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 insulation2. Tighter enclosures3. 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/suctionFunction of ht, shape, openings, etc.P = qz G Cp - qh G Cpi

Internal -- occurs through openings (windows)

External

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 VentilationBy: 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 BarriersTwo 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 TypesThere are many with different efficiency: 6-mil polyethylene film Aluminum foil Glass Foil-based insulation Foil-based gypsum board

Air BarriersAn 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 PenetrationSource Rain Melting snow Ground waterFactors 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 ProblemsIt 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 sealants2. Diversions by sloping surfaces/gutters3. Screens - projections

3. Prevent water accumulation by:1. Drainage2. Drying/evaporation3. Ventilation

4. Neutralize the physical force with:1. Capillary breaks 2. Drips3. Protected openings4. Rain screen

Barrier SystemsFace 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 SystemsCreate a pressure

equalized cavity, must include:

2. Structurally supported air barrier

3. Sealed and compartmental air chamber

4. External vents5. 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