Moisture Rules
Background / Moisture Management Concepts / Enclosure Control Functions and Strategies


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.

 


Florida house
House off the coast of Florida, ca. 1920

Canadian Building Digest quote

Mekong Delta house
House on the MeKong Delta, Viet Nam.


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.

Hydrophilic materials moisture content

The moisture content of hydrophilic materials is governed byrelative humidity and exhibits hysteresis between wetting and drying processes.

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

No matter how long you shake and dance,
the last drop always drips down your pants.


Popular graffiti above urinals in boys' washrooms.
 
 

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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BC condominium
Rotting and mold are normal outcomes of wood that is allowed to get wet and stay wet as observed in this forensic picture of a BC leaky condominium problem.


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.

1. Moisture must be present, in the liquid,     solid or vapourform.

2. Some path for transport of the moisture     must be  available.

3. A driving force to transport the moisture     must be applied.

4. The building enclosure must contain     materials that  are susceptible to     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.


Flashing overlap
Flashing - the understated effectiveness of the overlap.



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.

Mositure sources and sinks
Moisture sources, processes, storage and sinks - the whodunit of moisture management. [Adapted from J. Straube, Moisture in Buildings, ASHRAE Journal, January 2002.}

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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Managing moisture in building assemblies
Available mechanisms for managing moisture in building assemblies.
Bulk water control strategies
Bulk water control strategies: shedding, shielding, conveyance and drainage.
Drainage of rain
The drainage of rain is a fundamental moisture management strategy which deals with the most effective means of transporting the largest quantity of moisture away from and out of the building enclosure
Ventilation of drainage spaces
Ventilation of drainage spaces is a secondary method of moisture protection which dries out materials adjacent to the drainage space.

Having considered drainage and ventilation of the enclosure, it may also be prudent to evaluate the forgiveness of the system as it relates to imperfect workmanship and materials. The hygric buffer capacity of various envelope constructions has been estimated by Joe Lstiburek of Building Science Corporation in Boston. It is obvious from the data that steel frame construction is highly dependent on workmanship to avoid potential moisture problems. At the other end of the scale, masonry construction can easily absolve the builder's sins and compensate for substandard material substitutions. It also buys valuable time for facility managers who may fall behind on routine maintenance of the enclosure.

The safe storage of moisture represents a reliable means of increasing the factor of safety against moisture damage. This approach essentially extends the limit state for moisture penetration and accumulation and should be viewed in a similar manner to increasing load resistance factors in structural engineering.

Hygric Buffers
Hygric Buffer for 2000 Sq.Ft.(186 m2) Home [Source: J. Lstiburek, ASHRAE Journal, February 2002]

The next stage of design occurs at the level of the façade where the moisture management concepts must be translated and incorporated into the profile of the building. Recent enclosure performance disasters have repeatedly been premised on designers naively choosing form over function. In many areas of Canadian architectural practice this has led to the sad situation where enclosure design has been forfeited to engineering consultants and manufacturers' sales representatives. The future of this trend will be decided by the emphasis in architectural education on substance versus style, mastery versus abdication.

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Traditional building
Fashion or function? The eyebrows and moustaches of this traditional building have provided over a century of service. Facades with minimal relief must look to modern means of achieving comparable rain shedding and sun shading performance, all the while aging gracefully.


Enclosure Control Functions and Strategies
Moisture management is certainly a predominant consideration in enclosure design, but the other functions of the building envelope cannot be overlooked. Fundamental control strategies corresponding to physical phenomena influencing enclosures are summarized according to the primary control functions for moisture migration, heat transfer, air leakage and solar radiation. In a cold climate, it should be noted that relationships are somewhat hierarchical in that strategies for moisture management include strategies for control of heat flow and air leakage. Control of solar radiation is partially related to the previous three phenomena, but deals with a broader architectural perspective associated with insolation and fenestration.

The control of moisture migration is a fundamental requirement for well performing, durable building envelopes. Contemporary control measures utilize technologies which range in origin from previous millennia to the present, and provide designers with a broad choice of options. Irrespective of the options selected, the fundamental physical phenomena which drive moisture migration must be fully addressed by any control measure.

In some cases, a single material may control multiple mechanisms, such as in the case of polyethylene, which is normally used as both an air and a vapour barrier in residential construction. In other cases, several materials in an assembly may share the control functions. It should be recognized that the quality and consistency of materials and workmanship associated with single materials which resist multiple mechanisms, is far more critical than is the case for assemblies which are comprised of materials which share control functions and hence provide redundancy (higher factor of safety).

From as practical design perspective, well performing enclosures tend to:

shield the building envelope from exposure to rain and wind;

effectively shed and convey water away from the building; and

consistently drain away and dry out moisture which will invariably penetrate the building envelope.

Does this mean that only these types of enclosures should be considered in design? Returning to the previous discussion, it is important to reconcile site and occupancy influences, the intensity, duration and frequency of phenomena and the risks and consequences of failure. By examining building enclosures in a given locality, it is possible to determine what has worked and what has prematurely deteriorated and/or failed. High quality workmanship and materials may overcome inherent limitations from what may otherwise be considered inappropriate enclosure strategies. The responsibility for enclosure design rests with the architect and involves considerable experience and judgement. The next section on Enclosure Strategies takes a look at basic approaches to enclosure design primarily from a cold climate perspective, and aims to provide a comparative overview of enclosure alternatives.

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Tepees
 

 


Control strategies for critical enclosure
Control strategies for critical enclosure functions are derived from an understanding of the physical phenomena involved.