Air Leakage and Water Vapor Control

Chapter 6

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Air leakage - The leakage of conditioned air through cracks and unsealed joints in the building envelope

Factors that affect air leakage Area in envelope prone to leakage Air pressure differences between inside & outside

air

Solution: reduce air leakage area

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Wind-related pressure differential

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Temperature-related pressure differential

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Air leakage sites

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Air Retarder

Continuous membrane applied to exterior of building enclosure prior to finish 5-10 mil plastic sheet Micropores allow vapor to escape

but retard passage of moisture and air Applied with staples, taped joints

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Air retarder wrapped around exterior wood sheathing in wood light frame construction

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Temperature differentials

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Sealed window openings

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Indoor air quality

A well-sealed enclosure can subject interior space to air pollutants Dust, pollen, micro-organisms Carbon dioxide, carbon monoxide Volatile organic compounds (from sprays, cleaning

products) Formaldehyde (off-gassing from building products) Radon

Interior spaces must be ventilated with adequate amounts of fresh air to prevent health problems

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Water vapor in air

Almost always water vapor mixed in air Water vapor exerts pressure independent of

air pressure Related to amount of vapor present in air Saturated air has maximum amount of vapor

present Saturated vapor pressure exists when air is

saturated

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Relative Humidity

Amount of water vapor in air expressed as a relative term

RH = weight of water (as vapor) in airweight of water (as vapor) in saturated air

Vapor pressure of airVapor pressure of saturated airRH =

X 100

X 100

RH (vapor pressure of saturated air) = vapor pressure of air

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Calculate vapor pressure in 70º air at 45% relative humidity

Vapor pressure of saturated air @ 70º = 52.5 psf (Table 6)

RH (vapor pressure of saturated air) = vapor pressure of air

45100

( ) 5.5 = 23.6 psf

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Human sensation of relative humidity

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Water vapor moves (vapor drive) from warm to cold side of an assembly

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Dew Point - temperature at which air’s RH becomes 100%

Condensation - water vapor in air converts to liquid water

Condensation occurs when air temperature is decreased below dew point

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

R-value & location of dew point

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Control of condensation

Control entry of water vapor to wall cavity Reduce air leakage (air-moisture barrier on

exterior that allows vapor diffusion) Prevent vapor diffusion from warm side of

assembly by using a vapor barrier Allow any vapor that enters a wall or attic

cavity to exit the assembly Permeable air-moisture barrier on walls (see air

retarder) Attic ventilation

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Commonly used vapor retarders

Glass and metals Roof membranes Asphalt treated paper (kraft paper) Polyethylene sheet

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Proprietary vapor retarder

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Location of the vapor retarder (in cold climates)

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Attic ventilation

Control condensation Prevent formation of ice dams at projecting

eaves Snow & ice add unwanted loads to eaves Ice dams prevent water from draining off the roof,

resulting in leaks

Reduce heat transmission to interior of building during summer

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Soffit & gable end vents

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Soffit & ridge vent

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Soffit & turbine vents

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Temperature gradient across an assembly

Temperature drop across the entire assembly

∆t1 + ∆t2 + ∆t3 = t

Heat flow through an assembly

∆t1

Rt

= q

R1

Rt

∆t1 = ∆t

Temperature drop through a layer

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Temperature drop across a layer is proportional to the R-value of that layer

Temperature drop across the entire assembly

∆t1 + ∆t2 + ∆t3 = 60ºF

215

∆t1 = (60) = 8ºF

∆t2 = 48ºF ∆t3 = 4ºF

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Line of temperature gradient

Assume:

RH outside air = 80%

RH inside air = 50%

Assembly is vapor permeable

Dew (point per Table 1) = 50º

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Problem: Determine location of dew point in brick veneer wall assembly

Assume inside and outside temperatures are 70º and 10º respectively, and RH of inside air = 45%

Element R-Value

Inside Surface Resistance

0.7

1/2 in. thick gypsum board

0.5(0.60) = 0.30

3 1/2 in. thick fiberglass insulation

3.5(3.5) = 12.25

0.5 in. thick plywood

0.5(0.9) = 0.45

2 in. wide airspace 1.0

3 5/8 in. thick brick veneer

3.625(0.2) = 0.73

Outside surface resistance

0.2

Mehta, Scarborough, and Armpriest : Building Construction: Principles, Materials, and Systems

© 2008 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved.

Solution: approximately midway through insulation layer