Moisture & Moisture Movement

Moisture Fundamentals

 

In this section of the Moisture & Moisture Movement module, the properties of moist air, and moisture transport through building materials are examined.  Beginning with Dalton’s Law of partial pressures and the Ideal Gas Law, these fundamental gas relationships are used to assist in finding the capacity of air to hold moisture, the relative humidity of air, and the density of moist air. Next, the sensible and latent heat components of moist air are examined in order to find the total heat content of air/water vapour mixtures.  An aid for quantifying the properties of moist air, the Psychrometric Chart, is introduced and developed. Using this chart, various air conditioning processes are illustrated including dehumidification, humidification, heating, cooling and evaporative cooling.  A link is then provide between the interaction of moist air with hydrophilic and hydrophobic building materials.  Water transport mechanisms through materials are examined including vapour diffusion, bound water diffusion and capillary transport. Next, permeance properties of materials and how they relate to the water transport mechanisms are presented.   Then, Fick’s Law of diffusion is introduced and used to develop a one-dimensional steady-state transport equation for quantifying the rate of movement of water through building materials.  Illustrative sample problems are presented throughout in order to assist with the understanding of the fundamental properties of moist air and the related movement of moisture within building materials.

 

In this section of the Moisture & Moisture Movement module, the movement of moisture through building assemblies is examined. A method of calculating the composite permeance of an entire wall assembly consisting of a series of elements is presented.  Using the composite permeance and the one-dimensional steady state moisture transport equation, the amount of moisture transported through a wall assembly over a period of time is examined.  Methods of calculating the vapour pressure profile through various wall assemblies are also studied.  Using stated interface temperatures and the vapour pressure profile of a wall assembly, students are shown how the relative humidity at each material interface can be determined.  Knowing the resulting relative humidity profile of the wall students can resolve the critical question as to whether condensation has occurred within the wall assembly. Throughout the section, illustrative worked problems are presented to assist students with understanding moisture movement in building assemblies.

 

• Describe the properties of moist air
• Demonstrate how the psychrometric chart can be used to determine various properties of moist air
• Describe how moisture interacts with various materials