Water moves in, on, and through buildings through the following four paths.
Poetry used to be memorized, not written down, and handed from bard to bard, memory to memory, down through the generations. Perhaps folks out there have memorized poems back when schools taught such things, or for personal interest.
Not counting things I have written, I know only one poem by memory, written by David McCord. As proof, I will recite it for you here.
Epitaph on a Waiter
By and by
God caught his eye.
Water gets in through four pathways
Some things are inevitable. Like the Phillies beating the Red Sox in the World Series. And water getting into buildings. Water gets its way--or four ways, actually.
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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. That order is important because it helps us set management priorities.
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, 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.
(2011, July 12). Keeping Water Out of Buildings: Four Pathways. Retrieved from https://www.buildinggreen.com/blog/keeping-water-out-buildings-four-pathways
Good overview of the subject. Here are a few additions and amendments:
Bulk: Site features can also divert or deflect wind and water before it reaches the building, which is why height and site exposure is a determining factor in the water vulnerability of buildings.
Roof overhangs, or rather the ratio between overhang width and wall height, have been shown to be one of the most important factors in preventing water damage. Similarly, some method to drain roof water away from a building is key to durability. Every inch of rain on 1000 SF of roof will drain almost 625 gallons of water down toward the foundation.
Capillary: Capillarity requires two things: a "tube" and a reservoir to continue to supply that tube. So another essential capillary break is porous granular foundation backfill with a footing drain to prevent a reservoir next to the foundation wall. Lapped siding should be overlapped at least on inch to prevent local capillarity.
Vapor Diffusion: Water vapor movement is driven by vapor pressure and temperature differentials, not relative humidity (RH). For instance, most parts of the US have higher average RH in winter than in summer, yet the winter vapor drive is inside-to-out because the inside vapor pressure – even at lower RH – is much higher because of the more energetic environment. However, the sun shining on damp cladding even in the winter will drive water vapor inward against the dominant flow. RH, as well as temperature, drives liquid water diffusion – so the liquid diffusion drive in winter may be outside-to-in even while the vapor diffusion drive is in the opposite direction. Two streams passing in the night, as the poem goes.
And, actually, many old buildings do manage moisture well because they are so open to the flows of air, heat and humidity. That's why they're old and still standing. But an old building can lose its inherent resilience once we try to make it "better". As another old saying goes, "sometimes better to leave well enough alone".
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