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Glaser method under DIN 4108-3: what it calculates and where it ends

6 min read

The Glaser method answers exactly one question: under defined winter conditions, does condensation form in the building element through vapour diffusion, and can it dry out again in summer? It is the core of the calculation procedure in DIN 4108-3 for moisture protection. Anyone reading a result should know what goes into the calculation and what does not.

Two seasons, fixed boundary conditions

The calculation is steady-state, that is with constant conditions over the whole period. The condensation period runs for 90 days from December to February, the evaporation period for 90 days from June to August. For summer, no outdoor temperatures are specified, only vapour pressures.

Period Outside Inside
Condensation period (December to February) −5 °C, 80% relative humidity, 321 Pa 20 °C, 50% relative humidity, 1,168 Pa
Evaporation period (June to August) Vapour pressure 1,200 Pa Vapour pressure 1,200 Pa; in the condensation planes of external walls, saturation vapour pressure 1,700 Pa

BauNetz Wissen reproduces the values in the table as normative boundary conditions; the page does not name an edition. That such values change between editions of the standard is shown by a look back: in 1999, the Fraunhofer IBP (Fraunhofer Institute for Building Physics) still described −10 °C outdoor temperature over 60 days for the standard valid at that time. Today DIN 4108-3 is available in the 2024-03 edition, which according to DIN Media replaces the 2018-10 edition. For a proof, the values of the edition agreed in the project count.

From the layer build-up, the temperatures are determined from inside to outside and from them the saturation vapour pressure. This is compared with the profile of the actual vapour pressure. Depending on the position of the two curves, the standard distinguishes four cases, from no condensation at all to condensation over a whole zone.

The result is a balance. First it is determined whether condensation forms, in which layer and in what quantity; the evaporation quantity of the summer is then set against it. The test is whether the condensation accumulated can dry out completely again in the evaporation period and how much drying reserve remains against moisture introduced unintentionally.

The role of the sd value

The sd value (the water vapour diffusion-equivalent air layer thickness) is the product of the layer thickness in metres and the water vapour diffusion resistance factor μ of the material; it describes how strongly a layer resists the diffusion of water vapour. For multi-layer building elements it is determined layer by layer and added up. It thereby determines where in the cross-section the vapour pressure drops and whether the vapour pressure curve and the saturation curve touch. For timber flat roofs, BauNetz Wissen formulates the rule that in the sequence of membranes or sheets the sd values must decrease from inside to outside.

For practice this means: a layer with a high sd value on the outside slows drying back in summer, one on the inside slows vapour entry in winter. How a coating manufacturer states the sd value is shown in the article on the V and W classes in the data sheet.

What counts as harmless

  • The water that accumulates in winter must be released to the surroundings again in the evaporation period.
  • Building materials that come into contact with the condensation must not be damaged, for example by fungal growth or corrosion.
  • In roof and wall constructions, the mass of condensation should generally not exceed 1.0 kg/m².
  • If the building element contains layers that do not absorb water by capillary action, a maximum of 0.5 kg/m² applies. These include membranes, metals, normal-weight concrete as well as mineral wool and plastic foam.

Not every wall has to be calculated at all. According to BauNetz Wissen, the proof is not required, for example, for external walls plastered on the room side with an approved external thermal insulation composite system (ETICS) and for internal insulation with internal plaster up to R ≤ 1.0 m²K/W, if the build-up additionally has an sd,i value of at least 0.5 m. The text of the standard remains decisive. In § 11, the Building Modernisation Act (Gebäudemodernisierungsgesetz, GModG) still names the edition DIN 4108-3: 2018-10 for buildings to be constructed.

What the calculation does not see

The period balance method is a heavily simplified verification procedure for diffusion processes. BauNetz Wissen lists as not taken into account:

  • realistic, varying climatic boundary conditions,
  • increased moisture loads, for example from construction moisture or after building damage,
  • influences from radiant heat,
  • moisture-dependent material properties such as diffusion resistance and thermal conductivity,
  • moisture storage and transport through sorption and capillary conduction.

Liquid water transport is likewise left out. The diffusion calculation also does not cover convection: through leaks on the room side, warm, moist indoor air can flow into the construction. Equilibrium moisture content and user behaviour are also missing. For air-conditioned residential or residential-type uses, the calculation approaches of the standard expressly do not apply.

According to BauNetz Wissen, the standard values themselves, for example −5 °C over 90 days, do not appear realistic in all parts of Germany, but contain safety margins.

Existing walls: where the result gets thin

With an existing wall, the build-up and properties are often known only approximately, and the wall is rarely as dry as in the calculation model. The method works with fixed properties. Water that enters the wall as driving rain or through capillarity does not appear in the calculation; this follows from the missing liquid water transport.

If the simplified method is not sufficient for an adequately accurate assessment, DIN 4108-3 provides for hygrothermal simulation in accordance with DIN EN 15026. It simulates the behaviour of the building element over several years under realistic climatic boundary conditions. Where existing walls are to be insulated from the inside, the article on internal insulation in existing buildings explains how verifications and the WTA leaflets (WTA: Scientific-Technical Association for Building Preservation and Monument Conservation) work together.

A Glaser diagram that passes therefore proves that diffusion remains uncritical under standard conditions. It does not prove that the wall is dry, that the airtightness is right or that driving rain is kept out. For existing walls, the calculation is only as reliable as the assumption about the layers present.

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