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6. Moisture Design

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6. Moisture Design 6. Moisture Design Objectives: 3. Prevent vapor condensation (surface, concealed) 4. Prevent rain penetration Vapor Condensation 2. More new bldgs are susciptable to vapor condensation because of: 1. Increased thermal insulation 2. Tighter enclosures 3. More efficient h/c systems n To predict condensation, you need to understand and quantify: 1. Physical forces that affect the movement of water vapor and flow of heat within the wall section (psychrometrics) 2. Effect of insulation, air barrier, and vapor barrier and their interaction ---> function of properties and location Psychrometrics The higher the temp, the higher the amount of water vapor and vice versa. Psychrometrics When the air contains the max amount of vapor it can hold at a given temp, it is saturated. The temp corresponds to 100% saturation called the dew point. If temp goes below the dew point, condensation will occur in the form of liquid if the dp is above freezing and in the form of frost if the dp is below freezing. Psychrometric Chart Psychrometric Chart- Continue Properties of air: dry bulb temp, wet bulb temp, % RH, humidity ratio, vapor pressure, dp temp, effective temp Three parameters (variables): dry bulb temp, %RH and dp. Knowing two you can calculate the third using the chart Condensation in Buildings (surface) Winter condensation - insulated bldgs with high indoor humidity. Summer condensation - warm, humid climates in air- conditioning bldg. Condensation in Buildings (surface)- Continue To prevent condensation (surfaces), you have to control max interior humidity (see graph 5.5 in text). You can accommodate higher levels of interior RH by using double and triple glazing (see graph 5.4 in text). Another way to prevent condensation is to keep moist air away from windows by circulating dry air against it. Surface Condensation Permeance (M) Is the measure of building material ability to vapor flow. It is expressed in Perms defined as amount of water in grains (1/7000 lb) passing through one sq. ft in one hour under a vapor pressure difference of one inch of mercury Water flow resistance is the reciprocal of its pemeance. Permeability Permeability is the permeance of a one inch thick of material unit perm-inch. For building materials vapor permeance values, see table 5.4 in text. Examples: Concrete permeability 3.2 perm-inch. 4 mil polyethylene sheet 0.3 perms. Aluminum foil 0.0. Air (still) 1.30 perm-inch. Vapor Diffusion Relative Humidity ( RH) RH is the ratio of the amount of water vapor in air to the maximum amount of water vapor at the same temperature RH can also expressed as the ratio of actual vapor pressure (AVP) to saturated vapor pressure (SVP). i.e AVP=SVP * RH Note that saturated air has 100% RH Vapor Pressure Gradient Need: You need to calculate VPG to check if condensation occurs so that you may change the location of insulation ( so that it does not get wet) and/or provide a vapor barrier. Vapor Pressure Gradient- Contiune Calculation is based on linear relationship between drop in AVP and vapor resistance (Rv= 1/M) AVP is a function of temp and RH From temp you get SVP ( table) AVP= RH * SVP Concealed Condensation Perform VP gradient through wall section. If pressure for continuity is higher than SVP condensation will take place. Condensation Accumulation Analysis Control Strategies for Condensation Vapor movement by air flow Moving air carries water vapor with it. The infiltration and exfiltration of air through cracks and openings in a wall is greatly more than movement of water vapor by diffusion. Air moves through: Cracks Joints Windows/doors Utility penetration Porous materials Vapor movement by air flow- continued Uncontrolled movement of air can lead to serious problems of : Moisture leakage Condensation Undesirable heat loss or heat gain Air flow Through Building Envelope Air Flow- Low Rise Bldgs Stack effect Wind effect Chimney and exhaust fan Total effect = 0.2 psf Air Flow- Tall Buildings Stack Effect Wind Effect Fan Pressure Total effect= 42 psf Vapor movement by air flow- continued Air flow through openings can be induced by one or more of: 2. Wind pressure/suction (local). 3. Stack effect increases with ht. Also wind effects increase with ht. Therefore air flow is more critical in highrise bldgs than in lowrise bldgs. Wind pressure/suction Function of ht, shape, openings, etc. P = qz G Cp - qh G Cpi External Internal -- occurs through openings (windows) Internal pressure Net effect is an increase in design pressure for leeward wall and roof ---> watch structural exfiltration ---> increase air exfiltration through leeward wall and roof. Stack Effect in High Rise Bldgs Three key factors: -Temp differential Bldg height Opening distribution Stack Effect in High Rise Bldgs- Continue Stack Effect in High Rise Bldgs- Continue Mechanical Ventilation By: Fans - introduce or exhaust air Opening ( windows) If supply is greater than exhaust, a bldg is pressurized (positive pressure inside), which minimizes uncontrolled infiltration and pressure difference in lobby levels (highrise). This is desirable in highrise and in cold climates (lobby). If exhaust is greater than supply, negative pressure exists, which prevents exfiltration of moist interior air through bldg envelopes. This is desirable in swimming pools and industrial facilities. Vapor and Air Barriers Two ways of vapor transmission: 2. Air flow ---> air barrier ( always needed) 3. Vapor diffusion ---> vapor barrier. Vapor diffusion analysis determines whether air barrier is vapor permeable or vapor resistant. Vapor Retarders (barriers) What? A vapor retarder is a material with very low permeance to water vapor diffusion (less than one perm), ie. 6 mil polyethylene film. When? Is used when vapor diffusion is a problem ---> prevent condensation with high rate (cannot be accommodated by seasonal drying/wetting cycles). Vapor barriers- Continue Where? In cold climate vapor flow from inside to outside ---> v.b. be placed inside, In hot climate the reverse, In moderate climate no need of vapor retarders. Vapor Barrier Types There are many with different efficiency: 6-mil polyethylene film Aluminum foil Glass Foil-based insulation Foil-based gypsum board Air Barriers An air barrier is a system or network of materials which prevent air movement through a bldg enclosure. Air barrier is not necessarily a vapor barrier, i.e. Parging on masonry wall or concrete wall. Vapor barrier can be an air barrier if sealed to prevent air movement. Example: 6-mil polyethylene sheet. Requirements of an effective air barrier Air flow resistance Continuity Sealing of seams, joints, edges, etc. Vapor and Air Leakage Paths Determine the Need for Vapor Retarders Example of Combining Air and Vapor Barriers Water Penetration Source Rain Melting snow Ground water Factors Presence of water Opening A physical force Gravity Kinetic energy Capillary action Air currents Air pressure (differential) Hydrostatic pressure Physical Forces for Rain Penetration What’s a Leak? Water infiltration that is unintended, uncontrolled, exceeds the resistance, retention or discharge capacity of the system, or causes damage, or accelerated deterioration: Water penetration Water absorption Water permeation Water saturation Water infiltration Water leakage Q1: Is water absorbed into a porous cladding considered a leak? NO Q2: Is water in the drainage cavity behind a masonry veneer a leak? NO Preventing Water Problems It is possible through proper design and construction to mitigate the 3 factors which contribute to water penetration: 2. Limit water penetration with: 1. Barriers, membranes and joint sealants 2. Diversions by sloping surfaces/gutters 3. Screens - projections 3. Prevent water accumulation by: 1. Drainage 2. Drying/evaporation 3. Ventilation 4. Neutralize the physical force with: 1. Capillary breaks 2. Drips 3. Protected openings 4. Rain screen Barrier Systems Face seal or prime seal methods of moisture protection: Precast cladding Metal cladding Glass cladding Exterior insulation & finish system (EIFS) The system depends entirely on the skin as the only line of defense. Therefore, sealants are critical. “A bldg envelope that relies on joint sealants as its first and only line of defense will leak sooner or later.” Barrier systems are not forgiving and not realistic. They require frequent maintainance of jointing systems to assure satisfactory performance. Limiting Rain Penetration Limiting Rain Penetration- Continue Drainage Systems For cavity (1) walls, any water penetration through cracks/capillary suction of porous materials is interrupted by air space between the cladding and the backing. The cavity is then drained of penetrated water by flashing (2) and weep holes (3). Drainage Systems-continued This system has redundant protection and the internal collection and drainage system provides a second line of defense. This system is more expensive (initial cost), but requires less maintenance. Problems with: Pressure differential between ext. and cavity can draw substantial amount of water -- can cross cavity through ties/connectors. Drainage in Cavity Walls Proper drainage requires: At least 2” air space Continuous ( lapped & sealed at least 6”) flashing Flashing has to come through the wall with a drip edge Open drainage path (weep holes) Drainage Systems-continued Rain Screen Systems Create a pressure equalized cavity, must include: 2. Structurally supported air barrier 3. Sealed and compartmental air chamber 4. External vents 5. Adequate drains (flashing & weep holes) Rain Screen Systems- Continue As a result of pressure equalization, the force producing rian penetration is reduced to less than 1% of what it would have been in cavity system. The amount of water will be only 10% compared to cavity system. This significant reduction reduces the demand on the drainage system and reduces the chance of water penetration/water damage. Neutralizing Physical Force Neutralizing Physical Force- Continue.
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