Moisture Control
Moisture Control
Properly controlling moisture in your home will improve 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. Before deciding on a moisture control strategy for your home, you may first want to understand how moisture moves through a home.
Moisture control strategies typically include the following areas of a home:
- Attics
- Foundation
- Basement
- Crawl space
- 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 help 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:
- With air currents
- By diffusion through materials
- By heat transfer.
Moisture Migration
Of these three, air movement accounts for more than 98% 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 carefully and permanently air seal any unintended paths 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.
Source:U.S. DOE
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