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Wood and Moisture: Notes

References
Moisture-Related Publications, CBD

Manipulating a structure's temperature — through heating and cooling — is typically undertaken without considering humidification or dehumidification

Moisture content (MC)

MC is measured as the ratio of the weight of water in a given piece of wood to the wieght of the wood when it is completely dry: the ratio is typically expressed as percent moisture content (or the ratio of water lost over remaining weight of wood.

Humidity (water or moisture in the atmosphere)

  1. Air
    1. Air is a mixture of several gases, including
      1. nitrogen,
      2. oxygen, and
      3. water vapor.
    2. The total air pressure exerted by a volume of air in a given container on that container is the sum of the individual (partial) pressures of these gases.
    3. The vapor pressure is the partial pressure of the water vapor.
    4. The warmer air is, the more moisture it can hold.
  2. Absolute humidity
    1. absolute humidity is the ratio of the mass of water vapor to the mass of dry air in a given sample of air.
    2. the actual quantity of mositure in the atmosphere
    3. expressed in grains per cubic foot or grams per cubic meter
    4. the amount of water the air can hold varies with the temperature: the higher the temperature, the more moisture is able to be held in the air. At 70 degrees F the air can hold a maximun of 8 grains of moisture per cubic foot.
  3. Relative humidity (RH)
    1. The ratio of the amount of moisture in the air at a certain temperature to the maximum amount it would be able to hold at that temperature.
    2. Air is said to be saturated (at 100% relative humidity) when it contains the maximum amount of moisture possible at a specific temperature.
    3. Air holding half the maximum amount of moisture at a given temperature has a relative humidity of 50%.
    4. Relative humidity near surfaces is the single most important factor influencing moisture problems in buildings.
    5. If the air at 70 degrees F holds 4 grains of water per cubic foot, the RH would be 50% (because the air can hold 8 grains at that temperature.
    6. Dew point
      1. Dew point is the temperature at which water vapor condenses from the air.
      2. The cooler the air, the more water vapor condenses out as precipitation
      3. At 70 degrees F and 50% RH the dew point is 49.3 degrees F.

Nature of Water in Wood

  1. Free water
    1. liquid water present in lumen
    2. easy to remove
  2. Bound water
    1. present in cell wall
    2. as MC decreases below FSP removal of water is more difficult
    3. held by adsorption
      1. a chemical/physical attraction
      2. involves H-bonds between water and cellulos

Drying

  1. Removes water from lumens
  2. Removes some from cell wall
  3. If any water is present in lumens, cell wall is completely saturated
  4. Water in lumens does not change wood properties

Fiber saturation point (FSP)

  1. Point at which all "free" water is removed from lumens (cell cavities).
  2. Cell walls are still completely saturated with bound water, physiocally attracted to the cell wall. This leaves the cells in a weak condition.
  3. Bound water can only be removed when there is less than 100% RH.
  4. Wood is in an environment where the RH is between zero and 100% so only part of the bound water is lost.
  5. Wood in use must be in contact with liquid water to be greater than FSP

Equilibrium moisture content of wood

  1. Wood is hygroscopic - it responds to the changes in atmospheric humidity: it looses bound water as the RH drops and regains bound water as the RH increases.
  2. Water is gained or lost due to environmental changes with moisture content (MC) constantly changing to stay in equilibrium with environment: Wood is constantly trying to attain an Equilibrium Moisture Content (EMC)
  3. The equilibrium moisture content (EMC) is the amount of bound water contained in a piece of wood when a balance of moisture exchange is established at a particular RH level.
    1. Relative humidity appx. Equilibrium moisture content (at 70 degrees F)
      100% RH (FSP) = 31% EMC
      (total fiber saturation, which varies with species)
      75% RH = 14% EMC
      50% RH = 9% EMC
      25% RH = 5% EMC
      0% RH = 0% EMC
  4. MC is always < FSP (i.e. only bound water is present)
  5. As MC changes the way BOUND water is held will change
    1. RH < 0.2 monomolecular layer
    2. RH 0.2-0.9 polymolecular adsorption
    3. RH 0.9-0.99 capillary condensation (liquid water is held in minute capillaries in wood)
  6. Relationship between relative humidity & moisture content is not linear
  7. This is due to the ways that bound water is held in the cell wall.
  8. Temperature will effect EMC, with EMC 1% lower per 25 to 30 degree F rise in temperature.
  9. Hysteresis, which is the effect which causes the EMC curve to be slightly higher when wood is losing moisture (desorbing) than when the wood is picking up moisture (adsorbing).

Shrinkage and Swelling

  1. Below FSP wood shinks and swells as moisture content changes
  2. Shrinkage is proportional to amount of water lost
  3. Shrinkage also varies with S2 fibril angle (especially longitudinal shrinkage)
  4. Shrinkage & swelling are reported as a percentage of the original dimensions
    1. % shrinkage =decrease in dimension (or volume)/original dimension (or volume) *100
    2. % swelling=increase in dimension (or volume) /original dimension (or volume) *100
  5. Longitudinal shrinkage small <0.2%
  6. Tangential > radial
  7. Within a species shrinkage depends on
    1. size & shape of sample
    2. density (higher density gives greater shrinkage)
    3. drying rate
    4. allowances must be made for sawing and fitting of pieces

Building environment: Winter

  1. Winter produces a "dry" indoor environment.
    1. RH within a building will be dependent on:
      1. MC of outside air
      2. ventilation rate
      3. rate at which the moisture is lost through the building enclosure
      4. rate at which moisture is supplied to the air within the building
    2. Given an example where no moisture is generated from within: the outdoor environment may be 20 degrees F at 100 RH which, when heated to 75 degrees F on the interior, will produce a 12 % RH, or a 3% EMC.
  2. Residences typically generate significant moisture and tend to restrict ventilation (infiltration) to conserve energy. Moisture content of air within most houses in winter will be higher than that of the outside air. Here:
    1. Ventilation rate should be at least 0.3 changes per hour
    2. A family of four will produce at least 0.7 pounds of water per day from: bathroom sinks and showers/bathtubs, kitchen cooking, washing, watering plants and clothes washing (bringing it to 2.0 pounds per day).
    3. This will maintain a 40% RH unless there is abundant ventilation (as with historic structures) where the RH will decrease, requiring a humidifier (if the structure can manage such moisture).
    4. Condensation occurs on windows and walls in contact with the colder outside environment. This occurs where the lowering temperature increases the relative humidity and causes the environment to reach dewpoint.
    5. Water vapor can diffuse through interior finishes of walls and ceilings or leaking areas. The water enters wall cavities (when walls are without vapor barriers having a good perm rating). Here they will likely be on the site at which dewpoint, and condensation, will be reached, producing water and/or ice.
    6. For human comfort, a higher temperature is needed to offset a decrease in relative humidity.
    7. Structures, which are not designed to control high interior RH, should not exceed 20% RH at temperatures below zero degrees.
      1. Museums and libraries require a high RH (of at times 50%) as one of the required functions of the building
  3. Causes of moisture-related deterioration (condensation/dewpoint):
    1. Condensation where dewpoint is reached on the interior surface of a material.
      1. condensation on the inside of window surfaces
        1. reduces visibility
        2. damage surrounding construction: glazing, mullions, sash, sill woodwork
        3. as windows are the poorest component of the building enclosure, they determines the practical (and highest) level of humidity
      2. wall condensation on paints and plaster
    2. Concealed condensation where dewpoint is reached within the construction
      1. condensation in cavity walls and attics (insulation and frame and sheathing/siding) depending on temperature gradient and heat exchange and chimney effect (see below).
        1. This will depend on the thermal resistance (R=1/Conductance) of individual building materials and building assemblies: plaster, wood, insulation, metal, etc.
        2. Moisture will reduce the thermal resistance of a material .
      2. condensation below exterior finishes causing blistering of paint
      3. condensation and rusting of metal components, fasteners
      4. contribution to effloresence
      5. condensation on electrical components
      6. parapet and cornice failures
  4. Preventing condensation
    1. continuous vapor barriers to combat vapor diffusion caused by vapor pressure difference and dependent on the permeability of material(s)
      1. vapor barriers must have a low vapor permeability (or perm) rating: the transfer rate of one grain per square foot per hour under a vapor pressure difference of one inch mercury. For residential structures a perm of 0.75 or less is satisfactory.
      2. membranes: polyethylene, metal foils (copper or aluminum)
      3. coatings: oil-based paints, aluminum paints, asphalt coated papers and coatings
    2. Caulking to combat air leakage caused by air pressure differences and resulting in infiltration (of cold air) and exfiltration (of warm moisture-laden air). When moisture content is high, leakage may be more important (destructive) than diffusion. These may be caused by:
      1. wind pressure differences
      2. chimney effect caused by differences between indoor and outdoor temperatures, producing air infiltration at the lower levels and exfiltration at upper levels
      3. pressurizing of structures, especially where it opposes infiltration due to wind pressures.
    3. Ventilation
      1. interior living spaces which generate moisture: kitchens, bathrooms, laundry rooms
        1. conduct air from laundry dryer directly to exterior
        2. provide exhaust fans in bathroom, kitchen cooking area
        3. limit use of humidifier
      2. attics and cornices
        1. Vent gable roofs: one square foot per 150 square feet of floor space (one square inch per one square foot). Chimney effect producted by higher temperatures in attic induce ventilation
        2. Soffit vents or strips along cornices. Do not restrict circulation by insulating plate-rafter intersection. Use inconjunction with ridge ventilation.
        3. Flat roofs may be best insulated on top with rigid insulation applied over a vapor barrier on the roof deck.
      3. exterior cavity walls
      4. siding and soffit with drilled soffit vents
      5. brick with weep hole

Summer produces a "wet" indoor environment.

  1. Outdoor temperature 90 deg. F with 85% RH (with 12 grains), once inside with basements at 70 deg. F. releases 4 grains of moisture
  2. Architectural wood
    1. structural lumber is "kiln-dried" to a 19% MC
    2. cabinet wood is "kiln-dried" to 7.5 to 8% MC, or below 10% MC
    3. Equilibrium relative humidity (ERH), instead of EMC would emphasize that lumber should be at equilibrium with a 40% RH rather than lumber should be at 7.5% M