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32 Cards in this Set
- Front
- Back
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Average annual rainfall in the lower mainland |
• North Vancouver: 60 in (1500 mm) • Vancouver: 48 in (1200 mm) • chilliwack: 44 in (1100 mm) • Surrey: 41 in (1040 mm) • Richmond: 30 in (750 mm) • tsawwassen: 22 in (560 mm) |
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Wind driven rain in Canada |
Typically from the east |
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Changes to the BCBC |
(as a result of many building failures) • inclusion of climate specific requirements for cladding and moisture control • dependent on the amount of rainfall |
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moisture index |
• reflects the wetting and drying characteristics of a particular climate region (calculated from the wetting index and the drying index) • dependent on the annual rainfall, temperature, RH • measure of wall moisture load and helps to select an appropriate cladding type (rain screen/face seal) |
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moisture index by geographic region |
• rain screen walls now required in areas west of the coast mountains (in areas where the MI is between 0.9 and 1) • face seal may be used in other areas with lower moisture index ratings, but rain screen is still recommended |
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water movement is subject to: |
• gravitational forces • molecular forces and effects • forces that push and pull |
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exterior water can get into buildings if all three of these factors are present: |
• water is present in the outside surface of the wall • opening in the wall (crack, open joint, porous material) • a force (gravitational, capillary, or wind pressure) to push water through the opening |
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5 forces of water on buildings |
1. Gravity 2. capillary movement 3. surface tension 4. wind driven rain 5. air pressure/suction effect |
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Gravity effect on water |
water flows as falls in response to the pull of gravity |
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shedding water |
• overlap all exterior cladding material and sheathing papers to shed water • use umbrella effect |
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slope horizontal surfaces |
• never allow water to pool on horizontal surfaces • slope all horizontal surfaces to drain • minimum of 2% |
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differential shrinkage |
• reverse slope on decks can be caused by differential shrinkage if total wood depths are different or if other materials are used (water flows towards building on deck) |
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capillary movements |
• water can be pulled along through small openings (less than 1/8") by a molecular suction force called capillary action
examples: • porous materials like concrete and brick are subject to water absorption through capillary action • concrete foundation wall can absorb water through capillary action where it can be transmitted to the wood sill plate or evaporated into the interior air volume • solution: add a waterproof sill gasket between the concrete and wood sill plate |
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capillarity in foundations - reducing capillarity movement effect |
• fill openings in porous materials (damp proofing or water proofing on exterior surface of concrete foundation) • increase joint size to create a capillary break - tight fitting joints between materials can be subject to water leakage through capillary action - ensure joints are wide enough (3/8") to prevent this and wide enough to allow proper sealant application of required - tight fitting materials can induce capillary movement leading to water being transmitted further into the building envelope |
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surface tension |
molecules of water can cling together in a thin surface film which can spread along materials and resist the pull of gravity • to break surface tension angle joints upwards to change the direction of the surface |
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drip edge |
• designed to break surface tension and allow a droplet to form • as the droplet gets heavier, it will fall with the force of gravity |
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flashing and drip edge |
• create a location for a water droplet to form and fall away with gravity |
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wind driven rain |
• rain can fall with different intensities • when carried along with the force of wind (air pressure differential) the rain can be driven at the building • lower mainland wind carrying rain is typically from the east because of the proximity of the ocean and mountains and high and low daily pressure changes • wind adds a driving force to the rain and increases the momentum effect when the rain hits a surface |
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wetting effect |
• wind driven rain can be driven against a building at an angle • rain can be driven upwards in updrafts • the pattern of wetting indicates the different levels of exposure of the wall face from wind turbulence (more wetting at corners and roof) • rainwater can hit surfaces and bounce, creating a backsplash effect and wetting other surfaces (at bottom of wall) |
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responses to wind driven rain |
• design the performance level of the wall system to respond to the appropriate level of exposure (east facing walls of buildings are vulnerable to wind driven rain vs walls which are protected by large overhangs on sheltered faces) |
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air pressure/suction effects on water |
• water can be forced through openings by air pressure differentials • if there is a higher air pressure outside due to wind and a lower pressure inside, there can be a suction effect that will pull water inside |
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reducing pressure/suction effects |
• add an air barrier to separate pressure zones • add the air barrier on the interior |
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the 4 Ds to control water penetration |
1. deflection 2. drainage 3. drying 4. durability |
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deflection |
• first and most important line of defence to prevent water penetration in buildings • strategy: keep water off surface (deflect water away and let gravity do the rest) • use positive slopes on cladding materials (large overhangs, extended window sills, flashings and drip edges) |
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flashings |
direct water out and away from the building envelope with drip edges • change the direction of water flow and direct water out and away from vulnerable locations • thin, flexible, durable, waterproof material (metal, pvc, plastic membrane) ex. top of parapet walls, over Windows, doors, at changes in materials |
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roof overhangs |
• the top of the wall is the most vulnerable to rain wetting on the surface • overhangs protect this part of the wall with an umbrella effect
overhang ratio: overhang protection / wall height |
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drainage |
• 2nd line of defence • strategy: if water gets into the building envelope, give it a route out • sealing the exterior face of a building (face seal) will not prevent the passage of water into the building envelope • installation may not have been perfect, buildings move/settle over time, materials shrink and expand, cracks and gaps are created • this results in water leakage • without a drainage pathway, water will remain in the system, raising moisture content and encouraging the growth of fungus and decay |
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face seal wall |
• no path for water drainage if water enters wall system • all openings are sealed with sealants • relies on perfect construction |
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rain screen wall |
• water that gets past the cladding can drain out through the gap between cladding and the building paper/sheathing |
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design the exterior drainage system |
• all components of the building envelope can have drainage features (walls, roofs, decks, guardrails, Windows, skylights) |
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drying |
• 3rd line of defence • strategy: if building envelope materials get wet, allow for drying (most materials will continue to function well if allowed to dry) • drying will take place effectively if: - water has drained away (isn't being continually replaced) - materials are vapour permeable - there is heat present for evaporation to occur - evaporated water vapour can move to a lower vapour pressure area • drying will not take place through a vapour barrier - vapour barriers reduce vapour diffusion but do not allow drying back to the interior • drying can be encouraged with ventilation in roofs, overhangs, decks, walls (pressure equalized vented walls) |
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durability |
• final line of defence • strategy: use materials that have proven water resistance for use in vulnerable locations (wood should be pressure treated or rot resistant species in exterior locations) • ex. 1"x2" strapping for rain screen walls should be pressure treated • ex. pressure treated wood sill plate
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