The pressure of convection on construction
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Air currents can arise through either natural or forced convection.
Natural convection
Natural or ‘free’ convection results from pressure differences between one part of the building and another. Natural air movement can be either wind driven or buoyancy driven.
Wind driven air movement is caused by a difference in pressure between the inside and outside of the building caused by wind and is often referred to as ‘open loop’ convection.
Buoyancy driven air movement is caused by the tendency for warm air to expand, to become less dense and more buoyant, rising through the general air mass via the ‘stack effect.’ This is often termed ‘closed loop convection.’ Conversely cool air contracts, becoming denser, less buoyant and falling through the air mass. This creates circulating currents of rising and falling air.
Forced Convection
Forced convection occurs either where cold air moves along the surface of warmer material and at the surface the material temperature drops or where cold air penetrates the insulation, often due to poor function of the wind protection and airtightness. It can be driven by fans.
What is known as ‘wind washing’ can affect the thermal performance of low density insulation, short circuit the performance of insulating sheathing and cool down an air barrier system located towards the outside of a wall assembly, possibly below its dew point.
Thermal bypass
Air current or convection is a means of heat transportation or ‘thermal bypass’ and as warm air rises via the ‘stack effect’ it presses against a building envelope and uncontrollably escapes from it, unless both an air and wind tight building envelope is in place.
Concerns relating to ‘thermal bypass’ can have a significant influence upon the design of a residential and commercial developments.
Where can Convection processes occur?
In temperate climates, in highly insulated airtight buildings, there will be no internal draughts or convection currents. In less insulated and airtight buildings, there will inevitably be some convection as it is the result of poor jointing and inadequate sealing. If a building envelope is not airtight and lacking joint sealing tape or expanding foam tape, moist internal air can penetrate into the construction, condense and cause damage.
In hot, humid climates infiltration of humid external air can cause construction damage.
Convection can be noticeable above hot radiators where warm air rises, or next to windows where there may be a cold downdraught. These effects can be positively exploited at a larger scale in systems such as displacement ventilation, and through passive building design through the ‘stack effect’.
Convective heat transfer, or ‘thermal bypass’, typically occurs in hollow external walls, party walls and basements that are formed using multi-cell masonry and cavity construction.
Why is Convection important in airtight building envelope design?