture can eventually cause structural damage. New construction can include vapor retarders to accommodate indoor humidification. Depending on their construction, it might not be feasible to humidify existing buildings without risk of condensation and damage to the building structure.

Mold and Fungus Growth

Mold and fungus spores are difficult to eliminate from a building. The spores themselves are not much of a problem until they grow. To grow, mold spores need moisture and a food source.4 Neither moisture nor food necessarily comes from the air. Rather, they both more often come from the substrate where the spores land and germinate.5 Mold can grow inside air-handling units. In cooling systems, cooling coil condensate may be available as a moisture source. Although the relative humidity can be 95% or higher for months at a time, mold does not always grow in air-handling units. Mold will not grow even in high humidity environments unless it has food. When mold grows in air-handling units, the food source is accumulated dust and dirt. Keeping systems clean is the key to avoiding mold growth in air-handling units and ducts. Maintaining relative humidity below the oft-cited 60% level does not guarantee against mold growth. Mold can not only obtain food from a substrate, it can also obtain moisture from a substrate. Some substrates allow mold to germinate with fairly low moisture levels. Dirty surfaces and accumulated salts tend to deliquesce moisture out of the air. That moisture in the material promotes mold growth. Where moisture is unavoidable, as in a cooling system, the key to avoiding mold growth is to eliminate food sources. Materials that hold moisture can be sites for mold growth even in a room where the relative humidity is low. Like desiccants, some materials absorb moisture from the air even at low humidity. Other materials are slow to release moisture once they get wet. The literature suggests materials absorb moisture faster than they release it.6 If these materials are organic, they are ideal substrates for mold growth. Maintaining relative humidity below 60% at temperatures in the normal human comfort range may reduce mold growth. However, low relative humidity is no guarantee. Selecting materials and treating surfaces so they do not absorb or hold moisture appears to be a more effective strategy against mold growth.

Desiccants

Desiccants are materials that absorb moisture. Commercial desiccants generally absorb several times their own weight in April 1999

water. While desiccants are usually noted for their ability to absorb moisture, they also desorb moisture if the water vapor pressure of the ambient air is less than the vapor pressure of water in the desiccant. In that respect, desiccants can be a form of seasonal storage for latent cooling. Desiccants can be liquid or solid. Liquid desiccants ABsorb water vapor. Solid desiccants ADsorb water vapor. The difference is that the ABsorbed water goes into solution with the liquid desiccant. ADsorbed water attaches to the surface of solid desiccants. Solid desiccants have irregular surfaces with numerous pores that provide sites for water vapor molecules to attach. Liquid desiccants absorb water because they have a stronger attraction for water molecules than does water itself. Expressed scientifically, the vapor pressure of water in the desiccant is less than the vapor pressure of water in the air. The vapor pressure difference drives water molecules into the desiccant solution. The vapor pressure of water in the desiccant solution increases as the solution absorbs water and becomes more dilute. When the vapor pressure of water in the desiccant equals the vapor pressure of the ambient air, the desiccant stops absorbing water.7 Solid desiccants have numerous small passages or capillaries that attract water. Water is attracted to the surface of the desiccant, collects into droplets, and condenses in the capillaries. As with liquid desiccants, water sitting on the surface of the desiccant has a lower vapor pressure than water in the ambient air.8 Stated another way, the force attracting water vapor to the desiccant surface is greater than the force attracting water vapor into the air. Desiccants can achieve much lower specific humidity than mechanical refrigeration without over cooling the space or requiring a defrost cycle. As a practical matter, desiccant systems tend to be economical when the desired dew point is below about 40°F (4°C).

Static Electricity

Static electricity results when charges accumulate on a body. The problem occurs when those charges jump across an air gap on their way back to their source. People can pick up charges from walking across carpets. They carry those charges around with them until they get close to an object that has a conductive path back to the carpet. If the charges discharge through a computer or other electronic device, the discharge can scramble data or damage components. Indoor static electricity discharges are often associated with dry, winter weather. However, some of the biggest static electricity discharges in human experience occur during humid summer weather. They are thunderstorms. Even though people associate static electricity with low indoor humidity, broader observations show that static electricity discharges are not a function of relative humidity. The dielectric constant of a substance is a measure of its ability to hold a charge. The dielectric constant of air does not change very much with humidity. The reduction in static electricity discharges attributed to increasing humidity has little to do with moisture in the air. Rather, it is the influence of moisture on the electrical conductivity of materials.9 Static electric charges canASHRAE Journal

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Practical Guide not accumulate on conductive materials. The electrical conductivity of most common materials increases in proportion to their moisture content. Materials such as plastics, rubber, and machine drive belts that do not readily absorb moisture can accumulate static charges at 100% relative humidity.10 Previous editions of the ASHRAE Handbook implicitly recognize that increasing relative humidity does not necessarily eliminate static electricity. The 1983 and 1988 Handbooks state that “under some conditions, and with certain materials, maximum electrostatic charging occurs at relative humidities of 25% to 35% or higher.”11 That statement disappeared from the same chapters in the 1992 and 1996 editions of the Handbook. Adding moisture to the air affects static electricity only indirectly. If the materials in the room absorb moisture from the air and increase their conductivity, the risk of static electricity discharge decreases. However, simply adding moisture is not reliable. NFPA 99-1996, Health Care Facilities, calls for hospital operating rooms that utilize flammable anesthetics to be humidified to 50% relative humidity. Even with 50% relative humidity, the same standard calls for additional precautions against electrostatic discharge.12 The need for additional precautions demonstrates that room air relative humidity does not necessarily have a cause and effect relationship with static electricity discharges. Controlling static electricity discharges seems to depend on surface conductivity, static dissipating clothing, conductive flooring, and grounding as opposed to humidifying the air. 13

Figure 2: Equilibrium moisture content of wood.

Rust

Atmospheric corrosion (rust) is uncontrolled oxidation of a metal. In the case of stainless steels, oxidation produces a thin, protective coating on the metal surface. That oxidation is part of what makes stainless steel “stainless.” Aluminum and copper also form protective oxide coatings. On the other hand, carbon steel forms a loose oxide that readily separates from the base metal. The loose oxide particles fall off as scale, exposing new base metal to oxidize. The process continues until the metal rusts away. Plain carbon steel reportedly remains uncorroded when exposed to air at a relative humidity less than about 30%.14 The reference does not indicate whether 30% RH at 85°F (29°C) is any more aggressive to carbon steel than 30% RH at 25°F (–4°C). The increase in corrosion with increasing humidity is attributed to an increase in the electrical conductivity of the environment contacting the metal surface.15 All corrosion is electrolytic in nature, so the increase in conductivity almost certainly plays a part. However, moisture content does not affect the electrical conductivity of air. Any increase in conductivity associated with increased moisture can only be due to the interaction of water vapor with pollutants in the air. The ASM Metals Handbook describes the influence of surface condition on rust. Rust forms on surfaces with small pores at lower humidity than on surfaces with large pores. Small pores draw moisture out of the air by capillary condensation due to differences in vapor pressure.16 Vapor pressure and capillary condensation make more sense 88

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Figure 3: Dimensional change of wood with change in moisture content.

than relative humidity as a driving force for rusting. If the vapor pressure of water in the surrounding air is higher than the vapor pressure of water in small capillaries in the iron/iron oxide surface, the capillaries draw moisture out of the air. Moisture in the capillaries reacts with contaminants in the air or on the surface, increasing conductivity and resulting corrosion. Because rust tends to be irregular, more rust forms more capillaries, fostering even more rust. This analysis suggests that preventing corrosion appears to have more to do with surface finish and dew point than relative humidity environment. A smooth, polished surface provides few capillaries and few sites for capillary condensation. At high temperatures, low dew point results in a low relative humidity. However, as temperature goes down, relative humidity can increase without necessarily promoting rust if the vapor pressure of moisture in the air is below the vapor pressure required for capillary condensation. April 1999

Museums & Renovation Dimensional Changes Cellulosic materials like paper and wood readily take on and give up moisture from the air. Wood holds water in cell cavities and within its cell walls. Green wood can start out holding more moisture than the weight of the wood itself (more than 100% moisture content). When dried, wood first gives up water from cell cavities until the moisture content reaches about 30%. Further drying removes moisture from the cell walls. As the cell walls lose water, they shrink. The resulting stresses cause warping and checking.17 After the water in the cell cavities is gone, the cell walls give up moisture only until the wood reaches an equilibrium moisture content. The equilibrium moisture content depends on species, tempera- Figure 4: Influence of moisture content on dimensions of lithographic papers. ture, and relative humidity. Relative humidity is the strongest of those three influences. Figure 2 shows how the equilibrium moisture content for wood varies with temperature and relative humidity. Changing moisture content makes the wood expand or shrink. Figure 3 shows the magnitude of these changes. Like wood, paper also shrinks and grows with changes in moisture content. A 1933 study by Weber and Snyder for the National Bureau of Standards showed the effects of changing moisture content on the physical properties of printing papers.19 Figure 4 shows one of the findings from that study. Although the dimensional changes are small, they are enough to cause misalignment in multi-color printing processes. While the Weber and Snyder study confirms that relative humidity affects dimensions of wood and paper products, it Figure 5: Moisture isotherm of 194-year-old paper. is important to put these findings into perspective. First, the analysis relates to equilibrium moisture conFigure 5 shows the results of recent testing by the tent. Depending on size, thickness and how it is stored, the ar- Smithsonian Center for Materials Research and Education ticle may take hours or days to reach a new equilibrium moisture on a page from an 1804 law book. The paper was allowed to content when the ambient temperature and humidity change. reach equilibrium moisture content at various relative humidiFor these materials, temperature and humidity at any one mo- ties at constant room temperature. The dimensional changes ment or even over short periods are much less important than the were then measured. Figure 4 and Figure 5 taken together long-term average over time. relate room relative humidity to equilibrium moisture content Second, unless a process requires extreme precision, fairly broad for paper. In Figure 4, a 2 percentage point change in moischanges in temperature and relative humidity are required before ture content from 0.5% to 2.5% causes a dimensional change the dimensional changes become significant. Figure 2 shows that a of 0.18% or a strain of 0.0018. Figure 5 shows that a rather rather broad room temperature and humidity window of 59°F to extreme relative humidity change of 40 percentage points 87°F (15°C to 30°C) and 25% to 50% relative humidity results in (20% to 60%) to achieve that dimensional change. As a rea 4 percentage point change in equilibrium moisture content of wood. sult, unless extreme precision and dimensional stability are Figure 3 shows that a 4 percentage point change in moisture results required, paper and wood can tolerate fairly broad changes in less than 1% change in dimension. in environmental conditions with minimal impact. April 1999

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Practical Guide Summary and Conclusions

see CH2M Hill, Preventing Indoor Air Quality Problems in Hot, Humid Climates: Design and Construction Guidelines, Orlando, Fla, 1996.

Except for avoiding condensation, controlling indoor relative humidity does not necessarily protect materials. Relative humidity at best contributes indirectly to control of static electricity, and 4. See Technical Leaflet “Protecting Books and Paper Against mold growth. The moisture content of the materials exerts a much Mold,” Northeast Document Conser vation Center, greater influence and should be the parameter of interest for preAndover, Mass. serving books, papers and artwork. Other authors address the 5. See Motylewski, Karen, Insect and Fungus Management effects of moisture content on materials in greater detail. Conference Notes citing Florian, Mary-Lou, “Mold and Stored materials can take weeks or months to reach their its life cycles,” http://palimpsest.stanford.edu/bytopic/pest/ equilibrium moisture content. In a humidified environment, pestnote.html, Nov. 1994. books and papers do not release moisture 6. Ibid. during the winter, so they start the mechanical cooling season loaded with mois7. 1997 ASHRAE, Handbook— ture. If the environment is not humidified, Fundamentals, Chapter 21. Using humidity wisely stored books and papers give up moisture 8. Ibid. during the winter and go further into the 9. NFPA 921-1988, Guide for Fire cooling season before they have absorbed requires understandand Explosion Investigations, section enough moisture to support mold growth. 14-12.5.1. Also because hygroscopic materials take ing the operative time to absorb and desorb moisture from 10. Ibid. the air, fairly wide variations in tempera11. ASHRAE Handbook—Equipparameter: relative ture and relative humidity over the course ment, p. 5.1. of a day or even a week most likely do not 12. NFPA 99-1996, Health Care Fahave a significant impact on the stored mahumidity, specific cilities Annex 2, “Flammable anesterials. thetizing locations,” section 2-6.3.8: On the other hand, the risk of condenhumidity, or Reduction in Electrostatic Hazard. sation may make the building structure (including historic buildings) more sensitive to 13. K assebaum, J. H. and R. A . dew point. the effects of humidity than the stored maKocken, “Controlling static electricterials. In northern climates, winter humidiity in hazardous (classified) locafication adds moisture that can lead to contions,” IEEE Transactions on Indusdensation and increased mold growth. In try Applications, 33(1):209–215. hot, humid climates, over cooling can also result in condensation. Attempting to dehumidify without adequate vapor retard- 14. United States Steel. 1971. The Making, Shaping and Treating of Steel, 9th edition, p. 981. ers will be expensive and ultimately unsuccessful. Using humidity wisely requires understanding the operative 15. Ibid. parameter: relative humidity, specific humidity, or dew point. 16. American Society for Metals, Handbook Vol. 13—CorroOver cooling a room in the name of “dehumidification” raises sion, p. 82. relative humidity and may be counter productive for some materials. Allowing materials to absorb and desorb moisture slowly 17. Hoadley, R.B., “As dries the air, so shrinks the wood,” Fine Woodworking, The Taunton Press, 39(2):92–95. in response to seasonal climate changes may be a successful at maintaining long term stability in the materials and the build- 18. Weber, C.G. and Snyder, L.W., “Reactions of lithographic papers to variations in humidity and temperature,” National ings that house them. Bureau of Standards Journal of Research, vol. 12, paper References no. RP633, January 1934. 1. For a more detailed discussion of this topic, see 1997 ASHRAE, Handbook—Fundamentals, p. 22.19 and Acker, William G., “Water Vapor Migration and Condensation Control in Buildings,” Heating/Piping/Air Condition- Kenneth M. Elovitz, P.E., Member ASHRAE, is an engiing, 70(6):72–81. neering consultant and in-house counsel for Energy Economics, 2. The former term vapor “barrier” has fallen out of favor Inc., in Foxboro, Mass. Ken received a bachelor’s degree in metbecause “barrier” can imply an absolute block. Vapor re- allurgy and materials science from Lehigh University. He received tarders slow water vapor transfer just as thermal insulation a JD from Suffolk University Law School and has been admitslows, but does not eliminate, heat transfer. ted to practice in state and federal courts. He develops and edits 3. For a thorough discussion of design for hot, humid climates, these special supplements to ASHRAE Journal. = 90

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