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31 Cards in this Set
- Front
- Back
- 3rd side (hint)
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characteristics of water |
• a small molecule • can move through many materials that have pores or crystal spaces larger than the water molecule • a polar molecule: it is capable of dissolving substances, wetting surfaces, and incorporation into other chemical structures
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Phases of Water |
Solid (ice/snow) - crystalline form Liquid (water) - clumps of clingy molecules Gas (water vapour) - dispersed water molecules floating individually |
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Moisture balance |
a process of balancing moisture entry mechanisms (wetting) and moisture removal mechanisms (drying) |
definition |
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moisture imbalance results in...? |
deterioration of materials |
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wetting mechanisms |
exterior moisture • rain, driving rain • ground water and snow • moisture from humid air or wet materials via air movement or vapour diffusion interior moisture • occupant activities, equipment, laundry, plants, etc construction moisture • wet lumber • rain during construction
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3 mechanisms |
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condensation on windows |
1. warm interior air carrying water vapour touches the cold surface of a window 2. as the air cools, it cannot hold as much water vapour and reaches 100% RH. the dew point is reached and the water vapour condenses into the interior surface of the window 3. liquid water drips down the window and can cause damage to finishes and create an environment for fungal mould to grow |
3 steps |
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solutions to condensation on windows |
• keep interior RH levels low (35-55% depending on the season) • use double-glazed windows. the interior temperature of the inner glazed panel will be higher than a single glazed panel. the dew point may never be reached |
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air movement |
water vapour can be carried in convective warm air currents that rise above cooler air, or the vapour can move with the air under an air pressure differential |
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vapour diffusion |
(without air movement) - vapour will move from an area with a high concentration of vapour to an area of low concentration of vapour |
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vapour permeability |
vapour can spread through solid materials if there are spaces large enough (micro pores) to allow the vapour molecules to pass through |
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air movement vs vapour diffusion |
• bulk air moving under a pressure differential will carry more water vapour into a building envelope than through vapour diffusion • bulk air moving under a pressure differential also moved vapour much more quickly than through diffusion • a large amount of air will pass through a small hole in the building envelope assembly |
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material vapour permeability |
some materials are more permeable than others • wood • concrete • paper • water based paints some materials are less permeable than others • some plastics • asphalt and coated felts • exterior grade drywood some materials are impermeable • steel and aluminum • glass |
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vapour movement through materials |
• water vapour can move through solid but porous materials • when it reaches a colder material closer to the exterior, the temperature will each the dew point and the vapour will change to liquid and condense • this can cause deterioration of the materials in the wall (wood decay and rust/corrosion of metals) • regions with cold winters have a lower exterior air temperature • the dew point can be reached much more quickly • water will condense in the wall cavity or roof space more easily |
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condensation in milder climates |
• milder climates have a warmer exterior air temperature • the dew point may not be reached even if water vapour migrates across the wall assembly • condensation may not occur as frequently as in colder climates |
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drying mechanisms |
surface evaporation • dependent on energy and moisture content of the surrounding air (humidity) movement through solid materials via capillarity and diffusion • amount depends on the porosity/permeability of the materials drainage • water drains down and out ventilation • moist air is replaced with drier air |
4 mechanisms |
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components of air |
oxygen, nitrogen, carbon dioxide, gaseous water vapour |
4 parts |
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humidity |
the absolute amount of water vapour in the air |
definition |
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relative humidity |
the amount of water vapour relative to the maximum amount of water vapour the air can hold at that temperature • air can hold more water molecules at warmer temperatures • if the temperature decreases and it has the same volume of air with the same amount of vapour as before, it will produce a higher RH |
definition |
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Condensation |
Water condenses and forms droplets on a surface that is colder than the water vapour in the air (because the air next to the cold surface cools down) • RH increases since the air cannot hold as much moisture |
definition |
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dew point |
the temperature at which water vapour turns to liquid water when 100% RH is reached |
definition |
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exterior relative humidity levels change with: |
geographic region, precipitation level, temperature range, and seasonal changes |
4 |
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interior relative humidity levels in residences |
• activities • exterior water penetration (leaks, groundwater seepage, evaporation from foundation slabs) • air tight houses (interior humidity levels will rise unless properly ventilated) • wood framing moisture (wood that exceeded the 19% MC limit) |
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changes in interior relative humidity |
RH% of the interior environment is also affected by temperature. • cold outdoor air that is brought into the house and warms up will have a lower RH than previously • warm indoor air with a high RH can condense onto colder surfaces like windows which can lead to material damage and deterioration • the best indoor RH is 35-55% @ 21°C |
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reverse condensation |
• condensation can occur as the result of a higher exterior temperature and high exterior RH level • inside temperatures may be cooled and dried in the summer with air conditioning • condensation can occur in the wall cavity as the warm moist exterior air migrates through the wall until it reaches dew point and condenses |
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how to resist vapour diffusion in assemblies |
add a material to the wall assembly with a low vapour performance (vapour barrier) |
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how to resist vapour diffusion in assemblies |
add a material to the wall assembly with a low vapour performance (vapour barrier) |
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vapour barrier/retarder |
• must be applied to the entire interior wall/ceiling and sub-floor surface • vapour barrier must be located on the warm side of the insulation • vapour barrier must be continuous |
3 rules of installation |
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how to resist vapour diffusion in assemblies |
add a material to the wall assembly with a low vapour performance (vapour barrier) |
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vapour barrier/retarder |
• must be applied to the entire interior wall/ceiling and sub-floor surface • vapour barrier must be located on the warm side of the insulation • vapour barrier must be continuous |
3 rules of installation |
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types of vapour barriers |
1. polyethylene film (5 ng/Pa*s*m2) - most common - sheets are applied between the gypsum board and stud wall in front of the insulation - this is a very low vapour permeance material - 6 mil poly fun will meet BCBC requirements 2. vapour retarding paint (26 ng/Pa*s*m2) - paint which has a low vapour permeance - applied to interior surfaces over gypsum wall board - must meet BCBC requirements 3. vapour barrier on exterior of structure - other materials can act as a vapour barrier/retarder (sheathing membranes such as house wraps and peel + stick membranes) - vapour barrier must still be located on the warm side of the insulation - this can be a very successful strategy as it allows drying back to the warmer inside of the building |
3 types |
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if a wall has more than 1 vapour barrier.... |
• trapped moisture has nowhere to go • this can lead to material deterioration |
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