Building Physics II1
behaviour
• liquid water behaves in good approximation as an incompressible fluid with a density of ρl ≈ 1000 kg/m3
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• weather
– leaking pipes – broken seals – …
© howtobuildahouseblog.com 4
moisture content
• the moisture content is the mass of moisture per unit volume of material – symbol: w – SI unit: kg/m3
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moisture content at saturation
• the moisture content at saturation is the maximum mass of moisture per unit volume of material – symbol: wsat
– SI unit: kg/m3
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moisture ratio
• the moisture ratio is the mass of moisture per unit mass of material – symbol: X – SI unit: kg/kg
for wood-based materials
volumetric moisture ratio
• the volumetric moisture ratio is the volume of moisture per unit volume of material – symbol: ψ – SI unit: m3/m3
for highly porous materials
• moisture ratio vs. volumetric moisture ratio
• summary:
• use the moisture content w [kg/m3] for stone-like materials
• use the moisture ratio X [kg/kg] for wood-based materials
• use the volumetric moisture ratio ψ [m3/m3] for highly porous materials
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degree of moisture saturation
• the degree of moisture saturation is the ratio between the current and the maximum amount of moisture – symbol: Sm
– SI unit: dimensionless
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0 0.2 0.4 0.6 0.8
1
ee o
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ee o
• hygroscopic curve is a material characteristic
hygroscopic materials take up a lot of water before RH=1. Non-hygroscopic materials do not
BRICK MORTAR
capillary condensation
• consider a cylindrical pore with radius r, filled partially by gas and partially by liquid :
( )( ) ( )( )rrPP lg παγπ 2cos2 =−
gP lP2r γ α
PP lg =−
α = contact angle γ = surface tension Pg = gas pressure Pl = liquid pressure
Laplace’s law
capillary condensation
• the saturated vapor pressure above a curved meniscus differs from the one above a flat one
lP lP lP
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( )

−=
αγ cos2exp
capillary condensation
4.0E-09 m
capillary condensation
• capillary condensation – starts in the small pores – occurs in large pores only at very high RH
• consequences – a material with many small pores will take up
moisture at low RH → hygroscopic materials – a material with only large pores will only take up
water at high RH → non-hygroscopic materials
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1
ee o
BRICK MORTAR
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co nt
rib ut
io n
to S
m pe
r po
re si
ze [l
og -1
(P a)
1
ee o
• hygroscopic curve describes the «moisture buffering capacity» of a material
• hygroscopic materials can contribute to stabilize RH in the indoor climate – adsorb moisture when RH in environment is high – release moisture when RH in environment is low
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1
ee o
Question
• What is the impact of moisture (condensed water) on the thermal conductivity of insulation?
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1E-20
1E-18
1E-16
1E-14
1E-12
1E-10
1E-08
– due to gravity
transport due to capillarity
• imagine a tube with radius r, in contact with a water surface
2r
transport due to capillarity
• EXAMPLE: how much is the capillary rise of water in a tube with radius 0.001 m? (α=0°, ρl=1000 kg/m3, g=9.81 m/s2, γ=0.0725 J/m2)
( ) gr
transport due to capillarity
• EXAMPLE: how much is the capillary rise of water in a tube with radius 0.00001 m? (α=0°, ρl=1000 kg/m3, g=9.81 m/s2, γ=0.0725 J/m2)
( ) gr
transport due to capillarity
• capillary rise is most pronounced in materials with fine pores, such as many cementitious materials
© Ilan Kelman 2000
plaster/render – decrease of material
• solution: install/foresee a moisture barrier (aka DPC – damp proofing course)
inside outside
ground basement/ground
33 www.wbdg.org
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transport due to capillarity
• capillarity works in all directions! – in vertical direction, it is countbalanced by gravity – in horizontal direction, … not
-20
-15
-10
-5
brick
mortar
transport due to capillarity
(1) run-off
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liquid transport
– due to gravity
transport due to gravity
• in wide pores, gravitational forces dominate over capillary forces and cause flow
capillary forces
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plaster/render – decrease of material
• solution: – foresee a moisture barrier on horizontal surfaces – design architectural details carefully
outside
inside
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transport due to gravity
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– due to gravity
transport due to pressure
• the pressure at a point depends on the height of the water column above
• therefore, the water level in two connected water columns is always equal
ghPP lgl ρ+=
transport due to pressure
• solution: a water-thight barrier – at the outside: the only good solution – at the inside: only a cosmetic solution – most
likely the problems will get worse
© brehmann.de
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transport due to pressure ©
- one layer is simultaneously wind and rain barrier
- if the joint sealant degrades the pressure difference across the wall will cause leakage
− aP+
aP
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(1) water thightness (rain)
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summary transport
• capillary suction is connected to the pore width and is mainly active in small pores
• gravity is important in pores that show limited capillary suction, i.e. large pores
• air as well as water induced pressure differences are mainly important for sealants, material interfaces and fractures
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moisture content