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Energy Solutions

Four Ways That Water Gets Into Buildings

Water enters buildings by bulk transport, by capillarity, by piggybacking with infiltrating air, and by vapor diffusion. Once you get your priorities straight, you can implement strategies to prevent water damage.

Water gets into buildings through (1) bulk pathways, (2) capillarity, (3) being carried as vapor through air, and (4) vapor diffusion. Bulk water has by far more volume than the other three, and capillarity is second — and so on down the line.
Image Credit: Illustration: Peter Harris

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.

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.

Last week’s column: Recycled Content in Building Products: Should You Care?

Peter Yost at BuildingGreen contributed to this week’s post.

Tristan Roberts is Editorial Director at BuildingGreen, Inc., in Brattleboro, Vermont, which publishes information on green building solutions.


  1. Buildingwell .org | | #1

    Water management is just as important as air and temperature
    This is a great write up explaining the various ways water can get into your home/building. It emphasizes the importance of having a tight building shell - to protect from water as well as air/temperature issues.

  2. Bill Rose | | #2

    getzinta buildings?
    Very nice writeup, just a couple to add.

    You probably should begin by noting that buildings already contain a lot of moisture. For residences I figure about 5%. A 20 ton building already has a ton of moisture in it. Trapped in it, you might say. When somebody says "You're gonna trap moisture in there" the important riposte is "How much?"

    Construction moisture. 'Nuf said.

    Next, imagine having a block of wood on a scale, and having some way of changing the temperature of the block of wood. At the same vapor pressure, a chilled piece of wood gets heavier and a heated block of wood gets lighter. So another way to get water into building materials is simply to change their temperature. Your diffusion discussion above only talks about vapor pressure differences. In fact, building materials are always changing their wetness when their temperature changes. This temperature driven flux can be totally independent of any "indoor" vapor pressure, and totally independent of any vapor barriers or moisture control methods.

    Say you add insulation to an existing building, and you find that the exterior materials during cold weather are wetter than they were before. Reading your post you might think that the "new" wetness came from indoors and calls for vapor barriers. Wrong. The exterior materials are colder than they were before so they are simply wetter. Cold stuff is wetter than hot stuff. Make hot stuff cold and it gets wetter.

    There is a way of using both steady-state analysis and transient analysis to answer the question "where did this water come from anyway?" Anyone who thinks that water in materials always comes from the "high vapor pressure" side really needs to run the numbers. It doesn't. It comes primarily from the air immediately surrounding the materials, not the air four building components and a layer of paint away.

    Keep up the good work.

  3. Jeff Bennett | | #3

    walkways and window wells
    We are creating walkway along a building wall passing by 2 windows which are currently at ground level but which will below the walkway by about 3 inches how can we guard against water entering the building?

  4. GBA Editor
    Martin Holladay | | #4

    Response to Jeff Bennett
    I hope you aren't trying to raise your grade above your window sills. That would be a mistake.

    Are you building this walkway out of pressure-treated lumber?

  5. Robert Riversong | | #5

    This is the Comment I left at
    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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