Gravity Convection
Îoisture transfer mechanisms in buildings(2nd part)
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Read also: â€œMOISTURE MOVEMENT MECHANISMS (1st part)â€
Â Until now we have examined the transport mechanisms of liquid moisture e.g.
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* massive intrusion through discontinuities
Â (phenomenon governed mainly by wind action or gravity)
Â * capillary absorption which is evolving due to intermolecular forces and is the dominant mechanism in basements in the absence of gaps. Capillary absorption is responsible for rising damp phenomena.
Â Letâ€™s see nowâ€¦
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VAPOR TRANSPORT MECHANISMS
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Under this headline we can find:
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a) Air transport or convection mechanisms.
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b) The vapor diffusion mechanism which is of much less importance than the former.
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AIR TRANSPORT MECHANISMS
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There are three different cases here:
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1) Mechanical ventilation systems: Though this subject is out of the scope of this concise guide, I have to stress that this is the only totally controllable mechanism for vapor transfer.
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2) The stack or chimney effect which has to do with differences in temperatures in and out.
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3) The wind effect which has to do with the influence of wind action on buildings.
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THE STACK OR CHIMNEY EFFECT
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This is a similar phenomenon with the natural draught in chimneys or stacks.
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Here we have movement or air â€“ containing moisture donâ€™t forget â€“ through gaps due to internal and external temperature differentials which of course are translated into pressure differentials.
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The phenomenon is more intense when:
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 the buildings are higher
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 the temperature differentials are bigger.
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DURING WINTERâ€¦
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Air indoors is warmer and lighter and consequently rises up (buoyancy). At the top it exfiltrates through various gaps: cracks, holes, open windows etc.
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The laws of physics say that in the base of the building lower pressures will suck external air through gaps to compensate for the loss of air at the top. So during winter there are infiltrations from the lower part of the building.
Â DURING SUMMERâ€¦
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The direction of the air is reversed but the phenomenon is not as intense because temperature differences are quite smaller.
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Stack effect is not controllable and can cause serious interstitial condensation problems.
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Â WIND EFFECT
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Wind is a complicated phenomenon and owes its existence to air pressure differentials. These differentials are due to thermal effects taking place on both microclimate and macroclimate levels.
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Wind is air moving parallel to the earth surface.
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As wind falls on a building, it puts its windward side under a positive pressure and its leeward side under a negative pressure.
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Â This means that air infiltration â€“ carrying moisture with it â€“ takes place for equalization reasons through the gaps and discontinuities on the windward side.
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Conversely air exfiltration takes place through the gaps on the leeward side of the building.
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This means that air is sucked out of the building on the leeward side.
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Moisture loads infiltrating or exfiltrating through the gaps of the building depend mainly on:
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 the wind velocity
Â  the angle on wind incidence
Â  the existence of nearby buildings.
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Wind effect can be combined with stack effect. Both these phenomena are responsible for many grave problems due to interstitial condensation.
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DIFFUSION MECHANISM
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Diffusion phenomena are meant in relation to the buildingâ€™s envelope and have to do with water vapor moving from areas of higher vapor concentrations and therefore pressures  towards areas of lower pressures.
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This is not a powerful phenomenon as wind or stack effect but it needs to be taken into account because itâ€™s steady and persistent.
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Partial pressures of other gases e.g. oxygen or nitrogen are irrelevant for the phenomenon evolution.
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During winter
water vapors move from the pressurized interior towards the exterior environment.
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During summerâ€¦
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there is reverse movement from the exterior environment towards the air conditioned interior of buildings.
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This automatically means that control of diffusion with vapor barriers in mixed climates is difficult.
Â
Â
Chris Strogilis
Â
Who Added the Micro To Gravity  Part 1

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