Objectives

• Compare locations in North America• Potential for moisture problems• Different driving forces

• Analyze potential for moisture problems in cooling climates• Focus on wall assemblies

• Critically examine design solutions• Discuss example from ASHRAE Journal

article

BSC Approachhttp://www.buildingscience.com/designsthatwork/default.htm

http://www.buildingscience.com/designsthatwork/hygro-thermal.htm

• Divide nation (or world) into “zones” based on hygro-thermal parameters

• Design building codes and strategies to address the important sources and driving forces• Ignore unimportant driving forces

• Result should be buildings that are climate specific

Hot-Humid Climates

• More than 20” of rainfall and one of the following:• a 67°F (19.5°C) or higher wet bulb temperature for 3,000

or more hours during the warmest six consecutive months of the year; or

• a 73°F (23°C) or higher wet bulb temperature for 1,500 or more hours during the warmest six consecutive months of the year.

• What do we know about temperature and vapor pressure differences?

• What do we know about building construction?• What problems are likely?

Strategies for Hot-Humid Climates

• Interior and exterior air retarders

• Walls must be able to dry to the inside• No significant vapor retarders

• Don’t insulate edge of foundation. Why?

• Unvented attic

Disadvantages?

• Training construction workers• Every strategy involves air retarder system• Many strategies require exterior rigid insulation

• Risk of trying new ideas

• Defining appropriate zones without defining too many zones

• Relying on higher quality construction

Summary

• Compare locations in North America• Potential for moisture problems• Different driving forces

Diagnosing/Designing

• Consider• Moisture transport mechanisms• Driving forces• Moisture sources

Building in Austin

• Comparing options

1. Paint interior walls with vapor barrier paint.

2. Increase ventilation for dilution.

3. Seal building to limit infiltration.

4. Set thermostat to lower set point.

5. Balance HVAC to have positive pressure in building.

Paint interior walls with a vapor barrier paint

• Good idea or bad idea?

• What does VB paint do?

• What driving force is this addressing?

Data

• Austin ASHRAE 1% DB = 96 °F, WB =74 °F• RH = 35.7 %, w = 0.013 lb/lb • pw= 0.8417 psi × 35.7 % = 0.300 psi

• Inside 78°F 50% RH, w = 0.010 lb/lb• pw= 0. 4751 psi × 50 % = 0.237 psi

• Latex primer/sealer = 6.28 perms• perm = grain/(hr∙ft2∙in Hg)

• Vapor “barrier” paint = 0.45 perms• What is water vapor flux through paint?

• Do you want it to be high or low?

Governing Equation For Diffusion

• w water vapor flux [M/t/A, kg/s/m2]

• µ permeability [perms∙in, perm = grain/(hr∙ft2∙in Hg)]• Permeance [ng/(s·m2·Pa)]

• p is water vapor pressure

• x is distance along flow path

• Water diffuses from high vapor pressure to low vapor pressure

• Permeability is a function of temperature in materials• Very ugly non-linear relationship

x

pw

d

d

Increase ventilation for dilution

• What is driving force?

• What are indoor and outdoor humidity ratios?• w = 0.013 lb/lb out• w = 0.010 lb/lb in

• Dilution is not the solution to this pollution

Seal building to limit infiltration

• What are positive and negative consequences of air sealing?

• Assume going from 1 ACH to 0.35 ACH• How much does this reduce moisture source?• Assume 15000 ft3

building• Reduction in infiltration airflow of 163 CFM• Reduction in latent load of 2400 Btu/hr• Reduction in water vapor source of 2.2 lb/hr

• Does air sealing address driving force?

Set thermostat to lower set point

• Use more energy

• Provide more dehumidification

• Also, cool down surfaces and potentially raise dew point temperature

• Do we have a greater potential for condensation?

Balance HVAC to have positive pressure in building

• How do we achieve this?• Supply more air than

return

• Intentional return leak

• Seal supply ducts

• What driving force does this address?

• Also provides ventilation

When designing

• Always go back to first principles• Moisture sources• Driving forces• Material properties/geometry

Lstiburek Articles

• A little tongue-in-cheek

• Very good summary of material

• Elaborate on some examples in the article

Slabs?

• How do you pour a slab?

• Why do some places use sand?• Is sand a good capillary medium?

Rain on the Brick Sun Trick

Objectives

• Analyze moisture transport in buildings

• Assess conditions for condensation

• Diagnose mold growth

• Evaluate design strategies

• Key component of healthy building design• Particularly controlling moisture without

compromising energy efficiency