Introduction: Why Moisture Protection is So Important
Moisture is one of the most common causes of building damage. Whether it's mold, corrosion, or heat loss due to wet insulation – water in the construction costs money and impacts health. This article explains the most important basics of moisture protection, which standards apply, and how planners and contractors can safely verify or avoid condensation.
1. Types of Moisture and Their Impact on Buildings
Buildings are exposed to various moisture influences. The most important ones are:
- From outside: Precipitation, splash water, pressing and non-pressing water, soil moisture, groundwater
- From use: Cooking, showering, drying laundry, plants – a 4-person household releases approx. 10.4 kg of water into the indoor air daily
- From construction: Construction moisture, water vapor, condensation
All these types of moisture must be taken into account during planning and execution.
2. Relevant Standards at a Glance
| Standard | Scope of Application |
|---|---|
| DIN 4108-3 | Climate-related moisture protection, condensation verification |
| DIN 4108-7 | Airtightness of buildings |
| DIN 18195 | Waterproofing of structures |
| DIN 18531–18535 | Waterproofing of roofs, traffic areas, ground, interiors, containers |
| DIN 4095 | Drainage for the protection of building structures |
DIN 4108-3 is introduced by building authorities – its verifications must already be submitted with the building application.
3. Understanding Relative Humidity and Condensation
Warm air can absorb more water than cold air. If air cools down – e.g., on a poorly insulated wall – it releases moisture as condensate. This is the basis for mold formation.
The formula for relative humidity:
Φ = c / cs
(c = water vapor concentration, cs = saturation humidity at a given temperature)
At 20°C and 50% relative humidity, air contains 8.65 g/m³ of water. If the temperature drops, the relative humidity rises – until the dew point, where condensate precipitates.
Important for mineral building materials (concrete, plaster, masonry): They absorb moisture and release it again (sorption). Above 60% relative humidity, capillary condensation begins – free water in the pores, which promotes mold.
4. The sd-Value and the µ-Value – Understanding Diffusion Resistance
The µ-value (water vapor diffusion resistance factor) describes how strongly a building material inhibits water vapor – in relation to air (µ = 1).
- Mineral wool: µ ≈ 1 → diffusion-open → needs a vapor barrier on the room side
- Foam glass: µ → ∞ → vapor-tight → no connection between the cells
The sd-value (water vapor diffusion equivalent air layer thickness) relates the µ-value to the actual layer thickness:
sd = m × µ (Unit: meter)
For multi-layer components, the sd-values of all layers are added. Basic rule for flat roofs: The sd-values must decrease from inside to outside so that moisture can escape!
5. Condensation Verification: The Glaser Method
The Glaser method is the normatively prescribed calculation method according to DIN 4108-3. It evaluates if and where condensation occurs in a construction – based on temperature and vapor pressure distribution.
Boundary conditions for the dew period (Dec–Feb, 90 days):
- Inside: 20°C, 50% rel. humidity → vapor pressure 1,168 Pa
- Outside: -5°C, 80% rel. humidity → vapor pressure 321 Pa
Boundary conditions for the evaporation period (Jun–Aug, 90 days):
- Inside and outside: Vapor pressure 1,200 Pa each
The method distinguishes four cases: no condensation, precipitation in one plane, in two planes, or in one area.
Condensation is harmless if:
- It dries out again during the evaporation period
- The amount of condensation remains ≤ 1.0 kg/m² (for foils/metals ≤ 0.5 kg/m²)
- For wood: Moisture content ≤ 5%, condensation ≤ 0.25 kg/m²
6. Constructions Not Requiring Verification
Not every construction needs to be verified by calculation. Exempt from verification are, among others:
- Single-shell masonry and normal concrete walls with exterior plaster or ETICS
- Walls with interior insulation (R ≤ 1.0 m²K/W, sd,i ≥ 0.5 m)
- Timber walls according to DIN 68800-2 with a diffusion-retarding layer (sd,i ≥ 2 m)
- Earth-contacting walls with perimeter insulation according to DIN 4108-2
- Warm roofs with a diffusion-retarding layer (sd,i ≥ 100 m)
7. Damage from Condensation – and How to Avoid It
Condensation damage arises from:
- Thermal bridges (e.g., window frames, balcony slabs)
- Insufficient insulation
- Faulty vapor barriers
- Incorrect user behavior (too little ventilation, furniture against exterior walls)
Practical tip: The fRsi-value indicates whether a component surface is mold-proof. The limit value is fRsi ≥ 0.7 (corresponds to ≥ 12.6°C surface temperature under standard conditions). Modern insulated walls easily meet this value.
