What is the Stack Effect

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What is the Stack Effect?
The Stack Effect- is the pressure differences in
homes caused by the difference in air temperature
between inside and outside. This pressure
difference is directly proportional to the
temperature difference and the height of the
home. Multiple points of air leakage causes a
pressure gradient throughout the home, negative
pressure in the bottom and positive pressure near
the top of the home. Its the pressure that
causes the air movement, not vice versa. The
pressures are tiny, but constant (24/7 all winter
long).
1 gaps between floor joists and inner leaf of
external wall can connect with gaps throughout
the building 2 gaps and poorly sealed membranes
around, but especially beneath windows and window
sills, leak direct to the outside or into the
cavity 3 leakage through window openings due to
ineffective or missing draught proofing, through
hollow (plastic or metal) frames themselves 4
leakage through doors, especially the meeting
stiles of double doors 5 gaps beneath and around
doors 6 cracks around skirting boards linked to
gaps around the edges of suspended floors
7 leakage through suspended floors, typically
bare timber floor boards 8 gaps around loft
hatches 9 leakage from eaves into attics often
via cavities and behind plasterboard, indirectly
into rooms 10 gaps around skylights, e.g. where
the skylight frame is not sealed to the adjacent
rafters 11 cracks where dissimilar structural
elements such as columns meet floor slabs 12
leakage through porous masonry leafs
13 gaps in the external wall at services entry 14
leakage around ceiling roses, recessed spotlights
and pullcord switches between a warm room and
roof space or intermediate floor 15 gaps around
heater flues (in walls and roofs) 16 small gaps
where water / heating pipes enter rooms from
floors, walls and boxed in spaces 17 gaps around
waste pipe penetrations e.g. behind toilets,
baths and kitchen sinks 18 service entry points
19 ensure chimney flues are installed and sealed
20 large gaps where soil pipes / ventilation
flues penetrate the roof 21 other roof
penetrations e.g. overflow pipes 22 gaps between
heated spaces and a cold loft where water pipes
and cables pass between, often in airing
cupboards 23 poorly sealed wall mounted extract
fans, also ducted extract from cooker hoods,
tumble driers etc. allow air directly into and
out of the room, but also into the cavity 24
chimneys and flues, if not sealed properly will
allow leakage at all times
As shown in the diagram, the red arrows at the
top indicate positive pressure and the blue
arrows at the bottom indicate negative pressure.
As you get closer to the middle, or neutral
pressure area, the movement becomes less.
Where h is the distance to the neutral plane or
roughly half the height of the house and delta
T is the temperature difference between the
inside and outside of the house.
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