A building envelope firm is typically made up of a variety of engineering and architectural disciplines with a common goal of designing and maintaining a quality building enclosure. Understanding how water works on a general level allows us to predict and prepare for potential water problems.

Seven Fundamental Properties of Water

  1. Water molecules are very small: A cubic inch of water contains approximately 600 sextillion water molecules; a single snowflake contains 180 billion water molecules. They are very small!
  2. Water molecules are 'polar': In one water molecule there are opposite electric charges comprised of a larger, negatively charged oxygen atom and two smaller, positively charged hydrogen atoms.
  3. Water molecules are 'sticky': Like magnets, the positively charged hydrogen atoms are attracted to the negatively charged atoms of other polar molecules (not just water). This results in the polar molecules sticking to one another, an occurrence known as hydrogen bonding. For example, exhaling close to glass forms a wet fog on the glass. Why? The exhaled water molecules have bonded to oxygen atoms on the glass surface.
  4. Water molecules are little bundles of energy: Due to the sticky nature, water molecules can hold a large amount of heat (thermal energy). This heat can be transformed into a more organized type of energy such as speed (kinetic energy), making water molecules highly efficient when working together.
  5. Vapor, liquid and ice represent three distinct stages of energy transfer: Water molecules are constantly moving and colliding at varying speeds depending on temperature. At room temperature water molecules average speeds moving around 1,000 MPH!(1) At this speed the momentum is too much for water molecules to bond to other molecules, thus they remain dispersed as water vapor. At lower temperatures, slower speeds allow water molecules to bond, creating liquid...and eventually ice.
  6. Water is the 'universal solvent': The positively charged hydrogen atoms in water molecules are strong enough to pull apart a large number of substances (a good example is sugar cubes dissolving in water). While it often works at a far slower pace, water can dissolve more substances than sulfuric acid.
  7. Water and oil (or wax, or silicon...) don't mix: Materials comprised of nonpolar molecules, such as oil, are unaffected by the pull of the positive hydrogen atoms in a water molecule. When a chunk of wax is placed in water, not much happens. Some molecules, known as surfactants (i.e. soap) are polar (attractive to water) on one end and nonpolar (attractive to grease or oil) on the other. When warm water and soap is added to a sink filled with greasy pans, the soap molecules attach to both grease and water molecules taking all of it down the drain.

These properties have a direct effect on building envelope design and the performance of materials used in its construction. Understanding how water works on this basic level helps us better design the enclosure and better predict and identify problem created by water. Here are a few basic principles of water that are derived from the fundamentals above and are essential in the understanding of building envelope science.

Movement of Water
There are several forces that drive the movement of water; gravity pulls water towards the ground; air pressure typically pushes water down; wind can move water in nearly any direction. These are well-known forces that must be understood and addressed in any envelope design. Another force that moves water, and is just as relevant is capillary action. As a result of hydrogen bonding (remember, the negative hydrogen atoms bonding to positive atoms in neighboring molecules), water will attempt to travel 'uphill' unless stopped by air pressure or gravity. A good example is water in a glass trying to climb the edges of the glass, defeated by the air pressure pushing the water back into the glass.

Capillary action can move water without the benefit of wind or gravity moving through tiny voids. An example is metal roofing panels installed with a 2" unsealed overlap on a 3:12 roof slope, as detailed below. Water can be drawn into the tiny spaces between the overlapping panels by capillary action. The effects of air pressure and gravity are simply not strong enough to prevent capillary action with this sized lap at a 3:12 slope. A roof protected with asphalt shingles, installed at the identical slope, is not as vulnerable to capillary action due to the nonpolar molecules in the asphalt that prevent hydrogen bonding.

By considering all potentials for water movement and the relationship of each exterior material to water there is a better chance of keeping the enclosure dry.

Condensation, Evaporation and Relative Humidity
Maintaining a balance between condensation and evaporation within interior spaces is important to prevent moisture damage from occurring within the exterior walls. Evaporation takes place when water molecules gain enough energy (heat that is transformed into speed) to break their hydrogen bonds from other water molecules, allowing them to disperse as vapor. During condensation, cooler temperatures and decreased energy promote hydrogen bonding and the creation of free water.

Condensation Forms Only at Siding Areas Not Warmed by Underlying Steel Framing (Coe School, Seattle)

Relative humidity (RH) represents the ratio between the amount of water vapor in the air at a specific temperature and the amount the air could hold at that temperature(2). A relative humidity of 50% means half of the available thermal energy in a space must be used to reach a balance between condensation and evaporation. At 100% RH, there is no longer enough energy available to reach a balance and condensation occurs. Higher temperatures can saturate more moisture in the air. As the temperature drops the air can hold less water vapor resulting in condensation or free water. As the interior temperature drops at window glass or at points of air leakage, free water form on the colder surface eventually deteriorating the substrate material.

For hygroscopic(3) materials such as wood, RH values correspond to moisture content. At normal temperatures, RH is not temperature dependent. Whether the interior temperature is 30˚F or 90˚F, an RH of 80% will result in nearly the same moisture content (see Table 1). At RH values greater than 80%, no matter how hot it is, moisture content increases exponentially. The decreased supply of energy to power evaporation means more water molecules must be stored within the material.

Table 1: Moisture Content Values for Wood in a State of Equilibrium, 80% RH
Temperature (F°) Relative Humidity (%) Moisture Content (%)










It is quite common to find extended periods of 80% RH in many parts of North America. In this condition, even a small amount of additional vapor can raise the moisture content of hygroscopic materials.

Asphaltic Building Paper, Housewraps and Water
All Code-compliant building papers and housewraps must provide a level of vapor permeance from the interior while resisting water penetration from the exterior. This is more important is cooler climates where the interior temperature will be warmer than the exterior. Most asphaltic building papers are a barrier to water infiltration. These materials are also capable of absorbing some water until drying conditions return.

Housewraps typically have no absorptive capability. These products are usually manufactured with the creation of tiny holes or 'micro-perforations' that are too small for liquid water to penetrate, but allow water vapor to 'perm' through the layer. Similar products are used to manufacture waterproof clothings that are advertised to 'breathe'.

These materials generally work as designed because the liquid water molecule mass simply can't squeeze through the very small holes. For the same reason, water may become trapped behind the house wrap until the liquid can be heated sufficiently to evaporate.

Powerful surfactants, such as soap, can separate water molecules, making it possible for them to pass through the pores in the housewrap. For this reason, using soap on or around these products is not recommended. Consider the problems this can cause to a vinyl-sided exterior power washed with soapy water.

Water and Vapor Retarders
Vapor retarders are installed to prevent water vapor from diffusing into the wall and potentially causing damage to moisture sensitive components. The Second Law of Thermodynamics states wet always moves to dry; therefore, vapor retarders must be installed on the warm side of the insulation to prevent vapor from diffusing into the wall from the warmer interior (exterior in southern climates). In mixed climates this rule of thumb may not hold up creating challenges when determining the placement of a vapor retarder.

Further complicating matters are the roles played by other exterior wall materials preventing vapor movement. For instance, dry plywood sheathing can function as a vapor retarder if the joints are installed tightly; if wet, its ability to stop vapor movement decreases. Oriented strand board is typically more of a barrier than plywood, yet once water enters the panel it is slower to dissipate. Often, a building would be better served if no vapor retarder were installed, or if an interior paint with low vapor permeability was used. The building code limits options in this area.

Water and Wood
Water migration through wood occurs at the cellular level. In freshly harvested wood, water is in both the cell walls and the lumen (hollow space within cell). During lumber manufacturing, the wood is dried eliminating the liquid water within the lumen. Much of the water within the cell wall is eliminated as well.

As dry lumber(4) is exposed to moisture, water molecules bond within the cell walls. To accommodate this water, the cell structure swells. Eventually, the cell walls become saturated with water yet the lumens remain dry. This is known as the fiber saturation point (FSP). Depending on wood type, this usually occurs at moisture content of 28%-30%. Once the FSP is reached, free water begins to collect in the lumen. At this point moisture migration throughout the wood is accelerated.

Capillary action allows free water to migrate to adjacent cells that can accommodate the added moisture. The bonded water molecules force apart the wood strands resulting in swelling. Since the strands are vertically oriented, swelling typically occurs laterally. The accumulation of water and resultant swelling significantly diminishes the wood's structural performance. The higher levels of water can promote mold growth and decay.

Decay feeds on the organic materials in wood and uses the free water in the lumens as a means of transportation and to facilitate the feeding process. Limiting the available free water and keeping the wood below the fiber saturation point will prevent decay. Without free water, decay fungi cannot proliferate.

Moisture related problems require at least four conditions to occur: there must be a moisture source (i.e. condensation), a route for the moisture to travel, a driving force to cause movement, and materials susceptible to moisture damage.

Understanding the fundamental properties of water is a starting point for all our staff, no matter what the discipline. If you would like a full copy of Water 101, please drop me a note and a complete copy will be forwarded via email. Send requests to info@trinityerd.com.

May 2009
Issue Two, Volume Four

This article by Colin Murphy.

Colin Murphy is a founder and managing partner of Trinity | ERD.


"Pushing the Envelope: A Monthly Journal of Issues Concerning Building Design, Science, and Litigation" is a monthly publication of Trinity | ERD. This newsletter is intended as a thoughtful look into the issues of building construction.

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(1) The distance traveled by water molecules is substantially impacted by the continual collision with other molecules. 
(2) Source: Merriam-Webster Dictionary; www.merriam-webster.com
(3) Hygroscopic materials are porous materials that are capable of continually exchanging moisture with surrounding air and other materials to achieve balanced moisture content. Wood, gypsum, masonry and stucco are typical hygroscopic construction materials; in contrast, steel, vinyl, glass and bitumen materials are not hygroscopic.
(4) Source: Wood Handbook – Wood as an Engineering Material; Forest Products Laboratory, U.S. Department of Agriculture; Madison, WI; 1999
(5) Dry lumber is defined a lumber with a moisture content of 19%, or less.