Air layer thickness equivalent to water vapor diffusion

The air layer thickness equivalent to water vapor diffusion ( , sd value) is a building physics measure for the water vapor diffusion resistance of a component or component layer of a defined thickness and diffusion resistance number. ${\displaystyle s_{\mathrm {d} }}$

It describes the water vapor diffusion resistance clearly by specifying the thickness that a static layer of air must have so that the same diffusion current flows through it in the stationary state and under the same boundary conditions as the component under consideration.

The value of a component layer consisting of several successive layers is the sum of the values ​​of the individual layers . ${\displaystyle s_{\mathrm {d} }}$ ${\displaystyle s_{\mathrm {d} }}$

calculation

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The water vapor diffusion flow through a component layer can be described by the equation

${\displaystyle g\,=\,-{\frac {\delta }{\mu }}{\frac {\Delta p}{\Delta x}}\,=\,-{\frac {\delta }{\mu s}}\Delta p}$

${\displaystyle g}$

Water vapor diffusion flow density through the component in kg / (m ² s)

${\displaystyle \delta }$

Water vapor diffusion coefficient in air in kg / (m s Pa)

${\displaystyle \mu }$

Water vapor diffusion resistance coefficient

${\displaystyle \Delta p}$

Water vapor partial pressure gradient across the component in Pa

${\displaystyle \Delta x}$, ${\displaystyle s}$

Thickness of the component in m

If one writes the formula again for an air layer ( ) of the thickness and demands that the same diffusion flow should flow through it with the same partial pressure gradient as through the component under consideration, the comparison of both formulas results in the following condition for : ${\displaystyle \mu =1}$ ${\displaystyle s_{\mathrm {d} }}$ ${\displaystyle \Delta p}$ ${\displaystyle g}$ ${\displaystyle s_{\mathrm {d} }}$

${\displaystyle -{\frac {\delta }{\mu s}}\Delta p=-{\frac {\delta }{s_{\mathrm {d} }}}\Delta p}$,

from which it immediately follows:

${\displaystyle s_{\mathrm {d} }=\mu \cdot s}$

A static layer of air must therefore have the thickness to oppose the water vapor with the same diffusion resistance as the component under consideration with the water vapor diffusion resistance number and the layer thickness . ${\displaystyle s_{d}=\mu s}$ ${\displaystyle \mu }$ ${\displaystyle s}$

Temperature dependence

Most building materials are porous . Water vapor transport takes place in them mainly as diffusion in the pore air and is therefore subject to the same temperature dependence as diffusion in free air. Because the value compares the diffusion resistance in the building material with the resistance in free air, the temperature dependence is reduced and the value is a temperature-independent property of the material. ${\displaystyle s_{\mathrm {d} }}$ ${\displaystyle s_{\mathrm {d} }}$

But even if the temperature is not included as a direct factor in the formula for the equivalent air layer thickness, it should be borne in mind that the coefficient of resistance μ is dependent on the relative humidity and thus indirectly on the temperature.

application

According to DIN 4108-3 , the diffusion properties of certain components are classified as open, diffusion-inhibiting and -tight:

s d value	Degree of tightness	Resistance to water vapor diffusion
s d ≤ 0.5 m	open to diffusion	low
0.5 m <s d <1500 m	diffusion-inhibiting (vapor barrier)	medium
s d ≥ 1500 m	diffusion-tight (vapor barrier)	high

Components with an air layer thickness of 0.5 to 1500 m equivalent to water vapor diffusion are referred to as a vapor barrier , everything above that as a vapor barrier .

The air layer thickness equivalent to water vapor diffusion is also used as the abscissa axis in a Glaser diagram .

In general, the materials used in an outer wall should become more permeable from the inside to the outside.

From timber frame construction comes the rule of thumb that with planked cavity walls the value of the inner planking should be a factor of 7 to 10 higher than that of the outer air seal. This means that no condensate can form even under the most unfavorable conditions. ${\displaystyle s_{\mathrm {d} }}$

There is no need to install a highly diffusion-resistant layer (vapor barrier) on the inside of components if the building materials used are capable of capillary transport, e.g. B. capillary-active thermal insulation. If executed correctly, the condensation water that occurs in the component can be guided by capillary action to the inner and outer surface of the component and evaporate there or be stored by the building material and released back into the interior of the building. The same applies to wall structures in which sufficient ventilation is provided.

Examples

A 5 cm thick calcium silicate plate with a µ = 5 has a value of . So it is open to diffusion. ${\displaystyle s_{\mathrm {d} }}$ ${\displaystyle s_{\mathrm {d} }=\mu \cdot s=5\cdot 0{,}05\,\mathrm {m} =0{,}25\,\mathrm {m} }$

A 20 cm thick brickwork made of solid bricks with µ = 5 has one , i.e. This means that as much water vapor diffuses through a 20 cm thick solid brick wall as through a 1 m thick layer of static air. ${\displaystyle s_{\mathrm {d} }=\mu \cdot s=5\cdot 0{,}2\,\mathrm {m} =1\,\mathrm {m} }$

A 4 cm thick styrofoam plate with µ = 60 according to DIN EN ISO 10456 has a value of . ${\displaystyle s_{\mathrm {d} }}$ ${\displaystyle s_{\mathrm {d} }=\mu \cdot s=60\cdot 0{,}04\,\mathrm {m} =2{,}4\,\mathrm {m} }$

Wind brake foils, facade and sarking membranes should keep liquid water out, but let water vapor through. Sarking membranes for walls have a value of 0.2 m according to DIN EN ISO 10456 . ${\displaystyle s_{\mathrm {d} }}$

Common vapor retarders made of PE or PVC films have s _d values ​​of 2 to 50 m.

International use

In English-speaking countries, instead of the air layer thickness equivalent to water vapor diffusion, the Moisture Vapor Transmission Rate (MVTR) is predominantly used as a measure of the passage of water vapor.

literature

• Richard Jenisch, Heinz Klopfer, Hanns Freymuth, Karl Petzold, Martin Stohrer, Heinz M. Fischer, Ekkehard Richter: Textbook of Building Physics. Sound, heat, moisture, light, fire, climate. 5th edition. Teubner, Stuttgart 2002, ISBN 3-519-45014-3 (Section III, Chapter 3: Mechanisms of moisture transport).
• Moisture protection - Practical through extensive building material parameters, calculation aids & construction examples! , K. Schild, WM Willems
• DIN EN ISO 10456: 2010 . Building materials and building products - Thermal and moisture-related properties - Tabulated rated values ​​and methods for determining the thermal insulation nominal and rated values ​​(ISO 10456: 2007 + Cor. 1: 2009); German version EN ISO 10456: 2007 + AC: 2009. Beuth Verlag, Berlin 2010.

Individual evidence

1. ↑ according to DIN 4108 , part 3 (July 2001). Beuth-Verlag, Berlin 2001
2. ↑ WM Willems (ed.): Textbook of building physics. Sound - heat - humidity - light - fire - climate . 8th edition. Springer Vieweg, Wiesbaden 2017, ISBN 978-3-658-16073-9 .
3. ↑ Peter Cheret and Kurt Schwaner: Timber construction systems - an overview ; accessed in December 2016
4. ↑ U. Hestermann; L. Rongen: Frick / Knöll building construction theory 2 . 34th edition. Springer Vieweg, Wiesbaden 2013, ISBN 978-3-8348-1617-7 .
5. ↑ K. Bounin; W. Graf; P. Schulz: Handbook of building physics. Sound insulation, heat insulation, moisture protection, fire protection . 9th edition. Deutsche Verlags-Anstalt, Munich 2010, ISBN 978-3-421-03770-1 .
6. ↑ Less favorable value for a range of water vapor diffusion resistance μ: 5-20 z. B. for a two-shell masonry in the thaw period
7. ↑ M. Bonk (Ed.): Lufsky Bauwerkabdichtung . 7th edition. Vieweg + Teubner, Wiesbaden 2010, ISBN 978-3-8351-0226-2 .