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Water Vapor Migration and Condensation Control in Buildings 2

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Water Vapor Migration and Condensation Control in Buildings 2 HPAC Info-dex 2 Engineering Basics he articles in this section were selected by HPAC’s Engineering Editor based on their generic and fundamental nature. Engineering Basics T is intended to be used by engineers, contractors, and facility managers 2 to brush up on engineering fundamentals across a wide range of subjects pertaining to mechanical systems design, building science, and product selection. This year’s selections are as follows: 72 “Water Vapor Migration and Condensation Control in Buildings”—The basics of psychrometric analysis of moisture conditions, including evaluation of vapor barriers and other construction features, and internal and external moisture sources. Examples help guide the discussion of this complex topic. William G. Acker 89 “BACnet: Answers to Frequently Asked Questions”—Answers to frequently asked questions about BACnetE provide invaluable information for building automation system designers, owners, and operators. A primer on the revolutionary development in the building automation and controls industry. By H. Michael Newman Circle 350 on Reader Service Card June 1998 HPAC Heating/Piping/AirConditioning 71 MOISTURE CONTROL Water Vapor Migration and 2 Condensation Control in Buildings The basics of water vapor analysis and control By WILLIAM G. ACKER, pressure). Thus the water vapor lustrated in Equations 4 and 5. President, diffusion is from inside to outside. The inside and outside vapor Acker & Associates, In warmer climates with short pressures can be determined from Green Bay, Wis. heating seasons, the water vapor test data or by using typical psy- drive is from outside to inside due chrometric data for that region. If ater vapor is the gaseous to the drying effect of indoor air there is concern over the amount form of water and is an in- conditioning. If the water vapor of water vapor diffusion or con- W visible source of many condenses in the wall or roof sec- cern over condensation in the cav- problems in today’s buildings. tion and does not have the oppor- ity, the engineer may need to in- This article will provide a basic tunity to dry out, then over a pe- stall a vapor retarder. understanding of how water va- riod of time, there will develop an The ASTM definition of a vapor por transport occurs, when it con- accumulation of water that could retarder is a material with a va- denses, and when it can cause cause damage and/or mold damage and health problems. and mildew. Keep in mind TABLE 1—Typical vapor retarder Water vapor can travel through that this is a dynamic situ- materials. building structures by: ation where condensation Material Thickness, in. PERM* c Diffusion caused by vapor may occur during part of pressure differentials. the day and evaporation Aluminum foil 0.00035 0.05 c Air flow created by tempera- during another part. The 2-mil polyethylene 0.002 0.16 ture differentials. engineer’s job is to design 4-mil polyethylene 0.004 0.08 c Air flow created by mechani- the cavity so that conden- cal systems. sation does not occur, does 6-mil polyethylene 0.006 0.06 c Rain penetration through in- not occur frequently, or Butyl rubber elastomer 0.015 to 0.04 0.02 take louvers. only occurs in a safe region, To begin with, psychrometric such as an air space with Vapor retarder paint 0.0031 0.45 conditions on the inside of a build- drainage. Kraft facing on ing are usually different than the The terms and equations glass fiber batts 0.0118 0.40 outside conditions. The difference for water vapor transmis- *Typical PERM values can be obtained from the 1997 in psychrometric properties re- sion or diffusion are listed ASHRAE Handbook of Fundamentals or the ASTM sults in a vapor pressure differen- in the accompanying side- book, Moisture Control in Buildings. tial from inside to outside, which bar. Equations 6 and 7 are sets up the driving force for water the vapor diffusion equations por performance (PERM) of 1.0 or vapor diffusion. The direction of used to calculate the amount of less. The resistance to vapor water vapor flow is from high va- water vapor that passes through transmittance is illustrated in por pressure or high humidity to the wall or ceiling cavity. The Equation 1. The resistance to va- low vapor pressure or low humid- overall coefficient of vapor trans- por transmittance is the inverse of ity. In colder climates, such as the mission, M, is determined by the PERM value; therefore, the northern U.S. and Canada, the adding the resistances to vapor lower the PERM value, the amount of water vapor in the out- transmittence for the construc- greater the resistance to water va- side air is very low (low vapor tion materials (and air films) and por diffusion. Some state codes re- pressure) and is less than the then taking the inverse of that quire a PERM rating of less than building inside air (high vapor summation. This procedure is il- 1.0 to qualify as a vapor retarder. 72 HPAC Heating/Piping/AirConditioning June 1998 Water vapor diffusion ity walls and ceilings. In cold and season, the vapor retarder is Water vapor diffuses through moderate climates, the vapor re- placed on the outside of the cavity many building materials other tarder is placed on the indoor side because air conditioning of the in- than metals when a vapor pres- of the cavity. This is because the door air dries the air, thus lower- sure difference exists across the water vapor flow is from inside to ing the indoor vapor pressure be- construction. One consensus that outside (the inside is the higher va- low the outdoor vapor pressure. In seems to have been reached is that por pressure). In some warm cli- some warm climates, vapor re- vapor retarders are needed in cav- mates that have a short heating tarders are installed on both the 2 Water vapor diffusion through Psychrometrics used to materials of construction determine vapor pressures Terms Terms M = Overall coefficient of vapor transmission or MWdry air = Molecular weight of dry air, 28.9645 overall PERM value (some references also use M to lbm/lb-mole designate the PERM value for individual materials), grains water vapor/hr-sq ft-in. Hg MWwater vapor = Molecular weight of water vapor, 18.01534 lbm/lb-mole PERM = Permeance (this value varies with mate- rial thickness), grains water vapor/hr-sq ft-in. Hg Pabs = Total absolute pressure, in. Hg PERM-IN. = Permeability (this value can be found Pbar = Barometric pressure, in. Hg in tables without the knowledge of material thick- ness), grains water vapor-in./hr-sq ft-in. Hg Pgauge = Gauge pressure (sometimes called duct pressure or pipe pressure), in. Hg Pw1 – Pw2 = Difference of vapor pressure between ends of the flow path, in. Hg Pa = Partial pressure of the dry air in the mixture, in. Hg REP = Resistance to vapor transmittence, in. Hg- sq ft-hr/grains water vapor Pw = Partial pressure of water vapor in the mix- ture, in. Hg W = Total mass of water vapor transmission per unit area in unit time, grains water vapor/hr-sq ft Pws = Pressure of saturated pure water, in. Hg Equations RH = Relative humidity, percent 1 1 1) REP = = PERM PERM− IN./in. thick TDB = Dry bulb temperature, F T = Wet bulb temperature, F 1 PERM - IN. WB 2) PERM = = REP in. thick TDP = Dew point temperature, F 3) PERM - IN. = PERM× in. thick W = Absolute humidity, lb water vapor/lb dry air Equations = 1 4) M 8) Pabs = Pbar + Pgauge 111+++ 1 PERM1 PERM2 PERM3 PERM(etc.) 9) Pabs = Pa + Pw = 1 5) M +++ 10) RH = (Pw/Pws) 3 100 REP123 REP REP REPetc. 11) W = (Pw/Pa) 3 (MW /MW ) = =×() − water vapor dry air 6) W M Pw12 Pw (Pw/Pa) 3 0.62198 1 12) Pw can be determined using the equations or 7) WPwPw=×−() ∑ REP 12 can be found using the tested dew point temperature and the steam tables. June 1998 HPAC Heating/Piping/AirConditioning 73 MOISTURE CONTROL TABLE 2—Psychrometric computer analysis TABLE 3—Psychrometric computer analysis for 70 F air temperature. for 52 F air temperature. 2 sures. If the analysis involves an air remained unchanged. The rea- existing facility, the engineer can son is that cold air cannot hold as Exterior Interior test the actual conditions. The much moisture as warm air. In properties that must be deter- both cases, however, the absolute mined are barometric pressure, humidity, W (lb water vapor/lb gauge pressure, dry bulb temper- dry air), is the same. Water vapor ature, and something to identify To summarize, a change in the flow direction moisture in the air, such as wet air dry bulb temperature will bulb temperature, dew point cause a shift in the relative hu- temperature, or relative humid- midity even though the amount Given Calculated Material PERM values REP values ity. These properties and the of moisture in the air remains un- Outside air film 1000 0.00100 equations to calculate the vapor changed. A psychrometric com- 5 in. stone 0.64 1.56250 pressure are illustrated in the ac- puter analysis of these two condi- Air barrier 77 0.01299 companying sidebar. tions are illustrated in Tables 2 0.5 in. exterior plywood 0.5 2.00000 Relative humidity, RH, is a very and 3. You will also note that the 5 in. glass fiber insulation 21.818 0.04583 misunderstood term. It describes dew point temperature is the 0.5 in. gypsum board 40 0.02500 the amount of moisture the air same in both cases, which sup- 1.5 Paint 0.66667 holds relative to the maximum ports the theory that the mois- Indoor air film 160 0.00625 Total: 4.32024 it can hold at that temperature.
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