water 101: an introduction to the properties of water

1

Water 101An Introduction to the Properties of Water

Colin R. Murphy, RRC, LEED AP

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.

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.

4

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


5

Water molecules act much like magnets because opposites attract and likes repel.

3. Water molecules are ‘sticky’.


6

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.

7

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.



8



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.

9

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.

4. Molecules are little bundles of energy.


10

always moves toward

11

always moves toward

12

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.

5. Vapor, liquid and ice simply represent three distinct stages of energy transfer.


13

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.



14

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.



15

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.



16

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.



17

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

6. Water is the ‘universal solvent’.


18

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.

6. Water is the ‘universal solvent’.


19

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.

7. Water and oil (or wax or silicone) do not mix.


20

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.

7. Water and oil (or wax or silicone) do not mix.


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.

22

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

a. What is ‘surface tension’ and capillary action?

Key questions encountered in the field:

23

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.



24

Capillary Action

Surface Tension

Capillary Action

The molecules can climb only up to the point at which the opposing forces are in equilibrium.



25

Failure by a designer or contractor to consider these fundamental properties of water can result in ‘wicking’ water and resulting damage.



26

Capillary action can draw great quantities of rainwater into the small space between overlapping panels.

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.

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.

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.

30

Compared to plywood, OSB sheathing suffers a decreased ability to dry out – thus increasing the

likelihood of severe decay if it gets wet.

31

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.


b. What is evaporation and condensation?

32

Evaporation and condensation / adsorption occur continually around us as moisture strives to achieve the ‘energy balance’ required by Second Law of Thermodynamics.

Energy Balance

CondensationEvaporation



33

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.



34

Morning dew at Coe Elementary (Seattle) forms at the insulated spaces between the steel frames.

35

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.



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?

37

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.



38

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

c. What is relative humidity?


39

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.

Energy Balance

80% of Available Ambient Moisture Energy Is

Required To Maintain EMC Balance Between Room

and Surrounds



40

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.



41

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.

Energy Balance

At 100% RH, Liquid Moisture Forms To Relieve

The Unbalanced System



42

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.



43

Sorption isotherm for plywood published by Oak Ridge National Laboratory.

44

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

45

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.



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?

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)

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.

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)

53

Moisture sensitive? Paper product?

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.

55

...or can result in accelerated damage if water does breach the wall’s defenses.

New wall and cladding systems can provide even tighter barriers to moisture entry…

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.

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.

58

END OF PRESENTATION

THANK YOU!

water 101: an introduction to the properties of water

Education