Background
It should come as no surprise that on earth, known as the water planet of our solar system, water plays a dominant role in virtually all environmental phenomena. Addressing moisture protection measures is widely viewed as a primary enclosure design strategy. As was pointed out earlier for cold climates, experience indicates that when the requirements for the control of moisture migration have been satisfied, the other control requirements are either coincidentally satisfied, or more easily satisfied than if moisture migration is not addressed at the outset. The following insights are provided as background to this design heuristic.

First, lets begin with some important material properties related to moisture:

Hydrophilic or hygroscopic - materials that absorb water in their capillaries, such as wood, concrete, fabric, etc.

Hydrophobic - materials that do not absorb water such as glass, steel, plastic, etc.

In some cases, a hydrophobic material can be structured to behave like a hydrophilic material. Glass fibre and steel wools are examples of this phenomenon where hydrophobic materials form a material matrix with capillaries that can hold water.

Most building materials are hydrophilic and their moisture content is a function of the relative humidity to which they are exposed. A graph for brick, concrete and wood moisture content depicts hysteresis effects - differences between the wetting curves, where moisture content is increasing, and the drying curves, where it is decreasing. Looking at wood, it can be seen that the point of inflection in the curves occurs at approximately 50% relative humidity, at which point capillary flow becomes established and the moisture content begins to increase sharply. It should be noted that at 16% moisture content by weight, wood become susceptible to mold growth, corresponding to an exposure of about 80% relative humidity. Concrete is similar to wood, but smaller capillary diameters and the presence of aggregate cause a peculiar drying curve. Clay brick has even smaller capillaries and does not exhibit hysteresis until after 80% relative humidity.

The common characteristic of hygroscopic materials is that due to their capillary structures, they are easily wetted, but very difficult to dry. In practical terms, this means that if these materials become wet, it is unlikely they will completely dry out, potentially maintaining a moisture content that renders them susceptible to damage mechanisms.

Indeed, the difficulty of extracting liquid from capillaries has become part of North American popular culture as found in the following juvenile rhyme:

When designers forget this important characteristic of common building materials, the consequences are far from juvenile. Most recently in Canada, the leaky condominium problem in British Columbia attests to a major failure to understand and properly apply moisture protection fundamentals. The price tag for repair of damages has been estimated in the range of $500 - $800 million. For some sobering insights, view the report of the Commission of Inquiry into the Quality of Condominium Construction in British Columbia at: www.qp.gov.bc.ca/condo

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Moisture Management Concepts
The control of moisture in building enclosures is relatively straightforward, but based on the amount of research and publication devoted to the topic, it continues to elude common sense reasoning. There are basically 4 conditions that must all be satisfied to cause moisture-related problems.

In practice, it is not possible to completely eliminate any one of these conditions. Using materials that are moisture tolerant likely represents the easiest approach from a design perspective, but usually costs more than using conventional building materials. Normally, each of these conditions is addressed in design by moderating their effects. For example, large roof overhangs can shield walls from rain penetration thereby reducing the strength of the moisture source. Paths for moisture transport may be sealed to repel water. A pressure-moderated screen (air space behind the cladding) may reduce the amount of wind pressure driving rain through the enclosure. Protective or sacrificial coatings may be applied to materials to improve their resistance to rotting or corrosion. From an architectural science perspective, the probability of moisture-related problems can only be reduced, never completely eliminated. Nothing lasts forever.

Rather than dealing with the vast body of work on moisture control in buildings, this section presents fundamental approaches that are supplemented with useful references found under Related Resources.

Let's begin by examining the relationship between moisture sources, transport processes, moisture storage and sinks.

The chart adapted from the work of John Straube, University of Waterloo indicates that in the final analysis there are only three means of dealing with moisture: drainage, evaporation (drying by ventilation or convection), and diffusion. It is important to appreciate that these transport mechanisms operate at very different rates. Drainage can move the highest rate of moisture away from or out of the building enclosure. It is highly reliable because it is driven by gravity - a force that never takes a holiday and cannot be blocked. Evaporation is the next most effective drying mechanism, but unlike drainage, evaporation is not a guaranteed process. It relies on air velocity and temperature, much like the windshield defrost in motor vehicles, or hair dryers. For the ventilation drying of building facades, a clear air space with openings at the top and bottom is needed, and it is preferable if the sun can quickly heat the ventilation chamber and the air it contains to initiate stack action. Diffusion is significantly less effective than drainage or evaporation, but it has the advantage of being a fairly steady, continuous process on a seasonal basis.

Another important point to consider is that drainage and ventilation are a function of the articulation of enclosure elements (i.e., providing a drainage plane and drainage/ventilation space) whereas diffusion depends on the physical properties and arrangement of elements within an enclosure assembly. Practically speaking, this means that drainage and ventilation strategies remain largely constant among climate classes (based primarily on rain exposure), whereas the vapour permeability and location of membranes, coatings, sheathing papers and insulation may vary considerably.

Before selecting an enclosure design strategy, it is useful to take a whole building perspective on moisture management. The site and soil conditions, depth of water table, topography, building geometry and exposure are important determinants of bulk (free) water control strategies. HVAC systems must also be appropriately integrated to effectively control building temperature, humidity and air pressure.

Drainage spaces must be suitably ventilated to promote drying and guard against deterioration of materials that are susceptible to moisture damage and located adjacent to the drainage spaces. This simply requires that unobstructed openings are provided at the top and bottom of drainage spaces. Thus the air space is not only a pressure moderator and a capillary break, but also a ventilation stack.

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