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.
off the coast of Florida, ca. 1920
|House on the MeKong Delta, Viet Nam.
First, lets begin with some important material properties related
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
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
|No matter how long you shake
the last drop always drips down your pants.
Popular graffiti above urinals in boys'
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
back to top
|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
1. Moisture must be present, in the liquid,
solid or vapourform.
2. Some path for transport of the moisture must
3. A driving force to transport the moisture must
4. The building enclosure must contain materials
that are susceptible to moisture-related
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
|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
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.
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
back to top
|Available mechanisms for managing moisture
in building assemblies.
|Bulk water control strategies: shedding,
shielding, conveyance and drainage.
|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 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 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.
back to top
|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.
back to top
|Control strategies for critical
enclosure functions are derived from an understanding of the
physical phenomena involved.