Controlling moisture in your home improves the effectiveness of your air-sealing and insulation efforts and vice versa. Thus, moisture control contributes to a home’s overall energy efficiency.
The best strategy for controlling moisture in your home depends on your climate and how your home is constructed. Moisture-control strategies typically include the following areas of a home:
* Attics
* Foundation (basement, crawlspaces, and slab-on-grade floors)
* Walls
In most U.S. climates, you can use vapor diffusion retarders in these areas of your home to control moisture. Proper ventilation should also be part of a moisture-control strategy.
How moisture moves through a home
To understand the principles of moisture control, you need to understand the basics of how moisture can move through your home.
Moisture or water vapor moves in and out of a home in three ways:
1) With air currents
2) By diffusion through materials
3) By heat transfer.
Of these three, air movement accounts for more than 98 percent of all water vapor movement in building cavities. Air naturally moves from a high-pressure area to a lower one by the easiest path possible—generally through any available hole or crack in the building envelope. Moisture transfer by air currents is very fast (in the range of several hundred cubic feet of air per minute). Thus, you need to seal any unintended paths carefully and permanently to control air movement.
The other two driving forces, diffusion through materials and heat transfer, are much slower processes. Most common building materials slow moisture diffusion to a large degree, although they never stop it completely. Insulation also helps reduce heat transfer or flow.
The laws of physics govern how moist air reacts within various temperature conditions. The study of moist air properties is technically referred to as “psychrometrics.” A psychrometric chart is used by professionals to determine at what temperature and moisture concentration water vapor begins to condense. This is called the “dew point.” By understanding how to find the dew point, you will better understand how to avoid moisture problems in your house.
Relative humidity (RH) refers to the amount of moisture contained in a quantity of air compared with the maximum amount of moisture that air could hold at the same temperature. As air warms, its ability to hold water vapor increases; this capacity decreases as air cools.
For example, according to the psychometric chart, air at 68 degrees Fahrenheit with 0.216 ounces of water (H2O) per pound of air (14.8g H2O/kg air) has a 100 percent RH. The same air at 59 degrees F. reaches 100 percent RH with only 0.156 ounces of water per pound of air (10.7g H2O/kg air).
The colder air holds about 28 percent more moisture than the warmer air does. The moisture that the air can no longer hold condenses on the first cold surface it encounters (the dew point.) If this surface is within an exterior wall cavity, wet insulation and framing will be the result.
In addition to air movement, you also can control temperature and moisture content. Since insulation reduces heat transfer or flow, it also moderates the effect of temperature across the building envelope cavity.
In most U.S. climates, properly installed vapor diffusion retarders can be used to reduce the amount of moisture transfer. Except in deliberately ventilated spaces, such as attics, insulation and vapor diffusion retarders work together to reduce the opportunity for condensation in a house’s ceilings, walls, and floors. When exploring your moisture control options, you need to consider your climate first.
Information courtesy of EERE
