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(This page was automatically translated from German. Diffusion)

The description of the sd-Wertes (diffusion-equivalent air-layer-thickness)

The ability of construction-materials to be permeable for steam is described by the steam-diffusion-resistance-number. How a wall is diffusion-capable, depends on the materials and the thickness of its layers. As diffusion-resistance of a layer, one declares the air-layer-thickness in meters, that would put up the same resistance to the diffusion (exchange of steam and air-molecules), like the relevant layer. The lower the value, the the steam is braked less on the way of the warm one to the cold side. For with open pores constructions, a low μ-Wert is advantageous since the dehumidification can run out unhindered and fast. A statement about the effect of a material in a given construction is possible only with simultaneous consideration of the thickness of the material.

One gets the value of this diffusion-equivalent air-layer-thickness (abbreviated sd), if one the value of the steam-diffusion-resistance-number (μ) with the layer-thickness in meters multiplies.

It however only testifies this μ-Faktor how well water vapor (radius of 0,14-0,16 nm) diffuses in a material in the relationship to air (radius of 0,2-0,28 nm)[7], but nothing over the diffusivities of water. So, a cement-finery leaves (μ = 20, the water vapor relatively well through, so however it closes waters considerably well (relatively big water-molecules with a radius of 0,28 nm [6]). This nevertheless (steam -) is diffusion-openly applicable to materials, that don't let pass any water, however, is, for example plastic-cement-finery, Dispersion-paints. Clay or not too highly burned Bricks leaves steam, however, and also liquid Water diffuse. [5]

A 36,5 cm thick wall-layer from bricks (μ = 8) therefore would have an equivalent air-layer-thickness of sd = 8 x 0,365 m = 2,92 m.

With the plasterboard-disguise [4] yields a sd-Wert for itself = 8 x 0,0125 m = 0,1 m

Paint-materials have rather high values normally. Since however, the layer-thicknesses with few tenth are millimeters only very low, they hardly impair the diffusion-ability of the wall in general. However however, it becomes the Sorption (Steam-reception, influences, and that is important exactly for interiors to the reduction of the dampness-tops of the area-air.
So, the sd-Wert is a 0,5 corresponds mm thick glue-colorsstack [4] with. = 180... 215, sd = 200 x 0,0005 m = 0,1 m.
If an art-resin-dispersion-paint becomes [4] with. = 1800 uses, so the sd-Wert lies with 0,9 m. With an oil-pedestal-paint with. = 10.000 is this 5 m. It clearly becomes with it why a moisture-film (condensation) could refine exactly at this wall-coating in the kitchen earlier after cooking and that situated finery is usually more defective at old buildings for itself among them. With what should more material-particular qualities than only the steam-diffusion-resistance-number be taken into account, however.

Assess the steam-diffusion-resistance-number under

  • . = 10 shows a very good diffusion-ability for steam;
  • middle diffusion-values are 10 - 50;
  • with values of 50 - 500, the steam-diffusion is restricted;
  • with 500 - 15.000, it is restricted strongly;
  • a material works steam-closing from 15.000;
  • from 100.000, a material is steam-dense.
  • Into the direction of the diffusion it is determined by the absolute dampness-salary of the air and is directional with cold outside-air outside so. It is not dependent on in the direction of the heat-stream, she/it can be this inversely directional. Steam can therefore not out-diffuse any dew-water-formation of the surface despite strong cooling on the other side of the wall without changing its physical state. However, this changes if a dense layer is on the cold side and such a dust-empire refines where a steam-repletion-pressure is built. Then, a condensation can be caused. [3]
    With too low sd-Wert at an outside-wall, the dew point can be in the wall in clammy weather for the outside diffusing area-humidity already under relatively normal conditions. At the place, the to imbrue of the wall then begins all the more strongly the smaller its diffusion-ability is.
    To outside-walls, values therefore are applicable between sd = 4 and sd = 7 as especially favorable.

    To the avoidance of a steam-condensation with more-layer-y prefabricated parts, shall from indoors outside the Heat-river-resistance increases (Stack the insulation outside), and the Steam-resistances decrease. These conditions become lime-sand, full-stones, at the wall-construction, inner-finery, for example, Mineral-wool, Air-layer, facade-plate (wall with hinterlüfteter facade), filled. If the wall remains moist, this is to be heeded exactly at the reconstruction, so the water can behind a waterproof one or - escapes rejecting outside-layer, how it can be the case with the thermo-skin, for example, badly through diffusion, and the stonework is damaged over long time. [1] this problem appears also in the developed roof-projectile, here the condensation gathers between the inner-disguise of plasterboard or similar and this Steam-brake. It then drips into the area. It then is assumed that the roof is leaky. (Therefore, what could also be conceivably, is to be determined exactly the cause.)

    There plasterboard-construction-plates not very strong is, also the sd-Wert is low (see above). According to color-coating (type and strength), the sd-Wert possibly lies at most 10 m between 1. The steam-brake even has a sd-Wert of 100 according to type... 200 m and however, the steam-diffusion is stopped not restricted with it. Exactly if another high construction-dampness is available after the reconstruction, the Dämmung is moistened by diffusion more strongly than intended so. In the following picture, a roof-slant is shown with the Anschuss at a gable-wall. The steam-brake was not connected joint-densely, the same is applicable also to pushes. Despite this the plasterboard-plate joint-densely, with acrylic or joint-band, at which wall was connected, moisture (diffusion) reaches into the cavity, between plate and steam-brake. Through joints in the steam-brake, a hypotension is built, because of the existing steam pressure-inclination. The warm interior-air is striving to reach outside. With wind-load we the wind-suction still reinforces. This has an effect also on the diffusion. Through examinations from Wagner about steam-transportation because of from diffusion and convection was determined that with a sd-Wert = 10 m and middle diffusion of every 10 hours on a surface of 60 m2 for example the content one schnapps-glassful through this prefabricated part diffuses. [2]

    Picture steam-diffusion and convection at the dammed roof

    [1] Kur, Friedrich; Wohngifte, Handbuch für gesundes Bauen und Einrichtungen, 3. Aufl. Verlag Eichborn, 1993, S.568
    [2] Colling, Francois; Lernen aus Schäden im Holzbau, Deutsche Gesellschaft für Holzforschung Innovations- und Service GmbH, 2000, S. 132
    [3] Eichler, Arndt; Bautechnischer Wärme- und Feuchteschutz, Bauverlag 1989, S. 95
    [4] König, Holger; Wege zum gesunden Bauen, S. 95 ff
    [5] Oberrauch, Berhard; Bauphysikalische Daten über Lehm, Wohnung + Gesundheit 9/92 Nr. 64 S. 48/49
    [6] Zürcher, Christoph, Frank Thomas; Bauphysik Bau und Energie, 2.Aufl. 2004, vdf Hochschulverlag AG an der ETH Zürich, S. 53
    [7] Arndt, Horst; Wärmeschutz und Feuchteschutz in der Praxis, 2002, Verlag Bauwesen Berlin, S. 30

    Ingenieurbuero Peter Rauch

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