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 BACnetீ 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* ● Diffusion caused by vapor may occur during part of pressure differentials. the day and evaporation Aluminum foil 0.00035 0.05 ● 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 ● Air flow created by mechani- the cavity so that conden- cal systems. sation does not occur, does 6-mil polyethylene 0.006 0.06 ● 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 material 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 thickness), 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 mixture, 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) ϫ 100 REP123 REP REP REPetc. 11) W = (Pw/Pa) ϫ (MW /MW ) = =×() − water vapor dry air 6) W M Pw12 Pw (Pw/Pa) ϫ 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. ture in the air did not change. If, for example, the air tem- 1 Cross-section of wall perature is 70 F and the TABLE 4—Psychrometric conditions without vapor barrier. relative humidity is, say 50 for Fig. 1 case. percent, the air at that tem- Indoor air Outdoor air indoor and outdoor sides of the cav- perature contains only 50 ities. Some typical vapor retarder percent of the moisture it is Pbar = 29.2274 in. Hg Pbar = 29.2274 in. Hg materials are listed in Table 1. capable of holding. If the Pgauge = 0 in. Hg Pgauge = 0 in. Hg To calculate the amount of wa- temperature then drops ter vapor diffusion, the engineer from 70 to 52 F, the relative TDB = 68 F TDB = 9 F must define the design indoor humidity increases to 94.8 RH = 50 percent RH = 50 percent and outdoor psychrometrics to percent even though the obtain the needed vapor pres- amount of moisture in the Pw1 = 0.23563 in. Hg Pw2 = 0.03313 in. Hg

74 HPAC Heating/Piping/AirConditioning June 1998 Also, you will note that the vapor (Table 2), we will calculate the • Pw = 0.3684 in. Hg pressure is the same in both vapor pressure of air. The following example will il- cases. The vapor pressure, Pw ● Given: lustrate how to calculate the (partial pressure of water vapor • TDB = 70 F amount of water vapor diffusion in the mixture), of the outdoor • Pbar = 29.921 in. Hg through the wall section and will and indoor air can be calculated • Pgauge = 0 in. Hg be used to illustrate the impor- by using the equations in the • Pabs = 29.921 in. Hg tance of a vapor barrier in this sidebar and by using the steam ● Calculations: case. The diagram in Fig. 1 is a 2 tables to obtain the pressure of • Pws = 0.7369 in. Hg (from typical 2 by 6 in. wall section. The saturated pure water, Pws. Using steam tables) materials of construction and air the 70 F/50 percent RH condition • RH = 50 percent = Pw/Pws films are shown with their PERM

TABLE 5—Vapor migration calculation results for a 2 by 6 in. wall without a vapor barrier.

Resistance Surface Surface Calculated Saturated air Thermal to vapor dry bulb dew point surface vapor surface vapor Material Thickness, resistance, R, Permeance, transmittance, temper- temper- pressure, pressure, Px, description in. hr-sq ft-F/Btu PERM REP (1/PERM) ature, F ature, F Pw, in. Hg in. Hg

Outdoor air 9.00 -5.9089 0.03313 0.06627

Outdoor air film 0.1700 1000.0000 0.00100 Inner surface 9.4757 -6.3651 0.03318 0.06770

Stone 5.0000 0.1667 0.6400 1.56250 Inner surface 9.9421 20.2193**** 0.10642 0.06912

Air barrier 0.0040 0.0000 77.0000 0.01299 Inner surface 9.9421 20.3404**** 0.10703 0.06912

Exterior plywood 0.5000 0.6200 0.5000 2.00000 Inner surface 11.6768 34.8143**** 0.20077 0.7467

Glass fiber insulation 2.0000 6.9091 60.0000 0.01667 Inner surface 31.0083 34.9096**** 0.20155 0.16929

Glass fiber insulation 3.5000 12.0909 34.2857 0.02917 Inner surface 64.8383 35.0757 0.20292 0.60344

Gypsum board 0.5000 0.4500 40.0000 0.02500 Inner surface 66.0974 35.2173 0.20409 0.63044

Paint 0.0024 0.0000 1.5000 0.66667 Inner surface 66.0974 38.7642 0.23534 0.63044

Indoor air film 0.6800 160.0000 0.00625

Indoor air 68.0000 39.3304 0.23563 0.67324

Totals 11.5064 21.0867 4.3202370

Total water vapor transmission: 0.04687193430045 grains/sq ft-hr or 0.00000669599 lb/sq ft-hr. Sensible heat transfer through the roof (negative value indicates flow from outside to inside): 2.7980 Btu/sq ft-hr. Indoor air conditions: 68 F/35 percent RH. Outdoor air conditions: 9 F/50 percent RH. Elevation: 680 ft.

****If Pw is greater than or equal to Px, condensation will occur in that region. ****If the dew point temperature is greater than or equal to the dry bulb temperature, condensation will occur.

June 1998 HPAC Heating/Piping/AirConditioning 75 MOISTURE CONTROL

TABLE 6—Vapor migration calculation results for a 2 by 6 in. wall with a vapor barrier.

Resistance Surface Surface Calculated Saturated air Thermal to vapor dry bulb dew point surface vapor surface vapor Material Thickness, resistance, R, Permeance, transmittance, temper- temper- pressure, pressure, description in. hr-sq ft-F/Btu PERM REP (1/PERM) ature, F ature, F Pw, in. Hg Px, in. Hg

Outdoor air 9.00 -5.9089 0.03313 0.06627

2 Outdoor air film 0.1700 1000.0000 0.00100 Inner surface 9.4757 -6.3873 0.03315 0.06770

Stone 5.0000 0.1667 0.6400 1.56250 Inner surface 9.9421 3.1351 0.05196 0.06912

Air barrier 0.0040 0.0000 77.0000 0.01299 Inner surface 9.9421 3.2000 0.05211 0.06912

Exterior plywood 0.5000 0.6200 0.5000 2.00000 Inner surface 11.6768 11.6040 0.07619 0.7467

Glass fiber insulation 2.0000 6.9091 60.0000 0.01667 Inner surface 31.0083 11.6631 0.07639 0.16929

Glass fiber insulation 3.5000 12.0909 34.2857 0.02917 Inner surface 64.8383 11.7669 0.07674 0.60344

Poly vapor barrier 0.0040 0.0000 0.0800 12.50000 Inner surface 64.8383 37.8835 0.22723 0.60344

Gypsum board 0.5000 0.4500 40.0000 0.02500 Inner surface 66.0974 37.9167 0.22753 0.63044

Paint 0.0024 0.0000 1.5000 0.66667 Inner surface 66.0974 38.7875 0.23556 0.63044

Indoor air film 0.6800 160.0000 0.00625

Indoor air 68.0000 39.3304 0.23563 0.67324

Totals 11.5104 21.0867 16.8202370

Total water vapor transmission: 0.01203894247757 grains/sq ft-hr or 0.00000171985 lb/sq ft-hr. Sensible heat transfer through the roof (negative value indicates flow from outside to inside): 2.7980 Btu/sq ft-hr. Indoor air conditions: 68 F/35 percent RH. Outdoor air conditions: 9 F/50 percent RH. Elevation: 680 ft.

and REP values. You will note The diffusion of water vapor let’s recalculate the amount of wa- that this wall does not have an in- through this wall is 0.04687 ter vapor diffusion through the door vapor retarder. Psychromet- grains of water vapor per hr per cavity: ric conditions for this analysis are sq ft of wall surface. Now let’s de- W = (1/16.82024 ϫ (0.23563 – shown in Table 4. Using this in- termine what happens if we add a 0.03313) = 0.012039 formation and the summation of 4 mil vapor barrier between the You can see that without the the REP values in Fig. 1, we can gypsum board and glass fiber in- vapor barrier, the amount of wa- calculate the amount of water va- sulation. A 4 mil polyethylene va- ter vapor diffusion through the por flow: por barrier has a PERM of 0.08 or cavity is 3.89 times higher or, in W = (1/4.32024) ϫ (0.23563 – a REP of 12.5. This increases the other words, 289 percent higher 0.03313) = 0.04687 REP summation to 16.82024. Now than the wall with the vapor re-

76 HPAC Heating/Piping/AirConditioning June 1998 tarder. This is a significant the construction is shown in Fig. 2. cause the vapor can touch a sur- change, but is it needed? The problem was condensa- face that is below its dew point. To answer this question, we put tion in the false ceiling air cav- One solution might be to re- the parameters into a computer ity. The glass fiber insulation move the glass fiber insulation. program that calculates the sur- shown on top of the false ceiling In this case, however, it is not face temperatures of all the mate- would get so wet with moisture possible because the air cavity rials and also calculates the sur- it would collapse the false ceil- must be heated in winter to pre- face dew points to check for ing into the parking area. Sur- vent freezing of the pipes in the 2 condensation. If condensation oc- face temperatures based on heat air space. This is why the dia- curs, the program prints out stars loss calculations are also illus- gram shows radiant heat pipes. next to the calculated surface dew trated in Fig. 2. The solution used was to run the point temperatures. Table 5 From these temperatures, you radiant heat system in the sum- shows the results of the computer can see that during the summer, mer, which raises the surface analysis for the wall without the the glass fiber insulation was pre- temperatures above the dew vapor barrier, and Table 6 is with venting heat gain into the air point temperature. To save en- a vapor barrier. Without the vapor barrier, you can see that condensation does exist. Since water vapor is moving from inside to Walk-in cooler TDB = -10 F outside, you can see that it starts TWB = -11.2 F at the glass fiber insulation. The TDP = -23.62 F RH = 50 percent -8.35 F condensation may stop at the Pw = 0.01392 in. Hg glass fiber insulation due to the -7.62 F reduction of vapor flow through -7.62 F the remaining materials. If, how- 50.57 F Radiant ever, the condensation is fre- 4.5 in. concrete heat pipe quent, it could cause a loss of insu- lating value (R-value), which could cause the condensation to 4 in. polyurethane move into the adjacent materials. 0.75 in. sprayed fiber Water Condensation The wall with the vapor barrier 60 in. air space pipes problems (Table 6) has no condensation. 4 in. glass fiber insulation False ceiling Example cases 57.85 F Freezer and cold storage facilities 60.03 F require a lot of attention to design 91.56 F details. The indoor vapor pressures 94.59 F can be as low as 0.011 in. Hg, result- Outside air properties: ing in vapor pressure differentials TDB = 95 F T = 78 F as high as 0.78 in. Hg, which is WB TDP = 71.83 F three to four times higher than the RH = 47.39 percent differentials experienced in resi- Pw = 0.78668 in. Hg dential and commercial buildings. Vapor retarders are critical to re- Parking garage duce the moisture drive into the (open to atmosphere) freezer and to prevent condensation. One client had major conden- 2 Surface temperatures through the floor along with inside and outside air sation problems from the underside properties. of a supermarket floor. The supermarket was built with parking at space. This is a problem because ergy, one could install sensors to ground level; the cold freezer facili- the surface temperatures are well monitor the metal deck tempera- ties and supermarket were on the below the outside air dew point ture and the dew point tempera- second floor above the parking area. temperature. With no vapor bar- ture. A controller could then turn The parking area is totally open to rier and the poor air tightness of on the radiant heat to maintain the outdoor environment. One area the false ceiling, the air cavity will the surface at 5 F higher than the that had condensation problems have a dew point very close to the dew point temperature. was the underside of a walk-in outside air dew point. This, in The computer condensation cooler floor. An elevation sketch of turn, results in condensation be- analysis program revealed con-

June 1998 HPAC Heating/Piping/AirConditioning 77 MOISTURE CONTROL

densation problems inside the is over 4.0 in. Hg, which is tre- well above the temperature of the false ceiling. mendous as well as unacceptable. existing roof heating system. In One industry that has many The theory behind preventing roof this case, the roof heating system problems with water vapor diffu- surface condensation is to keep does not prevent condensation. sion and condensation is the paper the inside roof surface tempera- The cure to the condensation industry. The wet end section of a ture above the dew point tempera- problem is to find the hood leaks paper machine takes the water ture of the vapor rising to the ceil- and patch them. 2 and fiber and forms it into a sheet. ing. The surrounding air dry bulb The problems with water vapor The stock temperature is around temperature heats the roof sur- condensation inside the machine 100 to 120 F; therefore, the evapo- face, but due to heat loss through rooms are complex and require ration rate into the building is the roof, the surface temperature further review. The water vapor high. The dry end section, unlike will always be lower than the dry transmission is tremendous due the wet end section, has a hood to bulb temperature. One method of to the high vapor pressure differ- capture the water evaporated and raising the roof surface tempera- entials. The next problem is to de- discharges the water vapor out- ture is to add insulation to the sign the roof system so that the side. Some of these hoods, how- roof. If you look back to the first vapor can escape without conden- ever, have leaks that can raise the paper mill example, you can see sation or to have the vapor con- humidity excessively inside the that the dew point is 102 F and dense in a safe region. Over 45 building. The molecular weight of the dry bulb is 111 F. Therefore, percent of the roof failures are water vapor is less than the molec- in this case, you can expect the caused by poor roof design. Vapor ular weight of dry air. Therefore, roof temperature to be less than retarders are a necessity. the water vapor rises to the under- 111 F but hopefully not at or be- An example of a paper machine side of the paper machine building low 102 F. building roof is illustrated in Fig. roof, thus raising the dew point at To help prevent condensation in 3. The psychrometric test condi- the underside higher than the rest today’s machine rooms, one tions at the underside of the build- of the building. should supply a roof heating sys- ing roof were illustrated in the At one mill location, the air tem for the underside of the build- first example. This mill was expe- properties taken at the underside ing roof. Roof heating systems are riencing condensation and con- of the roof were as follows: air handling units that take in- crete deck spalling. The design

● Pbar = 29.8220 in. Hg door building air, heat it to 120 F, conditions were put into the con- ● TDB = 111 F ● TWB = 103.33 F ● T = 102.05 F DP Three-ply BUR membrane ● RH = 77 percent ● Pw = 2.0553 in. Hg 1 in. cork insulation This particular machine room 1 in. had little dryer hood leakage. Another survey for a machine room housing five paper machines showed a lot of condensation, cor- Two-ply vapor retarder rosion, and spalling of concrete Precast concrete deck units from the roof deck. A mass air and water vapor survey of the building Total R-value = 5.37 revealed a water vapor flow into the building of 123,175 lb per hr 3 Roof system. or 248 gpm. As you can imagine, this raised the building humidity and distribute it to the underside densation analysis program to to extreme levels. The worst area of the building roof. Depending check for problems. The results tested—at the underside of the upon roof insulation levels, this can be found in Table 7. roof—is illustrated below: process will then heat the roof to The program shows condensa-

● Pbar = 29.8220 in. Hg around 110 to 115 F. Certainly, tion in the concrete roof deck. This ● TDB = 159 F the building does not need any explains the roof spalling prob- ● TWB = 136.51 F more heat, but it is required until lems. In this case, there was in- ● TDP = 135 F the industry puts a hood on the sufficient insulation in the roof, ● RH = 54.81 percent wet end section. If you look back resulting in a low inside roof sur- ● Pw = 5.1651 in. Hg at the five paper machine building face temperature. The steel rein- The vapor pressure differential example, you will note that the forcement in pre-cast concrete across the roof system in this case dew point was at 135 F, which is decks will corrode when exposed

78 HPAC Heating/Piping/AirConditioning June 1998 to condensation and chemicals ures just a few years after instal- tion problems in buildings. It was such as chlorine. Test samples lation, and others have had roof kept simple so that the analysis taken from concrete decks have sections fall into the paper ma- would fit into this article. The shown acid-soluble chlorine-ion chines. To give you an idea of the building is a residential home contents of between 100 and 1100 costs, a mill in the south had to with severe attic condensation ppm of concrete. Special precau- replace the roof steel, concrete problems in the winter. The home tions need to be taken to prevent deck, and roofing materials at a has cathedral ceilings of tongue- the corrosion of steel reinforce- cost of over $1,000,000 or about and-groove maple panels. The 2 ment. $50 per sq ft of roof. The building home had condensation in the at- Poor conditions at the under- housed a tissue machine, and the tic that dripped through the ceil- side of a paper machine building replacement occurred 24 years af- ing. The contractor told the home- roof should not be taken lightly. ter installation. owner that the problem was the High humidities will result in The last example will illustrate storage of 22 face cords of fire- premature failure of the roof sup- blower door surveys, infrared sur- wood in the basement. port steel, concrete deck, and veys, the use of mass dry air and The first test conducted was a roofing materials. Some mills water vapor analysis, and adia- blower door test to determine the have experienced complete fail- batic mixing to solve condensa- building air change rate. The

TABLE 7—Vapor migration calculation results for a paper mill roof (Fig. 3).

Outdoor air 19.00 11.1422 0.07450 0.10493

Outdoor air film 0.1700 1000.0000 0.00100 Inner surface 21.9125 11.0681 0.07450 0.11876

3-ply BUR membrane 0.4000 0.3300 0.0100 100.00000 Inner surface 27.5661 16.3818 0.09406 0.15030

Cork insulation 1.0000 3.8500 2.1000 0.47619 Inner surface 93.5251 16.4043 0.09415 1.58068

2-ply vapor retarder 0.4000 0.3300 0.0001 10,000.00000 Inner surface 99.1788 101.4590**** 2.04965 1.87862

Pre-cast concrete deck 1.0000 0.0800 3.2000 0.31250 Inner surface 100.5493 101.4600**** 2.04972 1.95776

Indoor air film 0.6100 160.0000 0.00625

Indoor air 111.0000 102.0828 2.04972 2.66197

Totals 2.8000 5.3700 10,100.7959405

Total water vapor transmission: 0.00019555046821 grains/sq ft-hr or 0.00000002794 lb/sq ft-hr. Sensible heat transfer through the roof (negative value indicates flow from outside to inside): 17.1322 Btu/sq ft-hr. Indoor air conditions: 111 F/77 percent RH. Outdoor air conditions: 19 F/71 percent RH. Elevation: 90 ft.

****If Pw is greater than or equal to Px, condensation will occur in that region. ****If the dew point temperature is greater than or equal to the dry bulb temperature, condensation will occur.

June 1998 HPAC Heating/Piping/AirConditioning 79 Ridge MOISTURE CONTROL vent

4 Mass flow balance.

blower door air flow was 5200 cfm at a differential pressure of 50 pascals (0.201 in. WG). The calculated air change rate in air Cathedral ceiling with partial attic and 2 changes per hour (ACH) is tongue-and-groove ceiling as follows: ACH = (acfm ϫ 60 min per hr)/(Building volume, cu ft/1 air change) = (5200 ϫ 60)/(21,693/1) = 14.4 Soffit vent This is a high air change Water vapor Water vapor rate for a new home. from from Humidifier Water vapor The next step was to de- Air infiltrationtwo people new home Air exfiltration (15.74 lb 0 lb per hr from construction termine where the air flow per day) (Capacity: firewood materials T = 0 F T = 68 F DB 0.656 lb 5.83 lb storage 0.917 lb DB paths were. An infrared-im- T = -2.4 F T = 46.27 F WB per hr per hr) 8.40 lb per hr WB age heat-loss scan was con- TDP = -21.57 F per hr TDP = 14.9 F ducted with the blower on RH = 35 percent RH = 12.82 percent acfm = 1270.56 acfm = 1462.16 and with the blower door off scfm = 1427.66 scfm = 1429.88 to help accentuate the areas m = 6422.36 lb dry m = 6422.36 lb dry with air flow heat loss. air per hr air per hr m = 2.12 lb water m = 12.093 lb water The infrared-image scans vapor per hr vapor per hr showed air flow leaking through the tongue-and- Assumptions: groove ceiling panels. It 1. Furnace maintains home at 68 F was discovered later that 2. Calculations made with humidifier off the kraft-back vapor bar- 3. Blower door test at 50 pascals, acfm = 5200 4. Air flow rate at norm of 3 pascals, acfm = 1270.56 rier was not stapled against the inside face of the cathedral ceiling rafters. Instead, the is maintained at 68 F/35 percent and pounds of water vapor. There- vapor retarder was stapled to the RH. This amount of water vapor fore, the amount of dry air enter- inner sides of the rafters. If the flow is 1648 times higher than the ing must be equal to the amount vapor retarder is installed against water vapor diffusion into the at- of dry air leaving. This is not the the inside face of the rafter and tic. If the house was built with the case with acfms. The finalized the tongue-and-groove paneling is proper air tightness, the air flow analysis is illustrated in Fig. 4. nailed against the vapor retarder, at 3 pascals would be around 289 You will note that the storage of you can achieve air tightness. acfm. Therefore, the excess air firewood in the home did add a This was not done. Without air flow through this house comes to tremendous vapor load. What is tightness, you have a potentially 981 acfm. If we instead assume even more interesting is that the dangerous situation because air that only 1 percent of the excess calculated indoor humidity (air flow can carry much more mois- air flow goes into the attic, the leaving) is only 12.8 percent, even ture into the attic than diffusion amount of water vapor it would with all this water vapor load. through the materials. The carry is still 16 times higher than This condition was verified by the amount of water vapor diffusion the diffusion flow. What this illus- homeowner who said that the hu- through 1364 sq ft of ceiling in trates is that air tight construc- midifier had to run frequently be- this case is 0.01438 lb of water va- tion is extremely important. cause the house was so dry. The por per hr. The next step was to conduct a reason for the dry environment is The blower door air flow of 5200 mass flow analysis using the the excessive outside air flow into cfm was converted to air flow at tested air flow at 3 pascals, an in- the house, which was drying out normal building differential pres- door design temperature of 68 F, the house. Further investigations sure of 3 pascals (0.012 in. WG), and estimates of water vapor gen- revealed similar homes that were which is 1270 acfm. If all of this eration inside the home. The out- very hard to heat due to the excess air flow travels through the side winter air is assumed to be 0 air flow. tongue-and-groove ceiling into the F/35 percent RH. To conduct a To summarize, the condensa- attic, the water vapor it carries mass flow analysis, one must con- tion problem was due to the air will be 23.70 lb per hr if the home vert the flows to pounds of dry air and water vapor flow into the at-

80 HPAC Heating/Piping/AirConditioning June 1998 tic. The amount of water vapor en- tering the attic was too much for the attic to handle, which raised the air dew point, causing the condensation. This is why state codes emphasize air tightness. Conclusion 2 Due to space limitations, this article only covered some of the basics of water vapor analysis and control. The use of vapor retarders, for instance, gets profes- sionals into very heated and complex debates. Certainly, there are times when vapor retarders are needed and times when they are not. To help engineers with these decisions, the U.S. Army Cold Re- gions Research Laboratory (CR- REL) has developed a procedure, information about which can be obtained from the National Roof- ing Contractors Association in Rosemont, Ill. Also, there is a paper written by the staff of the Oak Ridge National Laboratory, Oak Ridge, Tenn., that provides new information about vapor retarder selection criteria. This is a difficult field that could use additional research work to as- sist the engineers’ efforts. It is also a field that is getting a lot of attention these days due to the concern of indoor air quality. For engineers who want to learn more about moisture transport modeling for buildings, the 1997 ASHRAE Handbook of Fun- damentalsrecommends the following but warns that most of the models are research tools that may be too complex for users other than researchers: ●Glasta (Physibel, Belgium) ●EMPTEDD (Trow, Toronto, Canada) ●Match (Technical University, Lyngby, Denmark) ●COND (Technical Univer- sity, Dresden, Germany) ●MOIST (NIST, Gaithersburg, Md.) HPAC

Questions or comments about this article may be directed to the author at 920-465-3548.

June 1998 HPAC Heating/Piping/AirConditioning 81