When looking at a home as a system, protection from the elements is a goal. Water and moisture can cause a great deal of damage to a structure and knowing all of the plains of potential pathways help to minimize damage and keep your home healthy. An article from Building Green helps to explain the pathways of water ingress and prevention of moisture to the home.
Water gets in through four pathways
Water moves in, on, and through buildings through the following four paths. I’ll go through these in order of magnitude–the most water is involved in the first path, and the least is involved in the fourth.
1) “Bulk” water: rain, runoff, and wind-driven water
Liquid or “bulk water”–rain, runoff, and other flows–is driven primarily by gravity but also by wind and pressure differences. Bulk water on the exterior of a building is managed by moving water down and off of the building, while site features move the water away from the building. A system of interconnected flashings, drainage planes or weather-resistive barriers, free-draining spaces, and claddings manage exterior bulk water.
Inside the building, we manage bulk water by preventing or containing plumbing leaks and condensation. Collection trays or pans, sensor-driven shut-offs, and routine maintenance defend against interior bulk water problems. Sprinkler systems introduce bulk water inside of a building in the event of a fire, but in addition to their benefits in quickly dousing a fire, they often prevent much larger magnitudes of water from being hosed in by the fire department.
2) Capillary water
Capillary water moves under tension through porous building materials or narrow channels between building materials that act like tubes. The porous nature of many building materials, and the incredible cohesion and adhesion of water means that liquid water can move against the force of gravity quite effectively.
The primary defenses against capillary water movement are capillary breaks in appropriate locations, such as the between the foundation and moisture-sensitive materials sitting on it. Capillary breaks are non-porous materials–such as sheet metal, impermeable membranes, closed-cell foams or plastics–or free-draining air spaces (sometimes referred to as rainscreen), generally 3/8″ (10 mm) or larger.
3) Air-transported moisture
Air-transported moisture is the vapor content of air as it leaks out of or into a building. Air leakage is driven by a combination of holes through the building envelope and one of three driving forces: wind, stack effect, or mechanically induced pressure differences (fans) between the inside and outside of the building.
The primary concern (other than the heat content of the escaping or entering air) of moisture-laden leaking air occurs when it is accompanied by a temperature drop, increasing condensation potential. For example, warm, humid air from a shower in the cold winter months can leak around the bathroom light fixture into the attic, condensing on the roof sheathing–eventually leading to rot.
We manage air-transported moisture with a continuous air barrier in the building envelope, built with interconnected air-impermeable sheet goods, caulks, sealants, and spray foams. To be completely effective, air barriers should be in contact with thermal barriers (insulation).
4) Vapor diffusion
Vapor diffusion is the movement of water as a gas according to relative humidity gradients or differences in vapor pressure. Water vapor moves from areas of high concentration to areas of low concentration.
You often hear about use of vapor barriers to restrict vapor movement in buildings, but anything that slows vapor movement is a double-edged sword: while we may want to control the movement of vapor into a building assembly, we should be much more interested in how the vapor permeability of individual building materials and assemblies affect the movement of vapor out of building assemblies. While building assemblies can get wet by all four forms of water movement, once water gets in, the main way it can get out is by diffusion, so it pays to make sure that assemblies can dry through diffusion in one or more directions.
Quite often the vapor drive of water into building assemblies is climate- and season-related: vapor drive is from the inside of heated buildings in the winter and from the outside of cooled buildings during the summer. We need to balance the restriction of this climate- and season-based vapor movement into building assemblies with the allowance for drying of the same assemblies. We do this by conducting a vapor profile analysis or hygrothermal (humidity plus temperature) modeling.
Water management and insulation
That’s a lot to digest, but it helps to understand these fundamentals when you are thinking about adding insulation to your building. Insulation restricts the flow of heat, which in turn reduces ability of building assemblies to dry out when wet. Lots of old buildings don’t manage moisture very well, but that’s not a problem for them because they are so poorly insulated that they dry out easily. Adding insulation to older buildings is a good idea for a lot of reasons, but we must think about moisture at the same time.