It reasonably can be argued that that most participants in the roofing, building and design trades tend to either take water for granted or lack the basic understanding of both the chemistry and physics that play such a large role in water accumulation within a building enclosure. Sure, people in the construction business fear the effects of excess water, but few have taken the time to understand why it acts the way it does.
The driving forces that cause moisture movement are fundamental properties of nature; therefore, preventing water infiltration and resulting damage at the typical building project requires close attention during the design and construction processes to all potential moisture sources and routes.
Seven fundamental aspects of water are presented in Water 101 to provide Trinity | ERD forensic personnel and our clients an essential knowledge base for the physical properties of water.
2. Participants in the roofing, building
and design trades:
• Tend to take water for granted
• Lack the basic understanding of
both the chemistry and physics that
play such a large role in water
accumulation within a building
enclosure.
2
3. Seven fundamental aspects of water
1. Water molecules are very small.
The water molecule (H2O) consists of
two hydrogen atoms and one oxygen
atom.
A cubic inch of water contains
600,000,000,000,000,000,000,000
water molecules.
A snowflake may be comprised of
180,000,000,000 water molecules.
3
4. Seven fundamental aspects of water
2. Water molecules are ‘polar’.
The water molecule is both asymmetrical and polar.
The electronic charge is unevenly distributed.
The large oxygen atom has a negative electrical field; the
two smaller hydrogen atoms have a positive field.
One large oxygen atom Two small hydrogen atoms
4
5. Seven fundamental aspects of water
3. Water molecules are ‘sticky’.
Water molecules act much like magnets because
opposites attract and likes repel.
5
6. Seven fundamental aspects of water
3. Water molecules are ‘sticky’.
Hydrogen Bonding - Molecules attract and stick to each other
unless stronger forces rip apart these adhesive bonds.
The asymmetrical shape of the water molecule provides a
variety of bonding formations, allowing diverse configurations
that include countless one-of-a-kind crystalline snowflakes.
6
7. Seven fundamental aspects of water
3. Water molecules are ‘sticky’.
What happens when you exhale
close to a glass surface?
The glass ‘fogs up’ – many of your
exhaled water molecules have
become stuck to the available
oxygen atoms at the glass surface.
7
8. Seven fundamental aspects of water
3. Water molecules are ‘sticky’.
The ability of these water vapor molecules to stick to many
material surfaces, such as the gypsum wallboard in your
living room, is called adsorption.
What does a nonsmoker notice
after departing a gathering of
cigarette smokers? The
nonsmoker smells of cigarettes
due to smoke molecules having
become adsorbed to the clothing.
8
9. Seven fundamental aspects of water
4. Molecules are little bundles of energy.
Energy is the capacity to do work, which consists of the
transfer or transformation of energy.
Increased thermal energy (heat) may be transformed
into increased kinetic energy (speed or momentum) of
water molecules or automobiles.
9
12. Seven fundamental aspects of water
5. Vapor, liquid and ice simply represent
three distinct stages of energy transfer.
Water molecules are moving at varying
speeds that increase or decrease as
the ambient temperature rises or falls.
Like ricocheting billiard balls, at such
high velocities most of the water
molecules simply have too much
momentum to ‘stick’ to other molecules
and thus remain dispersed as vapor. 12
13. Seven fundamental aspects of water
5. Vapor, liquid and ice simply represent
three distinct stages of energy transfer.
At slower molecular speeds some of the interacting
water molecules form groups.
These grouped molecules tend to have sufficient
momentum to avoid a fixed bond with other water
molecules within the group, but most cannot escape
from the pack.
13
14. Seven fundamental aspects of water
5. Vapor, liquid and ice simply represent
three distinct stages of energy transfer.
As water cools, the bonds of the slowing molecules are
maintained for longer before breaking and reforming
occurs.
The density of the cooling liquid increases as the
compressive forces of the stronger hydrogen bonds
condense the molecules tighter.
Cold water is denser than warm water. Similar to warm air
rising, warm water will float above cold water.
14
15. Seven fundamental aspects of water
5. Vapor, liquid and ice simply represent
three distinct stages of energy transfer.
Finally, at about 4°C (39°F), the liquid
reaches its maximum possible density.
The water molecules begin to reorganize into
the crystalline structures that comprise ice.
As liquid water freezes, its volume expands.
Many of us have experienced the destructive
freeze-thaw effects of expansion and
contraction between liquid and solid water.
15
16. Seven fundamental aspects of water
5. Vapor, liquid and ice simply represent
three distinct stages of energy transfer.
You can easily test this
expansion by putting a full
glass of water into a freezer –
the resulting ice will expand
above the top of the glass and
can break a closed container.
16
17. Seven fundamental aspects of water
6. Water is the ‘universal solvent’.
What happens when you put a cube
of sugar into liquid water?
The sugar dissolves because the positively
charged hydrogen ends of the water molecules
have sufficient strength to pull the slightly
polarized sugar molecules apart from the sugar
cube.
The result is a solution in which water is the
solvent and the sugar molecules are the solute.
17
17
18. Seven fundamental aspects of water
6. Water is the ‘universal solvent’.
The polar water molecule is stronger than the cohesive
forces holding the polar sugar molecules together.
Water often is called the universal solvent. It can pull apart
many substances, whether comprised of polar molecules
or charged ions (salt, for example).
Water can oxidize, melt, dissolve or otherwise destroy
more substances than sulfuric acid.
18
19. Seven fundamental aspects of water
7. Water and oil (or wax or silicone) do not mix.
What happens when you put a chunk of
wax into water?
The simple answer is ‘not much’.
The wax is comprised of nonpolar molecules that
remain unaffected by the pull of the hydrogen.
To dissolve the nonpolar wax you will need to use a
nonpolar solvent, such as gasoline.
19
20. Seven fundamental aspects of water
7. Water and oil (or wax or silicone) do not mix.
Molecules known as surfactants (surface
active agents) are polar on one end (and
thus attractive to water) and nonpolar on
the other end (and thus attractive to oil).
What happens when you add warm water
and soap to a sink filled with greasy pans?
The soap molecules attach themselves to
both the grease molecules and the water
molecules, allowing you to rinse the entire
mess down the drain.
20
21. Armed with our understanding of these seven
fundamental aspects of water, we can better
design and build structures, extending service
life and reducing maintenance, and better
evaluate the performance of existing buildings.
21
22. Key questions encountered in the field:
a. What is ‘surface tension’ and capillary action?
At the surface, a relatively taut ‘skin’ occurs as the
attractive forces of hydrogen bonding produce an inward
orientation of the outer molecules. It is this surface
tension that allows water bugs to walk across ponds…
WATERBUG
22
23. Key questions encountered in the field:
a. What is ‘surface tension’ and capillary action?
This also keeps water from climbing out of its container.
Liquid water will attempt to climb the walls as the water
molecules seek to form hydrogen bonds with available
oxygen molecules in the glass.
The only thing that prevents the water molecules from
climbing to the top of the glass in their quest for more
bonding is the mass of water molecules already bonded
to them.
23
24. Key questions encountered in the field:
a. What is ‘surface tension’ and capillary action?
Capillary Action Capillary Action
Surface Tension
The molecules can climb only up to the point at which the
opposing forces are in equilibrium. 24
25. Key questions encountered in the field:
a. What is ‘surface tension’ and capillary action?
Failure by a designer or contractor to consider these
fundamental properties of water can result in
‘wicking’ water and resulting damage.
25
26. Capillary action can draw great quantities of rainwater
into the small space between overlapping panels.
26
27. Now imagine asphalt shingles also installed with
a 2” unsealed overlap at a 3:12 slope. Is the
threat of capillary action as great?
No, because asphalt roofing (unlike metal) is
comprised of nonpolar molecules.
Water molecules simply cannot carry out the
hydrogen bonding process with bitumen
molecules.
Asphaltic materials are a natural choice for
weather protection at roofs.
27
28. While these two extreme examples greatly oversimplify
the issues a general conclusion can be drawn –
– the designer or contractor
who does not understand that
‘sticky’ water is attracted to
some building materials, but
not others, likely will someday
be sued by a dissatisfied
building owner.
28
29. Similarly, consider the difference between
OSB (oriented strandboard) and plywood
structural sheathing.
Wax is a key component of the OSB
manufacturing process
The addition of this nonpolar substance affects
the ‘moisture exchange’ of the wood elements
of the panel.
29
30. Compared to plywood, OSB sheathing suffers a
decreased ability to dry out – thus increasing the
likelihood of severe decay if it gets wet. 30
31. Key questions encountered in the field:
b. What is evaporation and condensation?
Evaporation is the transformation into vapor of a
hydrogen bonded water molecule that has gained
sufficient kinetic energy to break its bond.
Condensation and adsorption are the opposite.
Reduced levels of energy result in an increased amount
of hydrogen bonding of the water molecules – the de-
energized vapor molecules ‘stick’ to other materials or
condense into liquid form.
31
32. Key questions encountered in the field:
b. What is evaporation and condensation?
Evaporation and condensation / adsorption occur continually
around us as moisture strives to achieve the ‘energy balance’
required by Second Law of Thermodynamics.
Evaporation Condensation
Energy Balance
32
33. Key questions encountered in the field:
b. What is evaporation and condensation?
Increased thermal energy causes (by evaporation) an
increased number of vapor molecules in the air; the cooling
temperatures at night eventually reach a level at which the de-
energized vapor molecules must begin condensing onto the
nearest cool surfaces, producing liquid dew.
33
34. Morning dew at Coe Elementary (Seattle) forms at the
insulated spaces between the steel frames.
34
35. Key questions encountered in the field:
b. What is evaporation and condensation?
The same process of water vapor energization and
de-energization is occurring continually within this
room and within all of the porous building materials at
our construction projects…
…however, it often is not until the effects of this
process have resulted in mold growth or structural
deterioration that we pay it the attention it deserves.
35
36. What happens when wood and gypsum materials are
encased with stucco at both sides of an
unheated, unvented wing wall? Was it reasonable for
the Architect to be included in the ensuing lawsuit?
36
37. Key questions encountered in the field:
b. What is evaporation and condensation?
Heat is required to promote the drying of the wet
building materials. There is no alternative force with
sufficient power to sunder the strong bonding of the
water molecules.
The building professional who believes gravity will
remove excess moisture held within typical building
elements is courting disaster.
37
38. Key questions encountered in the field:
c. What is relative humidity?
The term relative humidity simply expresses the percentage
of the available energy that has been used to ‘free’
sufficient water molecules from the surrounding materials to
achieve the current energy balance (equilibrium).
Energy Balance
38
39. Key questions encountered in the field:
c. What is relative humidity?
A relative humidity (‘RH’) of 80% means all but 20% of the
available energy must be used to achieve the current energy
balance (equilibrium moisture content) between the moisture
bound within this room’s surrounds and the ambient vapor
molecules bouncing back and forth between its walls.
80% of Available Ambient
Moisture Energy Is Required To
Maintain EMC Balance
Between Room and Surrounds
Energy Balance
39
40. Key questions encountered in the field:
c. What is relative humidity?
The RH value expresses the operational efficiency of
the energy equilibrium process.
At 50% RH, the system is running efficiently; an energy
balance is achieved and a surplus of energy remains to
handle increased demands that may occur.
At 85% RH, the moisture balancing system is being
pushed to its limits; there is little margin for
accommodating any excess moisture.
40
41. Key questions encountered in the field:
c. What is relative humidity?
At 100% RH, there no longer is sufficient available
energy to maintain an energy balance; therefore, similar
to the operation of a relief valve, some of the excess
moisture must be condensed from the system.
At 100% RH, Liquid
Moisture Forms To Relieve
The Unbalanced System
Energy Balance
41
42. Key questions encountered in the field:
c. What is relative humidity?
For each porous building material, the relationship
between relative humidity and moisture content can be
expressed graphically with a sorption isotherm.
Note that at about 80% RH (no matter what
temperature) the moisture content of the plywood
begins to increase exponentially.
42
44. Key questions encountered in the field:
c. What is relative humidity?
Extended periods of ambient 80% RH are common here in
Vancouver. The moisture content of plywood exposed to this
humidity level remains satisfactorily low; however, a small amount
of additional moisture will soon raise the moisture levels to a
destructive level.
Energy Balance
44
45. Key questions encountered in the field:
c. What is relative humidity?
In short, the RH value tells us how much moisture
buffering capability we have in our exterior walls.
Here in the Pacific Northwest we often have a small
moisture buffer in our building materials and
therefore our design and construction practices must
be much more stringent than carried out in the non-
humid climate zones, such as Albuquerque, NM.
45
46. Diffusion is the movement of moisture due to vapor
pressure differentials.
Convection is the physical conveyance of moisture due
to air pressure differentials.
Remember that water, like all carriers of energy, always
moves from areas of high energy potential to areas of
low.
Which process – diffusion or convection – typically poses
the greatest risk for our buildings?
46
47. It is important to recognize that, “For a moisture-
related problem to occur, it is necessary for at least
four conditions to be satisfied:
1. A moisture source must be available.
2. There must be a route or means for the moisture to
travel.
3. There must be some driving force to cause
moisture movement.
4. The material(s) involved must be susceptible to
moisture damage.” (J.F. Straube)
47
48. Good building designs promote exfiltration of excess
moisture that may become trapped within the wall.
Consider an unvented wall assembly with wet framing
behind a polyolefin housewrap - designed to resist liquid
water penetration but to allow vapor passage.
Unless this trapped moisture is transformed into vapor,
the housewrap blocks the preferred escape route – unlike
asphalt building paper, which transfers moisture outward.
48
49. The necessary elements for successful
design, construction and long term
performance of building envelope systems
often are summarized by the 4 D’s:
Deflection
Drainage
Drying
Durability (or Decay Resistance)
49
54. Drying performance of
exterior walls has been
greatly affected by
design and material
innovations of the past
half-century.
The effect of new design advances, raised energy
standards and new construction systems often has
been a reduction in drying performance if water does
breach the barriers.
54
55. New wall and cladding
systems can provide
even tighter barriers to
moisture entry…
...or can result in
accelerated damage if
water does breach the
wall’s defenses.
55
56. Each new advance in cladding and wall design is
loudly proclaimed as the final solution…
… however, the voices of
those who analyze the
overall moisture exchange
effects of this ‘advance’ are
often overwhelmed by the
cheerleaders for the new
product or system.
56
57. At the fundamental level, building envelope
professionals must recognize that the tighter we
make our walls, the greater the potential for disaster if
‘leakage’ occurs.
The greater responsibility we all have to ensure good
project detailing and proper implementation of these
details.
Understanding the properties of water and the
‘moisture exchange’ mechanisms is an essential
component of this process.
57
